Conifers, Angiosperm Trees, and Lianas: Growth,
Whole-Plant Water and Nitrogen Use Efficiency, and
Stable Isotope Composition (d13C and d18O) of Seedlings
Grown in a Tropical Environment1[W][OA]
Lucas A. Cernusak2*, Klaus Winter, Jorge Aranda, and Benjamin L. Turner
Smithsonian Tropical Research Institute, Balboa, Ancon, Republic of Panama
Seedlings of several species of gymnosperm trees, angiosperm trees, and angiosperm lianas were grown under tropical field
conditions in the Republic of Panama; physiological processes controlling plant C and water fluxes were assessed across this
functionally diverse range of species. Relative growth rate, r, was primarily controlled by the ratio of leaf area to plant mass, of
which specific leaf area was a key component. Instantaneous photosynthesis, when expressed on a leaf-mass basis, explained
69% of variation in r (P, 0.0001, n5 94). Mean r of angiosperms was significantly higher than that of the gymnosperms; within
angiosperms, mean r of lianas was higher than that of trees. Whole-plant nitrogen use efficiency was also significantly higher in
angiosperm than in gymnosperm species, and was primarily controlled by the rate of photosynthesis for a given amount of leaf
nitrogen. Whole-plant water use efficiency, TEc, varied significantly among species, and was primarily controlled by ci/ca, the
ratio of intercellular to ambient CO2 partial pressures during photosynthesis. Instantaneous measurements of ci/ca explained
51% of variation in TEc (P , 0.0001, n 5 94). Whole-plant
13C discrimination also varied significantly as a function of ci/ca (R
2 5
0.57, P , 0.0001, n 5 94), and was, accordingly, a good predictor of TEc. The
18O enrichment of stem dry matter was primarily
controlled by the predicted 18O enrichment of evaporative sites within leaves (R2 5 0.61, P , 0.0001, n 5 94), with some residual
variation explained by mean transpiration rate. Measurements of carbon and oxygen stable isotope ratios could provide a useful
means of parameterizing physiological models of tropical forest trees.
Tropical forest ecosystems have been subject to ex-
tensive perturbations associated with anthropogenic
activity in recent decades, and such perturbations will
likely continue into the foreseeable future (Laurance
et al., 2004; Wright, 2005). Effective environmental
management requires knowledge of how such pertur-
bations impact upon cycling of carbon (C) and water
between forest trees and the atmosphere, and how
these C and water fluxes relate to plant nutrient status.
A sound, mechanistic understanding of the physio-
logical processes that control photosynthesis and tran-
spiration in tropical trees is therefore essential for
understanding and managing the human impact upon
tropical forests. In this study, we analyzed the phys-
iological controls over growth (the relative rate of C
accumulation), nitrogen (N) use efficiency (NUE; the
rate of C accumulation for a given N content), water
use efficiency (the ratio of whole-plant C gain to water
loss), and stable isotope composition (d13C and d18O) in
seedlings of a diverse suite of species grown side-
by-side in a tropical environment.
Conifers dominated the world?s forests prior to the
Cretaceous radiation in angiosperm diversity. How-
ever, conifers are largely absent from the lowland
tropical and subtropical forests of today. It has been
suggested that one means by which angiosperm tree
species are able to out-compete gymnosperm tree
species in tropical environments is through faster
seedling growth caused by improved hydraulic effi-
ciency (Bond, 1989; Brodribb et al., 2005). Angiosperm
xylem tissue contains vessels, specialized water-
conducting cells that are generally larger in diameter,
and therefore more conductive to water, than conifer
tracheids (Sperry et al., 2006). Conifer tracheid diame-
ters are biomechanically constrained because these cells
must perform the dual function of conducting water
and providing structural support to woody tissues,
whereas vessels need not perform the latter function in
angiosperm wood. Lianas are large woody vines that
occur predominantly in tropical forests; by attaching
themselves to neighboring trees, they have evolved an
additional means of freeing their xylem tissues from
structural constraints. Thus, angiosperm lianas may
achieve further increases in hydraulic efficiency com-
pared to angiosperm trees (Gartner et al., 1990).
1 This work was supported by the Smithsonian Tropical Research
Institute. L.A.C. was supported by a postdoctoral fellowship from
the Smithsonian Institution and a Tupper Research Fellowship from
the Smithsonian Tropical Research Institute.
2 Present address: School of Environmental and Life Sciences,
Charles Darwin University, Darwin, Northern Territory 0909, Australia.
* Corresponding author; e-mail lucas.cernusak@cdu.edu.au.
The author responsible for distribution of materials integral to the
findings presented in this article in accordancewith the policy described
in the Instructions for Authors (www.plantphysiol.org) is: Lucas A.
Cernusak (lucas.cernusak@cdu.edu.au).
[W] The online version of this article contains Web-only data.
[OA] OpenAccess article can be viewed onlinewithout a subscription.
www.plantphysiol.org/cgi/doi/10.1104/pp.108.123521
642 Plant Physiology, September 2008, Vol. 148, pp. 642?659, www.plantphysiol.org  2008 American Society of Plant Biologists
In this study, we grew seedlings of several species of
gymnosperm trees, angiosperm trees, and angiosperm
lianas in a tropical environment. We used this func-
tionally diverse range of species to quantify the phys-
iological controls over their C and water fluxes. We
also took advantage of the contrasting physiology of
the study species to test the theoretical basis for var-
iation in the C and oxygen (O) stable isotope compo-
sition of plant dry matter.
THEORY
Growth
Following Masle and Farquhar (1988), and based on
earlier treatments (Blackman, 1919; Evans, 1972), we
write the following expression to describe factors that
influence the relative rate of C accumulation of a plant:
r5
1
mc
 dmc
dt
5
Al?1 2fc?
r
?1?
where r is relative growth rate (mol C mol21 C s21), mc
is plant C mass (mol C), t is time (s), A is leaf pho-
tosynthetic rate (mol C m22 s21), l is the light period as
a fraction of 24 h, fc is the proportion of C gained in
photosynthesis that is subsequently used for respira-
tion by leaves at night and by roots and stems during
day and night, and r is the ratio of plant C mass to leaf
area (mol C m22). Equation 1 provides a useful tool
for examining sources of variation in r among plant
species and individuals within a species. It is similar to
the classical decomposition of r into net assimilation
rate (NAR; gm22 s21) and leaf area ratio (LAR; m2 g21),
but allows the assimilation term to be expressed as a
net photosynthetic rate, such as would be measured
using standard gas exchange techniques (Long et al.,
1996). Table I provides definitions of all abbreviations
and symbols used in this article.
NUE
Multiplying both sides of Equation 1 by the molar
ratio of plant C to N yields an expression for the NUE
of C accumulation:
NUE5
1
mn
 dmc
dt
5Anl?1 2fc?nl ?2?
where NUE is whole-plant NUE (mol C mol21 N s21),
mn is plant N mass (mol N), An is photosynthetic NUE
(mol Cmol21 N s21), and nl is the proportion of plant N
allocated to leaves. Equation 2 provides a basis for
linking An, a trait often quantified in ecophysiological
investigations, with NUE, an integrated measure of
NUE at the whole-plant level.
Transpiration Efficiency and C Isotope Discrimination
The ratio of C gain to water loss at the leaf level
during photosynthesis can be expressed as the ratio of
the diffusive fluxes of CO2 and water vapor into and
out of the leaf, respectively (Farquhar and Richards,
1984):
A
E
5
ca 2 ci
1:6n
?3?
where E is transpiration (mol H2O m
22 s21), ca and ci
are CO2 partial pressures in ambient air and leaf
intercellular air spaces, respectively, v is the leaf-to-
air vapor pressure difference, and 1.6 is the ratio of
diffusivities of CO2 and H2O in air. The v is defined as
ei-ea, where ei and ea are the intercellular and ambient
vapor pressures, respectively. The ratio of C gain to
water loss can be scaled to the whole-plant level by
taking into account respiratory C use and water loss
not associated with photosynthesis (Farquhar and
Richards, 1984; Hubick and Farquhar, 1989):
TEc5
?1 2fc?ca 1 2
ci
ca
 
1:6n?11fw?
?4?
where TEc is the transpiration efficiency of C gain, and
fw is unproductive water loss as a proportion of water
loss associated with C uptake, the former mainly
comprising water loss at night through partially
open stomata. Thus, fw can be approximated as En/
Ed, where En is nighttime transpiration and Ed is
daytime transpiration.
We suggest that the leaf to air vapor pressure
difference, v, can be written as the product of the air
vapor pressure deficit (D), and a second term, fv,
which describes the magnitude of v relative to D, such
that v 5 Dfv. This allows Equation 3 to be written as
D  TEc5
?1 2fc?ca 1 2
ci
ca
 
1:6fv?11fw?
?5?
Weighting TEc by D facilitates comparison of the
transpiration efficiency of plants grown under differ-
ent environmental conditions by accounting for vari-
ation due to differences in atmospheric vapor pressure
deficit (Tanner and Sinclair, 1983; Hubick and Farquhar,
1989). Thus, it accounts for variation in TEc that is
purely environmental. TheDTEc has units of Pa mol C
mol21 H2O.
