Genetic Legacies of 10,000 Years of Environmental Changes on Forest Trees

Global climate change and human land-use are two main threats affecting species and ecosystems, driving species extinction and biodiversity loss around the world. A critical issue at the forefront of these accelerating environmental changes is their impacts on long-term persistence and survival of species. Addressing this issue requires an understanding of how species, populations and communities have responded to different degrees of environmental perturbations in the past. Lessons from the past provide important evidence needed for a better characterization of the long-term consequences of these large-scale 20th century environmental changes and beyond.

My dissertation examines how environmental changes throughout the Quaternary (last 2.6 million years) impact patterns of genetic diversity through time. I examined two historical processes that have had the greatest impact in shaping genetic diversity among species and populations: past climatic oscillations since the last glacial period and the extensive regional forest clearance in the last 500 years. First, I assessed the coherence of the impacts of climate-driven range expansions on range-wide patterns of plant genetic diversity between Europe and Eastern North America. I showed that range-wide patterns of genetic diversity in plant taxa arising from these climate-driven range expansions markedly differed between Europe and ENA as a likely consequence of idiosyncratic features of each continent, for instance, biogeography e.g., presence of barrier to migration and climatic histories e.g. severity of climatic oscillations. Second, patterns of colonization can impact establishment and maintenance of genetic diversity in populations. I examined how presence and spread from small populations from farther north can influence maintenance of genetic diversity in a temperate tree species. I potentially provided the first empirical evidence of existence of ancient small populations farther north than commonly assumed and the early establishment of genetic diversity in these ancient populations. Finally, I showed that the extent of genetic impacts of the large-scale demographic declines due to extensive regional deforestations in the last 500 years varied between different habitats and between ecologically-similar forest tree species.

By explicitly examining these past processes at different temporal and spatial scale, my work provides a better and more complete picture of the long-term genetic consequences of past large-scale environmental changes on tree species. These insights are important and greatly needed in a better formulation of conservation and management strategies under accelerating environmental changes in the 21st century and beyond.