OIKOS 92: 160–168. Copenhagen 2001

Interspecific variation in sapling mortality in relation to growthand soil moisture

John P. Caspersen and Richard K. Kobe

Caspersen, J. P. and Kobe, R. K. 2001. Interspecific variation in sapling mortality inrelation to growth and soil moisture. – Oikos 92: 160–168.

To examine the causes of landscape variation in forest community composition, wehave quantified sapling mortality as a function of growth and soil moisture for sevendominant species in transition oak-northern hardwood forests of the northeasternUSA. We located saplings in sites that encompassed a wide range of variation in soilmoisture and light availability. In mesic conditions, the probability of mortalitydecays rapidly with increasing growth among shade tolerant species and moregradually among shade intolerant species: the rank order of survivorship at lowgrowth rates is Tsuga canadensis\Fagus grandifolia\Acer saccharum\Fraxinusamericana\Acer rubrum\Quercus rubra\Pinus strobus. The relationship betweenprobability of mortality and growth does not vary with soil moisture among speciesinsensitive to drought: Tsuga canadensis, Quercus rubra, and Pinus strobus. However,probability of mortality increases substantially with decreasing soil water availabilityfor the other four species. Acer saccharum and Fagus grandifolia have high mortalityrates under xeric conditions even when their growth is not suppressed. Acer rubrumand Fraxinus americana exhibited a steady but more gradual increase in the probabil-ity of mortality with decreasing soil moisture. Among the five deciduous hardwoodspecies we examined there is a weak inverse relationship between the ability to survivegrowth suppression, a measure of shade tolerance, and the ability to survive in xericconditions, a measure of drought tolerance. Tsuga canadensis, however, is tolerant ofgrowth suppression and exhibits high survivorship in xeric conditions, while Pinusstrobus is intolerant of growth suppression but insensitive to soil moisture. Speciesdifferences in water-dependent mortality are consistent with the species distributionsacross landscape gradients of soil water availability.

J. P. Caspersen, Dept of Ecology and E6olutionary Biology, Uni6. of Conneticut,Storrs, CT 06269-3042, USA (present address: Dept of Ecology and E6olutionaryBiology, Princeton Uni6., Princeton, NJ 08544, USA [jpc@eno.princeton.edu]). – R. K.Kobe, Dept of Forestry, Michigan State Uni6., Natural Resources Building, EastLansing, MI 48824-1222, USA.

Sapling mortality exerts a profound influence on forestsuccession and landscape patterns of forest communitycomposition. Forest succession is driven in large partby species differences in mortality; species capable ofsurviving extended periods of growth suppression assaplings gradually replace species unable to regeneratein the understory (Canham 1985, 1990, Kobe et al.1995, Walters and Reich 1996). Similarly, the spatialdistribution of species has been shown to reflect differ-

ential sapling mortality along environmental gradients(Kobe 1996).

One of the most striking and well-studied patterns offorest landscapes is species zonation along soil moisturegradients (Whittaker 1967, Oliver and Larson 1996),which presumably reflects species differences in droughttolerance. Most studies of drought tolerance have fo-cused on the functional ecology of carbon gain (Buell etal. 1961, Small 1961, Cook and Jacoby 1977, Hinckley

Accepted 1 September 2000

Copyright © OIKOS 2001ISSN 0030-1299Printed in Ireland – all rights reserved

OIKOS 92:1 (2001)160

et al. 1979, Bahari et al. 1985, Abrams 1988, Abrams etal. 1990). The few studies on drought tolerance thathave examined mortality have focused on eitherseedlings or canopy trees (Barton and Teeri 1993, El-liott and Swank 1994). In this paper, we examine howlandscape variation in soil moisture influences the spe-cies-specific relationship between mortality and recentgrowth in saplings.

