Differentiation of physiological traits among tropical live oak populations throughout dry forests of Central America

Background/Question/Methods

Local adaptation and physiological divergence of populations across a species range can result in differential vulnerability of populations to environmental change. Populations of the lowland evergreen live oak (Quercus oleoides), an ecologically important dry forest species of high conservation value in Central America, occur in contrasting climates that differ in dry season length throughout the species range. The Costa Rica population has been isolated from the rest of the range since the mid-Pleistocene due to volcanic activity, possibly permitting physiological differentiation and local adaptation unimpeded by gene flow. Here we ask whether populations of Q. oleoides from different rainfall regimes show genetic differentiation in water use traits. Specifically, we hypothesize that the Costa Rica population has evolved physiological traits important in local adaptation. Seeds from four Central American populations (Belize, Honduras, and two populations from contrasting elevations in Costa Rica) were sown into common gardens at Zamorano University in Honduras and the Guanacaste Conservation Area in NW Costa Rica.  Rainfall regimes were manipulated, simulating contrasting wet season and dry season rainfall patterns in a full-factorial block design, After two years of growth, physiological measurements, including water use efficiency (WUE) and whole plant hydraulic conductance (WPHC) were taken.

Results/Conclusions

Initial results indicate that populations do not show genetic differentiation in iWUE or WPHC under wet season conditions.  However, treatment had a significant influence on both WUE and WPHC (df=6, F=11.79, p<0.0001, and df=6, F=197.43, p<0.0001 respectively).  There was a significant population by treatment interaction for predawn leaf water potential (df=10, F=2.066, p<0.05) indicating that populations differ in either rooting depth or on individuals' capacity to equilibrate with the soil under different precipitation scenarios. Taken together these findings indicate that fragmented populations throughout the Central American tropical dry forest do not show differentiation in this set of physiological traits related to water use and may be vulnerable to changes in rainfall patterns.