Trees (2005) 19: 282?289
DOI 10.1007/s00468-004-0388-x
ORIGINAL ARTICLE
Jose? Luis Andrade ? Frederick C. Meinzer ?
Guillermo Goldstein ? Stefan A. Schnitzer
Water uptake and transport in lianas and co-occurring trees
of a seasonally dry tropical forest
Received: 7 April 2003 / Accepted: 4 October 2003 / Published online: 9 December 2004
C? Springer-Verlag 2004
Abstract Water uptake and transport were studied in eight
liana species in a seasonally dry tropical forest on Barro
Colorado Island, Panama. Stable hydrogen isotope compo-
sition (?D) of xylem and soil water, soil volumetric water
content (?v), and basal sap flow were measured during the
1997 and 1998 dry seasons. Sap flow of several neighbor-
ing trees was measured to assess differences between lianas
and trees in magnitudes and patterns of daily sap flow. Little
seasonal change in ?v was observed at 90?120 cm depth in
both years. Mean soil water ?D during the dry season was
?19? at 0?30 cm, ?34? at 30?60 cm, and ?50? at
90?120 cm. Average values of xylem ?D among the liana
species ranged from ?28? to ?44? during the middle of
the dry season, suggesting that water uptake was restricted
to intermediate soil layers (30?60 cm). By the end of the
dry season, all species exhibited more negative xylem ?D
values (?41? to ?62? ), suggesting that they shifted to
deeper water sources. Maximum sap flux density in co-
occurring lianas and trees were comparable at similar stem
diameter (DBH). Furthermore, lianas and trees conformed
to the same linear relationship between daily sap flow and
DBH. Our observations that lianas tap shallow sources of
soil water at the beginning of the dry season and that sap
flow is similar in lianas and trees of equivalent stem diame-
ter do not support the common assumptions that lianas rely
J. L. Andrade (


)
Centro de Investigacio?n Cient??fica de Yucata?n, A. C., Unidad
de Recursos Naturales, Me?rida 97310, Yucata?n, Mexico
e-mail: andrade@cicy.mx
Tel.: +52-999-9813914
Fax: +52-999-9813900
F. C. Meinzer
USDA Forest Service, Forestry Sciences Laboratory,
Corvallis, OR 97331, USA
G. Goldstein
Department of Biology, University of Miami,
Coral Gables, FL 33124, USA
S. A. Schnitzer
Department of Biological Sciences, University of
Wisconsin?Milwaukee, Milwaukee, WI 53211, USA
primarily on deep soil water and that they have higher rates
of sap flow than co-occurring trees of similar stem size.
Keywords Panama . Sap flow . Soil volumetric water
content . Stable hydrogen isotope ratio . Tropical forest
trees
Introduction
One important feature of tropical rain forests is the abun-
dance of climbing plants, especially woody climbers such
as lianas, which are present in both primary and secondary
forests and are conspicuous in gaps and clearings (Richards
1996; Schnitzer and Bongers 2002). Lianas typically con-
stitute about 25% of the woody stem abundance and species
richness in many tropical forests (Gentry 1991), where they
can utilize about 40% or more of the canopy tree species
for support (Putz 1984). The slender stems of lianas can
be more than 100 m long, potentially increasing the resis-
tance to water transport from the soil to the leaves. Nev-
ertheless, the water conducting capacity of lianas is gen-
erally thought to be greater than in trees of comparable
stem diameter because of their longer and wider vessels
(Zimmermann 1983; Carlquist 1985; Gartner et al. 1990;
Ewers et al. 1991, 1997).
The presence of large diameter vessels and high leaf area:
sapwood area ratios in lianas has led to the expectation that
lianas have higher sap flow rates per stem cross-sectional
area than co-occurring trees. However, this assumption is
supported by limited evidence and may not be universally
true. It has been reported that vines transpire more than trees
with similar stem diameter in an Amazonian secondary for-
est, although trees show smaller reduction in transpiration
from the wet to the dry season than do vines (Restom and
Nepstad 2001). Fichtner and Schulze (1990) reported high
sap flow and transpiration rates of three climbing plants
in a tropical deciduous forest, but no comparative data on
trees in the same site, measured with the same techniques,
under the same environmental conditions, were obtained.