The functional relationship between mortality andrecent growth has been used to quantify interspecificvariation in shade tolerance (Kobe et al. 1995, Kobeand Coates 1997). In these studies, radial growth isused as an integrated measure of whole-plant carbonand has been shown to be largely limited by lightavailability (Pacala et al. 1994, Wright et al. 1998).Thus, species differences in shade tolerance are charac-terized by the relationship between the probability ormortality and recent growth (Kobe et al. 1995). Thesestudies show that shade-tolerant species survive ex-tended periods of growth suppression whereas shadeintolerant species more readily succumb to a marginalcarbon balance. Extending this method to include soilmoisture has enabled us to address the following ques-tions: 1) how do species-specific rates of sapling mortal-ity vary with recent growth and soil moisture? 2) doesthe rank order of species survivorship vary with soilmoisture? and 3) are species differences in survivorshipconsistent with patterns of species distribution andabundance along soil moisture gradients?

Methods

Study area and species

The study area encompasses Great Mountain Forest(GMF) and adjacent land in northwestern Connecticut,USA (42°00%N, 73°15%W). Precipitation in northwesternConnecticut is well distributed throughout the year,with an annual total of 1150–1250 mm (Damman andKershner 1977). Topographic variation in soil moistureat GMF reflects variation in drainage (contributingarea) and soil depth (Gonick and Shearin 1970). GMFsoils are sandy to coarse loamy inceptisols derived fromshallow deposits of acidic glacial till overlaying gneissand schist bedrock (Gonick and Shearin 1970). Along atypical slope, soils vary from thin, lithic dystrochreptson ridgetops, to typic dystrochrepts on midslopes andaquic dystrochrepts in the valley bottoms (Gonick andShearin 1970). Associated with this variation in soilmoisture is variation in forest community composition(Damman and Kershner 1977). Xeric ridgetops aredominated by oak species, while the mesic midslopesand lowlands are dominated by northern hardwoodspecies. Most forest stands in the region regeneratednaturally at the end of the 19th century followinglogging or agricultural abandonment.

We examined seven tree species common in the tran-sition oak-northern hardwood region of the northeast-ern United States. These species are dominant in mid-to late-successional stands. In approximate order ofdecreasing shade tolerance (Kobe et al. 1995), the spe-cies we examined are: Tsuga canadensis (L.) Carr. (east-ern hemlock), Fagus grandifolia Ehrh. (beech), Acersaccharum Marsh. (sugar maple), Pinus strobus L. (east-ern white pine), Quercus rubra L (red oak), Acer rubrumL. (red maple), and Fraxinus americana L. (white ash).Among these seven species white pine and red oak aregenerally considered to be the most drought tolerant(Burns and Honkala 1990). White pine commonly oc-curs on sandy dry soils associated with glaciofluvialdeposits and red oak on shallow dry soils found onsteep slopes and ridgetops (Caspersen et al. 1999).Hemlock occurs across a wide range of conditions fromxeric ridgetops and steep slopes to mesic valleys (Kessel1979, Dunwiddie et al. 1996).

Field sampling and measurements

The direct way to quantify probability of mortality as afunction of growth is to measure the growth of eachindividual in a population of saplings and determinewhich live or die over some subsequent time interval.However, because the mortality rates of saplings can bevery low (Kobe et al. 1995), such a method requires along time interval and/or a very large sample size toinclude a sufficient number of dead individuals. Kobe etal. (1995) developed alternative methods to estimate theprobability of mortality as a function of growth whichwe have extended to estimate the probability of mortal-ity as function of soil moisture as well as growth.

Following Kobe et al. (1995), two primary objectivesguided our selection of field sites. First, we chose fieldsites to span a range of understory variation in lightavailability and thus a range of variation in saplinggrowth rates. Previous studies in GMF show that thegrowth rate of understory saplings is largely limited byvariation in light availability (Pacala et al. 1994). Sec-ond, we chose two sites for each species (one mesic andone xeric) that span a wide and continuous range ofvariation in soil moisture (Table 1). The area of thesesites varied from approximately 1000 to 20000 m2. Toestimate the numbers of live and recently dead saplingsof the focal species, we established belt transects in eachof the sites in July 1996. Depending on the size of thesite and the density of dead saplings, a different numberof belt transects were used to sample approximately onetenth to one fifth of the total area of the site. Weconsidered a sapling to be any individual \25 cm inheight that did not have foliage in the canopy of thestand. We used the same methods as Kobe et al. (1995)to determine whether individuals had died within thelast 30 mo (2.5 yr), and excluded individuals that had

OIKOS 92:1 (2001) 161

Table 1. Soil moisture conditions and proportion dead (observed and estimated) in each site.

