Date of Award

Spring 2025

Project Type

Thesis

Program or Major

Natural Resources

Degree Name

Master of Science

First Advisor

William H McDowell

Second Advisor

Alexandra R Contosta

Third Advisor

Wilfred M Wollheim

Abstract

Climate change is altering the depth and duration of winter across eastern North America, with implications for ecological processes dependent on the coldness and seasonality provided by it. Tree green-up is a critical ecosystem transition point being affected by changing climatic conditions. This study investigates how winter season severity- the cumulative depth and duration of the winter season- affects the timing of spring green-up across a dynamically changing region. Using site-level linear trend analysis, meta-analysis, and mixed-effects linear regression, I assessed spatio-temporal changes in winter severity and green-up timing, employing a precipitation-based winter season severity index and PhenoCam remote sensing technology. I assessed whether declining winter severity predicts regional green-up timing shifts, and how the effects of this change vary between northern and southern locations. I additionally assessed potential covariate drivers of green-up. Results showed significant declines in winter season severity in regions north and south of 41°N latitude. However, regions north of that latitude underwent more rapid and variable change than regions south. Green-up changes were weaker and more spatially heterogeneous. Southern locations experienced a marginally significant advancement in green-up timing, while northern locations experienced a non-significant delay in timing. Temperature severity scoring emerged as a significant predictor of green-up in northern regions, with total winter season precipitation as a weaker covariate. In southern locations, neither was related to the timing of green-up. My findings demonstrate the spatial and regional variability of tree green-up and the complex nuanced drivers that influence its timing. As winters continue to change unevenly across this region, it is imperative to understand the interacting mechanisms that drive this biologically important process.

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