Photosynthetic discrimination against 13C (D13C)
shares a common dependence with TEc on ci/ca. The
D13C in C3 plants relates to ci/ca according to the
following equation (Farquhar et al., 1982; Farquhar
and Richards, 1984; Hubick et al., 1986):
D13C5 a 2 d1 ?b 2 a? ci
ca
?6?
where a is the discrimination against 13C during dif-
fusion through stomata (4.4&), b is discrimination
against 13C during carboxylation by Rubisco (29&),
and d is a composite term that summarizes collectively
the discriminations associated with dissolution of CO2,
Growth, Water Use, and Stable Isotopes in Tropical Trees
Plant Physiol. Vol. 148, 2008 643
Table I. Abbreviations and symbols used in the text
A Area-based photosynthesis rate (mmol CO2 m
22 s21)
Am Mass-based photosynthesis rate (nmol CO2 g
21 s21)
An Photosynthetic NUE (mmol CO2 mol
21 N s21)
a Discrimination against 13C during diffusion through stomata
b Discrimination against 13C during carboxylation by Rubisco
C Molar concentration of water (mol m23)
ca Partial pressure of CO2 in ambient air (Pa)
ci Partial pressure of CO2 in leaf intercellular air spaces (Pa)
D Vapor pressure deficit of ambient air (kPa)
Dg Growth-weighted vapor pressure deficit of ambient air (kPa)
Di Average daytime vapor pressure deficit of ambient air during week i (kPa)
D18 Diffusivity of H2
18O in water (m2 s21)
d Discrimination against 13C during C3 photosynthesis not associated with a or b
E Transpiration rate (mmol m22 s21)
Ed Daytime transpiration rate (mmol m
22 s21)
Et Cumulative transpiration over the course of the experiment (mol)
En Nighttime transpiration rate (mmol m
22 s21)
ea Vapor pressure of ambient air (kPa)
ei Vapor pressure in leaf intercellular air spaces (kPa)
gs Stomatal conductance (mol m
22 s21)
L Scaled effective path length relating to 18O advection and diffusion (m)
LAR Leaf area ratio (m2 kg21)
LA1 Leaf area at the initiation of the experiment (m
2)
LA2 Leaf area at the conclusion of the experiment (m
2)
l Light period as a fraction of 24 h
lc Mass of C in leaf litter abscised during the experiment (mol C)
mc Plant C mass (mol C)
mc1 Plant C mass at the initiation of the experiment (mol C)
mc2 Plant C mass at the conclusion of the experiment (mol C)
mn Plant N mass (mol N)
MTR Mean transpiration rate over the course of the experiment (mol m22 d21)
NAR Net assimilation rate (g dry matter m22 s21)
NUE Whole-plant N use efficiency (mol C mol21 N d21)
n1 Leaf N as a proportion of whole-plant N
Parea Leaf P per unit area (mmol m
22)
pex Proportion of O atoms exchanging with medium water during cellulose synthesis
px Proportion of water in developing cells not subject to evaporative
18O enrichment
R The 18O/16O ratio of any water or dry matter component of interest
Ra The
13C/12C ratio of CO2 in ambient air
Rp The
13C/12C ratio of plant C
Rs The
18O/16O ratio of irrigation (source) water
r Relative growth rate (mol C mol21 C d21)
SLA Specific leaf area (m2 kg21)
TEc Whole-plant transpiration efficiency of C gain (mmol C mol
21 H2O)
v Leaf-to-air vapor pressure difference (kPa)
vg Growth-weighted leaf-to-air vapor pressure difference (kPa)
vi Average daytime leaf-to-air vapor pressure difference for week i (kPa)
wi Predicted dry matter increment for week i (g)
D13C Discrimination against 13C
D13Cp Discrimination against
13C in dry matter of the whole plant relative to CO2 in air
D18Oe The
18O enrichment of evaporative sites within leaves compared to source water
D18Oeg Growth-weighted prediction of D
18Oe over the course of the experiment
D18Oei Predicted average daytime D
18Oe for week i
D18OL The
18O enrichment of average lamina leaf water compared to source water
D18Op The
18O enrichment of stem dry matter compared to source water
D18Ov The
18O enrichment of atmospheric water vapor compared to source water
d13C The 13C/12C ratio relative to the PeeDee Belmnite international standard
d13Ca The d
13C of CO2 in ambient air
d13Cp The d
13C of plant C
d18O The 18O/16O ratio relative to Vienna Standard Mean Ocean Water
d18Op The d
18O of stem dry matter
d18Os The d
18O of irrigation (source) water
(Table continues on following page.)
Cernusak et al.
644 Plant Physiol. Vol. 148, 2008
liquid phase diffusion, photorespiration, and dark
respiration (Farquhar et al., 1989a). The term d may
be excluded from Equation 5, in which case the re-
duction in D13C caused by d is often accounted for by
taking a lower value for b. The D13C is defined with
respect to CO2 in air as D
13C 5 Ra/Rp 2 1, where Ra is13C/12C of CO2 in air and Rp is
13C/12C of plant
C. Combining Equations 5 and 6 gives
D  TEc5
ca?1 2fc??b 2 d 2D
13C?
1:6fv?11fw??b 2 a?
?7?
Equation 7 suggests a negative linear dependence of
TEc (or DTEc) on D
13C, although it can be seen that
there are many other terms in Equation 7 that have the
potential to influence the relationship between the two.
O Isotope Enrichment
It has been suggested that measurements of the O
isotope enrichment of plant organic material (D18Op)
can provide complementary information to that in-
ferred from D13C in analyses of plant water-use effi-
ciency (Farquhar et al., 1989b, 1994; Sternberg et al.,
1989; Yakir and Israeli, 1995). Specifically, D18Op could
provide information about the ratio of ambient to in-
tercellular vapor pressures, ea/ei, and thus about the
leaf-to-air vapor pressure difference, ei-ea, during pho-
tosynthesis. Note that ei-ea is equal to v in Equation 3.
In the steady state, water at the evaporative sites in
leaves becomes enriched in 18O relative to water en-
tering the plant from the soil, according to the follow-
ing relationship (Craig and Gordon, 1965; Dongmann
et al., 1974; Farquhar and Lloyd, 1993):
D18Oe5 e
1 1 ek1 ?D18Ov 2 ek?
ea
ei
?8?
where D18Oe is the
18O enrichment of evaporative site
water relative to source water, e1 is the equilibrium
fractionation between liquid water and vapor, ek is the
kinetic fractionation that occurs during diffusion of
water vapor out of the leaf, and D18Ov is the discrim-
ination of ambient vapor with respect to source water.
The D18O of any water or dry matter component is
defined with respect to source water (water entering
the roots from the soil) as D18O 5 R/Rs 2 1, where
D18O is the 18O enrichment of the component of inter-
est and R and Rs are the
18O/16O ratios of the compo-
nent of interest and source water, respectively. The e1
can be calculated as a function of leaf temperature
(Bottinga and Craig, 1969), and ek can be calculated by
partitioning the resistance to water vapor diffusion
between stomata and boundary layer, with the two
weighted by appropriate fractionation factors (Farquhar
et al., 1989b; Cappa et al., 2003). The D18Ov can be
calculated from measurements of the d18O of ambient
vapor and source water. If such data are not available,
a reasonable approximation is to estimate D18Ov as
2e1, whichmeans that ambient vapor is assumed to be
in isotopic equilibrium with soil water. An up-
to-date summary of equations necessary for parame-
terization of Equation 8 can be found in Cernusak et al.
(2007b).
Average lamina leaf water 18O enrichment (D18OL) is
generally less than that predicted for evaporative site
water (Yakir et al., 1989; Flanagan, 1993; Farquhar
et al., 2007), and carbohydrates exported from leaves
have been observed to carry the signal of D18OL rather
than D18Oe (Barbour et al., 2000b; Cernusak et al., 2003,
2005; Gessler et al., 2007). The D18OL has been sug-
gested to relate to D18Oe according to the following
relationship (Farquhar and Lloyd, 1993; Farquhar and
Gan, 2003):
D18OL5D
18Oe
?1 2 e2??
?
?9?
where ? is a Pe?clet number, defined as EL/(CD18),
where E is transpiration rate (mol m22 s21), L is a
scaled effective path length (m), C is the molar con-
centration of water (mol m23), and D18 is the diffusiv-
ity of H2
18O in water (m2 s21). The C is a constant, and
D18 can be calculated from leaf temperature (Cuntz
et al., 2007). The constancy of L, or otherwise, is
currently under investigation (Barbour and Farquhar,
2004; Barbour, 2007; Kahmen et al., 2008; Ripullone
et al., 2008). If L is assumed relatively constant, Equa-
tion 9 predicts that D18OL will vary as a function of
both D18Oe and E. To test for an influence of E on D
18OL,
it is necessary to first account for variation in D18OL
caused by D18Oe (Flanagan et al., 1994). To this end, the
relative deviation ofD18OL fromD
18Oe (12 D
18OL/D
18Oe)
can be examined, in which case Equation 9 can be
written as
1 2
D18OL
D18Oe
5 1 2
?1 2 e2??
?
?10?
Table I. (Continued from previous page.)
ecp The d
18O difference between plant dry matter and cellulose extracted from it
ek Kinetic H2
18O fractionation for diffusion through stomata and leaf boundary layer
ewc The
18O enrichment of cellulose compared to the water in which it formed
e1 Equilibrium H2
18O fractionation during the phase change from liquid to gas
fc Proportion of net photosynthesis subsequently used for respiration
fv Scaling factor to convert D to v (5 v/D)
fw Unproductive water loss as proportion of that associated with photosynthesis
? Pe?clet number
r Plant C mass per unit leaf area (mol C m22)
Growth, Water Use, and Stable Isotopes in Tropical Trees
Plant Physiol. Vol. 148, 2008 645
Equation 10 predicts that 1 2 D18OL/D
18Oe should
increase as E increases.
The transfer of the leaf water 18O signal to plant
organic material can be described by the following
equation (Barbour and Farquhar, 2000):
D18Op5D
18OL?1 2 pexpx?1 ewc1 ecp ?11?
where D18Op is
18O enrichment of plant dry matter, pex
is the proportion of O atoms that exchange with local
water during synthesis of cellulose, a primary constit-
uent of plant dry matter, px is the proportion of
unenriched water at the site of tissue synthesis, ewc is
the fractionation between organic oxygen and me-
dium water, and ecp is the difference in D
18O between
tissue drymatter and the cellulose component. For tree
stems, pexpx has been found to be relatively constant at
about 0.4 (Roden et al., 2000; Cernusak et al., 2005). The
ewc is relatively constant at about 27& (Barbour, 2007),
and for stem dry matter, ecp appears to be relatively
constant at about 25& (Borella et al., 1999; Barbour
et al., 2001; Cernusak et al., 2005). If these assumptions
are valid, variation in D18Op should primarily reflect
variation in D18OL. Thus, combining Equations 10 and
11 provides a means of testing for an influence of E on
D18Op, assuming that D
18Op provides a time-integrated
record of D18OL (Barbour et al., 2004):
1 2
D18Op 2 ewc 2 ecp
1 2 pexpx
 
D18Oe
5 1 2
?1 2 e2??
