| dc.description.abstract | Rapid environmental change in recent decades has challenged Ecologists to focus on understanding ecosystem response and physiological functioning in the face of increased disturbances. Understanding physiological responses of trees to disturbance and climatic variability can enable researchers to manage ecosystems to ensure continued ecological functioning in the future. In this dissertation, I use classic dendroecology, tree physiology theory, original stable isotope methodology, and a novel analytical model to explore the impacts of disturbance and climate variability on Quercus species in forested ecosystems in the Midwestern U.S. Anthropogenic land use changes along with increased occurrence of pathogen, pest, and climatic disturbance events are impacting forest ecosystems. A tree's susceptibility to decline following disturbance events must be assessed to understand changes to forest ecosystem function and distribution, especially at species boundaries, with predicted future increases in the frequency of disturbances such as drought in the Midwestern U.S. The oak ecosystems of eastern Kansas and the mixed oak-hickory forests of northwest Arkansas have experienced high levels of climatic variability in the past 5 decades, which have influenced differential physiological responses of co-occurring species. In Chapter 1, I investigate differential physiological response to pest and drought disturbances in co-occurring Quercus rubra. By examining growth, stable carbon, oxygen and nitrogen isotopes in tree-rings, and contemporary leaf nitrogen dynamics, I show that differential stable carbon and oxygen isotope relationships in tree-rings, along with leaf nitrogen relationships, suggest varied susceptibility to disturbance among well-interspersed, co-occurring trees. The differential responses of co-occurring species may provide insight into future forest composition under the prediction of increased disturbance events. In Chapter 2, I use two co-occurring species to explore the impacts of climate variability on physiological responses. I investigate climate relationships through growth, stable isotopes in tree-rings, and contemporary leaf data in an effort to understand the future of these species at their western range boundaries at the prairie-forest ecotone of North America. I suggest that a typically drought-vulnerable species exhibits stable carbon and oxygen isotopic values suggestive of greater water stress relative to a less drought-prone oak species and find evidence hinting at differences in factors influencing carbon source:sink dynamics related to response to vapor pressure deficit (VPD) and photosynthetic regulation. Comparing these data to the C source:sink dynamics of the more drought-tolerant, co-occurring oak leads me to explore nitrogen dynamics in an effort to understand the impacts of climate variability on these species' growth. The dynamics studied in this forest-prairie ecotone at the University of Kansas Field Station provide insight into the changing forest dynamics in coming decades under predicted increases in drought disturbance events. Nonlinear patterns in ecological systems can provide insight into capacity of a system to deal with variability. In Chapter 3, I use simplex and s-map forecasting models to assess nonlinearity in growth between healthy and dying trees to determine if nonlinear growth dynamics may relate to a tree's vulnerability to mortality following disturbance events for two forested regions in northwestern and west-central Arkansas, USA. By applying nonlinear forecasting models in a novel manner, I investigate the utility of discerning non-linear vs. linear dynamics in growth for understanding forest ecosystem dynamics and predictions of adaptability of trees to climatic variability. I also explore data concatenation, or stringing together of time series to increase statistical power, of tree-ring data sets to assess the presence of nonlinearity pre- and post-drought disturbance events, and the potential use of concatenation with nonlinear forecasting models as a tool for exploring the future of forest ecosystems with predicted increases in disturbance events. I suggest that nonlinear growth dynamics are linked to increased capacity to adapt to variability for trees, and discuss why this may be the case. Disturbance events are predicted to increase in frequency and duration under future climate change. Maintaining forest ecosystems, and their ability to cope with stress, is thus an increasing concern for forest managers. In Chapter 4, I explore forest management policy and its impacts on forest decline events in northwest Arkansas. I review the management policies of our nation's forests, and suggest adaptive management strategies and monitoring tools for decreasing the vulnerability of forests to future disturbance events. I suggest that management policies should address local goals for increasing biodiversity and adaptability of forests in the future and recommend ecosystem monitoring tools for forest managers. The results of this dissertation suggest that increasing frequency of forest disturbances will have significant ramifications for forest ecosystems through impacts on forest ecophysiological function, species distribution, and carbon and nutrient cycling. Detection of disturbance vulnerability may help in managers develop strategies for increasing forest adaptability to disturbances such as droughts and pest infestations. | |
