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The impacts of drought are expanding worldwide as a consequence of climate change. However, there is still little knowledge of how species respond to long-term selection in seasonally dry ecosystems. In this study, we used QST -FST comparisons to investigate (i) the role of natural selection on population genetic differentiation for a set of functional traits related to drought resistance in the seasonally dry tropical oak Quercus oleoides and (ii) the influence of water availability at the site of population origin and in experimental treatments on patterns of trait divergence. We conducted a thorough phenotypic characterization of 1912 seedlings from ten populations growing in field and greenhouse common gardens under replicated watering treatments. We also genotyped 218 individuals from the same set of populations using eleven nuclear microsatellites. QST distributions for leaf lamina area, specific leaf area, leaf thickness and stomatal pore index were higher than FST distribution. Results were consistent across growth environments. Genetic differentiation among populations for these functional traits was associated with the index of moisture at the origin of the populations. Together, our results suggest that drought is an important selective agent for Q. oleoides and that differences in length and severity of the dry season have driven the evolution of genetic differences in functional traits.

PREMISE OF THE STUDY Reproductive phenology is important for tree species that occur in seasonally dry environments, particularly for those with desiccation-sensitive, nondormant seeds. In this study, we compared germination, growth, and survival of seeds of the evergreen tropical live oak Quercus oleoides produced at different times during the wet season at two sites that differ in rainfall along an elevation gradient. Our goal was to determine the effects of reproductive timing on germination and juvenile fitness for this widespread species in seasonally dry forests of northwestern Costa Rica. METHODS We collected seeds early and late in a single wet season from two populations with contrasting rainfall and reciprocally planted them into common gardens. Two watering treatments (ambient and supplemental watering) were established at the drier low-elevation garden. Seeds were exposed to ambient rainfall at the wetter high-elevation garden. We conducted selection analyses using aster models to examine variation in selection on seed size and timing of germination. KEY RESULTS Trees of Q. oleoides had higher fitness when seeds were produced, dispersed and germinated late in the wet season. Postgermination, water limitation during the dry season reduced seedling fitness by decreasing survival but not growth. CONCLUSIONS In contrast to studies in temperate climates where earlier germination is typically favored, we show that selection on days to germination is temporally and spatially heterogeneous. Selection was found to favor either rapid or delayed germination depending on seed cohort and habitat.

• It has long been hypothesized that species are limited to the north by minimum temperature and to the south by competition, resulting in a trade-off between freezing tolerance and growth rate. We investigated the extent to which the climatic origins of populations from four live oak species (Quercus series Virentes) were associated with freezing tolerance and growth rate, and whether species fitted a model of locally adapted populations, each with narrow climatic tolerances, or of broadly adapted populations with wide climatic tolerances. • Acorns from populations of four species across a tropical-temperate gradient were grown under common tropical and temperate conditions. Growth rate, seed mass, and leaf and stem freezing traits were compared with source minimum temperatures. • Maximum growth rates under tropical conditions were negatively correlated with freezing tolerance under temperate conditions. The minimum source temperature predicted the freezing tolerance of populations under temperate conditions. The tropical species Q. oleoides was differentiated from the three temperate species, and variation among species was greater than among populations. • The trade-off between freezing tolerance and growth rate supports the range limit hypothesis. Limited variation within species indicates that the distributions of species may be driven more strongly by broad climatic factors than by highly local conditions.