However, despite the latest recognition that lianas play an
283
integral role in several aspects of tropical forest dynamics
(Schnitzer and Bongers 2002), studies on water transport
and belowground acquisition of water by liana species are
uncommon. Such studies are required to understand the
interactions of lianas with other plant life forms and their
contribution to whole-forest transpiration.
Stable isotope techniques have been used non-invasively
to study spatial and temporal partitioning of soil water up-
take in both temperate and tropical regions (Ehleringer and
Dawson 1992; Flanagan et al. 1992; Jackson et al. 1995,
1999; Le Roux et al. 1995; Dawson 1996; Dawson and Pate
1996). Stable hydrogen (1H, 2H or D) isotope composition
of the soil water usually varies with depth because of evap-
orative fractionation, which results in an enrichment of the
heavier isotope in the upper soil layers. In tropical regions,
evaporative fractionation is one of the major factors leading
to variations in the isotope composition of soil water dur-
ing prolonged dry periods. A number of studies have used
xylem water isotope data to demonstrate partitioning of soil
water resources among different species of tropical woody
plants (Jackson et al. 1995, 1999; Meinzer et al. 1999),
as well as among different sized individuals of the same
species (Dawson 1996; Meinzer et al. 1999). In addition,
seasonal differences in xylem water isotope ratios support
the idea that exploitation of progressively deeper sources of
water may permit some species to maintain relatively high
rates of sap flow during a prolonged dry season (Dawson
and Pate 1996; Meinzer et al. 1999).
Although lianas are widely believed to tap deep sources
of soil water and have high rates of water use, there is
a paucity of published studies in which water uptake and
transport have been characterized simultaneously in lianas
and co-occurring trees. In the present study, we investigated
diurnal patterns of sap flow and its response to environmen-
tal variables in several liana and co-occurring tree species
in a seasonally dry tropical forest on Barro Colorado Island,
Panama. We also employed stable hydrogen isotope analy-
ses to assess the degree of spatial and temporal partitioning
of soil water among the liana species.
Materials and methods
Field site and species
Field measurements were conducted from January through
April in 2 consecutive years (1997 and 1998) on Barro
Colorado Island (BCI), Panama (9? 09?N, 79? 51?W). The
island is covered by semi-evergreen moist tropical forest
with a canopy height of 35?40 m and has a mean annual
rainfall of 2,600 mm, with a marked dry season from mid-
December through April (Leigh and Wright 1990). The
study site was located on a plateau near a permanent 50-ha
plot, established in 1980 (Hubbell and Foster 1983). Fifteen
circular plots (30 m in diameter) were established during
1996 in which trees greater than 20 cm DBH and lianas
greater than 5 cm DBH were marked and identified. Liana
species represented by at least three different individuals
with their crowns in the canopy were chosen for measure-
ments of xylem sap ?D and sap flow. A subset of trees was
also selected in the same plots and used to measure sap
flow at the same time as in the neighboring lianas. A list of
the eight liana species studied, their size, and their relative
abundances are presented in Table 1.
Soil volumetric water content (?v) was monitored at
0?15 cm, 15?30 cm, 30?60 cm, 60?90 cm, and 90?120 cm
with 1.2-m multi-segment profiling TDR probes (type A,
Environmental Sensors, Victoria, BC, Canada) installed
permanently in four randomly chosen study plots during
the dry seasons of 1997 and 1998. The probes were read
with an MP-917 TDR unit (Environmental Sensors).
Xylem and soil water hydrogen isotope composition
Xylem tissue samples from three to four individuals of six
liana species were obtained by extracting small cylinders
of wood with an increment borer at a height of about 0.1 m
above the ground. The outer bark and other non-xylem
tissue were removed and the active xylem tissue was im-
mediately placed in glass containers, sealed with a rubber
stopper and Parafilm, and kept frozen until vacuum distil-
lation in the laboratory. Each individual was sampled twice
(21 March and 18 April 1997). Soil samples were collected
with an auger at 10-cm intervals down to 100 cm in two to
three sites with the highest density of the liana species on
the same dates that xylem tissue samples were collected.
