Do differences in understory light contribute to s

Authors: T BrenesArguedas A B Roddy P D Coley Thomas A Kursar

Publish Date: 2010/12/01

Volume: 166, Issue: 2, Pages: 443-456

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Abstract

In tropical forests regional differences in annual rainfall correlate with differences in plant species composition Although water availability is clearly one factor determining species distribution other environmental variables that covary with rainfall may contribute to distributions One such variable is light availability in the understory which decreases towards wetter forests due to differences in canopy density and phenology We established common garden experiments in three sites along a rainfall gradient across the Isthmus of Panama in order to measure the differences in understory light availability and to evaluate their influence on the performance of 24 shadetolerant species with contrasting distributions Within sites the effect of understory light availability on species performance depended strongly on water availability When water was not limiting either naturally in the wetter site or through water supplementation in drier sites seedling performance improved at higher light In contrast when water was limiting at the drier sites seedling performance was reduced at higher light presumably due to an increase in water stress that affected mostly wetdistribution species Although wetter forest understories were on average darker wetdistribution species were not more shadetolerant than drydistribution species Instead wetdistribution species had higher absolute growth rates and when water was not limiting were better able to take advantage of small increases in light than drydistribution species Our results suggest that in wet forests the ability to grow fast during temporary increases in light may be a key trait for successful recruitment The slower growth rates of the drydistribution species possibly due to tradeoffs associated with greater drought tolerance may exclude these species from wetter forestsChanges in species composition along environmental gradients increase species diversity at regional scales Chave 2008 For that reason a central question in plant ecology is how biotic and abiotic interactions determine why species grow where they do and to what extent species’ adaptations to environmental niches can limit their spatial distributions Suding et al 2003 Gaston 2009 Sexton et al 2009 At the regional scale an important correlate of species turnover is annual rainfall which in tropical ecosystems can vary tenfold between wet and dry forests Change in forest composition along rainfall gradients has been well documented in the literature Clinebell et al 1995 Swaine 1996 Bongers et al 1999 Pyke et al 2001 Davidar et al 2007 However there are many environmental variables that covary with annual rainfall and may contribute to determining species geographic distributions Because the interactions of these variables are complex and also differ among locations understanding the mechanisms that limit species distributions along this gradient is challengingHere we address the mechanisms that promote species turnover along a rainfall gradient as two questions What prevents wetdistribution species from colonizing dry forests and what prevents drydistribution species from colonizing wet forests Recent evidence suggests that intolerance to seasonal drought is the main factor limiting the distributions of wetforest species Drought tolerance correlates with species distributions along rainfall gradients Engelbrecht et al 2007 Also wetdistribution species have fewer adaptations to cope with water stress Baltzer et al 2008 Kursar et al 2009 and in addition suffer higher dryseason mortality if experimentally transplanted to a dry forest BrenesArguedas et al 2009 In contrast the mechanisms that prevent drydistribution species from establishing in wet forests are less clear Among other possibilities wetter sites tend to have poorer soils ter Steege et al 1993 Santiago et al 2005 higher pestpressure Coley and Barone 1996 Givnish 1999 BrenesArguedas et al 2009 and lower light availability see “Discussion” Our studies suggest that soil and herbivore effects are subtle relative to the effects of water limitation and if present may only be demonstrated in longterm experiments BrenesArguedas et al 2008 2009 The present study focuses on whether light availability influences the distribution of species along a rainfall gradient on the Isthmus of PanamaDry forests may have higher light availability in the understory for a number of reasons Dry forests can have fewer trees and less basal area per hectare than wetter forests Murphy and Lugo 1986 Losos and the CTFS Working Group 2004 Adaptations for water balance and temperature control can favor small leaves Givnish 1984 and this may result in lower leaf area index Deciduousness during the dry season should result in more canopy openness during part of the year Condit et al 2000 Finally lower rainfall may correlate with lower cloudiness and higher canopylevel sunlight Wright and van Schaik 1994 While each of these factors alone or in combination with others is likely to result in decreasing understory light availability with increasing rainfall to our knowledge the magnitude of this light gradient has not been measuredThe importance of light limitation and adaptations to contrasting light environments within a site are well documented Bloor and Grubb 2003 Balderrama and Chazdon 2005 Baltzer and Thomas 2007 Here we ask if a similar mechanism of niche partitioning based upon light availability contributes to the turnover of shadetolerant species along a rainfall/light gradient To address this we measured understory light along a rainfall gradient and studied the responses of shadetolerant plants that occur in understory light environments as these are the most common species and microhabitats in tropical forestsDrydistribution species may have adaptations that allow them to take advantage of higher understory light in drier forests and due to tradeoffs these may be less shadetolerant than wetdistribution species Smith and Huston 1989 Givnish 1999 Hence drydistribution species may be excluded from wetter forests by their inability to tolerate lower understory light However more lightdemanding species tend to have faster growth rates and superior competitive ability relative to shadetolerators Kitajima 1994 and previous analyses in our study system suggest that growth rates are faster for wetdistribution species BrenesArguedas et al 2009 Thus an alternative hypothesis is that competition for a limiting resource light or nutrients may be a major determinant of individual success in wetter forests Goldberg 1990 In tropical rainforests competition has received much attention from the perspective of shadetolerant versus gaprequiring species at a single site with limited consideration of how competition among shadetolerant species may change along a light gradient

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