670
BIOTROPICA 32(4a): 670?676 2000
Sources of Variability in Isoprene Emission and Photosynthesis in
Two Species of Tropical Wet Forest Trees1
Manuel Lerdau and Heather L. Throop
Department of Ecology and Evolution, State University of New York at Stony Brook, Stony Brook, New York
11794-5245 U.S.A.
ABSTRACT
Isoprene (2-methyl-1,3-butadiene), a volatile organic compound produced by many plants, is the principal source of
photochemically active reduced compounds in the troposphere. Emission from tropical forest trees accounts for . 70
percent of the annual global ?ux of isoprene, and under certain environmental conditions, trees may lose a large
fraction of their ?xed carbon to isoprene production. It is not known, however, if the production and emission of
isoprene serve an adaptive role, or are factors that control isoprene emission from tropical trees well understood. We
present results from a study investigating patterns of variability in isoprene emission and photosynthesis in two tropical
wet forest tree species, Brosimum utile (H. B. K.) Pitt (Moraceae) and Dussia munda (Leguminosae/Fabaceae). We
used leaf-level measurements to investigate the within- and among-individual variability in isoprene emission and
photosynthetic rates, and looked at how these rates changed with light intensity. Leaves of both species showed similar
responses to increases in light intensity, with photosynthesis appearing closer to saturation than isoprene emission at
high light intensities. There was a large difference in both the photosynthetic and isoprene emission patterns of canopy
and subcanopy leaves, with canopy leaves consistently showing much greater isoprene ?ux and photosynthetic rates
than subcanopy leaves. This difference was smaller, although still discernible, when differences in speci?c leaf weight
(SLW) were considered. These results suggest that tropical trees exhibit biochemical as well as structural responses to
canopy position, with patterns of vertical variability that resemble those seen in temperate trees. These data have
important implications for understanding the possible functional signi?cance of isoprene emission and for creating
accurate canopy-level isoprene ?ux models.
Key words: Brosimum utile; canopy; Dussia munda; isoprene; Panama; photosynthesis; tropical wet forest; vertical var-
iation; volatile organic compounds.
THE PRODUCTION AND EMISSION BY PLANTS of volatile
organic compounds (VOCs) is one of the most im-
portant ways in which plant processes in?uence the
structure and functioning of the atmosphere (Ler-
dau et al. 1997). Atmospheric photochemistry
models have shown that VOC oxidation has major
impacts on the residence times and concentrations
of tropospheric ozone, carbon monoxide, and
methane (Fehsenfeld et al. 1992, Pierce et al.
1998). In addition, studies of plant environmental
physiology have demonstrated the major roles of
VOCs in such diverse plant functions as herbivore
defense, pollinator attraction, and stress tolerance
(Lerdau et al. 1997). One of the most important
VOCs in terms of both atmospheric impacts and
rate of emission relative to photosynthesis is iso-
prene, 2-methyl-1,3-butadiene (Fehsenfeld et al.
1992). Global models of VOC emissions have
identi?ed tropical wet forests as the single most
important source of isoprene, making understand-
ing emission controls paramount for biosphere/at-
mosphere research (Guenther et al. 1995).
1 Received 3 May 1999; revision accepted 10 November
1999.
Isoprene is produced in chloroplasts and emitted
through stomata immediately after production. Iso-
prene-emitting plants typically expend ca 0.1?2.0
percent of their net photosynthesis on isoprene
emission (Monson et al. 1994, Sharkey et al. 1996).
Under conditions of water stress, however, plants
can emit isoprene at a greater rate than they ?x car-
bon through photosynthesis (Tingey et al. 1979,
Lerdau & Keller 1997). This carbon cost suggests
that there may be a bene?t to isoprene production
and emission, and recent papers have suggested a
role for isoprene emission in thermal stress tolerance
(Sharkey & Singsaas 1995, Singsaas et al. 1997; cf.
Logan & Monson 1999). The pattern of variability
observed within and among plants can offer insights
into the functional importance of isoprene emission
and is essential for developing predictive models of
emission that can be used in atmospheric chemistry.
Previous research on isoprene emission variabil-
ity has found a number of environmental factors
that govern emission rates on varying timescales.
From a given leaf at any one time, light and tem-
perature are the two most important controls
(Guenther et al. 1993, Sharkey & Loreto 1993).
Other studies have demonstrated that isoprene emis-
Sources of Variability in Tropical Isoprene Fluxes 671
sion at constant light and temperature depends upon
the developmental stage of the leaf measured and its
nitrogen concentration (Harley et al. 1994, Litvak
et al. 1996). Experiments in controlled environ-
ments have shown that the light intensity in which
a leaf develops has a strong in?uence on its isoprene
emission capacity (Sharkey et al. 1991, Litvak et al.