?
?12?
RESULTS
Growth, Photosynthesis, and Elemental Concentrations
Daytime meteorological conditions over the course
of the experiment are shown in Table II. Dates of
initiation of transpiration measurements and harvest
for each species are shown in Table III. Table III also
shows the initial and final dry masses, in addition to
root/shoot ratios. Variation in relative growth rate, r,
among species is shown in Figure 1A; variation in the
components of r, A, and 1/r, is shown in Figure 1, B
and C, respectively. The r varied significantly among
functional groups (P , 0.0001), and among species
within functional groups (P , 0.0001). Gymnosperm
trees had the lowest mean value of r, whereas angio-
sperm lianas had the highest mean value; angiosperm
trees had a mean value of r intermediate between that
of gymnosperm trees and angiosperm lianas (Fig. 1A).
In contrast, there was less variation among species and
functional groups in instantaneous photosynthesis
rates expressed on a leaf area basis (Fig. 1B), although
the species Pinus caribaea and Stigmaphyllon hypargyreum
were notable for having relatively high values of A.
Variation in r tended to be more closely associated
with variation in 1/r than with variation in A. Gym-
nosperm trees had the lowest mean value of 1/r,
whereas angiosperm trees had an intermediate mean
value, and angiosperm lianas had the highest mean
value (Fig. 1C). The liana species S. hypargyreum pos-
sessed swollen, tuberous roots, which caused it to have
a root/shoot ratio much higher than any other species
in the study (Table III), and to have a reduced 1/r
relative to the other two liana species (Fig. 1C).
Variation in instantaneous photosynthesis, when
expressed on a leaf mass basis, was a good predictor
of variation in r (Fig. 2). The former was measured
over several minutes, whereas the latter was measured
over several months. Mass-based photosynthesis, Am,
is the product of A (mol m22 s21) and specific leaf area
(SLA; m2 kg21). The correlation between Am and r was
almost entirely driven by variation in SLA, because A
on a leaf area basis was not significantly correlated
with r (P 5 0.14, n 5 94).
The C and N concentrations of leaves, stems, roots,
and whole plants for each species are given in Sup-
plemental Table S1. For whole-plant C concentration,
there was significant variation, both among functional
groups (P , 0.0001), and among species within func-
tional groups (P , 0.0001), as shown in Figure 3A.
Gymnosperm trees had a mean C concentration of
49.6%, significantly higher than angiosperm trees and
lianas. Angiosperm trees and lianas did not differ from
each other in whole-plant C concentration, and had
mean values of 45.4% and 44.9%, respectively. For
whole plant N concentration, there was also significant
variation among functional groups (P , 0.0001) and
among specieswithin functional groups (P, 0.0001), as
shown in Figure 3B. Angiosperm lianas had the highest
mean whole-plant N concentration at 1.22%, followed
by angiosperm trees at 1.01%, then by gymnosperm
trees at 0.82%. Accordingly, there was significant
variation among functional groups (P , 0.0001) and
Table II. Average daytime meteorological conditions at the study site over the course of the experiment
Values are monthly means of measurements taken every 15 min between the hours of 7 AM and 5:30 PM local time. We focused on daytime hours to
characterize conditions during photosynthetic gas exchange.
2005 2006
June July Aug Sept Oct Nov Dec Jan Feb March April May
Air temperature (C) 28.0 26.7 28.6 29.6 30.0 27.7 30.7 28.1 27.9 29.1 28.7 28.2
Relative humidity (%) 81.3 82.9 82.6 83.3 79.3 84.2 75.0 72.5 68.8 68.3 74.1 80.1
Vapor pressure deficit (kPa) 0.73 0.62 0.71 0.70 0.89 0.60 1.12 1.06 1.18 1.31 1.03 0.78
Wind speed (m s21) 0.33 0.26 0.31 0.33 0.45 0.29 0.50 0.61 0.87 0.82 0.75 0.38
Photon flux density (mmol m22 s21) 685 655 649 628 750 602 743 808 859 925 854 685
Cernusak et al.
646 Plant Physiol. Vol. 148, 2008
among species within functional groups (P , 0.0001)
in whole-plant C/N mass ratio (Fig. 3C). Gymno-
sperm trees had the highest mean whole-plant C/N at
64.7 g g21, followed by angiosperm trees at 49.6 g g21,
then by angiosperm lianas at 39.0 g g21.
Concentrations of phosphorus (P), calcium (Ca), and
potassium (K), and the N/P mass ratio in leaf dry mat-
ter for each species are shown in Table IV. There was
significant variation among functional groups (P ,
0.0001) and among species within functional groups
(P, 0.0001) for all elements and for N/P. Angiosperm
lianas tended to have higher mean concentrations of P,
Ca, and K in their leaf dry matter than angiosperm and
gymnosperm trees. The mean leaf N/P was higher in
angiosperm trees than in gymnosperm trees or angio-
sperm lianas; mean values were 9.3, 4.9, and 4.7 g g21,
respectively.
When expressed on a leaf area basis, the leaf P
concentration was significantly correlated with mean
transpiration rate (MTR) across all individuals (R2 5
0.24, P , 0.0001, n 5 94). The equation relating the
two was Parea 5 0.096MTR 1 2.75, where Parea is in
mmol m22, and MTR is in mol m22 d21.
NUE
Equation 2 presents a means for analyzing variation
among functional groups and species in whole-plant
NUE (mol C mol21 N s21). We calculated NUE as the
product of r and mc/mn, the whole-plant C to N molar
ratio; thus, a relatively high C/N has the effect of
increasing NUE. Figure 4A shows variation among
species in NUE. There was significant variation among
Table III. Experimental time period, initial and final plant dry mass, and root to shoot ratio for each species in the study
Values for final dry mass and root to shoot ratio are given as the mean, with the SD in parentheses. An SD is not given for P. guatemalensis because
only one plant survived for this species. Full species names are given in Figure 2. NA, Not applicable.
Species Family Start Date End Date
No. of
Plants
Initial Dry
Mass
Final Dry
Mass
Root to Shoot
Ratio
g g g g21
Gymnosperm tree species
C. lusitanica Cupressaceae May 23, 2005 Dec. 13, 2005 8 5.8 60.2 (19.0) 0.36 (0.06)
P. caribaea Pinaceae May 23, 2005 Dec. 13, 2005 8 5.6 64.6 (30.9) 0.27 (0.09)
P. guatemalensis Podocarpaceae May 23, 2005 May 11, 2006 1 0.8 107.3 (NA) 0.16 (NA)
T. occidentalis Cupressaceae April 26, 2004 Dec. 13, 2005 8 4.7 41.4 (16.5) 0.47 (0.08)
Angiosperm tree species
C. longifolium Clusiaceae July 11, 2005 March 10, 2006 6 3.1 62.6 (27.0) 0.39 (0.08)
C. pratensis Clusiaceae Aug. 22, 2005 May 11, 2006 6 0.2 112.2 (62.3) 0.76 (0.26)
H. alchorneoides Euphorbiaceae June 20, 2005 Dec. 13, 2005 6 0.2 36.6 (30.2) 1.10 (0.13)
L. seemannii Tiliaceae Aug. 29, 2005 March 10, 2006 7 0.1 39.8 (7.3) 0.61 (0.10)
P. pinnatum Fabaceae Nov. 7, 2005 March 10, 2006 2 0.6 53.9 (16.6) 0.58 (0.20)
P. pinnatum Fabaceae Nov. 7, 2005 May 11, 2006 3 0.6 21.6 (13.3) 0.36 (0.14)
S. macrophylla Meliaceae Nov. 7, 2005 May 11, 2006 7 0.8 24.3 (11.8) 0.34 (0.10)
T. rosea Bignoniaceae June 20, 2005 Dec. 13, 2005 6 0.6 64.8 (11.0) 0.92 (0.12)
T. grandis Verbenaceae May 17, 2004 May 11, 2006 7 0.1 45.5 (11.3) 0.93 (0.17)
Angiosperm liana species
G. lupuloides Rhamnaceae Aug. 29, 2005 March 10, 2006 6 0.01 31.5 (15.6) 0.47 (0.20)
M. leiostachya Asteraceae Aug. 29, 2005 March 10, 2006 3 0.1 21.2 (4.3) 0.37 (0.27)
M. leiostachya Asteraceae Nov. 14, 2005 May 11, 2006 3 0.3 23.2 (4.8) 0.25 (0.06)
S. hypargyreum Malphigiaceae Aug. 29, 2005 March 10, 2006 7 0.1 50.3 (20.3) 1.99 (0.21)
Figure 1. A to C, Variation among spe-
cies in mean relative growth rate (A),
net photosynthesis, expressed on a leaf
area basis (B), and leaf area per unit
plant C mass, 1/r (C). Error bars repre-
sent 1 SE. Sample sizes for each species
are given in Table III.
Growth, Water Use, and Stable Isotopes in Tropical Trees
Plant Physiol. Vol. 148, 2008 647
functional groups (P , 0.0001) and among species
within functional groups (P , 0.0001). However, un-
like results for r, angiosperm trees and lianas did not
differ from each other with respect to NUE (P 5 0.84).
In contrast, NUE of gymnosperm trees was lower than
that of both angiosperm trees (P , 0.0001) and angio-
sperm lianas (P , 0.0001). Figure 4, B and C, shows
variation among species in the NUE components, An
and nl. Variation in NUE among species tended to
reflect variation in An, the photosynthetic NUE (Fig.
4B). The An was also higher in angiosperm trees and
lianas than in gymnosperm trees (Fig. 4B). This vari-
ation in An was offset to a lesser extent by variation in
nl, the proportion of plant N allocated to leaves (Fig.
4C). Gymnosperm trees had highest mean nl, at 0.69,
followed by angiosperm trees at 0.59, then by angio-
sperm lianas at 0.50. Thus, a higher allocation of N to
leaves in gymnosperm trees compensated to some
extent for their much lower An. However, the An was
still the dominant control over NUE (Fig. 5).