Species Site Estimated proportion deadMean soil moisture (range) (%) Proportion dead

Red maple 0.23Xeric 25.5 (10.1–34.8) 0.24Mesic 38.0 (30.3–63.9) 0.25 0.23

Sugar maple Xeric 0.3725.2 (14.7–33.6) 0.39Mesic 39.2 (30.5–63.9) 0.11 0.16

Beech Xeric 26.5 (19.2–36.4) 0.42 0.43Mesic 0.0939.65(33.4–60.2) 0.09

White ash Xeric 23.9 (14.3–31.21) 0.43 0.40Mesic 0.2235.2(25.2–41.7) 0.22

White pine 0.35Xeric 27.5 (21.1–34.2) 0.24Mesic 0.4138.8 (28.3–59.1) 0.50

Red oak Xeric 0.2827.0 (19.9–34.4) 0.27Mesic 30.7 (26.1–36.9) 0.31 0.28

Hemlock Xeric 0.3224.15 (13.6–35.7) 0.30Mesic 0.1237.3 (20.4–64.6) 0.11

died more than 30 mo before the sampling. The 1994–1996 period as a whole was neither extremely dry norwet compared to the entire climate record for north-western Connecticut, though it did include a significantlate-season drought in 1995 (minimum monthly PalmerDrought Severity Index= −2.14 in August 1995, Na-tional Climatic Data Center).

In July and August 1996, we harvested a randomsample of 25–50 living saplings and a random sampleof 25–50 recently dead saplings. We removed stemcross sections at 10 cm above ground level from eachsapling. For each of the harvested stem sections (liveand dead), we measured the radius and recent annualrings using a Velmex digital ring analyzer (0.01 mmresolution) and a 10× stereo microscope. Radius wasestimated as the average of the two radii bisecting theangles formed by the longest and shortest diameter ofthe cross section. The width of the five most recentannual rings was measured along these two radii tocalculate the average annual radial growth.

For both the live and dead saplings, we measured soilmoisture using a time domain reflectometer (TDR,Trase model 605X01, Soil Moisture Equipment Corpo-ration) following Topp et al. (1980). The TDR waveg-uides were inserted into the soil at the base of eachsapling to measure volumetric water content (%) to adepth of 15 cm. We minimized the sampling effort byusing a single pair of waveguides for each measurementand by limiting the number of measurements to one ortwo per sapling, on 17 July and/or 3 August. Themajority of saplings were measured twice, in which casewe report the average of these two dates. The values wereport must be viewed as a relative index of wateravailability for several reasons. First, one or two mea-surements do not provide a fully integrated measure ofthe daily and seasonal variation in water availability.However, in a parallel study in which soil moisture wasmonitored throughout the growing season using thesame methods we found that a single mid-season mea-

surement was sufficient to characterize the large differ-ences in soil moisture between sites spanning landscapegradients in GMF (Fig. 1). Second, we used a singlefactory calibration equation (Topp et al. 1980) ratherthan separate calibrations for different soil types be-cause the saplings spanned a continuous range of soilcharacteristics. The error due to using a single calibra-tion is small (1–10%) relative to the range of variationin soil moisture across sites (Gray and Spies 1995).Finally, comparisons with a Troxler-Sentry soil mois-ture probe showed that our Trase TDR consistentlyoverestimated water content by 10–15%. Nevertheless,the TDR measurements serve as an adequate index ofrelative soil moisture content because this difference isconsistent across the range of conditions we sampled inGMF.

Fig. 1. Correlation between a single mid-season soil moisturemeasurement (15 July) and the average soil moisture for thegrowing season (average of 11 measurements from 5 June to17 August).

