Dissertation

The Phenology of Aboveground Tree Growth and Forest Carbon Cycling

Teets, Aaron

2022

Overview

Phenology is the study of recurring biological events and how they are affected by interannual variations in climate. Phenology has a direct effect on the annual productivity of forest ecosystems because it determines the length of the growing season and physiological activity. Longer growing seasons tend to have positive effects on forest ecosystem productivity in areas not limited by water, in others, longer growing seasons could amplify water stress and have a net negative effect on forest productivity. This dissertation entails three studies that investigate how forest phenology responds to changing environmental conditions. Each of the projects utilizes high-frequency measurements of forest phenology across different forest types to address knowledge gaps and better understand how forests cope with a changing climate. The first chapter investigates how the phenology of aboveground tree growth varies along an elevation gradient in the semi-arid southwestern United States (US). The timing of growth in these semi-arid ecosystems showed high variability among years, particularly at low elevations, indicating growth phenology in water-limited ecosystems is more variable than temperate forest ecosystems not limited by water. The second chapter utilizes digital repeat photography to extract phenology dates and model future changes in spring and fall transition dates in temperate deciduous forests in the eastern US. The models suggested season length will be more affected in the fall in response to future climate change than spring, but spring will have a larger positive effect on the forest productivity. The third chapter investigates the link between low temporal resolution tree growth measurements (i.e., annual tree rings) and high-resolution forest carbon uptake measurements (i.e., net ecosystem productivity) to identify potential lags in allocation of carbon to structural aboveground biomass. Lags in allocation of previous year carbon uptake to structural growth were detected at half of the sites studied, suggesting trees use stored carbon from previous years, but the role of stored carbohydrates for growth may not be as important in slow growing ecosystems due to other limitations on growth. By using multiple different sites in each of the outlined chapters, forest phenology was shown to respond strongly to the seasonality of most limiting factors. Phenology of forest ecosystems in the southwestern US had stronger links to water limitation, while the phenology of northeastern US was more strongly temperature limited. The widespread linkage of phenology to temperature in the literature suggests water-limitation is an undervalued driver of phenological change across the US, and globally.

• Publisher: ProQuest Dissertations & Theses
• Subject: Ecology
• ISBN: 9798368417592