|dc.description.abstract||Maintaining tree vigor and forest health is critical as forest health risks increase over time. Eastern US forests have been losing quality in recent decades due to forest health risks such as tree mortality and nonnative invasive plant species (NNIPS) under extreme climate and disturbance regimes. These risks are often dynamic in space and time, taking specific spatiotemporal patterns. As such, there is a great need to integrate statistical and mathematical models to develop a multiscale geospatial modeling framework to address forest health risks affecting forest ecosystem sustainability. This study focused on two major leading forest health risks: 1) NNIPS in the Southern forestlands and 2) oak decline and mortality in the Ozarks in the Central US.
First, this study developed methods to quantify invasion severity indices and invasive stages, identified the best modeling scale by evaluating spatial autocorrelation, and investigated influencing factors associated with invasive severity and the invasibility of forests. Invasion severity increased over time and space in the region. County-level modeling unit best explained spatial autocorrelation and was chosen as the best modeling unit for the spatial analysis. The spatial lag model showed positive and statistically significant lag coefficients, implying neighbors' importance in explaining the invasion severity. Road length, neighbors' population density, and the number of households are likely to increase the invasion severity.
Second, this study explored species composition change, evaluated the effect of biotic and abiotic stresses on oak mortality, and examined species' response to extreme drought in terms of resistance, recovery, and resilience. The tree species composition in the Ozarks has changed; oak and pine dominance has reduced, and less commercially valuable species such as hickory, maple, and other eastern hardwood proportions have increased. The red oak group was at serious risk because of the higher mortality and lower ingrowth. Further, species responses to drought were significantly different among species groups. Fast-growing species, such as red oaks, had a lower resistance, recovery, and resilience; in contrast, slow-growing species, such as white oaks and hickories, had a higher resistance, recovery, and resilience.
In summary, this study prescribed tools to quantify and classify the severities of NNIPS; those will help develop and implement policies to control and mitigate the negative consequences of NNIPS. This study also provides insight into species' response to severe drought, which will help better understand potential species composition changes in the future and develop the adaptive mechanism to increase stand productivity.||en_US