1996). These studies of controls over variability have
led to the concept of `` instantaneous'' emission con-
trols (controls such as light and temperature that
affect the emission rate from any one leaf at any one
time) and `` basal'' emission controls (those factors
that explain differences in emission rates from leaves
which are in identical light and temperature envi-
ronments; Monson et al. 1995).
While the instantaneous controls over isoprene
emission variability have been investigated for more
than 20 years and appear to apply similarly to
plants from many different taxa and many different
ecosystems, basal variability controls are not well
understood, especially in plants from tropical eco-
systems (Zimmerman 1979, Keller & Lerdau
1999). Research in other aspects of plant function,
such as carbon gain and water use, have demon-
strated that one of the most important potential
sources of variability is vertical position in the can-
opy. Canopy position is often tied to the light en-
vironment in which leaves develop, and thus to leaf
physical structure and biochemical function (Field
1983). Trees from two temperate tree genera, Liq-
uidambar (Hamamelidaceae) and Quercus (Faga-
ceae), have been examined and found to show sub-
stantial amounts of vertical variability in isoprene
emission capacity expressed on a leaf area basis.
This variability has been partially, although not ful-
ly, explained as resulting from the impacts of can-
opy position on speci?c leaf weight (SLW; Harley
et al. 1996, 1997; Sharkey et al. 1996). There,
however, have not been any examinations of the
patterns of vertical variability in isoprene emission
from tropical trees. Studies of variability controls
in tropical taxa are important to evaluate the gen-
erality of patterns seen in temperate taxa, because
of the importance of isoprene emission in the trop-
ical atmosphere, and to develop predictive models
of isoprene emission.
Previous research has suggested that isoprene
?uxes from tropical ecosystems may control daily
concentrations of the hydroxyl radical, and thus the
redox state of the tropical troposphere (Jacob &
Wofsy 1988). Earlier studies on isoprene emission
from these systems have focused on instantaneous
rather than basal controls (Lerdau & Keller 1997,
Keller & Lerdau 1999, Lerdau & Throop 1999).
This paper presents the ?rst estimates of vertical
variability in isoprene emission rates within and
among individuals from trees in a tropical ecosys-
tem. Based on the patterns of variability observed,
we were able to gain insight into the sources of
variability and thus into the functional signi?cance
of isoprene emission. Such insight is also a ?rst step
toward incorporating variability into predictive
models of isoprene emission.
MATERIALS AND METHODS
Research was conducted at the Fort Sherman can-
opy crane, which is managed by the Smithsonian
Tropical Research Institute for the United Nations
Environmental Program. The canopy crane is lo-
cated within Fort Sherman near the Atlantic coast
of the Republic of Panama in a forest preserve site
of ca 120 km2. The average annual rainfall at the
site is ca 3500 mm, and all species are evergreen.
The exact forest age is not known, but historical
estimates suggest that it has not received intensive
logging within the last 200 years. The canopy av-
erage height is ca 40 m, with the tallest trees . 44
m. The crane is 52 m high and has a radial length
of 54 m. This length gives a horizontal area cov-
erage of ca 9000 m2. The true area coverage was
slightly higher because the crane tower was located
at a low point within the circle; i.e., the tower was
in a streambed, and hills rose up on either side.
The crane has a 1.4-m2 gondola, the position of
which is controlled by a crane operator working at
the top of the tower. The crane operator maintains
contact with the gondola occupants by means of a
two-way radio and can position the gondola any-
where within the reach of the crane. The gondola
is large enough for two people and the analytical
equipment (described below).
Measurements of photosynthesis and isoprene
emission were made following the methods de-
scribed by Lerdau and Keller (1997), using a LI-
6400 gas exchange system (LI-COR, Lincoln, Ne-
braska) with a temperature and light controlled cu-
vette and a Voyager gas chromatograph with a pho-
toionization detector (Photovac-PE, Norwalk,
Connecticut). The LI-6400 system incorporates
thermoelectric heat exchangers mounted on the sides
of the cuvette to control temperature, and a variable
intensity red LED light source with a peak irradi-
ance at 670 nm. Tenneson et al. (1994) have shown
that the red light has no effect on isoprene emission
relative to white light sources, as long as all mea-
surements are made under steady-state conditions.
Carbon dioxide and water vapor were measured us-
672 Lerdau and Throop
ing the LI-COR open-path infrared gas analyzers.