Soil and xylem samples were sent to Mountain Mass
Spectrometry Laboratory (Evergreen, Colo., USA) for
preparation and analysis. Water was extracted by cryogenic
vacuum distillation and the stable hydrogen isotope com-
position was analyzed using mass spectroscopy. The stable
hydrogen isotope composition was expressed in conven-
tional delta (?) notation as the D/H ratio relative to the
V-SMOW standard (Ehleringer and Osmond 1989; Smith
and Ziegler 1990):
?D? = [(D/Hsample ? D/Hstandard) ? 1] ? 100
data for soil ?D were grouped into three intervals: 0?30 cm,
30?60 cm, and 90?120 cm, which showed the highest ?D
differences and also correlated with ?v values.
Sap flow measurements
Sap flow was measured using the constant heat dissi-
pation method described by Granier (1985, 1987). Pairs
of 20-mm-long, 2-mm-diameter temperature probes (UP,
Munich, Germany) were installed in the sapwood near the
base of the liana and tree stems, approximately at 1.5 m
from the ground. The upper (downstream) probe was con-
tinuously heated with a constant current power supply (UP,
Munich) while the lower unheated probe measured the ref-
erence temperature of the sapwood. The protruding por-
tions of both probes were insulated with a layer of foam
rubber surrounded by an outer shield of reflective material
and transparent plastic. Probe temperatures were recorded
continuously with a datalogger (CR21X, Campbell
284
Scientific, Logan, Utah, USA) equipped with a 32-channel
multiplexer (AM416, Campbell Scientific) and 10-min av-
erages were stored in a solid-state storage module (SM192,
Campbell Scientific). Concurrent measurements were
made in a total of 12?18 individuals of lianas and trees
distributed between two different plots. Sap flow was mea-
sured continuously during a 5?7 day period in each indi-
vidual on five occasions between January and April 1998.
Sap flux density was calculated from the temperature dif-
ference between the probes using an empirical relationship
developed by Granier (1985) and revalidated by Clearwater
et al. (1999). Mass flow of sap was obtained by multiply-
ing sap flux density by the sapwood cross-sectional area.
Sapwood cross-sectional area was determined by dye in-
jections (0.1% indigo carmine) in the main stem. Upstream
cores were extracted after injection and the width of colored
sapwood measured to calculate the cross-sectional area of
active xylem.
Environmental variables were recorded as 10-min aver-
ages with an automated weather station installed on a 40-m
canopy tower on BCI. Photosynthetic photon flux density
(PPFD) was measured with a quantum sensor LI190SB
(Li-Cor, Lincoln, Neb., USA) and atmospheric saturation
deficit (ASD) was calculated from measurements of air
temperature and relative humidity made with shielded sen-
sors (HMP35C, Campbell Scientific).
To compare sap flow in tree stems with dimensions sim-
ilar to those of lianas, upper canopy branches of Ficus
insipida and Luehea seemannii trees were accessed with
the Smithsonian Tropical Research Institute canopy crane
in the Parque Natural Metropolitano near Panama City dur-
ing the dry season of 1997 (for details on the site and
methodology see Andrade et al. 1998). Sap flow in trees
and lianas was assumed to be constant across the entire sap-
wood depth. However, for some Panamanian tree species,
sap flux density is maximal in the outermost 4 cm of sap-
wood (where we actually measured it) and declined with
increasing distance into the sapwood (James et al. 2002).
Statistical analysis
For both liana and tree species, linear correlations were
obtained to describe the relationships of maximum sap flux
density and both daily duration of maximum sap flux den-
sity and the initial slope of the sap flux velocity versus
air saturation deficit (ASD) relationship, and also between
these two last variables. Maximum sap flow density was
determined as the maximum constant value that lasted for
more than 30 min with a 5% variation during clear days.