Transpiration Efficiency
Mean values for each species for TEc, the whole-
plant transpiration efficiency of C gain, are shown in
Table V. Also shown in Table V are the growth-
weighted estimates of the daytime vapor pressure
deficit, Dg, by species. There was significant variation,
both among functional groups (P, 0.0001), and among
species within functional groups (P , 0.0001), in both
TEc and Dg. However, across the full data set, TEc and
Dg were not significantly correlated (P 5 0.11, n 5 94),
suggesting that Dg was not a primary control over TEc.
Taking the product of Dg and TEc allows analysis of
variation in TEc independently of variation in Dg, as
articulated in Equation 5. The DgTEc also varied
significantly among functional groups (P , 0.0001)
and among species within functional groups (P ,
0.0001). Angiosperm trees had the highest mean
DgTEc at 1.58 Pa mol C mol
21 H2O, followed by an-
giosperm lianas at 1.30 Pa mol C mol21 H2O, then by
gymnosperm trees at 1.11 Pa mol C mol21 H2O.
Among all species, there was a 3.7-fold variation in
DgTEc (i.e. the largest species mean was 3.7 times the
smallest species mean).
Table V summarizes for each species the compo-
nents of DgTEc that we quantified: fv, the ratio of leaf-
to-air vapor pressure difference to air vapor pressure
deficit; fw, the ratio of unproductive to productive
water loss; and ci/ca, the ratio of intercellular to
ambient CO2 partial pressures during photosynthesis.
Although there was a 1.8-fold variation among species
in fv, this parameter did not appear to be a primary
control over DgTEc: the term 1/fv explained only 13%
of variation in DgTEc (R
2 5 0.13, P 5 0.0004, n 5 94);
moreover, the slope of the relationship between DgTEc
and 1/fv was negative, opposite to that predicted by
Equation 5. The parameter fw similarly did not ap-
pear to exert a strong control over DgTEc: although
DgTEc was positively correlated with 1/(11 fw) (R
25
0.18, P , 0.0001, n 5 92), there was only a 1.1-fold
variation in this term among species, suggesting that it
could only account for a variation in DgTEc of ap-
proximately 10%. In contrast, the ci/ca appeared to be
the primary control over DgTEc. Among species, there
was a 2.3-fold variation in instantaneous measure-
ments of 12 ci/ca, and ci/ca explained 46% of variation
in DgTEc. Regression coefficients are given in Table VI.
Furthermore, instantaneous measurements of 12 ci/ca
explained 64% of variation in the composite term
vgTEc(1 1 fw) (R
2 5 0.64, P , 0.0001, n 5 94). Taking
this product means that only the variables fc, ca, and
ci/ca remain on the right side of Equation 5.
Variation in instantaneousmeasurements of ci/ca was
largely driven by variation in stomatal conductance,
gs, rather than by variation in photosynthesis, A. If gs
Figure 2. Mean relative growth rate plotted against instantaneous
measurements of photosynthesis expressed on a leaf mass basis. White
symbols with internal cross-hairs refer to gymnosperm tree species;
completely white symbols refer to angiosperm liana species; black
symbols and black symbols with internal cross-hairs refer to angio-
sperm tree species.
Cernusak et al.
648 Plant Physiol. Vol. 148, 2008
controls variation in ci/ca, then ci/ca should decrease as
1/gs increases. The 1/gs is equivalent to the stomatal
resistance. In contrast, if A controls ci/ca, then ci/ca
should decrease as A increases. Figure 6A shows that
instantaneous ci/ca decreased as a linear function of
1/gs. In contrast, Figure 6B shows that there was a
weak tendency for ci/ca to increase as A increased, op-
posite to the trend that would be expected if A were
controlling ci/ca.
We used measurements of leaf temperature, taken
with a hand-held infrared thermometer, to calculate
values of fv for the species harvested on the second
and third harvest dates (Table III). We then compared
these instantaneous measurements of fv with our
time-integrated estimates for each plant based on
leaf energy balance predictions and meteorological
data. The time-integrated estimates of fv compared
favorably with the instantaneous measurements of fv
(R2 5 0.42, P , 0.0001, n 5 55), thus providing some
validation of the former.
Stable Isotope Composition
The C isotope composition of leaves, stems, roots,
and whole plants is shown for each species in Table
VII. Also shown is the difference in d13C between
leaves and the sum of stems plus roots, the heterotro-
phic component of the plant. Across all individuals,
leaf d13C was more negative than stem d13C (P ,
0.0001, n 5 94) and root d13C (P , 0.0001, n 5 94),
whereas stem d13C was more negative than root d13C,
but by a much smaller amount (P 5 0.0008, n 5 94);
mean values for leaf, stem, and root d13C were 229.4,
228.1, and 227.8&, respectively.
Table IV. Leaf P, Ca, and K concentrations, and N/P ratios of experimental plants
Values are given as the mean for each species, with the SD in parentheses. No SD is given for
P. guatemalensis because only one individual of this species survived. Sample sizes for the other species
ranged from five to eight individuals, as shown in Table III. NA, Not applicable.
Species P Ca K N/P
g kg21 g kg21 g kg21 g g21
Gymnosperm tree species
C. lusitanica 3.10 (0.76) 8.3 (0.9) 20.9 (2.8) 3.6 (0.8)
P. caribaea 1.43 (0.47) 3.6 (1.4) 9.6 (1.6) 7.6 (1.4)
P. guatemalensis 2.48 (NA) 8.1 (NA) 16.9 (NA) 5.9 (NA)
T. occidentalis 4.24 (0.86) 10.5 (2.2) 16.8 (1.5) 3.4 (0.8)
Angiosperm tree species
C. longifolium 0.94 (0.23) 7.7 (0.8) 8.4 (1.4) 12.4 (1.5)
C. pratensis 1.38 (0.47) 13.6 (1.1) 14.4 (5.2) 9.7 (2.1)
H. alchorneoides 2.22 (0.59) 12.5 (3.9) 24.0 (4.3) 6.8 (1.5)
L. seemannii 3.59 (0.81) 18.0 (2.1) 14.2 (1.6) 5.0 (1.4)
P. pinnatum 1.68 (0.23) 10.2 (2.1) 16.7 (4.3) 18.2 (5.7)
S. macrophylla 1.33 (0.35) 13.4 (1.5) 20.3 (3.0) 12.0 (2.8)
T. rosea 1.43 (0.04) 14.4 (4.3) 15.1 (4.4) 11.1 (2.2)
T. grandis 5.96 (0.42) 8.6 (1.0) 13.3 (2.1) 2.0 (0.2)
Angiosperm liana species
G. lupuloides 5.33 (1.15) 14.4 (2.1) 24.5 (2.4) 4.4 (0.9)
M. leiostachya 2.66 (0.30) 13.2 (1.4) 29.5 (3.5) 6.6 (1.2)
S. hypargyreum 6.23 (1.09) 23.4 (3.3) 24.6 (4.0) 3.5 (0.5)
Figure 3. A to C, Variation among spe-
cies in the C concentration of dry matter
on a whole-plant basis (A), the N con-
centration of dry matter on a whole-
plant basis (B), and the C/N mass ratio
of dry matter on a whole-plant basis (C).
Error bars represent 1 SE. Sample sizes
for each species are given in Table III.
Growth, Water Use, and Stable Isotopes in Tropical Trees
Plant Physiol. Vol. 148, 2008 649
We converted plant d13C values to 13C discrimina-
tion by assuming d13C of atmospheric CO2 to be 28&.
Whole-plant D13C, D13Cp, covered a range from 18.8&
to 22.9& among species, corresponding to d13Cp values
ranging from226.3& to230.2& (Table VII). There was
significant variation in D13Cp among functional groups
(P 5 0.002) and among species within functional
groups (P, 0.0001). TheD13Cp was lower in angiosperm
trees than in gymnosperm trees, whereas angiosperm
lianas did not differ significantly from angiosperm or
gymnosperm trees. Mean values were 21.0&, 21.2&,
and 21.5& for angiosperm trees, angiosperm lianas,
and gymnosperm trees, respectively.
The D13Cp was significantly correlated with instan-
taneous measurements of ci/ca (Fig. 7), as predicted by
Equation 6. We estimated the term d of Equation 6 by
least-squares regression by assuming fixed values for a
and b of 4.4& and 29&, respectively. This resulted in
an estimate for d of 3.1&; the regression equation
explained 57% of variation in D13Cp. Thus, the predic-
tive power of this relationship was equivalent to that
obtained with a standard linear regression, in which
both the slope and intercept are free to vary (Fig. 7).
Using the mean estimate of 3.1& for d, and values of
4.4& and 29& for a and b, respectively, we calculated a
D13Cp-based estimate of ci/ca for each plant. Mean
values of these estimates for each species are given in
Table V. There was a 2.4-fold variation among species
in the D13Cp-based estimates of 1 2 ci/ca.
Variation in D13Cp was significantly correlated with
variation in DgTEc (Fig. 8); the former explained 49%
of variation in the latter. Regression coefficients and
the coefficient of determination for least-squares linear
regressions of TEc, DgTEc, and vgTEc against D
13C of
leaves, stems, roots, and whole plants are given in
Table VI. In general, whole-plant D13C was a better
predictor of variation in TEc than D
13C of leaves, stems,
or roots individually. Additionally, weighting of TEc
by Dg or vg tended to result in modest increases in the
proportion of variation explained by the regression
models (Table VI).
Correlations between D13Cp and 1/gs and A further
supported the conclusion that variation in ci/ca was
largely driven by variation in gs. The D
13Cp decreased
as a linear function of 1/gs (Fig. 6C). In contrast, the
D13Cp showed a weak tendency to increase as a func-
tion of A (Fig. 6D), opposite to the trend that would be
expected if A controlled variation in ci/ca.
Variation among species in the O isotope composi-
tion of stem dry matter is given in Table VII. We
calculated the 18O enrichment above source water of
stemdrymatter,D18Op, from themean d
18O of irrigation
water of 24.3&. The observed D18Op was significantly
correlated with the predicted 18O enrichment of evap-
orative site water, D18Oe, weighted by predicted
weekly growth increments (Fig. 9). We tested whether
the residual variation in D18Op, after accounting for var-
iation in D18Oe, was related to transpiration rate by
plotting 1 2 [(D18Op 2 ewc 2 ecp)/(1 2 pexpx)]/D
18Oe
against the MTR. As shown in Equation 12, this
term should increase with an increasing transpiration
rate if there is a significant Pe?clet effect. Our analysis
detected a significant relationship between the two
Figure 4. A to C, Variation among spe-
cies in whole-plant NUE (A), photosyn-
thetic NUE (B), and nl, the leaf N
content as a proportion of whole-plant
N content (C). Error bars represent 1 SE.