Sample air from the chamber-mounted detectors in
the LI-COR could be routed directly to the gas
chromatograph. A Te?ony valve was used to direct
air?ow either to the gas chromatograph or to the
LI-6400 reference and sample analyzers. The LI-
6400 gas exchange system uses a CO2-cartridge de-
livery system to maintain constant CO2 concentra-
tions in the cuvette. Tests showed that this system
contained no contaminants that interfered with iso-
prene measurements (data not shown).
The air?ow through the LI-6400 cuvette was
held constant at 200 mmol/sec. The gas chromato-
graph contained a small pump that drew air at 70
mmol/sec through a 1-ml sample loop for ten sec-
onds. Sample gas in the loop was valve-injected
onto a 0.53-mm internal diameter CP-Sil-5 col-
umn 10 m in length. The column was maintained
at 408C and hydrocarbon-free air was used as the
carrier gas. The detector output was linear across a
broad range of isoprene concentrations, from 0 to
600 ppb, with an intercept through zero. The de-
tection limit on this system was ca 2 ppb, and the
precision and accuracy both were within ten per-
cent of the measured value. These conditions al-
lowed us to identify phytogenic isoprene emission
rates of $4 nmol/m2/sec. Single-point calibrations
were performed daily using a gas phase isoprene
standard prepared by vaporizing and diluting pure
liquid isoprene (obtained from Sigma/Aldrich, St.
Louis, Missouri) according to a method recom-
mended by Photovac (Anonymous 1990). These
standards were compared to commercially available
gravimetric standards (Scott-Marrin, Irvine, Cali-
fornia) and were always found to be within ten
percent. Linearity was determined for both the
commercial and static dilution standards by con-
ducting serial dilutions. For the daily single-point
calibrations, saturated headspace of a vial contain-
ing pure isoprene at known temperature (08C) was
diluted once in a known volume of air to obtain a
gas phase concentration within the range typically
seen during emission studies, usually 100 ppb.
For two species (Brosimum utile [H. B. K.] Pitt
[Moraceae] and Dussia munda [Leguminosae/Faba-
ceae]), variations within and among individuals for
isoprene emission and photosynthetic rate were mea-
sured. Both Brosimum and Dussia are common,
widespread genera found throughout South and
Meso-America (Gentry 1993). These two species are
typical of wet forests, but other members of these
genera are found in seasonal dry forests as well
(Croat 1978). Characterizations were made on in-
dividuals that were growing underneath the boom
of the crane. Within each individual, simultaneous
photosynthetic and isoprene ?ux measurements were
taken on three sun leaves (in full sun at the top of
the canopy) and three shade leaves (shade leaves se-
lected were those in the darkest possible location
easily accessible from the gondola and were at least
3 m below the top of the canopy; LAI data were
not collected). Measurements were made on three
individuals of B. utile and on two individuals of D.
munda. Light response measurements were made on
all six leaves used for each individual and were con-
ducted at 308C, with light levels varying from 125
to 1760 mmol photons/m2/sec. The CO2 concen-
tration in the cuvette was held at 360 ppm and the
relative humidity was maintained between 60 and
85 percent. Measurements were collected between
0900 hours and 1500 h. SLW calculations were
made on all six measured leaves on both D. munda
individuals and on all six measured leaves on two of
the three B. utile individuals. SLW was determined
for each leaf measured by collecting a known area
of leaf (excluding the midrib), drying it at 608C
until no weight change was observed, and then
weighing the leaf disks on an analytical balance.
Both photosynthetic and isoprene emission
rates were calculated for each leaf on both an area
basis and on a per unit mass basis. The means of
these rates for both sun and shade positions on
each individual were calculated. For each species at
each canopy position, a grand mean was calculated
by determining the mean of the within-individual
tree means. The change in photosynthesis and iso-
prene emission between 1000 mmol photons/m2/
sec (equivalent to half of full sunlight) and 1760
mmol photons/m2/sec (the maximum light inten-
sity possible with the light source) was calculated
by dividing rates at 1760 mmol photons/m2/sec by
rates at 1000 mmol photons/m2/sec and multiply-
ing by 100. These data were arcsine-square root
transformed and analyzed with a paired t-test.
Mean SLWs of sun and shade leaves for each spe-
cies were compared using a t-test.
