As human disturbances increasingly cause tropical forests worldwide to shift towards earlier successional states and climate change increases the severity and frequency of drought events, it is important to better understand how disturbed forests might respond to drought and whether early- and late-successional tree species consistently differ from one another in their water relations. Tropical pioneer tree species are traditionally thought to have greater tolerance to drought than late-successional species, since early successional environments are sunnier and experience greater variation in temperature and humidity than mature forest. However, mature tropical rainforests exhibit a high degree of niche partitioning with regard to light environment, evident in the stratification of the canopy and understory. The objective of this study was to determine whether variation in foliar water stress tolerance can be linked to successional habitat specialization in an extremely diverse tropical rainforest. In this study, we compared leaf pressure-volume curves, pre-dawn and midday water potential measurements in situ, as well as wood density and leaf mass per area (LMA), in five early- and five late-successional tree species. We hypothesized that early-successional (shade intolerant) species would have more negative turgor loss point, lower bulk modulus of elasticity, greater osmotic potential, lower leaf mass per area (LMA), and lower wood density than late-successional (shade tolerant) species.
Results/Conclusions
Pressure-volume curve parameters showed significant differences between species, but no clear pattern linking water-relations traits to successional habitat specialization. Both LMA and wood density showed some correlation to successional position, although phylogeny could also be important. The findings suggest that high functional diversity in terms of leaf-level water relations within the forest community in different successional environments can outweigh functional differences across succession. This could have implications for the community-wide resilience of secondary tropical forests in the face of increasing severity of drought events associated with a warming climate.