Sample sizes for each species are given
in Table III.
Figure 5. Whole-plant NUE plotted against photosynthetic NUE.
Whole-plant NUE was calculated from mean relative growth rate,
measured over several months, whereas photosynthetic NUE was
calculated from instantaneous photosynthesis measurements, taken
over several minutes. Different symbols refer to different species, as
detailed in Figure 2.
Cernusak et al.
650 Plant Physiol. Vol. 148, 2008
parameters (R2 5 0.14, P5 0.0002, n5 94), supporting
the notion of a significant Pe?clet effect, although there
was considerable scatter in the relationship. Using the
MTR and En/Ed, we calculated a daytime MTR, then
used the nonlinear regression routine in SYSTAT to
solve for an average value of L, the scaled effective
path length, across the full data set. This analysis
estimated a mean value of L for the full data set of
53 mm, with the 95% confidence interval ranging from
43 to 62 mm.
DISCUSSION
In this article, we present a comprehensive compar-
ison of physiological processes in seedlings of conifers,
angiosperm trees, and angiosperm lianas under trop-
ical field conditions. The comparison yielded novel
insights into physiological differences among these
functional groups, when grown in a tropical environ-
ment. For example, we observed that liana species, on
average, had higher 1/r than tree species, and that this
trait was associated with faster growth. We also ob-
served that gymnosperm trees had significantly lower
whole-plant NUE than both angiosperm trees and
angiosperm lianas. In addition, the results provided an
integrated account of the physiological controls over
growth, whole-plant water and NUE, and stable iso-
tope composition across the full range of species.
Relative growth rate, r, was mainly controlled by
variation in 1/r, the amount of assimilative surface
area for a given plant biomass (Fig. 1). Of the compo-
nents of 1/r, the SLA played a key role, such that the
product of SLA and instantaneous photosynthesis, A,
was a strong predictor of variation in r (Fig. 2). The
whole-plant NUE was mainly controlled by An, the
photosynthetic rate for a given amount of leaf N (Fig.
5). An increase in the proportional allocation of N to
leaves, nl, in species with low An was observed; how-
ever, the increased nl compensated to only a relatively
modest extent for low An (Fig. 4). The primary control
over the transpiration efficiency of C uptake, TEc, was
ci/ca, the ratio of intercellular to ambient CO2 partial
pressures during photosynthetic gas exchange (Tables
V and VI). The ci/ca was also the primary control over
whole-plant 13C discrimination, D13Cp (Fig. 7), such
that variation in D13Cp was closely correlated with
variation in TEc (Table VI; Fig. 8). The ci/ca, in turn,
was largely controlled by stomatal conductance, gs
(Fig. 6). The 18O enrichment of stem dry matter, D18Op,
was primarily controlled by the predicted 18O enrich-
ment of the evaporative sites within leaves, D18Oe,
during photosynthetic gas exchange (Fig. 9). Variation
in leaf transpiration rate further explained some of the
residual variation in D18Op not accounted for by vari-
ation in D18Oe.
Growth and NUE
We observed that the term 1/r was the primary
control over variation in r, and that A was a relatively
conservative parameter among species (Fig. 1). These
Table V. Transpiration efficiency and related parameters for each species
Symbol definitions are as follows: transpiration efficiency of C uptake (TEc); growth-weighted daytime vapor pressure deficit (Dg) and leaf-to-air
vapor pressure difference (vg); the ratio of nighttime to daytime transpiration (En/Ed); and the ratio of intercellular to ambient CO2 partial pressures
(ci/ca). The ci/ca is given as the value measured with a portable photosynthesis system (instantaneous), or as the value estimated from whole-plant
13C discrimination (D13Cp-based). Values are given as the mean for each species, with the SD in parentheses. No SD is given for P. guatemalensis
because only one individual of this species survived. Sample sizes for the other species ranged from five to eight individuals, as shown in Table III. NA,
Not applicable.
Species TEc Dg vg fv (5 vg/Dg) fw (5 En/Ed)
Instantaneous
ci/ca
D13Cp-Based
ci/ca
mmol C
mol21 H2O
kPa kPa
Gymnosperm tree species
C. lusitanica 1.29 (0.20) 0.76 (0.01) 0.90 (0.07) 1.17 (0.08) 0.11 (0.02) 0.78 (0.04) 0.80 (0.02)
P. caribaea 1.20 (0.19) 0.76 (0.01) 0.72 (0.05) 0.95 (0.07) 0.04 (0.02) 0.86 (0.03) 0.88 (0.02)
P. guatemalensis 3.48 (NA) 1.04 (NA) 1.71 (NA) 1.64 (NA) 0.04 (NA) 0.63 (NA) 0.63 (NA)
T. occidentalis 1.85 (0.19) 0.76 (0.01) 0.92 (0.03) 1.21 (0.05) 0.03 (0.03) 0.80 (0.02) 0.78 (0.01)
Angiosperm tree species
C. longifolium 1.83 (0.24) 1.05 (0.02) 1.53 (0.12) 1.46 (0.10) 0.04 (0.04) 0.76 (0.04) 0.78 (0.05)
C. pratensis 2.16 (0.22) 1.02 (0.01) 1.42 (0.06) 1.39 (0.06) 0.01 (0.01) 0.76 (0.03) 0.73 (0.01)
H. alchorneoides 1.50 (0.20) 0.83 (0.01) 1.37 (0.04) 1.65 (0.05) 0.08 (0.01) 0.80 (0.02) 0.85 (0.01)
L. seemannii 0.98 (0.15) 1.20 (0.00) 1.43 (0.20) 1.19 (0.16) 0.05 (0.01) 0.80 (0.03) 0.81 (0.02)
P. pinnatum 2.76 (0.62) 1.12 (0.10) 1.53 (0.28) 1.39 (0.36) 0.02 (0.01) 0.73 (0.09) 0.71 (0.06)
S. macrophylla 1.07 (0.22) 1.05 (0.01) 1.21 (0.03) 1.15 (0.03) 0.07 (0.01) 0.86 (0.02) 0.88 (0.02)
T. rosea 1.92 (0.24) 0.82 (0.00) 1.37 (0.18) 1.67 (0.21) 0.11 (0.02) 0.75 (0.06) 0.79 (0.03)
T. grandis 0.84 (0.07) 0.98 (0.01) 1.20 (0.08) 1.23 (0.09) 0.08 (0.01) 0.88 (0.03) 0.81 (0.03)
Angiosperm liana species
G. lupuloides 0.96 (0.34) 1.25 (0.01) 1.37 (0.08) 1.10 (0.06) 0.04 (0.01) 0.84 (0.03) 0.80 (0.02)
M. leiostachya 0.89 (0.23) 1.11 (0.10) 1.39 (0.20) 1.25 (0.12) 0.07 (0.02) 0.79 (0.05) 0.80 (0.06)
S. hypargyreum 1.35 (0.14) 1.21 (0.01) 1.18 (0.06) 0.97 (0.05) 0.03 (0.01) 0.82 (0.03) 0.82 (0.02)
Growth, Water Use, and Stable Isotopes in Tropical Trees
Plant Physiol. Vol. 148, 2008 651
results agree with those presented previously for 24
herbaceous species (Poorter and Remkes, 1990; Poorter
et al., 1990), and for many woody species (Cornelissen
et al., 1996; Atkin et al., 1998; Wright and Westoby,
2000). For the herbaceous species, SLA was also a key
component of 1/r, such that variation in r was not
correlated with variation in A, but was strongly cor-
related with variation in Am, the product of A and SLA
(Poorter et al., 1990). It should be noted that A was
measured near the end of the experiment, and our
results thus do not preclude the possibility that vari-
ation in A may have modulated r earlier in plant
development. Nonetheless, our measurements of Am
explained more than two-thirds of total variation in r
(Fig. 2). Among the species that we grew, the gymno-
sperm trees generally had lowest 1/r and r, whereas
angiosperm lianas had highest 1/r and r, with angio-
sperm trees intermediate between the two (Fig. 1). This
pattern suggests that variation in 1/r could be related
to hydraulic efficiency, with the tracheid-bearing gym-
nosperm species constrained by a lower hydraulic
conductance for a given plant mass, and thereby
requiring a greater plant mass to support a given
amount of assimilative, and thus evaporative, surface
area. On the other hand, the angiosperm liana species,
having freed themselves from the constraint of struc-
tural self-sufficiency, and possessing hydraulically ef-
ficient vessels, might then have required a smaller
plant mass to deliver water to a given leaf surface area.
Measurements of whole-plant hydraulic conductance
per unit plant mass would be necessary to confirm this
hypothesis. However, it would be consistent with
differences in hydraulic conductivity observed previ-
ously between gymnosperm and angiosperm seed-
lings (Brodribb et al., 2005).
As shown in Equation 1, the term fc, the proportion
of net C fixation used for respiration, has potential to
influence r. It was previously observed for 24 herba-
ceous species that fc ranged from about 0.5 to 0.3, and
that r was negatively correlated with fc, as predicted
by Equation 1 (Poorter et al., 1990). Although we did
not measure fc in our study, we can speculate that the
slower-growing species had higher values, because
they generally had higher whole-plant C concentra-
tions (Fig. 3A), which would correlate with higher
tissue construction costs (Vertregt and Penning de
Vries, 1987; Poorter, 1994). The r was negatively cor-
related with whole-plant C concentration across the
full data set (R2 5 0.35, P , 0.0001, n 5 94).
