Rainfall seasonality and pest pressure as determinants of tropical tree species' distributions

Abstract

Drought and pests are primary abiotic and biotic factors proposed as selective filters acting on species distributions along rainfall gradients in tropical forests and may contribute importantly to species distributional limits, performance, and diversity gradients. Recent research demonstrates linkages between species distributions along rainfall gradients and physiological drought tolerance; corresponding experimental examinations of the contribution of pest pressure to distributional limits and potential interactions between drought and herbivory are limited. This study aims to quantitate differential performance and herbivory as a function of species range limits across a climatic and floristic transition in Southeast Asia. Khao Chong Botanical Garden, Thailand and Pasoh Forest Reserve, Malaysia straddle the Kangar-Pattani Line. A reciprocal transplantation across a seasonality gradient was established using two groups of species (“widespread” taxa whose distributions include seasonally dry forests and “aseasonal” taxa whose distributions are limited to aseasonal forests). Growth, biomass allocation, survival, and herbivory were monitored for 19 months. Systematic differences in performance were a function of species distribution in relation to rainfall seasonality. In aseasonal Pasoh, aseasonal species had both greater growth and survivorship than widespread species. These differences were not a function of differential herbivory as widespread and aseasonal species experienced similar damage in the aseasonal forest. In seasonally dry Khao Chong, widespread species showed higher survivorship than aseasonal species, but these differences were only apparent during drought. We link this differential performance to physiological mechanisms as well as differential tolerance of biotic pressure during drought stress. Systematic decreases in seedling survival in aseasonal taxa during drought corresponded with previously documented physiological differences and may be exacerbated by herbivore damage. These results have important implications for tropical diversity and community composition in light of predicted increases in the frequency and severity of drought in hyperdiverse tropical forests.

Growth responses to soil water potential indirectly shape local species distributions of tropical forest seedlings

“…We also found little evidence for species having lower mortality rates “at home” (Table ) and we found no evidence for a “home advantage” (Table ). These results indicate that the role of first‐year mortality responses to SWP in shaping species distributions was relatively minor, even though experiments suggest that first‐year mortality influences distributions with respect to variation in water, nutrient, and light availability (Baltzer & Davies, ; Engelbrecht & Kursar, ; Engelbrecht, Kursar, & Tyree, ; Lucas, Bruna, & Nascimento, ). Contrasting with our results, larger seedlings (20–50 cm tall) of dry‐associated species had lower mortality than wet‐associated species in the same 50‐ha plot, particularly on the dry plateau (i.e., “home advantage”) in a severe dry season (Comita & Engelbrecht, ).…”

“…As we found that mortality rates declined rapidly with seedling height for all species (Green, Harms, & Connell, ; Rose & Poorter, ), a growth advantage allows seedlings to escape the vulnerable small seedling stage more rapidly (Kitajima & Fenner, ) than seedlings of other species. Experiments have documented species‐specific responses of seedling growth to water availability (Ashton et al, ; Baltzer & Davies, ; Born et al, ; Bunker & Carson, ; O'Brien, Ong, & Reynolds, ; O'Brien, Philipson, Tay, & Hector, ; Yavitt & Wright, ). Our study is the first to show that differential growth responses of naturally regenerating seedlings contribute to local species distributions along a gradient of soil water availability.…”

Contrasting patterns of insect herbivory and predation pressure across a tropical rainfall gradient

“…Enhanced density‐ and distance‐dependent insect herbivory in less seasonal and more humid forests has been suggested to contribute to this pattern (Janzen , Connell , Coley & Barone , Leigh et al . , Baltzer & Davies ).…”

Seasonal drought limits tree species across the Neotropics

“…Whilst drought‐tolerant species tend to have a higher capacity for water conductance and CO 2 assimilation under water‐limiting conditions, they grow more slowly in the scarce understory light of wet forests where shade‐tolerant species have a competitive advantage (Brenes‐Arguedas et al , , Gaviria and Engelbrecht ). Drought‐tolerant species are also apparently more vulnerable to pest damage in moist areas (Baltzer and Davies , Spear et al ). Thus, in less physiologically stressful environments, tropical tree species' occurrences could be limited by stronger biotic interactions, both with competitors and natural enemies (MacArthur , Normand et al ).…”