Introduction / Background / Justification

Future droughts in the Caribbean are predicted to intensify, which may influence the functional diversity of tropical tree communities. Tree species with traits that buffer the physiological stress of low soil moisture (e.g., deep roots, dense wood) may have a demographic advantage, potentially impacting forest composition and carbon dynamics.

Objective(s)/Hypothesis(es)

We grew seedlings of eight common tree species spanning a range of life history strategies under experimentally manipulated soil moisture conditions in the Luquillo Experimental Forest, Puerto Rico. The objectives were to address: (1) Are species’ demographic response to soil moisture variability related to their mean trait values? And (2) Is phenotypic plasticity in seedling functional traits related to soil moisture?

Methods

Using rain-out shelters (1.2 m x 1.2 m), we created a gradient in average soil moisture conditions, ranging from 9-49%, over an eight-month study period. Rain-out shelters were placed on both experimental and control plots (N=30 ea.). Rainfall was collected from control shelters and placed on the plots following each rain event. We monitored seedling survival weekly and growth (total leaf length) monthly. A final harvest was used to measure total biomass, root, stem, and leaf traits of the surviving individuals.

Results

Survival of all species responded positively to higher soil moisture. However, species with more conservative strategies (i.e., higher leaf mass per area, and higher stem density) were able to survive even under low soil moisture conditions, compared to species with more-acquisitive strategies which suffered higher mortality under drier conditions. Leaf area growth and survival of species with more conservative functional strategies also tended to be more sensitive to soil moisture, in that their survival and growth increased more rapidly per unit of soil moisture, compared to species with more acquisitive strategies. Intraspecific trait variation was related to soil moisture for most traits and species, suggesting that phenotypic plasticity may play an important role in moderating drought response of seedling populations.

Implications/Conclusions

Our results show how drought selects for individual seedlings with traits that confer greater survival and growth at low soil moisture. Over time, individuals more conservative resource acquisition and use strategies may become more common under increasingly frequent and intense droughts in the Caribbean.