Additionally, differences between the liana and tree species
were tested for the variables: duration of maximum sap flow
and the slope of the sap flux density versus ASD relation-
ship, using a Student t-test.
Results
The eight liana species were relatively abundant in the
study site. Three species, Combretum decandrum, Maripa
panamensis, and Prionostemma aspera, had seven or more
individuals per hectare that had already reached the canopy
(DBH > 5 cm; Table 1). On the other hand, Hiraea
reclinata, M. panamensis, and P. aspera had the greatest
densities of small individuals (>200 per ha). The species
E. monostachya and H. volubilis had the largest individ-
uals in our sample (average DBH 11.9 cm and 7.5 cm,
respectively) but the lowest densities (Table 1).
Total rainfall on BCI from January through late April
1997 was 99.6 mm, about 46% of the 68-year average value
for the period (Windsor 1990). Two significant precipita-
tion events occurred during the 1997 dry season; 41.2 mm
of rainfall on 5 and 6 February, and 31.2 mm on 22 April
(Fig. 1). Soil volumetric water content (?v) in the upper
30 cm was about 15% prior to the February rainfall, then
increased to 32%, after which it declined steadily to a min-
imum value of 11% before the rain on 22 April (Fig. 1).
Between 30 and 60 cm depth, ?v did not increase signifi-
cantly following the rains in early February and late April,
and remained at about 30% during the entire dry season.
At 90?120 cm depth, ?v remained at about 50% during
much of the dry season, then declined slowly to 40% over
a 40-day period, but the rains of late April and early May
increased it again to 50% (Fig. 1).
During March and April 1997, soil water ?D values de-
creased from ?19.2? in the upper 30 cm to ?34.2? at
30?60 cm depth, and to ?49.9? at 90?120 cm depth
(Fig. 1). Water samples from springs on BCI during this
Table 1 Abundance of liana
species used for xylem water
samples and sap flow
measurements. The size-class
density distribution data are
from 16 (24?36 m) plots
(13,824 m2 total area) located in
intact old-growth forest stands
on BCI. All individuals >50 cm
height and >0.3 cm DBH were
included. Data are reported on
number of individuals per ha
(Schnitzer and Carson,
unpublished information)
Species Family Density (per ha)
<5 cm dbh 5 to 10 cm dbh Total
Abuta racemosa (Thunb) Tr.
& Planch
Menispermaceae 33 0 33
Clitoria javitensis H.B.K Fabaceae 36 1 37
Combretum decandrum Jacq Combretaceae 38 7 45
Entada monostachya DC Fabaceae 0 0 0
Hippocratea volubilis L. Hippocrateaceae 9 0 9
Hiraea reclinata Jacq Malpighiaceae 337 1 338
Maripa panamensis Hemsl Convolvulaceae 364 9 373
Prionostemma aspera
(Lam.) Miers
Hippocrateaceae 214 23 237
285
Fig. 1 Soil volumetric water content at different depths during the
1997 dry season on Barro Colorado Island, Panama (n = 2?4 soil
probes). Arrows show rainfall events greater than 10 mm. Average
delta deuterium (?D) is shown for each soil depth in March and April
period yielded an average ?D value of about ?60? . In all
six liana species from which xylem sap was obtained, ?D
values were more negative at the end of the dry season
(day 108) than near the middle (day 80) of the dry season
(Fig. 2). For example, Entada monostachya had the least
negative xylem sap ?D value (?27? ) at day 80, implying
that soil water uptake was largely restricted to the soil layer
between 0 and 60 m (Figs. 1, 2). At day 108, the xylem sap
?D value for this species was ?52? , suggesting that most
of the water was obtained from depths greater than 100 cm.
In Hippocratea volubilis, on the other hand, the average
xylem sap ?D value was ?43? at the middle of the dry
season, suggesting that soil water uptake may have been
restricted to soil layers deeper than 60 cm. By day 108, its
xylem sap ?D value had decreased to ?62? , a value close
to that of ground water.
