Historical disturbance regimes exert selective pressures on species traits. In turn, these traits determine the biological legacies that connect pre- and post-disturbance communities, including the pattern of surviving organisms, propagules, and other materials that shape future composition and structure. Anthropogenic activities have altered disturbance regimes, including their frequency, severity, extent, type, and interaction, and these shifts may challenge existing suites of life history strategies. The chapters of this dissertation examine the mechanisms by which changing disturbance regimes and disturbance interactions may alter or erode biological legacies, with important consequences for system recovery.
In the fire-prone coastal forests of California, an emerging infectious tree disease, sudden oak death (SOD), represents a novel biotic disturbance. This disease, caused by the introduced oomycete Phytophthora ramorum, produces acute mortality in susceptible host tree populations, impacts forest fuel profiles, and alters subsequent fire behavior. I conducted this research within a long-term forest plot network established in the early 2000s to monitor the nascent SOD epidemic across the Big Sur region. In 2008, wildfire burned across this preexisting network, creating a rare opportunity to examine ecological impacts of compound disturbance events. In this work, I leverage pre- and post-fire data to examine the potential for disease-fire interactions to: 1) challenge important biological legacies maintained by contrasting asexual and sexual life history traits, 2) shift forest regeneration trajectories, and 3) impact future SOD epidemiology.
These forests are dominated by woody plant species with the capacity to both asexually resprout and facultatively sexually regenerate, but belowground tree survival and vegetative regrowth represent the most abundant form of post-fire regeneration in this system. In Chapter 1, I assess the extent to which SOD-fire interactions challenge biological legacies maintained by the persistence of resprouting species, using data describing pre-fire fuels, disease occurrence, fire severity, and patterns of post-fire belowground survival and regrowth. I found that the accumulation of SOD-related surface fuels and increased substrate fire severity were associated with decreased probability of belowground survival for typically highly-persistent resprouting trees. However, surviving individuals resprouted more vigorously, potentially due to reduced stand competition, and indicated evidence of stand-level resilience, despite changes in individual-level vulnerability.
The evolution and occurrence of resprouting and seed-producing traits is thought to have been shaped by disturbance regimes’ impacts on patterns of adult survival, competition, and propagule availability. In Chapter 2, I hypothesized that compounded disease and fire disturbances may alter opportunities for facultative sexual regeneration in this resprouter-dominated system, due to its impacts on these mechanisms. I found that diminishing availability of mature seed sources constrained post-fire seedling recruitment; however, where seed sources persisted, pre-fire disease impacts increased opportunities for seedling establishment and survival, likely due to gap formation and decreased competition associated with the SOD-related mortality of typically long-lived host species.
Phytophthora ramorum is not completely eradicated from burned areas, and in turn, wildfire has the potential to impact the future epidemiology of this disease via changes to host composition, forest structure, and microclimatic conditions. In Chapter 3, I examine the influence of recurring wildfire on SOD epidemiology, including the occurrence, infestation intensity, and mortality impacts of P. ramorum. Here, I demonstrate that wildfire can alter forest disease dynamics via its influence on host composition and structure. Results from this analysis suggest that more frequent fire reduces P. ramorum occurrence, probability of host infection, and the density of important host species. In recently-burned environments, the persistence and reinvasion of the pathogen is strongly related to the legacy of surviving aboveground host vegetation. Further, recent wildfire reduces the impacts of P. ramorum on host stem mortality. Thus, the biological legacies determined by preceding disease-fire interactions are expected to shape future epidemiological pattern.
In a variety of communities impacted by altered disturbance regimes, resprouting species are expected to be more competitive than obligate seeding species, due to resprouters’ abilities to persist locally without relying on dispersal, successful germination, and development to maturity. Together, the results of this dissertation work indicate mixed signs of resilience in this resprouter-dominated system. Following disease-fire interactions, density-dependent increases in resprouting vigor and increased opportunities for sexual recruitment may facilitate structural recovery, gradual compositional shifts to non-susceptible tree species, and opportunities for increased genetic variability. However, increased belowground mortality of typically highly-persistent asexual species and the gradual removal of susceptible hosts will have important impacts on stand turnover, population structure, and compositional shifts. Further, though recurring wildfire may reduce inoculum pressure and diminish mortality impacts, SOD epidemics will gradually reemerge in burned areas. This work provides important insight to the vulnerability and resilience of biological legacies maintained by asexual and sexual regeneration.
|Advisor:||Rizzo, David M.|
|Commitee:||Metz, Margaret R., North, Malcolm P., McRoberts, Neil|
|School:||University of California, Davis|
|School Location:||United States -- California|
|Source:||DAI-B 81/1(E), Dissertation Abstracts International|
|Subjects:||Ecology, Plant Pathology|
|Keywords:||Coast redwood, Disturbance ecology, Emerging infectious disease, Forests, Sudden oak death, Wildfire|
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