Date of Award

2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Plant Biology

First Advisor

Stephen R. Keller

Abstract

The composition of forest communities and the distributions of individual tree species are both strongly tied to climatic conditions through species-specific physiological tolerances to the abiotic environment. As a result, spatial and temporal variation in climate, both natural and anthropogenically induced, exert strong influence on tree species distributions and their adaptations to local conditions. In order for trees, which are sessile, to persist in a rapidly changing environment, genetic variation and/or phenotypic plasticity must be maintained to facilitate adaptive evolution. While strong local adaptation to current climate has been reported for trees sampled across broad spatial landscapes (e.g., latitude), few studies have investigated microgeographic adaptation, or adaptation occurring within the dispersal neighborhood, despite the common occurrence of tree populations distributed across steep fine-scale environmental gradients (e.g., elevation). Understanding the spatial scale of local adaptation and the capacity for adaptive evolution is a key issue under ongoing climate change, as many forest tree species become exposed to climate conditions outside of their current adaptive optima.

In this dissertation, I used multidisciplinary approaches to investigate how climate shapes biodiversity across and within forest tree species. I utilized a long-term forest tree inventory dataset to examine how species composition along an elevational climate gradient in the northeastern United States has responded to anthropogenic environmental change. I found that complex species-specific responses have led to an overall reduction in beta diversity in recent years, yielding a more homogeneous community, with the combined effects of sulphate deposition and warming temperatures being the two main drivers of this change. To assess how intraspecific diversity responds to this elevational climate gradient, I focused on red spruce (Picea rubens Sarg.), a coniferous tree abundant in high elevation spruce-fir forests of Vermont and other cool, mountainous locales throughout eastern North America. Utilizing population genetic techniques, I found limited genetic structure in red spruce populations along elevational gradients, pointing to extensive gene flow. However, divergent selection between elevations has been strong enough to overcome high gene flow, allowing for local climatic adaptation in quantitative traits such as bud phenology and cold tolerance. Finally, I established a common garden study replicated along an elevational gradient of planting sites to test the spatial scale at which local adaptation to climate and phenotypic plasticity occurs and quantified genetic variation for these processes. Significant heritable genetic variation was found for both local adaptation and phenotypic plasticity in families collected from fine- and broad-spatial scales for bud phenology and growth-related traits. Using the transfer distance between family source and planting site climates to predict the response of functional traits, I found strong evidence of local adaptation to source climate shaping bud phenology traits among broad-scale families yet impacts of transfer distance on overall early-life fitness were weak at both spatial scales. The magnitude of performance and bud phenology plasticity was similar between spatial scales, and plasticity in phenology traits (from either scale) did not confer a performance advantage.

Altogether, this work advances our understanding of how climate influences both the forest and the trees, at timescales spanning decades, and at spatial scales from hundreds of kilometers to the bottom versus the top of the same mountain. Understanding the drivers of forest community structure and the evolutionary mechanisms that trees can implement to counter the effects of a rapidly changing environment are imperative to help predict species responses to future climatic and environmental change.

Language

en

Number of Pages

180 p.

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