Geographic patterns of genetic diversity under climate change: linking genes and ecosystems

Climate change is having profound effects on species distributions. However, much less is understood about how climate change may alter the distribution of genetic variation within species across landscapes. Maintaining genetic diversity within populations is essential for the survival of species in the face of rapid climatic changes, but importantly, losses of genetic variation will also have significant consequences on entire ecosystems. The objective of this dissertation is to understand how genetic variation in a riparian cottonwood species, Populus angustifolia, affects mass and energy exchange between the land and atmosphere across ~1700 km of latitude of the western United States, and how genetic variation may respond to climate change. Specifically, I examine: (1) the potential for large-scale land-atmosphere feedbacks in hydrologic processes driven by geographic differences in plant population traits; (2) the extent to which including genetic population structure into species distribution models alters predictions of suitable conditions and geographic distributions; and (3) the extent to which genetic trait variation in bud break phenology is predicted to change in response to novel climatic conditions. The findings of this dissertation suggest that populations from landscapes with different hydrologic histories will differ in their ability to maintain favorable water balance with changing atmospheric demands for water; that species-range distribution models that do not include genetic information provide overly-broad projections of suitable conditions on the landscape compared to models that include population genetic structure; and, that a net loss of trait variation in future climates is predicted across all populations of P. angustifolia, with the trailing-edge population is predicted to lose the most. These ideas have broad implications for predicting population-level ecological and evolutionary responses to climate change as decreases to genetic variation reduce a species’ ability to adapt to novel conditions and can have profound effects on ecosystem processes like resource cycling (e.g., water, carbon, nitrogen) and on species interactions within ecological communities.