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Assessing the impact of sudden oak death on crown fire potential in Tanoak forests of California
The introduction of non-native pathogens can have profound effects on forest ecosystems resulting in tree mortality, changes in species composition, and altered fuel structure. The 1990’s introduction of Phytophthora ramorum, the pathogen recognized as causing the tree and plant disease known as sudden oak death, has caused rapid decline and mortality of tanoak (Lithocarpus densiflorus) in forests of coastal California, USA. To understand the potential effect that mortality could have on fuel structure and fire behavior, foliar moisture content of uninfected tanoaks, sudden oak death-infected tanoaks, sudden oak death-killed (dead) tanoaks, and surface litter was tracked for 12 months. Foliar moisture content of uninfected tanoaks averaged 82.3% for the year whereas foliar moisture content of infected tanoaks had a lower average of 77.8%. Dead tanoaks had significantly lower foliar moisture content than uninfected and infected trees, averaging 12.3% for the year. During fire season (June through September), dead tanoak foliar moisture content reached a low of 5.8%, with no significant difference between dead canopy fuels and surface litter. Remote automated weather station (RAWS) 10-hour timelag fuel moisture data corresponded to foliar moisture content of dead leaves, holding promise as a predictor of seasonal crown fire hazard. Decision support tools, based on Van Wagner’s (1977) crown ignition equation, can predict canopy base height values to escape crown ignition, however the Van Wagner equation was developed for conifers, not broadleaf trees (such as tanoak). No empirical data exist to corroborate ignition thresholds for extremely low foliar moistures found in dead foliage. To quantify crown base height ignition thresholds, a laboratory experiment was employed to measure foliar ignition and consumption at crown base heights from 0.5 m to 1.5 m across the range of foliar moistures found in healthy, sudden oak death-infected, and sudden oak death-killed tanoaks. Results from laboratory burning showed all foliage was consumed at the lowest simulated crown base heights in the laboratory, however consumption of live foliage dropped off quickly with increasing crown base height, with minimal consumption occurring at 1 m and above. Consumption of dead foliage declined with increasing simulated crown base heights, with some consumption still occurring at the highest crown base height tested (1.5 m). Using logistic regression, variables of crown base height, temperature, and duration of temperatures above 320 ºC or 410 ºC were used to predict crown ignition probabilities for all foliar moisture treatments tested (80%, 70%, 9%, and 5% foliar moisture content). Crown base height performed well as a predictor of crown ignition with correct predictions 87% to 91% of the time. Minimizing the probability of live tanoak foliage ignition results in a crown base ignition threshold lower than the Van Wagner model, while the dead tanoak foliage ignition threshold is considerably lower than an extrapolated Van Wagner equation. This suggests that tanoak will resist crown ignition at a lower threshold than conifers across the crown base heights tested. Results from this study will help refine the decision support tools for fire managers in sudden oak death-affected areas as well as serve as a model for other forests where diseases and insect epidemics have altered forest crown fuels.