Typical daily courses of sap flow for co-occurring liana
and tree species on clear days with comparable PPFD and
air saturation deficit regimes are shown in Figs. 3, 4, 5.
Lianas and trees appeared to differ with respect to two
notable features of their daily courses of sap flow. The
initial rate of increase in sap flow in the morning was usually
greater in lianas than in trees, and maximal or near-maximal
rates of sap flow were maintained for a longer period of time
in lianas than in trees. Comparisons of the mean duration
of maximum sap flux density and the mean initial slope of
the relationship between sap flux density and air saturation
deficit for all liana and tree species in which sap flow was
measured (Table 2), confirmed the apparent trends seen
in Figs. 3, 4, 5. The mean duration of maximum sap flux
density for lianas (3.4 h) was twice that of trees (1.7 h; P <
0.001), and the mean slope of the morning increase in sap
flow with increasing air saturation deficit was more than
twice as great in lianas than in trees (P < 0.001).
Maximum sap flux density decreased sharply and then
more gradually with increasing DBH for both lianas and
tree branches and stems (Fig. 6a,b). For a stem size of about
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Fig. 2 Average xylem sap delta deuterium (?D) sampled on 21 March
(day 80) and 18 April (day 108), 1997 for six liana species (n=4
plants; upper panel) and soil and springs ?D sampled on the same
dates (n=6 samples; lower panel) on Barro Colorado Island, Panama
5 cm DBH the maximum sap flow was about 0.4 m h?1 and
decreased exponentially to about 0.15 m h?1 for a stem size
of about 20 cm DBH. All lianas and trees conformed to the
same linear relationship between daily sap flow and DBH
(Fig. 6c).
Discussion
We found that six liana species utilized water from soil lay-
ers deeper than 30 cm during the middle of the dry season
(average xylem ?D = ?35? ) and all species switched to
deeper water sources (more than 60 cm deep) by the end of
the dry season (average xylem ?D = ?48? ). The largest
seasonal changes in xylem water ?D occurred in relatively
large individuals of E. monostachya and H. volubilis. This
pattern was not observed in larger trees, which tend to ex-
tract water from shallower sources than do smaller trees as
the dry season progresses (Meinzer et al. 1999).
The results in Fig. 1 suggest that soil water at depth
remains readily available during the dry season on Barro
Colorado Island. There was little change in soil volumetric
water content (?v) at 90?120 cm depths during the dry
286
Fig. 3 Diurnal courses of a photosynthetic photon flux density
(PPFD, solid line) and air saturation deficit (dotted line), b basal
sap flux density for the liana Abuta racemosa, and c basal sap flow
for the tree species Luehea seemannii and Platimiscium pinnatum,
on 11 February 1998 (day 42)
season of both 1997 and 1998. Shallower water content
differed between the 2 years, however. During the 1998
dry season (El Nin?o year), three main precipitation events
did not permit ?v at 0?30 cm depth to drop below 15% (data
not shown), although the total amount of rainfall for that
period was similar to the 1997 dry season. During the dry
season of 1997, however, there was a significant decrease in
?v at 0?30 cm depth due to evapotranspiration (Fig. 1). The
duration and severity of the dry season in lowland seasonal
Panamanian forests can vary considerably (Windsor 1990).
Though El Nin?o lowers total annual precipitation, it does
not necessarily affect the length of the dry season.
Pronounced droughts may affect shallow-rooted under-
story plants and even some trees in tropical forests (Becker
et al. 1988; Chiarello et al. 1987; Oberbauer et al. 1987).
Soil water potentials measured at 20 cm depth during a dry
season on the plateau of Barro Colorado Island dropped to
?2.3 MPa, which affected the predawn water potentials of
the shallow-rooted Psychotria horizontalis but not those of
Fig. 4 Diurnal courses of a photosynthetic photon flux density
(PPFD, solid line) and air saturation deficit (dotted line), b basal
sap flux density for the liana Entada monostachya, and c basal sap
flow for the tree species Alseis blackiana and Quararibea asterolepis,
on 20 February 1998 (day 51)
the deep-rooted Trichilia tuberculata (Becker et al. 1988).