RESULTS
All individuals of both species showed similar re-
sponses in both photosynthesis and isoprene emis-
sion to light intensity. The general response was a
hyperbolic or rectangular hyperbolic change in iso-
prene emission and photosynthesis to a linear in-
crease in light intensity (Fig. 1). In general, pho-
tosynthesis appeared to be closer to saturation at
higher light intensities than isoprene. The mean
percent change in photosynthetic rate between
Sources of Variability in Tropical Isoprene Fluxes 673
FIGURE 1. Light response curves for isoprene ?ux
and photosynthetic rates in (a) Brosimum utile and (b)
Dussia munda. Each point represents the rates (x? 6 SE)
for all measured leaves of each species. Rates are calcu-
lated on a per-area basis. Open symbols represent iso-
prene emissions, and closed circles represent photosyn-
thesis.
FIGURE 2. Light response curves of among-individ-
ual and canopy position variability for isoprene ?ux and
photosynthetic rates. Each point represents the x? 6 SE
for all sun or shade leaves of a given individual. Rates
were calculated on a per-area basis. (a) Brosimum utile
photosynthesis. (b) Dussia munda photosynthesis. (c) B.
utile isoprene ?ux. (d) D. munda isoprene ?ux. Open
symbols are sun leaves; closed symbols are shade leaves;
different shapes represent different individuals.
light intensities of 1000 and 1760 mmol photons/
m2/sec (98.09% for B. utile and 117.67% for D.
munda) was less than the mean percent change in
isoprene emission (143.64% for B. utile and
130.49% for D. munda). For both species, the
change was signi?cantly greater for isoprene than
photosynthesis after an arcsine-square root trans-
formation (for B. utile, transformed photosynthesis
x? 5 5.67, isoprene x? 5 6.86, N 5 12, t 5 5.05,
P , 0.01; for D. munda, transformed photosyn-
thesis x? 5 6.22, isoprene x? 5 6.56, N 5 12, t 5
3.82, P , 0.01). Although there were notable dif-
ferences among individuals in their isoprene emis-
sion and photosynthetic rates at any one light in-
tensity, the differences between sun- and shade-de-
veloped leaves overwhelmed the among-individual
variability for both processes (Fig. 2). When the
mean rates were plotted together, the differences
became even more apparent, with sun leaves having
higher ?ux and photosynthetic rates than shade
leaves on a per unit area basis (Fig. 3). By 500
mmol photons/m2/sec light, the sun leaves had sig-
ni?cantly greater photosynthetic and isoprene
emission rates for both species (Fig. 3).
Differences were seen between the SLWs
(where SLW 5 biomass:leaf area ratio) for sun and
shade leaves of each species. For D. munda, sun
leaves had a signi?cantly greater SLW than shade
leaves (shade leaf SLW x? 5 192 g/m2, sun leaf SLW
x? 5 258 g/m2; N 5 6, t 5 2.39, P , 0.05), while
there was no signi?cant difference for B. utile
(shade leaf SLW x? 5 180 g/m2, sun leaf SLW x? 5
169 g/m2; N 5 6, t 5 0.220, P . 0.05). It is
674 Lerdau and Throop
FIGURE 3. Light response curves for sun and shade
leaves. Each point represents the x? 6 SE for all sun or
shade leaves for each species. Rates were calculated on a
per-area basis. (a) Brosimum utile photosynthesis. (b) Dus-
sia munda photosynthesis. (c) B. utile isoprene ?ux. (d)
D. munda isoprene ?ux. Open symbols are sun leaves;
closed symbols are shade leaves. For both species, all
shade means differ signi?cantly from sun means at 500
mmol photons/m2/sec for both photosynthesis (for B.
utile, t 5 3.56, df 5 10, P , 0.01; for D. munda, t 5
2.20, df 5 10, P , 0.05) and isoprene ?ux (for B. utile,
t 5 2.65, df 5 10, P 5 0.01; for D. munda, t 5 5.11,
df 5 10, P , 0.001).
FIGURE 4. Light response curves for sun and shade
leaves. Each point is the x? 6 SE for all sun or shade
leaves of each species. Rates were calculated on a mass
basis. (a) Brosimum utile photosynthesis. (b) Dussia mun-
da photosynthesis. (c) B. utile isoprene ?ux. (d) D. munda
isoprene ?ux. Open symbols are sun leaves; closed sym-
bols are shade leaves. For both species, shade and sun
means do not differ signi?cantly at 500 mmol photons/
m2/sec for both photosynthesis (for B. utile, t 5 1.73, df
5 10, P . 0.05; for D. munda, t 5 0.48, df 5 10, P .
