, Bombacaceae) are widely thought to store water intheir stems for use when water availability is low. We tested this hypothesis byassessing the role of stored water during the dry season in three baobab species inMadagascar.• In the dry season, leaves are present only during and after leaf flush. We quantifiedthe relative contributions of stem and soil water during this period through measuresof stem water content, sap flow and stomatal conductance.• Rates of sap flow at the base of the trunk were near zero, indicating that leaf flushingwas almost entirely dependent on stem water. Stem water content declined by upto 12% during this period, yet stomatal conductance and branch sap flow ratesremained very low.• Stem water reserves were used to support new leaf growth and cuticular transpi-ration, but not to support stomatal opening before the rainy season. Stomatal openingcoincided with the onset of sap flow at the base of the trunk and occurred only aftersignificant rainfall.
Key words:
Adansonia
, Bombacaceae, leaf flushing, phenology, sap flow, stomatalconductance, tropical dry forest, water storage.
Stem-succulent trees are prominent in arid tropical ecosystemsaround the world, and baobab trees (
Adansonia
L.; Bombacaceae)are among the best-known examples. Because of the highwater content and large volume of their stems and branches,and their occurrence in seasonally dry environments, it haslong been assumed that baobab trees depend on stored waterduring periods of low water availability, and replenish thestored water during periods of rainfall or reduced water demand(Newton, 1974; Owen, 1974; Wickens, 1983; Baum, 1996;Sorg & Rohner, 1996). The actual role of water storage inbaobab trees, however, had not been examined until recently,and had simply been assumed from their physiognomy. In arelated study we found no evidence that stored water was used
to maximize daily stomatal opening during the rainy season;instead, baobab trees appear to prioritize conduit safety andturgor maintenance (Chapotin
et al
., 2006). The goal of thepresent study was to understand the role of stem water duringthe dry season. Baobab trees, like all stem-succulent trees,are leafless during most of the dry season, and new leavesare flushed only towards the end of the dry season. As thisphenological pattern considerably shortens their growingseason, the ability to use stored water for early leaf flushingcould be advantageous.
Stem water storage plays a physiological role in manydifferent plants, and contributes to their ability to survive in adiverse range of ecosystems. Arborescent palms can use waterstored in the stem to buffer xylem potentials and supportleaves when water availability is limited (Holbrook & Sinclair,
., 2000; Cedeño, 2001). Manydeciduous, dry forest trees have large, swollen stems with ahigh water content (Holbrook
et al
., 1995), and the propor-tion of species with this growth form increases in drier areas(Medina, 1995). Contrary to expectation from their highwater-storage capacity, stem-succulent trees shed their leavesearlier than co-occurring, nonsucculent trees, often before orat the onset of the dry season, a pattern suggestive of a ‘drought-avoidance’ strategy (Owen, 1974; Fenner, 1980; Holbrook,1995; Borchert & Rivera, 2001; Lobo
et al
., 2003). Additionally,during short periods of drought, stem succulents may closetheir stomata to prevent transpiration and turgor loss (Nilsen
et al
., 1990).Baobab trees, like many stem succulents, flush new leaves
before the rainy season. (Daubenmire, 1972; Frankie
., 2002). The end of the dry seasonis when soil water availability is lowest, and as baobab trees arethought to be shallow-rooted (Fenner, 1980; Guy, 1971), anygrowth during this time should be dependent on stored water.Our major objective in this study was to learn to what extentstored water is relied on during the dry season and for leafflushing. A second goal was to learn whether the use of storedwater allows baobab trees to extend their growing season,either by initiating stomatal opening before the onset of therainy season, or simply by having the leaves present when therainy season begins.
Materials and Methods
Field site
This study was conducted at the Kirindy Forestry Reserve,one of the largest remaining tracts of tropical dry forest inwestern Madagascar (longitude 44
°
49
′
E, latitude 20
°
27
′
S)(Rakotonirina, 1996). The majority of precipitation occursduring the rainy season (December–March) and rainfall isabsent during the main part of the dry season (April–October).Mean annual rainfall is 770 mm, but can be highly variableand ranges from 300 to 1400 mm yr
−
1
(Ganzhorn
et al
., 1990).Soils are sandy with low water retention, except near seasonalwatercourses where increased clay content confers a higherwater-retaining capacity (Sorg & Rohner, 1996).
