Date of Award

Summer 8-15-2025

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Forest Resources (MFR)

Department

Forest Resources

First Committee Advisor

Jay Wason

Second Committee Member

Nicole Rogers

Third Committee Member

Yong-Jiang Zhang

Abstract

Forests are increasingly at risk to climate changes. Extreme climate events are occurring on faster timescales and are of larger magnitudes, yet are difficult to predict. Additionally, regionally novel extreme climate events and combinations of extreme events (i.e., heat and drought), may profoundly alter species distribution ranges especially at ecotones, where forested ecosystems merge. Ecotones may be more sensitive to change due to high levels of species biodiversity and many species at their range margins, thus serving as indicators of change. In the Northern Forest of the northeastern United States, the temperate-boreal forest has a mix of tree species with southerly and northerly range distributions, making it highly susceptible to extreme climate events. Therefore, in this thesis, we tested how winter warming, heatwaves, and drought events can affect tree regeneration in the Northern Forest using two experimental studies.

Our first study aimed to determine how spring phenology and cold tolerance of ten tree species in the northeastern U.S. responded to winter warming events and early springs. We used a greenhouse experiment to expose 300 seedlings of ten tree species to winter and spring warming events of different frequencies and durations while tracking leaf phenology. In addition to monitoring leaf phenology, we measured how two weeks of winter or spring warming impacts the cold tolerance of each species. We found that the timing of leaf out naturally varied among species, with deciduous species leafing out earlier than evergreen species in our control treatment. Overall, longer warming treatments significantly advanced leaf out, whereas the shorter warming treatments did not. The timing of leaf out was best predicted by photoperiod for deciduous species and growing degree days for evergreen species. For most species, cold tolerance was reduced when exposed to two weeks of warming, regardless of the time of year when warming exposure occurred. Our findings underscore the links between environmental cues and phenological sensitivities, which are critical in understanding future phenological shifts under a changing climate.

In our second study, we aimed to determine how growth, physiology, and survival of seedlings of the same ten tree species are impacted by extreme drought, a summer heat wave, and a combined drought and heat wave. We exposed 760 seedlings to month-long drought, heat, and compounded drought and heat events across six sites representing a wide climate gradient in Maine, U.S. We measured the growth and survival of seedlings, as well as the degree to which species functional traits explained growth and survival. Seedlings exposed to the compounded heat and drought treatment experienced the most mortality, especially among northern conifer species. Across all sites, the treatments did not affect the overall growth during the treatment year. Our results highlight that the location within a species range is a key determinant of survival rates when exposed to compounded heat and drought events, which may be due to their underlying stress tolerance traits. These findings have broader implications for managing future forest compositions, especially, at the temperate-boreal ecotone.

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