Comparative population genetics of tropical trees across a rainfall gradient in Central Panama: Implications for species' responses to climate change

Background/Question/Methods

Pronounced shifts in tropical dry season length and intensity are predicted as a result of climate change. Predicting how populations and communities will respond is a fundamental challenge for ecologists. Previous work has demonstrated large interspecific variation in co-occurring tropical tree species in their response to seasonal drought. However, we know less about intraspecific variation in drought responses.  How strongly individuals and populations vary in their drought responses, the extent this variation is determined by genetic and environmental factors, will in part determine how populations, species, and tropical forest communities will respond to climate change.  We present preliminary results from a project that is assessing intraspecific variation in drought responses for 16 species of tropical trees across a strong rainfall gradient in Central Panama (1700 - 3000 mm annual rainfall in ~ 70 km). Our project combines reciprocal seedling transplants, common garden drought experiments, physiological measurements, and genetic analyses to assess the extent to which species show intraspecific variation in response to moisture and drought.  In this symposium talk, we present preliminary results on how those species examined to date vary in genetic diversity and differentiation across the rainfall gradient and the implications our results have for the movement of adaptive genetic variation.  We choose 16 focal species that differ in their distribution across the rainfall gradient, with 5 species occurring across the entire gradient, 7 species are dry side restricted, and 5 species have predominantly wet side distributions.  We used a genotype by sequencing approach to identify unique genetic variation within and among populations and species.

Results/Conclusions

We find significant genetic differentiation among populations of our focal species across the rainfall gradient.  In the widespread canopy tree Anacardium excelsum, we find no evidence of inbreeding within populations (F_IS = -0.02, P = 1.00) compared to the dry side restricted Chrysophyllum cainito (F_IS = 0.05, P < 0.001), with the former showing less genetic differentiation between populations (F_ST = 0.027, P < 0.001) than the latter (F_ST = 0.047, P < 0.001). Our results suggest that, despite the small geographic distances that separate our populations, enough genetic differences exist between populations to suggest restricted gene dispersal distances, and that variation exists among species. Our results have important implications for how these species will respond to changing environmental conditions.