Study species
The eight species in the genus
Adansonia
, commonly referredto as baobabs, have a disjunct distribution across Africa,
Madagascar and Australia (Armstrong, 1983). The six Malagasyspecies, which probably form a monophyletic group (Baum,1998), are restricted to the drier regions in the north, west andsouth of the island, where they are found in deciduous forests,spiny forests, scrublands and savannas. Baobabs range inheight from 5 to 30 m, and all have huge, swollen, cylindricalor tapered trunks (Baum, 1995). Rooting depth is generallyshallow, and roots can access a large area around the tree, withindividual roots traveling along the surface for up to 70 m(Guy, 1971; Fenner, 1980). Their low-density wood has largevessels, few fibers, and abundant parenchyma throughout thestem (Fisher, 1981; Newton, 1974).
The three species of baobab examined in this study were
Adansonia rubrostipa
Jum. & H. Perrier;
Adansonia za
Baill.;and
Adansonia grandidieri
Baill. (the latter was included onlyfor measures of water content and phenology).
Adansoniarubrostipa
and
A. za
are found over a large range of habitattypes in western and southern Madagascar, and exhibit a widevariety in overall morphology, while
A. grandidieri
has anarrower range and a more consistent morphology (Baum,1996). Study trees ranged from 1 to 3.5 m in diameter and15–25 m in height. Additional trees smaller than 1 m indiameter were included for phenological observations. Accessto the crown of the tree for installation of sap flow probes andstomatal conductance measurements was through use of thesingle-rope technique (Laman, 1995).
Phenology
Vegetative phenology was assessed during several seasonsbetween September 2001 and January 2003. Leaf bud breakand crown flush during the 2001 and 2003 leaf-flushingperiods, and extent of leaf drop during the 2002 leaf-sheddingperiod, were monitored on a weekly basis. Observations weremade on 45 trees of
A. rubrostipa
, 28
A. za
and nine
A.grandidieri
.
Soil water content
Soil cores were taken from a depth of 10–30 cm below theleaf-litter layer at four representative sites in the KirindyForest to quantify soil water content during the dry-to-rainy-season transition. The soil cores were placed in sealed plasticbags in the field, then brought to the laboratory, weighed,dried at 75
°
C and reweighed. Water content was calculated asa percentage of dry weight. Three cores were taken per siteevery 7–14 d from September to December 2003.
Stem water content
Cores 12 mm in diameter and 50 cm in length were extractedfrom the stem, between 0.5 and 1.5 m from the ground, usingan increment borer. Three sections 5 cm in length and centeredat 2.5, 15 and 30 cm beneath the bark were immediately
sealed into preweighed plastic bags. Fresh weight and volumewere determined before drying the cores at 75
°
C to a constantweight (24 h). Water content was calculated as percentage offresh volume (Domec & Gartner, 2002). Cores were extractedfrom five trees each of
A. rubrostipa
,
A. za
and
A. grandidieri
four times between October 2001 and February 2002, thenagain in January, February and March 2003. Two moremeasurements were made in September and December 2003.Five additional trees of
A. rubrostipa
and
A. za
were includedto increase the sample size for these last two measurements.Stem diameter at breast height was measured monthly with adbh tape to assess stem shrinkage and swelling in response touptake or use of stem water. Permanent guides were used toensure repeated measurements with the dbh tape were madeat the same height each time. Total water content in thestems of several representative trees at different times of yearwas estimated by taking the diameter of the tree at severalpoints along the stem and the height at the point of branchdivergence, and using the above stem water content values.
Sap flow measurements
Rates of sap flow were measured using the thermal dissipationtechnique (Granier, 1985), with probes similar in design tothose described by James
et al
. (2002). Unheated referenceprobes revealed large underlying temperature gradients in thestem and branches, which led to significant errors when thetraditional Granier calibration was applied to continuouslyheated probes. The probes were therefore heated discontinuously,following Do & Rocheteau (2002a, 2002b), and rates of sapflow were calculated by taking the difference between heatedand unheated measurements and then applying their calibrationformulas. Probes were inserted to a depth of 1 cm beneaththe bark, and were placed at the base of the trunk (0.5–1.0 mabove ground) to measure sap flow from the roots to the stem;at the top of the trunk (within 1 m of the point of mainbranch divergence) to measure sap flow from the main stemto the branches; and on distal branches (8–13 cm in diameter)to measure sap flow from the branches to the leaves. Twoprobes were installed at each location, and calculated sap flowrates from probes at the same location were averaged.Dimensions of the stem and branches at the locations of probeinsertion are presented in Table 1. Sap flow rates were measuredin two trees of each species during each measurement time(leaf-flushing period and transitional period).
