Sudden Oak Death

Kerri sent me this memo. I reproduce only a part of it, as it is quite long. Also, I want to note, that I was one of the landowners who volunteered for this study, however, I found out from Kerri, they are not following through, or didn’t this last time anyway, on my property.

The University of California, Davis, has active and ongoing research in the Big Sur region to understand the interactions between Sudden Oak Death (SOD) and local ecosystems. In collaboration with more than 40 landowners, we established 280 ecological monitoring plots throughout Big Sur in 2006-07, and continue to survey these areas each year since. We seek to document the range of Phytophthora ramorum (the pathogen responsible for SOD), what forest characteristics encourage its establishment and spread, and the impacts the disease has on all aspects of the ecology of our local forests. P. ramorum is a non-native pathogen that is causing widespread tree mortality throughout coastal California forests, and the Big Sur study is important and unique for its wide geographic scope.
To address these research questions, we collect a variety of ecological data including the size, health, growth and survival of trees and shrubs, the presence of P. ramorum and other pathogens or pests, and environmental characteristics like canopy openness or soil chemistry. Following the 2008 Basin and Chalk fires, we have also directed a lot of research effort towards understanding how SOD and wildfire may interact in impacting the forests. The major data collection efforts in this study have included:
2006-07: establishment of plots in the forest monitoring network and collection of baseline data
2008: survey of burn severity indicators in a subset of plots that burned in the Basin Complex
2009: widespread survey of tree mortality in plots affected by the fire, by SOD, by both or neither.
2010-11: detailed surveys in redwood and mixed-evergreen habitats of tree growth, and fuel accumulation to compare separate and joint impacts of SOD and fire and forest recovery from disturbance.
Many results from this study are making important contributions to management efforts and to our scientific understanding of the ecology of Big Sur’s forest. Some of the major results include:
• SOD is selective in the trees it kills: Coast live oak (Quercus agrifolia), Shreve Oak (Quercus parvula), and tanoak (Notholithocarpus densiflorus) die from the disease. Tanoak dies in the highest numbers, and large tanoak stems dying faster than small stems.1 These results document a forest composition shift as we lose susceptible oaks and tanoaks leaving non-susceptible species like California bay laurel (Umbellularia californica) and coast redwood (Sequoia sempervirens).
• Diversity is important: plots that have a higher diversity of tree species seem to be at a lower risk of disease.2
• The relationship between SOD and fire is complex: Fuels are much greater in infested plots than uninfested plots, but burn severity depends on the progression of the disease and the ways the fuels change through time. Especially problematic are the brief windows where there are many standing, newly dead trees or areas where logs have accumulated a deep fuelbed.3 Firefighters reported difficulty fighting the Basin Fire because of these fuel accumulations,4 and elsewhere in CA, SOD fuels have been linked to how firefighting resources may need to be allocated differently than in areas without SOD.5 Although the disease does not directly affect coast redwoods, fewer redwoods survived the fires in areas where the disease was present, perhaps because increased fuels caused the fire to burn longer.6
• The 2008 fires suppressed, but did not eliminate, P. ramorum: Immediately following the fires, we found the pathogen in only about 20% of the burned areas it had previously been known to occupy. Since then, two wet years with late spring rains have helped the pathogen spread through its former range and expand into new areas. Sites were more likely to contain the pathogen after the fire if they’d had a higher prevalence of SOD pre-fire and if there were surviving bay laurel trees, the main source of P. ramorum inoculum.7 The most recent expansions of the pathogen are surprisingly in hotter, drier areas and include parts of the Stone Ridge Trail below Cone Peak, upper Dolan Ridge just north of Big Creek Reserve, the South Coast Ridge road just below Chalk Peak, the south side of the Prewitt loop and high up on the ridges of Plaskett ridge and Los Burros road.

What’s next? The Rizzo Plant Pathology lab at UC Davis was recently awarded a National Science Foundation grant that will allow us to continue our research on the effects of disturbance by Phytophthora ramorum and interactions with wildfire, the natural background disturbance in Big Sur. We will continue to work with landowners and managers to collect these valuable data and to apply our findings to forest management problems so that we may provide scientific support for informed management decisions. We are grateful to the many residents of Big Sur and managers at various agencies that have given us permission to work on their land and make this important research possible.

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