162 OIKOS 92:1 (2001)

Statistical analyses

We used maximum likelihood estimation and a simu-lated annealing search algorithm to parameterize mor-tality models that quantify the probability of saplingmortality as a function of recent growth and soil mois-ture (Szymura and Barton 1986, Edwards 1992,Hilborn and Mangel 1997). The number of deadsaplings in a population, which was estimated from thetransect counts of live and dead stems at each site,follows a binomial probability distribution. Thus, theprobability that D dead saplings are found in a transectof N total individuals is a series of Bernoulli trials (liveor dead):

UN(1−U)N−D (1)

The probability of obtaining a dead sapling withgrowth g and soil moisture w is determined by the jointdistribution of growth rates h(g) and soil moisture f(w)and the probability of mortality m(g, w) for a saplingwith growth g and soil moisture w :

YD(g, w �dead) =h(g) f (w)m(g, w)&�

0

&�0

h(g) f(w)m(g, w) dg dw

(2)

Similarly, the joint distribution of growth rates and soilmoisture for live saplings is specified by:

YL(g, w �live)=h(g) f(w)[1−m(g, w)]&�

0

&�0

h(g) f(w)[1−m(g, w)] dg dw

(3)

The joint distribution of growth and water was spe-cified as the product of h(g) and soil moisture f(w)because they vary independently. To test for covariancebetween growth and soil moisture for all individualswithin each site (both live and dead), we calculated thePearson product moment correlation coefficient foreach species. To test for significance we conductedmultiple comparisons t-tests using a Dunn-Sidak cor-rection for the experimentwise error rate. To correct formultiple comparisons among species the critical valuefor significance is adjusted by the number of species.Growth was not significantly correlated with soil mois-ture for any of the species (R2B0.05 and p\0.18 forall relationships).

The likelihood of obtaining the entire data set for aspecies, including all sites, is a function of the parame-ters of the underlying probability distributions h(g),f(w), and the mortality function m(g, w):

L= 5csites

j=1

ÆÃÃÃÃÃÈ

·

UjDj (1−Uj)

Nj−Dj

5cdead

i=1

hj (gij) fj(wij)m(gij, wij)Uj

· 5c live

i=1

hj (gij) fj(wij)[1−m(gij, wij)]1−Uj

ÇÃÃÃÃÃÉ

(4)

where j denotes sites, i denotes individual stems, and Dj

and Nj−Dj are the counts of dead and live individualsat site j. Note that the mortality model m(g, w) iscommon to all sites but that N, D, U, h(g) and f(w) aresite-specific. Since growth and soil moisture were inde-pendently distributed, we used separate gamma densityfunctions for h(g) and f(w).

To test for an effect of soil moisture on mortality, wecompared a null model specifying mortality as a func-tion of growth alone with several models specifyingmortality as function of soil moisture as well as growth.The null model is a negative exponential model:

m(g, w)=e− (Ag) (5)

where A is a fitted parameter. The effect of soil mois-ture on mortality was tested with two different models:

m(g, w)=e− (Ag+Bw) (6)

m(g, w)=e− (Ag+Cgw) (7)

where A, B and C are fitted parameters that mayassume negative values, but m(g, w) is constrained tovary between 0 and 1.

The difference in log likelihood between these modelsand the null model, Dl,lw=2(Ll,w−Ll), is distributed asx2 with 1 df, as specified by the difference in thenumber of parameters between the models. In additionto computing a likelihood ratio test for each speciesindividually, we computed an aggregate likelihood ratiotest for all seven species as a group to find the a singlemodel that best fits the data for all species. We calcu-lated the aggregate likelihood by summing the maxi-mum log likelihood obtained for each of the sevenspecies, then used likelihood ratio tests with sevendegrees of freedom (one parameter for each species). Tocompare the two two-parameter models to one anotherwe used Akaike’s Information Criterion (AIC) for com-paring non-nested models (Hilborn and Mangel 1997).We calculated an aggregate AIC for all species bysumming the AIC for each individual species. Themodel with the smallest aggregate AIC statistic pro-vides the best fit to the data.