Our measurements indicated that the conductive sapwoodin some places was probably < 1 cm in depth, which weestimated by placing probes at different depths to measurepresence or absence of sap flow at that point. As probe tipswere themselves 1 cm in length, it was not possible to resolve theactual conductive sapwood depth. Clearwater
et al
. (1999)discuss the implications of using sap flow probes longer orshorter than conductive sapwood depth and the variouscorrections that may be applied when the exact sapwood depth
is known. In the absence of accurate sapwood depth estima-tions, we focus on the relative timing and overall pattern ofsap flow at the different points in the tree, and we present ourdata as sap flux densities.
Stomatal conductance
Stomatal conductance of
A. rubrostipa
and
A. za
trees wasmeasured with a Li-Cor 1600 steady-state porometer.Measurements were taken every 30–60 min from dawn todusk to quantify the diurnal pattern of stomatal conductanceduring the 2002 rainy season ( January and February) in threetrees of
A. rubrostipa
and one
A. za
. During the 2003 dryseason, representative morning time-courses were made onnewly flushed leaves from one tree of each species. Ten to 15leaves per tree were measured at each point in time, and theaverage value calculated. Further measurements were taken intwo separate years (2001 and 2003) during the dry season tocompare levels of stomatal conductance before and after therainy season began. These measurements were made between09:00 and 10:00 h on three trees of each species, using 12–15leaves per tree. Measured leaves were distributed, to the extentpossible given the inherent difficulties with access, throughoutthe outer crown of the tree. Air saturation deficit (ASD) ateach measurement time was calculated from the relativehumidity and temperature of the air as measured by theporometer, according to the formula:
ASD =
e
s
−
e
a
where
e
s
is the saturated water vapor pressure and
e
a
is theambient water vapor pressure of the air.
Results
Phenology
Leaf break was initiated in early October and leaf shedding inearly March. All trees had a full crown of mature leaves by
Table 1 Dimensions of main stem and small branches at the location of probe insertion for each tree on which sap flow measurements were made
Parameter AR1 AR2 AR3 AZ1 AZ2 AZ3
Diameter at base (cm) 127 147 110 119 128 112Diameter at top (cm) 80 92 116 82 94 87Diameter of branch 1 (cm) 10 11 8 11 10 11Diameter of branch 2 (cm) 13 11 8 9 11 13Height to main branches (m) 13 12 11 13 13 17
Adansonia rubrostipa (AR); Adansonia za (AZ).Sap flow measurements on AR1, AR2, AZ1 and AZ2 were made during the leaf-flushing period; measurements on AR3 and AZ3 during the transitional period between leaf-flushing period and rainy season.
mid-December, and were completely leafless by mid-April(Fig. 1).
Soil water content
Soil water content varied between the different sites at theKirindy Forest, but remained constantly low within each sitefrom September to December, and did not begin to increaseuntil after significant rainfall had occurred. During the leaf-flushing period, water content values ranged from 3.8
±
0.1to 8.8
±
0.4 at the sites measured (percentage of dry weight,mean
±
SE). Note that the last measurements (28 December2003) were made at the beginning of the rainy season and donot reflect the maximal water content values that are expectedonce the surface soil water has replenished fully.
Stem water content
In
A. rubrostipa
, stem volumetric water content did not changesignificantly during the dry season (March–September), butdecreases in stem water content of 10.4 and 12.2% weremeasured during the leaf-flushing period (September–December) in two separate years (Fig. 2). There was a slightdecrease in stem water content in
A. za
and
A. grandidieri
, butthe difference was not significant. All species experienced avery small decrease in diameter during the dry season, but
A. za
, and to a lesser extent
A. grandidieri
, shrank considerablyduring the leaf-flushing period compared with the dry season(Fig. 3). The total volume of water lost was calculated forrepresentative
A. rubrostipa
and
A. za
trees. Despite the factthat the majority of water lost was through shrinkage for
A. za
and through water extraction with little size change for
A.rubrostipa
, the total volume of water for similarly sized trees ofeach species was approximately the same (1000 l). Measurementsof total stem volume were not available for
A. grandidieri
,which generally has a larger diameter, a greater height, and lesstaper than
A. rubrostipa
. Stem shrinkage in
A. grandidieri
would correspond on average to a four- to fivefold greatervolume of water loss for each meter of stem height than in
A. rubrostipa
.