The leaf N/P mass ratios that we observed (Table
IV) were generally low for tropical vegetation (Reich
and Oleksyn, 2004). However, they are consistent with
a previous study conducted under similar conditions,
but with variable amounts of rice (Oryza sativa) husk
mixed into the experimental soil (Cernusak et al.,
2007b). At a similar rice husk/soil mixture as used in
this study, we previously observed a mean leaf N/P
ratio of 5.9 for Ficus insipida (Cernusak et al., 2007b),
whereas the overall mean for all species in this study
was 7.2. The generally low leaf N/P ratios suggest
that plant growth in this study was constrained pri-
marily by N availability, rather than by P availability
(Koerselman and Meuleman, 1996; Aerts and Chapin,
2000). This is consistent with the addition of rice husks
increasing the C/N ratio of the experimental soil,
thereby favoring microbial immobilization of soil N,
Table VI. The proportion of variation in transpiration efficiency explained by 13C discrimination and
instantaneous ci/ca
Linear regression equations were fitted with the following parameters alternatively used as dependent
variables: transpiration efficiency of C uptake (TEc); the product of TEc and growth-weighted vapor pressure
deficit (DgTEc); and the product of TEc and growth-weighted leaf-to-air vapor pressure difference (vgTEc).
Independent variables were whole-plant 13C discrimination (D13C); D13C of leaves, stems, or roots
individually; and instantaneous ci/ca. For each analysis, n5 94. All regression coefficients were significant
at P , 0.0001.
Dependent
Variable
Regression Coefficient
Model R2
Intercept
Whole-Plant
D13C &
Leaf
D13C &
Stem
D13C &
Root
D13C &
Instantaneous
ci/ca
TEc 7.76 20.298 0.45
DgTEc 8.40 20.330 0.49
vgTEc 14.4 20.594 0.53
TEc 7.04 20.253 0.38
DgTEc 8.03 20.300 0.48
vgTEc 12.9 20.501 0.45
TEc 6.87 20.262 0.40
DgTEc 7.42 20.290 0.45
vgTEc 12.8 20.528 0.50
TEc 6.66 20.255 0.36
DgTEc 6.53 20.251 0.32
vgTEc 11.8 20.487 0.40
TEc 7.30 27.27 0.51
DgTEc 7.21 27.20 0.46
vgTEc 13.6 214.6 0.62
Cernusak et al.
652 Plant Physiol. Vol. 148, 2008
and thus reducing N availability to plants. This pro-
vided a useful experimental basis for comparing NUE
among the species in our study, because N was likely
the nutrient most limiting plant growth.
We calculated whole-plant NUE as the product of r
and mc/mn, the whole-plant molar ratio of C to N.
Although the gymnosperm species had higher C/N
than the angiosperm species (Fig. 4), they were still at a
marked disadvantage with respect to NUE. Such dis-
advantage resulted primarily from a lower An (Figs. 4
and 5). This difference between gymnosperm and
angiosperm seedlings, whereby the former employed
Nmuch less efficiently than the latter for accumulating
C, could be decisive in determining competitive out-
comes between the two. Although productivity in
tropical forests is generally considered P limited, it
has also been reported that N availability can constrain
tree growth in both montane (Tanner et al., 1998) and
lowland (LeBauer and Treseder, 2008) tropical forests.
Thus, a higher NUE may contribute to angiosperm
dominance in tropical environments. The lowAn in the
gymnosperm species compared to the angiosperm
species resulted primarily from lower SLA, because
A was generally similar between the two groups, and
leaf N concentration was lower in gymnosperms than
in angiosperms (Supplemental Table S1).
Transpiration Efficiency
The species included in the study exhibited a large
variation in the transpiration efficiency of C uptake,
TEc (Table V). This is consistent with previous results
showing large variation in TEc among seven tropical
tree species (Cernusak et al., 2007a). When the TEc for
each individual plant was normalized according to its
growth-weighted mean daytime vapor pressure defi-
cit, Dg, the variation among species was still apparent,
suggesting that Dg was not a primary control over TEc.
Additionally, the relative ranking among species in
this study was consistent with results for three species
that were also measured previously (Cernusak et al.,
2007a); in this study Platymiscium pinnatum had the
highest DgTEc, Swietenia macrophylla an intermediate
value, and Tectona grandis the lowest value (3.05, 1.12,
and 0.82 Pa mol C mol21 H2O, respectively). Previ-
ously, we observed that TEc for these three species was
3.97, 2.88, and 1.63 mmol C mol21 H2O, respectively
(Cernusak et al., 2007a).
In this study, we were able to confirm that ci/ca was
the primary control over DgTEc. The fv also showed a
moderate variation among species, suggesting that it
could be an important source of variation in DgTEc
(Table V); however, the fv tended to be negatively
correlated with ci/ca, due to a mutual dependence of
the two parameters on gs. Thus, it appeared that
variation in fv mostly served to dampen what would
have been the full effect of variation in ci/ca on DgTEc.
For example, plants with low gs tended to have low
ci/ca (Fig. 6), which would increase DgTEc, as shown
in Equation 5. All else being equal, the low gs would
also cause leaf temperature to increase, thereby in-
creasing fv, which would then cause a counteracting
Figure 6. A to D, The top two panels show instanta-
neous measurements of ci/ca plotted against stomatal
resistance (A) and photosynthesis (B); the bottom two
panels show whole-plant 13C discrimination plotted
against stomatal resistance (C) and photosynthesis (D).
Stomatal resistance is the inverse of stomatal conduc-
tance. Different symbols refer to different species, as
described for Figure 2.
Growth, Water Use, and Stable Isotopes in Tropical Trees
Plant Physiol. Vol. 148, 2008 653
decrease in DgTEc. However, it is clear from the large
variation in DgTEc and its correlation with ci/ca (Table
VI) that variation in fv only dampened, and did not
completely cancel, the effect of ci/ca on DgTEc. Of the
other terms in Equation 5, we found that fw, the un-
productive water loss as a proportion of that associ-
ated with photosynthesis, played only a minor role in
modulating DgTEc, in agreement with previous
results (Cernusak et al., 2007b). Finally we consider
variation in 1 2 fc: although this term is an important
functional trait, and may play an important role in
modulating r, it likely plays a lesser role in controlling
DgTEc than ci/ca. For example if fc varied among
species from 0.3 to 0.5 (Poorter et al., 1990), the term
1 2 fc would vary from 0.5 to 0.7, whereas we ob-
served variation in 12 ci/ca from 0.12 to 0.27 (Table V).
Thus, the formerwould be associatedwith a 1.4-fold var-
iation in DgTEc, and the latter with a 2.3-fold variation
in DgTEc.
There was significant variation in DgTEc among the
plant functional groups that we studied, such that
angiosperm trees had highest DgTEc, on average,
and gymnosperm trees lowest, with angiosperm lianas
intermediate between the two. Angiosperm trees also
had an advantage over gymnosperm trees if water-use
efficiency was analyzed as TEc, vgTEc, ci/ca, or D
13Cp.
Thus, in addition to having an advantage over gym-
nosperm seedlings in NUE, angiosperm seedlings
may also have a competitive advantage in terms of
water-use efficiency, when grown in tropical environ-
ments. However, it should be emphasized that our
experiment was carried out under well-watered con-
ditions, and thus may not necessarily be indicative of
trends when water availability is limiting to plant
growth.
Of the gymnosperm species that we grew, only one,
Podocarpus guatemalensis, occurs naturally in the trop-
ical forests of Panama. Although generally associated
with highland forests, this species also occurs on low-
lying islands off both the Pacific and Atlantic coasts.
Unfortunately, only one individual of P. guatemalensis
survived in our experiment, and we therefore ex-
Table VII. The C and O isotope composition of experimental plants
Whole-plant d13C values were calculated by weighting the d13C for each tissue by the fraction of C in that tissue relative to the whole plant. The d13C
of stems plus roots was calculated similarly to show the difference between the d13C of leaves and heterotrophic tissues. Values are given as the mean
for each species, with the SD in parentheses. No SD is given for P. guatemalensis because only one individual of this species survived. Sample sizes for
the other species ranged from five to eight individuals, as shown in Table III. NA, Not applicable.
Species
Carbon-Isotope Ratio (d13C) & Oxygen-Isotope Ratio (d18O) &
Leaves Stems Roots Whole Plant
Leaves 2
(Stems 1 Roots)
Stems
Gymnosperm tree species
C. lusitanica 229.7 (0.3) 227.8 (0.4) 226.8 (1.0) 228.5 (0.5) 22.4 (0.5) 20.1 (0.4)
P. caribaea 230.7 (0.6) 230.0 (0.6) 229.4 (0.4) 230.2 (0.5) 20.9 (0.2) 20.0 (0.3)
P. guatemalensis 225.4 (NA) 223.3 (NA) 223.5 (NA) 224.5 (NA) 22.0 (NA) 24.7 (NA)
T. occidentalis 228.9 (0.2) 227.5 (0.3) 227.2 (0.6) 228.0 (0.3) 21.6 (0.5) 20.7 (0.4)
Angiosperm tree species
C. longifolium 228.4 (1.3) 227.2 (0.8) 228.1 (1.2) 228.0 (1.1) 20.7 (0.5) 23.8 (0.6)
C. pratensis 227.1 (0.3) 226.4 (0.4) 226.9 (0.3) 226.9 (0.3) 20.4 (0.1) 23.0 (0.3)
H. alchorneoides 231.0 (0.1) 229.8 (0.3) 228.7 (0.4) 229.6 (0.3) 22.0 (0.3) 20.6 (0.3)
L. seemannii 229.3 (0.4) 228.1 (0.8) 228.2 (0.4) 228.6 (0.4) 21.1 (0.3) 22.8 (0.2)
P. pinnatum 227.3 (1.1) 225.7 (1.6) 225.4 (2.0) 226.3 (1.3) 21.7 (0.8) 24.8 (1.3)
S. macrophylla 231.6 (0.4) 229.3 (0.3) 229.1 (0.4) 230.3 (0.4) 22.4 (0.1) 23.9 (0.5)
T. rosea 229.6 (0.8) 228.0 (0.6) 227.5 (0.8) 228.1 (0.7) 22.0 (0.5) 23.2 (0.3)
T. grandis 229.5 (0.5) 228.0 (0.5) 227.9 (0.6) 228.6 (0.6) 21.5 (0.2) 22.9 (0.3)
Angiosperm liana species
G. lupuloides 229.2 (0.5) 228.0 (0.5) 228.3 (0.3) 228.4 (0.4) 21.1 (0.5) 24.8 (0.4)
M. leiostachya 229.3 (1.3) 228.0 (1.1) 227.0 (1.3) 228.3 (1.3) 21.7 (0.2) 23.8 (1.0)
S. hypargyreum 229.5 (0.7) 229.2 (0.4) 228.7 (0.5) 228.9 (0.5) 20.7 (0.3) 22.4 (1.1)
Figure 7. Whole-plant 13C discrimination plotted against the ratio of
intercellular to ambient CO2 partial pressures determined from instan-
taneous gas exchange measurements. Different symbols refer to differ-
ent species, as defined in Figure 2.