0.1) and isoprene ?ux (for B. utile, t 5 1.68, df 5 10,
P . 0.05; for D. munda, t 5 1.26, df 5 10, P . 0.1).
possible that, particularly for D. munda, observed
differences in physiological rates may have been
due to variations in SLW. To test for this possibility,
the mass-based overall sun vs shade light response
curves were plotted; i.e., Figure 3 was replotted
with units in nmol/g/h for isoprene and mmol/g/h
for photosynthesis (Fig. 4). The differences be-
tween sun and shade leaves, although smaller, were
still apparent, indicating that SLW does not com-
pletely explain the physiological differences be-
tween sun and shade leaves.
Sources of Variability in Tropical Isoprene Fluxes 675
DISCUSSION
Differences in light environment between the can-
opy and subcanopy of trees have long been asso-
ciated with differences in leaf development and
photosynthetic rates (Field 1983, Hirose & Werger
1987). Leaves that develop in darker subcanopy en-
vironments are known to have lower SLW, and in
some cases, reduced allocation of resources per unit
biomass to carbon ?xation (Bjo?rkman 1981, Hi-
rose & Werger 1987, Ellsworth & Reich 1993).
Previous research on vertical variations in area-
based isoprene emission and photosynthesis for
Quercus and Liquidambar has demonstrated that
most, although not all, of the variation in physio-
logical rates as one moves through the canopy can
be explained by changes in SLW (Harley et al.
1996, Sharkey et al. 1996). Of the differences that
remain, the effect of canopy position on isoprene
emission always has been found to be larger than
the effect on photosynthesis.
The two tropical taxa measured in this study
showed similar patterns in the effects of canopy po-
sition on isoprene emission and photosynthesis. The
decrease in SLW of subcanopy leaves relative to can-
opy leaves in D. munda mirrored a pattern com-
monly observed in temperate trees. The high vari-
ance of SLW in B. utile and the small sample size
of this study may have obscured a similar pattern in
that species. The sun/shade differences for all leaves
of both species in isoprene emission rate were greater
on an area basis than on a mass basis (177% greater
in sun leaves at 1000 mmol photons/m2/sec for the
area-based measurements and 126% greater on a
mass basis). Similar, but smaller, mass- and area-
based differences also were seen for photosynthesis
(154% greater in sun leaves at 1000 mmol/m2/sec
for the area-based measurements and 132% greater
on a mass basis). Although generalizations based on
so few taxa and only two vertical points are risky,
these results suggest that tropical trees show the same
patterns of vertical variations in gas ?uxes as tem-
perate trees, perhaps to an even greater extent.
Recent studies on the controls over gas ?uxes
into and out of leaves have emphasized the impor-
tance of SLW in determining physiological rates.
In this context, SLW appears to serve as a master
structural integrator of leaf-level processes and con-
trols (Reich et al. 1997). The results presented here
on gas exchange in B. utile and D. munda highlight
the need to examine biochemical as well as struc-
tural adaptations. These trees appear to show ad-
justments to light availability that go beyond what
can be explained by changes in SLW. It is worth
noting that these adjustments were larger for iso-
prene production than for photosynthesis and thus
support the suggestion that isoprene emission may
serve an adaptive role in helping leaves tolerate the
greater thermal stress that occurs in the top of the
canopy compared to the subcanopy (Sharkey et al.
in press). These results, however, do not exclude
the possibility that isoprene emission occurs as a
nonadaptive consequence of the high light-induced
up-regulation of another metabolic pathway such
as the carotenoid pathway (Lerdau et al. 1997).
These results also have important implications
for efforts to model isoprene emission in tropical
ecosystems. Emissions from these ecosystems are a
major source of reduced photochemically active
compounds to the troposphere. Understanding var-
iations in basal emissions rates is essential to de-
veloping realistic models of canopy-scale emissions
(Geron et al. 1997). If the pattern observed for B.
utile and D. munda is found in other tropical iso-
prene-emitters, then ecosystem-scale ?ux models
will need to incorporate a basal emission rate that
declines with canopy depth, irrespective of changes
in SLW. Clearly, an important next step will be to
examine vertical variations in trace gas ?uxes for
more taxa of tropical trees. The recent deployment
of canopy cranes in several tropical forests now
makes this strategy feasible (Lowman 1998).
ACKNOWLEDGMENTS
This research was supported by a NASA New Investigator
Program award to ML. We thank S. J. Wright and I.
Verleye for logistical support. We thank S. J. Wright, M.
Garcia, and M. Samaniego for assistance in identifying
plant taxa and analyzing samples, and E. Andrade and J.
Herrera for operating the crane. The Smithsonian Trop-
ical Research Institute and the United Nations Environ-
mental Program jointly sponsor the Canopy Biology Pro-
gram. Special thanks are due to the government of Den-
mark for contributing the Ft. Sherman crane.
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