Sap flow rates
During the leaf-flushing period, sap flow rates at the bottomof the stem were low, indicating little to no water entry fromthe roots and soil into the stem during this time. The probesat the top of the stem had the highest sap flow rates of thethree positions, and those in the small branches had moderaterates. Positive sap flow rates through the upper stem andbranches while rates at the base of the tree were close to zeroconfirmed that water reserves in the stem were used to supportleaf flushing (Fig. 4). Water-use rates in the branches and thetop of the stem increased as leaf flushing progressed, thengradually decreased after the leaves were expanded. Occasionalrainfall events after the leaves had expanded caused sap flowrates at all points in the tree (base, top and branches) toincrease, although sap flow rates decreased quickly as soilwater was depleted (Fig. 5). Sap flow rates were highest in themorning, when ASD of the air was lowest, and declined tolower values over the course of the day. After the onset of therainy season, coincident with greater soil water availability,sap flow rates at all points in the tree reached higher values and
Fig. 1 (a) Percentage of study trees of each species with a full crown of leaves in two separate seasons (Adansonia rubrostipa, n = 45; Adansonia za, n = 28; Adansonia grandidieri, n = 9). (b) Daily rainfall during the same periods.
generally remained high throughout the day (0.5–1 l dm−2 h−1
at base; 2–3 l dm−2 h−1 at top; Chapotin, 2005).
Stomatal conductance
Stomatal conductance during the leaf-flushing period was closeto zero and remained low until after the rainy season began(Fig. 6). Some trees exhibited a small amount of stomatalopening shortly after sunrise, although the degree of stomatalopening was lower than during the rainy season by over anorder of magnitude, but stomata closed as the morning progressedand air-saturation deficit increased (Fig. 7). However, not alltrees exhibited early morning stomatal opening, and in somecases values of stomatal conductance were zero even duringthe morning after leaves were fully flushed (data not shown).
Discussion
Phenological patterns in African tropical dry forests varyconsiderably from site to site. Leaf flushing in certain forests
Fig. 2 Stem water content at three depths in the tree before and after the dry season (left), and before and after the leaf-flushing period (right). (a) Adansonia rubrostipa (AR); (b) Adansonia za (AZ); (c) Adansonia grandidieri (AG). Values are mean of five trees for all species during the dry season and AG during the leaf-flushing period, and for 10 trees of AR and AZ during the leaf-flushing period ± 1 SE. An asterisk between two data points indicates a significant difference over that time period (paired t-test, P < 0.05). Values from before the dry season are maximum values in stem water content obtained at the end of the rainy season.
Fig. 3 Diameter decrease at breast height during the dry season and leaf-flushing period for Adansonia rubrostipa (AR); Adansonia za (AZ); Adansonia grandidieri (AG). Values are mean diameter decrease ± 1 SE. A paired t-test (P < 0.05) indicated significant differences in stem diameter before and after each time period for AR and AZ during the dry season, and for all species during the leaf-flushing period.
Fig. 4 Sap flow rates in (a) two Adansonia rubrostipa (AR) trees; (b) two Adansonia za (AZ) trees during the leaf-flushing period. Leaves are fully flushed by the times indicated by arrows. Each line is the average of values from two individual probes in similar locations (except one probe only for the top probe on the first AZ tree; data are not available for any top probe on the second AZ tree).
can be entirely opportunistic with respect to water availability(Lieberman, 1982), while in others, such as the Kirindy Forest,many trees flush new leaves before the rainy season (Sorg &Rohner, 1996). The leaf-flushing period in baobab trees atKirindy occurs towards the end of the dry season, and a fullcrown of mature leaves is often maintained for several weeksbefore the onset of the rainy season. The end of the dry seasonis generally when soil water content is at its lowest andevaporative demand at its highest, and we therefore hypothesizedthat the water required for early leaf flushing and transpirationin baobab trees derives from stored reserves in the tree.Furthermore, relying on stored water might allow baobab treesto maintain stomatal opening even when soil water is unavailable,thereby increasing the length of the growing season.
Stem water usage
Stem water content was high in all species (71–75% byvolume in the outer sapwood), and generally decreased with
increasing depth into the stem. When expressed as a percentageof dry weight, water-content values in baobab trees (500–800%) are much higher than published values for othertree species (30–300%, Tsoumis, 1991) as well as otherstem-succulent trees (120–410%, Borchert, 1994a); althoughit should be noted that these values are high also becausethe solid fraction in baobab stems is quite low (Chapotin,2005).