Cernusak et al.
654 Plant Physiol. Vol. 148, 2008
cluded it from all species-level analyses. However, the
lone surviving individual was interesting in that it had
the highest DgTEc and lowest ci/ca of any plant in
the study (Figs. 7 and 8). The ci/ca of 0.63 that we ob-
served for this individual of P. guatemalensis is similar
to a ci/ca of about 0.60 observed previously for well-
watered Podocarpus lawrencii (Brodribb, 1996). Further
research is necessary to determine whether high
water-use efficiency is a common trait within the genus
Podocarpus, and whether this trait contributes to the
ability of Podocarpus to persist in otherwise angiosperm-
dominated tropical forests.
Stable Isotope Composition
Whole-plant 13C discrimination, D13Cp, showed a
strong correlation with instantaneous measurements
of ci/ca (Fig. 7), suggesting that in general D
13Cp was a
faithful recorder of ci/ca, as predicted by Equation 6.
The mean value for d that we estimated for the full
data set was 3.1&, reasonably similar to a value of
4.0&, recently estimated for Ficus insipida (Cernusak
et al., 2007b). The D13Cp was also a reasonably good
predictor of variation in TEc, DgTEc, and vgTEc (Table
VI). We previously observed that the relationship
between D13Cp and TEc broke down at the species
level, appearing to reflect species-specific offsets in the
relationship between the two parameters (Cernusak
et al., 2007a). Whereas there was some evidence of
similar behavior in this study, as can be seen in Figure 8,
the species-level relationship between D13Cp and
DgTEc was generally much stronger in this study.
For example, in a least-squares linear regression be-
tween D13Cp and DgTEc using species means, the for-
mer explained 57% of variation in the latter (R25 0.57,
P 5 0.002, n 5 14); if P. guatemalensis was included in
the regression, the D13Cp explained 77% of variation in
DgTEc (R
2 5 0.77, P , 0.0001, n 5 15). The main dif-
ference between the earlier study (Cernusak et al.,
2007a) and this study was likely the range of variation
in D13Cp exhibited by the particular species that com-
prised the experiments. In the earlier study, mean
values for D13Cp at the species level ranged from only
20.3 to 21.7&, whereas in this study, species means
ranged from 18.8& to 22.9&; including the individual
of P. guatemalensis would further extend the lower
range to 16.9&.
Although our results show a generally strong cor-
relation between D13Cp and DgTEc, we suggest that it
is best to err on the side of caution when interpreting
variation among species in the former as indicative of
variation among species in the latter. As shown in
Equation 7, there are many terms with potential to
influence the relationship between D13Cp and DgTEc ,
not the least of which is variation in d, which could be
associated with variation among species in mesophyll
conductance to CO2 (Lloyd et al., 1992; Warren and
Adams, 2006; Seibt et al., 2008). Moreover, as was
previously the case (Cernusak et al., 2007a), we ob-
served significant variation among species in the dif-
ference between d13C of leaves and that of stems and
roots (Table VII). The mechanistic basis for such var-
iation among species in the d13C difference between
leaves and heterotrophic tissues is not well under-
stood (Hobbie and Werner, 2004; Badeck et al., 2005).
The 18O enrichment of stem dry matter, D18Op,
varied significantly among species, and much of the
observed variation inD18Op could be explained by vari-
ation in D18Oe, the predicted
18O enrichment of evap-
Figure 8. The product of transpiration efficiency of C gain (TEc) and
daytime vapor-pressure deficit of ambient air (Dg) plotted against
whole-plant 13C discrimination. Different symbols refer to different
species, as defined in Figure 2. Daytime air vapor-pressure deficit was
weighted according to the predicted weekly growth increment for each
individual plant.
Figure 9. The 18O enrichment of stem dry matter relative to irrigation
water plotted against the predicted 18O enrichment of water at the
evaporative sites in leaves. The predicted D18Oe was weighted accord-
ing to the predicted weekly growth increment for each individual plant.
Different symbols refer to different species, as defined in Figure 2.
Growth, Water Use, and Stable Isotopes in Tropical Trees
Plant Physiol. Vol. 148, 2008 655
orative sites within leaves (Fig. 9). Because plants were
grown over different time periods throughout the year,
and due to predicted differences in leaf temperature,
there was a reasonable variation among species in
predicted D18Oe. Species means for growth-weighted
D18Oe ranged from 6.9& to 13.0& (14.6& for P.
guatemalensis), and explained 73% of variation in the
observed species means for D18Op (R
2 5 0.73, P 5
0.0001, n 5 14), or 75% if P. guatemalensis was included
(R2 5 0.75, P , 0.0001, n 5 15). These correlations
suggest that the assumptions described in the theory
section for the D18Op model are reasonable. Addi-
tionally, we observed that the term 12 [(D18Op2 ewc2
ecp)/(1 2 pexpx)]/D
18Oe was significantly related to the
daytime MTR across the full data set, providing evi-
dence for a Pe?clet effect, as articulated in Equation 12.
This result is consistent with an experiment involving
three temperate tree species (Barbour et al., 2004). In
this study, we estimated a mean scaled effective path
length, L, for the full data set of 53 mm, similar to a
value of 54 mm estimated previously for Eucalyptus
globulus (Cernusak et al., 2005). Analysis of D18Op in
stem dry matter likely provides an advantage over
analysis of leaf dry matter for data sets such as ours,
which comprise diverse sets of species, because ecp for
stem dry matter tends to be less variable within and
among species than ecp for leaf dry matter (Borella et al.,
1999; Barbour et al., 2001; Cernusak et al., 2004, 2005).
Given a sound theoretical understanding of sources
of variation in d13C and d18O in plant dry matter, it
should be possible to use such isotopic data to con-
strain physiological models of tropical forest trees.
Data from the present experiment support the sug-
gestion that measurements of d13C can be used to make
time-integrated estimates of ci/ca at the tree or stand
scale. The photosynthetic rate, A, can then be pre-
dicted from ci, assuming ca is known or can be pre-
dicted. Finally, gs can be calculated from A, ci, and ca.
An example of this modeling approach was recently
provided (Buckley, 2008), along with a discussion of its
advantages and disadvantages. In the case of d18O, it
should be possible to use this signal to reconstruct
the ratio of ambient to intercellular vapor pressures,
ea/ei, during photosynthesis (Farquhar et al., 1989b;
Sternberg et al., 1989). The strong relationship that we
observed between stem dry matter D18Op and the
predicted D18Oe (Fig. 9) supports this idea. However,
our analysis also confirmed that the relationship be-
tween D18Op and D
18Oe was further modified by tran-
spiration rate, E, suggesting that it may be necessary to
obtain information independently about E to calculate
ea/ei from D
18Op. Such information might be obtained
from sap flux measurements or eddy covariance data,
for example. Assuming ea is known or can be pre-
dicted, an estimate of ei based on D
18Op might then be
particularly valuable, as it was recently suggested that
the leaf-to-air vapor pressure difference, ei-ea, will
likely be an important control over productivity in
tropical forest trees in the face of changing climate
(Lloyd and Farquhar, 2008).
CONCLUSION
We observed that 1/r was an important control over
relative growth rate, r, in a diverse group of seedlings
grown under tropical field conditions, including gym-
nosperm trees, angiosperm trees, and angiosperm
lianas. The gymnosperm trees generally had lower
1/r and r than the angiosperm species, and this may
have reflected differences in the hydraulic efficiency of
plant biomass among functional groups. Additionally,
we observed that An, the photosynthetic NUE, was the
primary control over whole-plant NUE, and that the
gymnosperm species appeared to be at a significant
disadvantage with respect to this trait compared to
angiosperm species. Variation in whole-plant water
use efficiency among species was primarily controlled
by ci/ca, which in turn varied as a function of stomatal
conductance. Whole-plant 13C discrimination was also
controlled by ci/ca, and thus correlated with whole-
plant water use efficiency. The 18O enrichment of stem
dry matter of the experimental plants varied primarily
as a function of the predicted 18O enrichment of
evaporative site water within leaves, and secondarily
as a function of the daytime MTR. Results provided
quantitative information about the mechanisms con-
trolling fluxes of C and water between forest trees and
the atmosphere, and the coupling of these processes to
plant N status. Moreover, our data set enabled rigor-
ous testing of the theoretical basis for variation in 13C
and 18O of plant dry matter; measurements of these
stable isotope ratios could prove useful for parame-
terizing forest ecosystem process models.
MATERIALS AND METHODS
Study Site and Plant Material
The study was carried out at the Santa Cruz Experimental Field Facility, a
part of the Smithsonian Tropical Research Institute, located in Gamboa,
Republic of Panama (907# N, 7942# W), at an altitude of approximately 28 m
above sea level. Average meteorological conditions at the study site during the
experiment are shown in Table II. These values were calculated from data
collected on site every 15 min by an automated weather station (Winter et al.,
2001, 2005). Seedlings of Cupressus lusitanica, Pinus caribaea, and Thuja
occidentalis were obtained from a commercial nursery in Chiriqui Province,
Republic of Panama. All other species were grown from seed collected in the
Panama Canal watershed, or obtained as seedlings from PRORENA, a native
species reforestation initiative operated through the Center for Tropical Forest
Science at the Smithsonian Tropical Research Institute. Familial associations
for each species are shown in Table III. The species C. lusitanica and P. caribaea
are conifers, with native distributions extending from Mexico to Nicaragua. T.
occidentalis is a conifer native to northeastern North America, and Tectona
grandis is a timber species native to south and southeast Asia. All other species
included in the study occur naturally in Panama.