We used likelihood profiling to estimate asymptotic95% support limits for the parameter estimates of themortality models (Hilborn and Mangel 1997). We thenpropagated the uncertainty in the parameter estimatesthrough the model by calculating the mortality function

OIKOS 92:1 (2001) 163

Table 2. Likelihood ratio tests.

Model Effect pNull model D.F.*

B0.005Eq. 6 Growth+Soil moisture Growth (Eq. 5) 7Eq. 7 Growth+Soil moisture Growth (Eq. 5) 7 B0.005

*Likelihood ratio tests were calculated by summing the maximum log likelihood obtained for each of the seven species and using1 df per species.

for 10000 combinations of parameter values at theboundary of the 95% support regions. The calculatedminimum and maximum probability at each level ofgrowth and moisture bound the 95% confidence limitsof the mortality model.

Results

The models specifying the probability of mortality as afunction of both growth and soil moisture explainedsignificantly more variation than the models specifyingmortality as a function of growth alone (Eqs 5–7, Table2). Of the two two-parameter models, the model m(g,w)=e− (Ag+Cgw) provided a better fit to the data(AIC= −3329 vs −3804). Since growth occurs in bothterms of this model, the two parameters do not indepen-dently govern how growth and soil moisture influencemortality. As a consequence, the parameters cannot beinterpreted simply in biological terms. However, as Capproaches zero, the risk of mortality becomes lessdependent on soil moisture. When A is negative and Cis positive, the risk of mortality increases steeply withdecreasing soil moisture, as is the case for sugar mapleand beech (Table 3). This feature of the model results ina significantly better fit to the data, as the model is notsignificantly different from other 2-parameter modelswhen A is constrained to be positive.

The seven species exhibit considerable variation in theprobability of mortality with respect to both growth andwater availability (Fig. 2). Confidence intervals (95%)for the mortality functions show significant differencesamong species (Fig. 2) and the bivariate support limitsfor the parameters of the mortality function show thatspecies are well segregated in parameter space (Fig. 3).

The relationship between the probability of mortalityand growth in mesic conditions reflects species differ-ences in shade tolerance. Species less tolerant of growthsuppression exhibit a steep increase in the probability ofmortality with decreasing growth. More shade tolerantspecies, such as hemlock, beech and sugar maple, arecapable of surviving growth suppression and exhibit amore gradual increase in the probability of mortalitywith decreasing growth. In mesic conditions, the rankorder of survivorship is hemlock\beech\sugarmaple\white ash\red maple\red oak\white pine(Fig. 4).

The relationship between the probability of mortalityand water availability reflects species differences in

drought tolerance. Species tolerant of drought exhibitthe same growth-dependent probability of mortalityacross the full range of soil moisture conditions.Drought tolerant species include hemlock, red oak andwhite pine. In contrast, species intolerant of drought,such as sugar maple and beech, show a steep increase inthe probability of mortality with decreasing wateravailability. The rank order of survivorship inxeric conditions is hemlock\red oak\whiteash\redmaple\sugarmaple\white pine\beech (Fig. 5).White pine is relatively insensitive to drought but showspoor survivorship across all soil moisture conditions.

The relationship between shade and drought toler-ance is illustrated by plotting the probability of mortal-ity at low growth in mesic conditions versus theprobability of mortality at high growth in xeric condi-tions (Fig. 6). Among the broadleaf species there is anweak inverse relationship between shade tolerance anddrought tolerance: the most shade tolerant broadleafspecies, sugar maple and beech, are the least droughttolerant. Hemlock, however, is both shade tolerant anddrought tolerant and white pine is insensitive to droughtbut very intolerant of growth suppression.