The measured sap flow patterns were consistent withthe hypothesis that the water necessary for leaf flushing wasderived almost entirely from the stem and branches, and notfrom the soil. The presence of sap flow at the top of the stemindicated substantial amounts of water moving from the steminto the branches, and moderate flow rates in smaller branchesreflect water moving into the leaves to support leaf expansionand cuticular water loss. Increasing water use during leafflushing and decreasing water use thereafter probably reflectchanging leaf area and cuticular properties of the leaves duringmaturation (Fig. 4).
Fig. 5 Sap flow rates in (a) one Adansonia rubrostipa (AR) tree; (b) one Adansonia za (AZ) tree during the transitional period between dry and rainy seasons. Leaves were fully flushed by the time measurements were initiated. Sap flow rates responded quickly to small rainfall events (arrows) ranging from 1.5 to 8.5 mm total precipitation. Rainfall amount (mm) given next to each arrow. Each line is the average of values from two individual probes in similar locations (except one probe only for top of AZ).
Adansonia rubrostipa and A. za differed in the degree towhich stem diameter shrank with water withdrawal, althoughboth species lost a comparable volume of water during thisperiod. In A. rubrostipa the uniformly distributed fibers, vesselsand parenchyma cells appeared to maintain the volume of thewood as air replaced withdrawn water. In A. za, however, thegrouping of the parenchyma cells into tangential bands,alternating with bands comprised primarily of vessels andfibers, allowed the stem to shrink when water was withdrawn
without an increase in the air fraction. These results under-score the importance of not relying solely on water content ordiameter changes when evaluating stem water loss, as evenclosely related species can differ greatly in wood compositionand the extent of shrinkage with water loss.
Stem water does not support stomatal opening
There was no evidence that stem water reserves were beingused to support stomatal opening, as stomatal conductancewas consistently low until after the rainy season began and soilwater was replenished (Fig. 6). This was similar to the patternobserved in another dry forest deciduous tree, Enterolobiumcyclocarpum, where electron-transfer rates in newly flushedleaves were low until after the rainy season began (Brodribbet al., 2002). Comparing the relationship between stomatalopening and ASD during the dry season with that during therainy season demonstrates that greater stomatal closure is notsimply a factor of increased ASD during the dry season, andthus stomata must also be independently regulated by anotherfactor (Fig. 8). Measured leaf-water potentials before the rainyseason reached a minimum of c. −1.0 MPa during theafternoon, which was near the turgor-loss point for leaves(unpublished data for A. rubrostipa). We propose that limitedwater availability in the upper soil layers during this timerestricts stomatal opening and, as a consequence, photosyntheticactivity.
Whole-tree water use increased rapidly after isolated rain-fall events (Fig. 5). This increase in water flow was noticeableat all points measured in the tree, but particularly so in thesmall branches as a result of increases in transpirational waterloss from the leaves, and at the base of the stem, indicating theseasonal onset of water uptake by the roots. The rapidity withwhich water uptake followed individual rainfall events issignificant in that it indicates that fine roots were present bythe time leaves were flushed or produced quickly upon soil
Fig. 6 Leaf stomatal conductance in Adansonia rubrostipa and Adansonia za during leaf flushing (November 2003); after leaves are fully mature (December 2003); just after the rainy season has begun (January 2004); and in the middle of the rainy season (January/February 2002). All measurements were made between 09:00 and 10:00 h. Values are mean of three trees per species ± 1 SE (except one tree only for A. za in the rainy season of 2002), with five to 15 leaves measured per tree. Significant differences between months within a species are indicated by different letters (t-test, P < 0.05). Difference between the values marked by a′ and b′ had only a tendency towards significance (P = 0.06).
Fig. 7 Stomatal conductance during early morning on newly flushed leaves for one Adansonia rubrostipa and one Adansonia za tree in December 2003. Values are mean of 10–15 leaves per tree ± 1 SE. Air saturation deficits (ASD) during each measurement period are also shown.
hydration, as they were in another dry forest site (Kummerowet al., 1990). These increased rates of water use were not sus-tained, however, and rates of sap flow in the branches andstem declined gradually over the several days following therainfall event, in response to declining soil water availability.Sap flow rates then remained low until the next rainfall event.It was not until after the onset of the rainy season, when rainfallevents were frequent enough to prevent soil drying, that sapflow and stomatal conductance reached their maximal valuesand remained consistently high (Chapotin et al., 2006).