Although it has been suggested that shrubs and tree saplings
may have roots at depths between 30 to 120 cm on Barro
Colorado Island, an irrigation experiment did not show any
change in tree phenology of the Barro Colorado Island
forests (Wright 1996), suggesting that most species have
access to the water table during the dry season. Indeed,
some canopy trees in this forest have been shown to extract
water from deeper and more reliable sources to maintain
similar sap flow rates during the dry season (Meinzer et al.
1999).
Lianas are considered among the most deep-rooted
species in tropical forests and there is evidence that their
roots can grow belowground to a depth of several meters
(Holbrook and Putz 1996; Tyree and Ewers 1996). Using
stable hydrogen composition of xylem water, Jackson et al.
(1995) found that the liana Bauhinia sp. had access to water
from much deeper portions of the soil profile compared to
sympatric tree species.
287
Table 2 Sap flow characteristics for seven liana species and eight tree species from Barro Colorado Island. Values are means ? SE (n =
sample size)
Duration of maximum sap flux
density (h)
Slope of sap flux density versus
ASD (m h?1 kPa?1)
n
Liana species
Abuta racemosa 2.38 ? 0.26 0.13 ? 0.02 8
Clitoria javitensis 6.83 ? 0.44 0.52 ? 0.04 3
Combretum decandrum 1.83 ? 0.46 0.22 ? 0.02 6
Entada monostachya 4.80 ? 0.50 0.39 ? 0.06 8
Hippocratea volubilis 4.60 ? 0.76 0.40 ? 0.06 7
Hiraea reclinata 1.25 ? 0.25 0.03 ? 0.01 4
Prionostemma aspera 2.14 ? 0.57 0.15 ? 0.05 7
Mean ? SE 3.40 ? 0.31 0.26 ? 0.03 43
Tree species
Alseis blackiana 1.35 ? 0.51 0.10 ? 0.02 7
Guatteria dumetorum 0.56 ? 0.03 0.11 ? 0.00 3
Hura crepitans 2.93 ? 0.43 0.10 ? 0.02 3
Luehea seemannii 1.46 ? 0.25 0.19 ? 0.02 5
Platimiscium pinnatum 2.57 ? 0.41 0.13 ? 0.02 8
Quararibea asterolepis 0.98 ? 0.20 0.09 ? 0.02 6
Trichilia tuberculata 1.73 ? 0.33 0.11 ? 0.01 9
Virola surinamensis 1.86 ? 0.33 0.06 ? 0.02 4
Mean ? SE 1.68 ? 0.12 0.11 ? 0.01 45
A notable feature of daily courses of sap flow in some of
the liana studied was a rapid early morning increase to a
plateau of several hours duration (e.g. Figs. 4b, 5b). Strong
stomatal limitation of water loss apparently allowed sap
flow to remain nearly constant despite increasing air satu-
ration deficit. The daily courses of basal sap flow for lianas
showed two distinct patterns (Figs. 3, 4, 5 panel b). Similar
patterns were observed in the daily courses of sap flow for
two liana species in a dry tropical forest in Mexico (Fitchner
and Schulze 1990). Sensitivity of stomata to humidity has
been documented for some temperate and tropical liana
species (Castellanos 1991; Teramura et al. 1991), where
stomatal closure with increasing ASD resulted in a little or
no increase in the rate of water loss. Sustained maximum
transpiration rates can also be maintained by a large water
storage capacity, as observed in some large tropical trees
(Goldstein et al. 1998). Teramura et al. (1991) suggested
that the ability of temperate lianas to maintain relatively
high leaf water potentials despite high transpiration rates
may be associated with considerable amounts of stored
water, possibly located in large tuberous roots (Forseth and
Teramura 1987). Large tubers have been observed in some
tropical lianas (Tyree and Ewers 1996). However, there
was no evidence that the liana species in this study possess
this type of underground organ, and we therefore suspect
that the flat profile of sap flow time courses results from
stomatal control. Because maximum stem diameter and sap
wood volume are typically smaller in lianas than in trees,
maximum stem water storage capacities in lianas are ex-
pected to be smaller than in trees. However, the architecture
of many lianas, consisting of leafy stems in the canopy sup-
ported by long sections of thick, leafless stem, may enhance
their stem water storage capacity relative to transpiring leaf
area. Our results raise a number of questions regarding the
water storage capacity, stomatal behavior, and regulation
of water balance among different liana species. For exam-
ple, relatively high transpiration rates are maintained over
an extensive portion of the day in C. javitensis, which has
very slender stems and therefore potentially low stem water
storage capacity. This species is among the 20 most abun-
dant liana species in the BCI forest (Table 1; Putz 1984),
and would be a good candidate for future ecophysiological
studies.
