Past and future disturbances are linked by their feedbacks with forest structure-- the size, species, and spatial distribution of vegetation in a forest. Disturbances like wildfire and bark beetle activity can alter forest structure, which then influences the outcomes of future disturbances. The long-term persistence of forest ecosystems hinges on these feedbacks, which promotes resilience-- the ability of a system to absorb disturbances and still retain its essential identity and functions. I explore these feedbacks by measuring disturbance severity as well as local-scale forest structure at broad spatial extents in the yellow pine/mixed-conifer forest system of the Sierra Nevada, California. I bring new tools, such as massively parallel cloud-based GIS and drone remote sensing, to bear on questions about how forest structure affects wildfire and bark beetle disturbance in this region. I introduce a new framework to describe how wildfire suppression biases burning conditions and thus observed fire effects in large fire events to be more extreme than would be expected if all ignitions were allowed to burn. With this selection bias of large fires in mind, I generate a new dataset of fire effects in the Sierra yellow pine/mixed-conifer system that captures outcomes from smaller fire events. I use this new fire effects dataset and also measure variability in horizontal forest structure using the computer vision approach of texture analysis for nearly 1000 fires that burned in the system between 1984 and 2017. I find that greater variability in forest structure reduces the probability of high severity wildfire, which increases forest resilience in this system ill-adapted to recover from large high-severity events. Finally, I use drone-captured imagery and structure from motion (SfM) techniques to recreate complex forest structure of over 9 km2 of western pine beetle-attacked forest along a 350 km latitudinal gradient and a 1000 m elevation gradient. I found that availability of the host tree for the western pine beetle, ponderosa pine, increases the probability of ponderosa pine mortality and average host size plays a different role depending on the climatic water deficit (a proxy for tree moisture stress) at each site: at cool wet sites, more small hosts drive mortality; at hot dry sites, more large hosts drive mortality. Overall, this work demonstrates how an understanding the complexities of local forest structure, including the size, species, and spatial distribution of trees, can generate new insights into how broader-scale patterns of tree mortality arise during wildfire and bark beetle disturbance.
|Advisor:||Latimer, Andrew M., North, Malcolm P.|
|Commitee:||Millar, Constance I.|
|School:||University of California, Davis|
|School Location:||United States -- California|
|Source:||DAI-A 81/3(E), Dissertation Abstracts International|
|Subjects:||Ecology, Forestry, Geography|
|Keywords:||Bark beetle, Disturbance, Forestry, Remote sensing, Resilience, Wildfire|
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