Ecological Processes and Genomic Variation Associated with Seedling Performance in Tropical and Temperate Trees

Understanding the response of plants to natural selection is an important topic in plant evolutionary ecology, both in terms of understanding what structures plant populations spatially and genetically and how plants will be able to respond to changing conditions. We investigate three means by which seedlings respond to selection pressure: spatial escape, evolutionary response via local adaptation, and phenotypically plastic responses measured as changes in gene expression. First, in tropical forest in Ecuador, we test the classic Janzen-Connell hypothesis that seed dispersal is advantageous because it provides escape from host-specific soil pathogens associated with high seedling densities near maternal trees. In support of this hypothesis, we find that survival of Pentagonia macrophylla seedlings in the field increases and disease incidence decreases with decreasing density of conspecifics of various life stages. However, a greenhouse experiment suggests that trade-offs between dispersal and local adaptation, along with spatial heterogeneity in soil properties, shape seedling establishment. Second, we propose a method to test for evidence of selection in seedling populations, using two oak species (Quercus douglasii and Q. kellogii) planted in common gardens in California as case studies. We test for genetic differentiation between multivariate genotypes of seedling populations that survive or die and find that selection acts more strongly at distant than local sites and that the signal of selection increases over time. Employing SNP-survival association tests, we further identify 31 variants that may be responsible for observed genetic divergence between seedlings that lived versus died. Finally, we test the effects of one pervasive selective pressure, drought stress, on variation in gene expression among seedlings within a population of Q. douglasii seedlings. We identify a suite of genes that consistently respond to drought stress across all maternal families tested, in addition to a set of 70 genes whose transcriptional response to drought varies with family, providing adaptive potential to respond to changing conditions. This work suggests that selection by pathogens shapes the spatial structure of a tropical tree population and that standing variation in a temperate tree population may allow adaptive evolutionary and plastic responses to changing climate