Discussion

Most studies that examine tree species responses todrought seek to address whether species differ in tissue-water relations (Abrams 1988), gas exchange (Hinckleyet al. 1979, Bahari et al. 1985, Abrams et al. 1990) orgrowth (Buell et al. 1961, Small 1961, Cook and Jacoby1977, Hinckley et al. 1979). Relatively few studies haveaddressed whether species differ in mortality (Bartonand Teeri 1993, Elliott and Swank 1994), and mostfocus on seedlings or canopy trees. These studies showthat water availability can have a large effect on the

Table 3. Maximum likelihood estimates and 95% supportlimits of the parameters of the mortality model (Eq. 16).

Species A C

0.11 (0.07, 0.12) 0.11 (0.08, 0.12)Red maple−2.88 (−3.39, −2.15) 0.20 (0.16, 0.23)Sugar maple

0.40 (0.31, 0.45)−8.27 (−9.18, −6.15)BeechWhite ash 0.07 (0.03, 0.07) 0.13 (0.12, 0.19)

0.007 (0.005, 0.011)1.69 (1.13, 1.86)White pine0.0014 (0.001, 0.002)3.93 (3.108, 4.93)Red oak0.009 (0.005, 0.013)12.49 (10.31, 16.41)Hemlock

164 OIKOS 92:1 (2001)

Fig. 2. Probability of mortality (over a 2.5-yr period) as a function of average radial growth and soil moisture for seven species.

survival of seedlings and canopy trees and that there isconsiderable interspecific variation in survivorship. Inthis study, we show that soil moisture also has adramatic effect on sapling survivorship and that speciesdifferences in survivorship may in part explain speciesdistribution patterns.

The seven species exhibited substantial variation insurvival that is consistent with both the amplitude oftheir distribution and patterns of dominance along soilmoisture gradients. Sugar maple and beech, the mostshade tolerant broadleaf species, showed a steep in-crease in the risk of mortality at low soil moisture.Accordingly, these species are narrowly distributedacross soil moisture gradients and dominate maturestands only under mesic to intermediate conditions

(Damman and Kershner 1977, Host et al. 1987, Whit-ney 1991, Caspersen et al. 1999). Red maple and whiteash were intermediate in drought tolerance, exhibiting asteady but gradual increase in the probability of mor-tality with decreasing soil moisture. These species aremost common in mesic and intermediate conditions,but exhibit a broader amplitude along moisture gradi-ents than sugar maple or beech (Damman and Kersh-ner 1977, Caspersen et al. 1999). Red maple is a minorcomponent of ridgetop oak forests occurring on dryacidic soils while white ash is a subdominant species inoak and hickory forests on dry rocky soils enrichedwith limestone till (Damman and Kershner 1977). Redoak showed no variation in mortality across the rangeof soil moisture conditions we sampled and exhibits a

OIKOS 92:1 (2001) 165

Fig. 3. Bivariate 95% confidence regions of the mortalityfunction parameters (A, C).

Fig. 5. Probability of mortality as a function of growth at 23%soil moisture.relatively broad amplitude across soil moisture gradi-

ents (Damman and Kershner 1977, Host et al. 1987,Whitney 1991, Caspersen et al. in 1999).

White pine also showed no variation in mortalitywith respect to soil moisture which indicates that it isrelatively insensitive to drought. However, white pinewas the least tolerant of growth suppression among theseven species. In a previous study (Kobe et al. 1995),white pine was found to be intermediate in shadetolerance in a site on sandy soils derived from icecontact deposits rather than glacial till as in the presentstudy. The cause of this difference in survivorship ontwo different soil types is unknown. However, thehigher survivorship of white pine on sandy soils isconsistent with the common association of white pinewith sandy soils (Caspersen et al. 1999).

Hemlock exhibited the highest survival in mesic andxeric conditions alike, consistent with both its broaddistribution and late-successional dominance across awide range of conditions. In New England and

throughout much of its range, the distribution of hem-lock along moisture gradients is notably bimodal. It iscommonly found in mesic habitats in ravines, draws,and stream bottoms, as well as xeric habitats on steepslopes and ridgetops (Kessel 1979). Hemlock is also adominant species in old-growth stands in both mesicand xeric habitats, including xeric sites on steep slopesand ridgetops in western and central Massachusetts(Nichols 1913, Dunwiddie et al. 1996).