Limited use of stem water despite early flushing
Early leaf flushing in baobab trees does not appear to allowstomata to open before the rainy season when soil wateravailability is still low. This raises two main questions: whydoes leaf flushing in baobab trees precede the onset of therainy season; and why is stored water not drawn upon moreextensively to maximize carbon assimilation? The first questionis pertinent because the use of stored water for leaf flushingappears to necessitate some investment in water-storagetissue. Furthermore, daily temperatures reach their yearlymaximum during this time, and the inability to reduce leaftemperatures through evaporative cooling exposes the leavesto potentially damaging temperatures. Nontranspiring canopyleaves frequently reached 40°C, a temperature that inducesheat-stress responses in many plants ( Jones, 1992).
It has been suggested that a physiological advantage of earlyleaf flushing is that leaves are ready to begin photosynthesiz-ing when the rainy season begins (Rivera et al., 2002). Havingthe leaves present by the end of the dry season allowed baobabtrees to open their stomata as soon as soil water became avail-able, and eliminated the time lag that would otherwise haveexisted between the onset of the rainy season and the onset ofphotosynthetic activity. Additionally, early leaf flushing allowsbaobab trees to take advantage of scattered or early rainfallevents, although they may be of limited duration or intensity.
Even very small rainfall events occurring before the full onsetof the rainy season were accompanied by immediate increasesin sap flow, not only at the base of the tree, indicating wateruptake from the soil, but also in the branches, indicatingtranspirational water loss caused by stomatal opening. In anenvironment where the rainy season is short and its length canvary by several months, maximizing photosynthesis in thisway would be advantageous.
With regard to the second question, as to why more exten-sive use of stored water does not occur, results from tworelated studies indicate that stored water may be unavailablebecause of tissue water relations, transport limitations andbiomechanical considerations. Baobab stem wood is highlyvulnerable to cavitation and has a turgor-loss point near actualfield water potentials (Chapotin et al., 2006). In addition, theconductive portion of the sapwood is restricted to the 1–2 cmjust beneath the bark, so that radial transport of water in thestem to the transpiration stream is through a relatively highresistance pathway, and is likely to occur very slowly. This rateof water movement may be sufficient for the needs of growingtissues, but is unlikely to meet the demand of actively tran-spiring leaves. Furthermore, the wood is soft and weak, andcomprised primarily of living tissues which may exhibitdecreased mechanical stability under excessive water withdrawal(Niklas, 1988; Chapotin, 2005).
Water use in baobab trees before the onset of the rainy seasonappears surprisingly conservative, given the vast amounts ofwater in the trunk. However, the constraints described above,and the highly unpredictable climate to which baobab treesare subject, may require that use of stored water be limited inthis way. The timing of the rainy season can vary by severalmonths, and the total amount of precipitation in any givenrainy season is similarly variable (Fenner, 1980). By restrictingwater use to leaf flushing rather than stomatal opening, thebaobab tree is able to maintain leaf turgor until the onsetof the rainy season, and to produce a new crop of leaves thefollowing year if current-year precipitation levels were not
Fig. 8 Stomatal conductance as a function of air saturation deficit (ASD) in Adansonia rubrostipa trees during the dry and rainy seasons. All measurements were made between 07:00 and 15:30 h. Each point is the mean of five to 15 leaves from one tree ± 1 SE. Three to five trees per curve.
sufficient to replenish the water content in the wood. Theresults of this study appear to contradict general ideas aboutwater use in baobab trees by indicating a limited role forstored water. To our knowledge there are no similar studies onstem-succulent trees that focus specifically on water use dur-ing the period between leaf flushing and the onset of the rainyseason. Stem-succulent trees occur in many distantly relatedplant taxa, yet exhibit striking convergence in physiog-nomy, ecology and habitat. Although this study focuses onjust one genus, the results suggest that water-use patterns instem-succulent trees may need further study.
Acknowledgements
This project was made possible through funding by theSinclair Kennedy Traveling Fellowship, the Arnold Arboretumand the Department of Organismic and Evolutionary Biologyat Harvard University, the Garden Club of America, andthe National Science Foundation. We are thankful to LalaoAndriamahefarivo and other Missouri Botanical Garden staffin Antananarivo for acquiring research permits and providinglogistical support, and to the German Primate Center (DPZ)for permission to work at the Kirindy Research Station.Michael Burns provided invaluable technical expertise andRandol Villalobos assisted with equipment assembly. RodolpheRasoloarivony, Heather Patt and Mario Ramohavelo providedfield assistance. We also thank three reviewers for their helpfulcomments and suggestions.
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