Conservation prioritization based on past cascading climatic effects on genetic diversity and population size dynamics: Insights from a temperate tree species

Abstract

Aim: The biogeographic history affects adaptive evolution by altering the pattern of genetic variation and provides the background upon which other evolutionary processes are operating. Assessment of the consequence of such eco-evolutionary interactions is crucial to population's genetic diversity maintenance. Yet, considering how biogeographic and multiple evolutionary processes interact in complex environments to alter genetic diversity in conservation planning remains unresolved. Location: The Pacific Northwest. Methods: Herein I document the impact of geographic landscapes and past climatic fluctuations on the evolutionary processes that drive population divergence of Populus trichocarpa by assembling genomic, climatic and species occurrence data. Results: Based on inferences of demographic history of the species and tests for niche divergence, results show that the British Columbia (BC)-North and BC-South populations are ancestral and subject to more recent bottlenecks than the Oregon population. Consistently, ecological niche modelling illustrates that compared to the Last Glacial Maximum (~21 Ka), the present niche suitability of the BC-North and -South populations is overall marginally inferior to that of the Oregon population. However, genomic analysis demonstrates that the Oregon harbours the lowest genetic diversity, possibly due to an extended bottleneck experienced by the population. Model prediction indicates that in the 2050s, the impact of climate change is minor compared to the niche suitability of the species as a whole, but climate change will likely result in considerable genetic shifts over two generations especially in the Oregon and BC-South populations, which will attenuate along a northward gradient. Main conclusions: This study underpins the long-lasting impact of demographic histories and natural selection on genetic diversity, rather than highlighting the consequence of recent demographic events (e.g. bottleneck in the Holocene epoch; ~12 Ka) potentially cascading to eroding genetic diversity and yielding conservation urgency.