The initial dry mass at the commencement of transpiration measurements
for each species is shown in Table III; these dry masses were estimated by
harvesting three to five individuals judged to be similar in size to the seedlings
retained for the experiment. Seedlings were transplanted individually into
38-L plastic pots (Rubbermaid Round Brute; Consolidated). Each pot contained
25 kg of dry soil mixture, which comprised 60% by volume dark, air-dried top
soil, and 40% by volume air-dried rice (Oryza sativa) husks. The rice husks
were added to improve soil structure and drainage. The pot water content was
brought to field capacity by the addition of 8 kg of water. The soil surface was
covered with 2 kg of gravel to minimize soil evaporation, and the outer walls
of each pot were lined with reflective insulation to minimize heating by
Cernusak et al.
656 Plant Physiol. Vol. 148, 2008
sunlight. Ametal trellis was added to each pot containing a liana seedling. The
pots were situated under a rain shelter with a glass roof, such that there was
essentially no interception of rain by the pots in the otherwise open-air
conditions. The initiation of measurements varied among species, due to the
temporal variation in the availability of seed and seedlings. Harvest dates also
varied, depending on the date of initiation of measurements and growth rates;
there were three harvests in total. The start and end dates of transpiration
measurements for each species are given in Table III.
Growth and Transpiration Efficiency Measurements
Pots were weighed at a minimum frequency of once per week to the
nearest 5 g with a 64-kg capacity balance (Sartorius QS64B; Thomas). After the
mass was recorded, water was added to each pot to restore it to its mass at
field capacity. As plant water use increased with increasing plant size, the pots
were weighed and watered more frequently. We endeavored to maintain pot
water content above 5 kg at all times, such that the range of soil water contents
experienced by the plants ranged approximately from field capacity to 60% of
field capacity. Control pots without plants were deployed among the pots with
plants in a ratio of one control pot to each six planted pots. The control pots
wereweighed each week to estimate soil evaporation. Cumulative plant water
use was calculated as the sum of pot water loss over the course of the
experiment minus the average water loss of control pots for the same time
period. Shortly before plant harvest, the pots were weighed at dawn and dusk
for 2 d to calculate nighttime transpiration separately from daytime transpi-
ration. Immediately following plant harvest, total plant leaf area was mea-
sured with a leaf area meter (LI-3100; LI-COR). Leaves, stems, and roots were
separated at harvest and oven-dried to a constant mass at 70C; theywere then
weighed to the nearest 0.02 g.
The mean relative growth rate, r, of each plant was calculated as r 5
[ln(mc2)2 ln(mc1)]/t, where ln(mc2) and ln(mc1) are natural logarithms of the C
mass at the end and beginning of the experiment, respectively, and t is the
duration of the experiment (Blackman, 1919). The MTR over the course of the
experiment was calculated as the cumulative water transpired divided by
the leaf area duration (Sheshshayee et al., 2005): MTR5 Et/[(LA1 1 LA2)0.5t],
where Et is cumulative water transpired, and LA1 and LA2 are the leaf area at
the beginning and end of the experiment, respectively. The transpiration
efficiency of C gain, TEc, was calculated as TEc5 (mc22mc11 lc)/Et, where lc is
the C mass of leaf litter abscised during the experiment.
To compare the D experienced by species that were grown over different
time periods (Table III), we calculated a growth-weighted D for each individ-
ual plant. Dry matter increments were predicted at weekly time steps for each
plant using relative growth rates calculated over the full experiment. Thus, the
dry matter increment for week 1,w1, was calculated asw15m12m0, wherem0
was initial plant dry mass, and m1 was calculated as m1 5 m0e
rt, with t 5 7 d;
the r was calculated as described above. The dry matter increment in week 2,
w2, was then calculated as w2 5 m2 2 m1, where m2 was calculated as m2 5
m1e
rt, with t again set at 7 d, and so on. For each week during the experimental
period, we also calculated an average daytime D from the meteorological data
collected at 15-min intervals. Growth-weighted D, Dg, was then calculated as
Dg 5
+
n
i51
Diwi
+
n
i51
wi
?13?
whereDi is the average daytime D for week i (kPa), and wi is the predicted dry
matter increment for week i (g).
In addition to calculating a growth-weighted D for each species, we also
predicted a growth-weighted v. Average weekly v for each plant was predicted
using a leaf energy balance model developed by D.G.G. dePury and G.D.
Farquhar (unpublished data), and described by Barbour et al. (2000a). The
model was parameterized with weekly average daytime values for air tem-
perature, relative humidity, irradiance, and wind speed taken from the data
collected by the automated weather station. Stomatal conductance for each
plant, measured as described below, was further used to parameterize the
model. The model predicted average weekly daytime leaf temperature for
each plant. The intercellular water vapor pressure, ei, was then calculated as
the saturation vapor pressure at leaf temperature, and this value was used to
calculate an average weekly value of v for each plant. The growth-weighted v,
vg, was then calculated as in Equation 13, but replacing Di with vi, the average
daytime v for week i.
In a similar fashion, we predicted a growth-weighted D18Oe for each plant.
For each weekly time step, the predicted value of ea/ei was used with Equation
8 to calculate average weekly daytime D18Oe. The D
18Ov was assumed equal to
2e1, calculated from air temperature (Bottinga and Craig, 1969). Growth-
weighted D18Oe, D
18Oeg, was then calculated for each plant as in Equation 13,
but replacing Di with D
18Oei, the average daytime D
18Oe for week i.
Leaf Gas Exchange and Leaf
Temperature Measurements
Wemeasured leaf gas exchange on three to five leaves per plant in the week
preceding plant harvest with a Li-6400 portable photosynthesis system (LI-
COR). Leaves were illuminated with an artificial light source (6400-02B LED;
LI-COR) at a photon flux density of 1,200 mmol m22 s21. Measurements were
made during both the morning and afternoon for each plant, and the mean of
the two sets of measurements was taken for each individual. The mean leaf
temperature during measurements was 32.7 6 1.5C (mean 6 1 SD), and the
mean v was 1.55 6 0.46 kPa (mean 6 1 SD).
Several days prior to the harvests that took place on March 11, 2006 and
May 10, 2006 (Table III), we made measurements of leaf temperature with a
hand-held infrared thermometer (Raytek MT Minitemp; Forestry Suppliers).
Measurements were repeated two to three times on three to five leaves per
plant under clear-sky conditions near midday. Values for each plant were
averaged, and v was calculated from measurements of relative humidity and
air temperature, assuming ei was at saturation at the average leaf temperature
for each plant. Leaf temperature was not measured for the plants harvested on
December 13, 2005 (Table III).
Stable Isotope and Elemental Analyses
Leaf, stem, and root dry matter were ground to a fine, homogeneous
powder for analysis of isotopic and elemental composition. The d13C, total C,
and total N concentrations were determined on subsamples of approximately
3 mg, combusted in an elemental analyzer (ECS 4010; Costech Analytical
Technologies) coupled to a continuous flow isotope ratio mass spectrometer
(Delta XP; Finnigan MAT). The d18O of stem dry matter was determined on
subsamples of approximately 1 mg (Delta XP; Finnigan MAT), following
pyrolysis in a high-temperature furnace (Thermoquest TC/EA; Finnigan
MAT). Analyses were carried out at the Stable Isotope Core Laboratory,
Washington State University. The d13C and d18O values were expressed in d
notation with respect to the standards of PeeDee Belemnite and Vienna
Standard Mean Ocean Water, respectively. The 13C discrimination of plant
dry matter (D13Cp) was calculated as D
13Cp 5 (d
13Ca 2 d
13Cp)/(1 1 d
13Cp),
where d13Ca is the d
13C of CO2 in air and d
13Cp is that of plant dry matter. We
assumed a d13Ca of 28&. The oxygen isotope enrichment of stem dry matter
(D18Op) was calculated as D
18Op 5 (d
18Op 2 d
18Os)/(1 1 d
18Os), where d
18Op
is d18O of stem dry matter, and d18Os is that of irrigation (source) water.
Irrigation water was drawn from two 800-L tanks, sealed to prevent evapo-
ration, which were periodically refilled with tap water, to buffer against
short-term variation in d18Os. The tank water had a mean d
18O of 24.3 6 0.5&
(mean6 1 SD, n5 6); we therefore calculated D18Op assuming a d
18Os of24.3&.
Leaf dry matter was further analyzed for P, K, and Ca concentrations by
acid digestion and detection on an inductively coupled plasma optical-
emission spectrometer (Perkin Elmer). Leaf samples were prepared by
digesting approximately 200 mg of sample material under pressure in
polytetrafluoroethylene vessels with 2 mL of concentrated nitric acid.
Statistical Analyses
We analyzed relationships between continuous variables using least-
squares linear regression. Variation among species and among functional
groups (gymnosperm trees, angiosperm trees, and angiosperm lianas) was
assessed with a nested design in the general linear model routine of SYSTAT
11 (SYSTAT Software); the functional group and species nested within the
functional group were taken as independent factors. For these analyses, the
number of observations was 93, the degrees of freedom for the functional
group was 2, the degrees of freedom for species nested within the functional
group was 11, and the degrees of freedom error was 79. Pairwise comparisons
among species or functional groups were then carried out according to
Tukey?s method. Among the study species, there was one, Podocarpus
guatemalensis, for which only one individual survived. All other species
Growth, Water Use, and Stable Isotopes in Tropical Trees
Plant Physiol. Vol. 148, 2008 657
comprised between five and eight individuals, as shown in Table III. Because
there was only one individual of P. guatemalensis, we excluded this species
from analyses aimed at assessing variation among functional groups and
species. However, we included the individual in linear regression analyses of
continuous variables. We considered it important to report data for this
individual, as it represents the only gymnosperm species in the study native to
the tropical forests of Panama.
Supplemental Data
The following materials are available in the on-line version of this article.
Supplemental Table S1. The C and N concentrations of experimental
plants.
ACKNOWLEDGMENTS
We thank Milton Garcia and Aurelio Virgo for technical assistance, and
Ben Harlow for carrying out isotopic and elemental analyses.
Received May 26, 2008; accepted June 23, 2008; published July 3, 2008.
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