There are numerous factors other than soil moisturethat may influence sapling mortality and species distri-bution patterns. For example, the availability of nitro-gen and other mineral nutrients may covary with soilmoisture and may contribute to some of the variationin mortality that we observed. In addition, sapling sizeand species differences in rooting depth may contributeto some of the variation in mortality that we observed.Thus, further experimental studies are needed to firmlyestablish the underlying causes of inter- and intra-specific variation in sapling mortality. Nevertheless, ourresults show interspecific variation in mortality–soilmoisture relationships that are consistent with qualita-

Fig. 4. Probability of mortality as a function of growth at 40%soil moisture.

Fig. 6. The probability of mortality when light is most limitingversus the probability of mortality when water is most limit-ing.

166 OIKOS 92:1 (2001)

tive observations of drought tolerance (Burns andHonkala 1990).

Species differences in survival during drought areprobably due to a diverse range of physiological andmorphological adaptations. The contrast between thegas exchange rates of broad leaves and needle leavessuggests that leaf morphology may in part explain whythe conifers showed no variation in survival with re-spect to soil moisture. Broad leaves achieve muchhigher rates of photosynthesis than needle leaves be-cause the specific leaf area of broad leaves is muchhigher (Reich et al. 1992). Yet, because a larger leafarea entails greater evaporative demand, broad leaftrees incur a trade-off between maximizing carbon gainand minimizing water loss during drought (Ellsworthand Reich 1992). In contrast, needle leaves have lowerspecific leaf areas and lower evaporative demand, whichmay allow them to better survive during drought.

The contrast in carbon-water relations between oaksand other broad leaf species suggests that there are ahost of other adaptations that may foster survivalduring drought. Compared to other broad leaf species,oaks exhibit a variety of drought adaptations, includingdeep root systems, osmotic adjustment, ring-porousxylem anatomy, and thick leaves with small stomata(Abrams 1991). Thus, species differences in survivalduring drought likely reflect trade-offs at all levels ofplant function, such as trade-offs between photosynthe-sis and water-use efficiency, hydraulic conductance ver-sus cavitation resistance, and allocation to root versusshoot tissue.

Constraints on balancing carbon gain and water lossare also thought to impose a trade-off between shadeand drought tolerance (Smith and Huston 1989). Theproposed trade-off is premised on the assumption thatshade tolerance is achieved by maximizing low-lightcarbon gain (Smith and Huston 1989), which increasesstomatal conductance of both carbon dioxide and watervapor. Thus, tree saplings may face inevitable con-straints on growth when light and water are simulta-neously limiting. However, considering saplingsurvivorship, we find only weak support for a trade-offbetween shade and drought tolerance. A possible expla-nation for our results is that the ability to survivelimitation by one resource may be associated with aconservative strategy that confers tolerance of multiplelimiting resources. Indeed, there is a common suite oftraits that characterize plants found in low resourceenvironments, including low maximum rates of re-source acquisition, photosynthesis, and growth (Chapin1991). This suggests that there is a common suite ofmechanisms that allow plants to survive resource limi-tation (Chapin 1991), and perhaps multiple resourcelimitation as well.

Acknowledgements – This research was initiated by the leadauthor as a dissertation chapter at the Univ. of Connecticut

and was made possible by grants from NASA (Earth SystemScience Fellowship) and the Dept of Ecology and Evolution-ary Biology (NSF Graduate Traineeship in Evolution, Ecol-ogy, and Conservation of Biodiversity-BIR-9256616). The leadauthor is indebted to his dissertation committee, John Silan-der, Charlie Canham and Robin Chazdon, and to StevePacala. We thank the Childs family for their hospitality andfor use of the facilities at Great Mountain Forest. We thankBeth Hobby, Brandon Maio, Paul Barten Ed Roy, KristinGondar, and Ginger Pollack for help in the field and lab. Wethank Ben Bolker, George Hurtt, Helene Muller-Landau andMiguel Zavala for their advice and assistance.

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