Population genetic structure, pollen dispersal, and local adaptation in Quercus oleoides forests of Costa Rica

Abstract

Recent and ongoing anthropogenic land use has altered natural landscapes and resulted in isolated patches of native vegetation across the globe. This process of habitat fragmentation reduces continuous habitat into small remnants in a matrix of altered terrain. The impetus for this research was to contribute to the growing body of work on the effects of habitat fragmentation while simultaneously gaining a better understanding of the specific role that recent fragmentation played in the evolution and demography of the most ubiquitous species in one particular region. My goal was to understand the evolutionary history of Quercus oleoides in Costa Rica in order to more effectively conserve and possibly restore the region’s seasonally dry forest in the future. How has the conversion of the seasonally dry forest of Costa Rica to an agricultural mosaic affected Quercus oleoides (live oak), the dominant tree species of remnant forest fragments? Although studies addressing the genetic consequences of habitat fragmentation are becoming more common, assessments of genetic structure and population viability that inform management decisions for conservation and restoration are rare. This study combined analyses of genetic diversity, pollen dispersal, and the growth and survival of various seedling families to provide an integrated evaluation of the response of a critical dry forest species to fragmentation and will help guide management and restoration efforts in the Aréa de Conservación Guanacaste (ACG). v The Q. oleoides forests of Guanacaste province, Costa Rica are something of a biological enigma: they are geographically disjunct and genetically distinct from conspecifics and similar species, and geographically quite restricted within Costa Rica while spanning a broad range of environments and associations within that range. Quercus oleoides is ectomycorrhizal in a habitat dominated by endomycorrhizal associations, possesses an atypical developmental process with regard to germination and emergence system, produces a fruit type that is extremely rare in the tropics, is wind pollinated in a habitat dominated by insectpollinated species, is evergreen in a habitat where most species are deciduous or semi-deciduous, and its reproductive phenology is largely mismatched to the seasonally dry environment of Guanacaste, producing large crops of dessicationsusceptible acorns at the beginning of a dry season more severe than what the species encounters anywhere else in its range. Despite this seeming mismatch between traits and environment, Q. oleoides is by far the most common large tree wherever it occurs. As such it is an extremely important structural species in Guanacaste dry forest. Its seeds are consumed by a wide range of mammalian and avian seed predators and its evergreen habit undoubtedly has a large effect on the abiotic environment experienced by many dry forest organisms. The subsequent chapters describe three previously unanswered questions about the past, present, and future status of Q. oleoides in the ACG. In Chapter 1, I characterized the standing genetic diversity of 13 Q. oleoides populations vi and the geographic structuring of that diversity. The pattern of that diversity was compared to geographic distance, flowering time similarity, and environmental similarity among populations. The structuring of genetic diversity was also compared between two age cohorts representing pre-fragmentation individuals and post-fragmentation individuals. I found that Q. oleoides in Costa Rica contained a high level of genetic diversity as well as genetic variation that is geographically structured across the landscape. The degree to which this structuring is due to fragmentation, however, is small in comparison to the genetic structure that has existed prior to fragmentation. This is somewhat counterintuitive due to the expectations provided from population genetic theory that can be applied to fragmented landscapes. If habitat fragments are isolated from one another such that gene flow no longer occurs among them, inbreeding may reduce offspring fitness and limit the viability of populations in those fragments. Isolated habitat fragments then become genetically differentiated over time due to the random process of genetic drift. Genetic diversity may also be affected because the amount of genetic variability in a population decreases due to the loss of rare alleles when the individuals carrying them are removed. This is termed a genetic bottleneck because the genetic variability of future generations is contained in the few surviving individuals. Small populations are vulnerable to stochastic environmental and demographic occurrences because adaptation by an organism to a changing environment depends on the genetic variability present in the population. The loss of genetic diversity reduces future evolutionary options and can lead to extinction. Population genetic variation consists of the sum of all genetic variation among individuals within the population. It can be measured by parameters including allelic richness (A) and expected heterozygosity (He). Allelic richness is the average number of alleles per locus and observed heterozygosity is compared to expected heterozygosity under Hardy-Weinberg equilibrium conditions. Wright’s F-statistics are means of describing how genetic diversity is partitioned in a population. High values for FST indicate that subpopulations have very different gene frequencies than the total population. A loss in heterozygosity can occur with inbreeding due to the higher chance that offspring of a mating event between two individuals with the same common ancestor may share the same alleles. One method for quantifying genetic variation within species is to assay highly variable regions of repeated DNA units called microsatellites. Individuals of a population were characterized by the differences in length of 11 of these non-coding genetic units. Although I observed no significant correlations between genetic distance and geographic distance, flowering time similarity, or environmental similarity in Chapter 1; I analyzed pollen dispersal more rigorously in Chapter 2 in order to better calculate contemporary pollen dispersal distance estimates. It is not unusual for studies of plant populations in fragmented landscapes to report few of the negative consequences predicted by theory, and that is because pollen may actually disperse father in fragmented landscapes. My results from two separate molecular analyses of pollen dispersal distance using 8 of the microsatellite markers from Chapter 1, however, indicated that the average pollen dispersal that resulted in viable offspring predominately occurred over very short distances. Both the paternity exclusion and two-generation methods yielded similarly short dispersal distance estimates. Evidence from the physical trapping of pollen in one location indicated that pollen was capable of moving much farther, however, so the importance of long distance pollen dispersal may rely more on phenology. I observed staminate and pistillate flowering times in 10 sites over two years, but the lack of strong seasonality in flowering obscured any obvious patterns. The geographic structuring of genetic diversity and the short average pollen dispersal distance provide a sound foundation for testing for local adaptation in Q. oleoides populations. In Chapter 3, I compared the growth and survival of upland and lowland maternal families in their native and foreign environments. The native environment of the populations of families differs most notably in their elevations and the lack of precipitation during the 4-5 month dry season in the lowlands. Seedlings planted in the lowland garden from both populations experienced a much higher level of mortality than seedlings planted in the upland garden, but using the aster models approach for comparing the likelihood of various models of combined growth and survival data, we did not identify evidence for local adaptation. Overall, these experiments indicate that contemporary Q. oleoides in Costa Rica have a rich and complicated population genetic history that despite obvious and extensive habitat fragmentation has not severely affected genetic variation or demographic processes. The long term outlook for the recovery of the tropical dry forests in general and the Q. oleoides stands, in particular, is good. Little direct action by managers is required and any active planting efforts do not seem to be encumbered by site-specific seed requirements. I do recommend local seed sources, however, out of an abundance of caution. These results not only add to the field fragmentation studies by examining a common, tropical tree over multiple habitats; this work also provides applicable information to an actively managed region that is in a transitory successional state.