The xylem sap flux density (sap flow rate per cross-
sectional area of sapwood) in vines of a Mexican dry forest
was several times greater than rates reported for temper-
ate trees (Fichtner and Schulze 1990). Nevertheless, maxi-
mum sap flux densities in lianas on Barro Colorado Island
were similar to those in tree branches with comparable
cross-sectional area (Fig. 6), consistent with recent univer-
sal allometric scaling models for plant vascular systems
(Meinzer et al. 2001). According to the Hagan?Poiseulle
equation describing water transport in conduits, larger ves-
sels should result in higher sapflow velocities at a given
pressure gradient (Nobel 1991). However, high specific hy-
draulic conductivity associated with larger vessels in lianas
could result in smaller water potential gradients in lianas
than in trees, and therefore similar sap velocities in trees
and lianas (Ewers et al. 1991; Tyree and Ewers 1996). It is
important to note that because sap velocity cannot be mea-
sured directly with techniques commonly used to measure
sap flux density, sap flux density is an unreliable indicator
of true sap velocity as shown by tracer studies (James et al.
2003).
In summary, our observations that lianas tap shallow
sources of soil water at the beginning of the dry season
288
Fig. 5 Diurnal courses of a photosynthetic photon flux density
(PPFD, solid line) and air saturation deficit (ASD, dotted line), b
basal sap flux density for the liana Hippocratea volubilis, and c basal
sap flow for the tree species Guatteria dumetorum and Trichilia tu-
berculata, on 20 March 1998 (day 79)
and that sap flux density is similar in lianas and trees of
equivalent stem diameter do not support the common as-
sumptions that lianas rely primarily on deep soil water and
that they have higher rates of sap flux density than co-
occurring trees of similar stem size. Maximum sap flow
rates with deuterated tracers should be measured in lianas
and co-occurring trees to assess if predictions made based
on xylem anatomy are correct.
Acknowledgements This research was supported by National
Science Foundation grant IBN-9419500. S.S. was partially supported
by NSF grant DEB-9527729 and the Garden Club of Allegheny
County. We thank Smithsonian Tropical Research Institute for pro-
viding facilities and logistical support, and Sandra Bucci, Christian
Cruz, Jose? Luis Justavino, and Luis Mej??a for field assistance. Dr. Eric
Graham and one anonymous reviewer provided helpful comments on
an earlier version of the manuscript
Fig. 6 a, b Maximum sap flux density and c maximum daily sap flow
for lianas (open symbols) and branches and trees (closed symbols)
as a function to diameter at breast height. Open circle Hippocratea
volubilis, open square Entada monostachya, open diamond Clitoria
javitensis, open triangle Abuta racemosa, diamond with a dot within
Combretum decandrum, open inverted triangle Prionostemma as-
pera, black circle Trichilia tuberculata, black square Luehea seeman-
nii, black triangle Platimiscium pinnatum, black inverted triangle
Quararibea asterolepis, black diamondAlseis blackiana, dark bor-
dered circle Ficus insipida, dark bordered square Guatteria dumeto-
rum, hexagon Ochroma pyramidale, circle with a dot within Cecropia
obtusifolia. Values for tree branches from 4 to 6 cm diameter were
measured in 1997 and other values were measured in 1998
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