Implications of sudden oak death for wildland fire management

Human activities and climate change have altered historical disturbance regimes, introduced disturbances, and encouraged novel interactions between multiple disturbances. Ecosystems and the species that comprise them may be poorly equipped to withstand or recover from these altered disturbance regimes. In the fire-prone coastal forests of California and Oregon, sudden oak death (SOD), caused by the pathogen Phytophthora ramorum, is an emerging, non-native plant disease that causes widespread tree mortality and associated implications for fire regimes. Disease-related tree mortality alters fuel loads, with patterns of fuel accumulation varying depending on stand composition, disease severity, and time since pathogen invasion. Simulations and observational studies suggest these altered fuel profiles can impact subsequent fire behavior, and the extent of this interaction may depend on the severity and timing of disease impacts. Initial tree death can elevate the risk of crown ignition, while latter stages can increase surface fuel loading and have been linked to increased fire severity in wildfires. Further, disease history can also influence fire severity with cascading effects leading to unexpected increases in mortality of non-susceptible tree species and changes in nutrient cycling. The longer-term impacts of SOD-fire interactions on system resilience and recovery remain to be seen, but increased fire severity, changed stand structure, and altered biogeochemical cycling may have important consequences for post-fire regeneration and future ecosystem function. Fuels management strategies that diminish crown fire hazards at early stages and mitigate surface fuel hazards at later stages offer some promise, but have yet to be tested in large landscapes. Given SOD-wildfire interactions, further integration of disease- and fire-related management plans will be essential to minimizing impacts of these compounded disturbances

An Experimental Comparison of Stand Management Approaches to Sudden Oak Death: Prevention vs. Restoration

Many coastal forests stretching from central California to southwest Oregon are threatened or have been impacted by the invasive forest pathogen Phytophthora ramorum, the cause of sudden oak death. We analyzed a set of stand-level forest treatments aimed at preventing or mitigating disease impacts on stand composition, biomass, and fuels using a before–after-control-intervention experiment with a re-evaluation after 5 years. We compared the effects of restorative management for invaded stands and preventative treatments for uninvaded forests with two stand-level experiments. The restorative treatments contrasted two approaches to mastication, hand-crew thinning, and thinning with pile burning with untreated controls replicated at three distinct sites (N = 30), while the preventative treatments were limited to hand-crew thinning (N = 10) conducted at a single site. Half of the restoration treatments had basal sprouts removed 2 and 4 years after treatment. All treatments significantly reduced stand density and increased average tree size without significantly decreasing total basal area, both immediately and 5 years after treatments. Preventative treatments did not reduce the basal area or density of timber species not susceptible to P. ramorum, suggesting the relative dominance of these species increased in accordance with host removal. Follow-up basal sprout removal in the restoration experiment appears to maintain treatment benefits for average tree size and may be associated with small decreases in stand density 5 years after initial treatment. Our study demonstrates that for at least 5 years, a range of common stand management practices can improve forest conditions threatened or impacted by sudden oak death

Wildfire Alters the Disturbance Impacts of an Emerging Forest Disease via Changes to Host Occurrence and Demographic Structure

Anthropogenic activities have altered historical disturbance regimes, and understanding the mechanisms by which these shifting perturbations interact is essential to predicting where they may erode ecosystem resilience. Emerging infectious plant diseases, caused by human translocation of nonnative pathogens, can generate ecologically damaging forms of novel biotic disturbance. Further, abiotic disturbances, such as wildfire, may influence the severity and extent of disease-related perturbations via their effects on the occurrence of hosts, pathogens and microclimates; however, these interactions have rarely been examined. The disease ‘sudden oak death’ (SOD), associated with the introduced pathogen Phytophthora ramorum, causes acute, landscape-scale tree mortality in California\u27s fire-prone coastal forests. Here, we examined interactions between wildfire and the biotic disturbance impacts of this emerging infectious disease. Leveraging long-term datasets that describe wildfire occurrence and P. ramorum dynamics across the Big Sur region, we modelled the influence of recent and historical fires on epidemiological parameters, including pathogen presence, infestation intensity, reinvasion, and host mortality. Past wildfire altered disease dynamics and reduced SOD-related mortality, indicating a negative interaction between these abiotic and biotic disturbances. Frequently burned forests were less likely to be invaded by P. ramorum, had lower incidence of host infection, and exhibited decreased disease-related biotic disturbance, which was associated with reduced occurrence and density of epidemiologically significant hosts. Following a recent wildfire, survival of mature bay laurel, a key sporulating host, was the primary driver of P. ramorum infestation and reinvasion, but younger, rapidly regenerating host vegetation capable of sporulation did not measurably influence disease dynamics. Notably, the effect of P. ramorum infection on host mortality was reduced in recently burned areas, indicating that the loss of tall, mature host canopies may temporarily dampen pathogen transmission and ‘release’ susceptible species from significant inoculum pressure. Synthesis. Cumulatively, our findings indicate that fire history has contributed to heterogeneous patterns of biotic disturbance and disease-related decline across this landscape, via changes to the both the occurrence of available hosts and the demography of epidemiologically important host populations. These results highlight that human-altered abiotic disturbances may play a foundational role in structuring infectious disease dynamics, contributing to future outbreak emergence and driving biotic disturbance regimes

Interacting Disturbances: Wildfire Severity Affected by Stage of Forest Disease Invasion

Sudden oak death (SOD) is an emerging forest disease causing extensive tree mortality in coastal California forests. Recent California wildfires provided an opportunity to test a major assumption underlying discussions of SOD and land management: SOD mortality will increase fire severity. We examined prefire fuels from host species in a forest monitoring plot network in Big Sur, California (USA), to understand the interactions between disease-caused mortality and wildfire severity during the 2008 Basin Complex wildfire. Detailed measurements of standing dead woody stems and downed woody debris 1–2 years prior to the Basin fire provided a rare picture of the increased fuels attributable to SOD mortality. Despite great differences in host fuel abundance, we found no significant difference in burn severity between infested and uninfested plots. Instead, the relationship between SOD and fire reflected the changing nature of the disease impacts over time. Increased SOD mortality contributed to overstory burn severity only in areas where the pathogen had recently invaded. Where longer-term disease establishment allowed dead material to fall and accumulate, increasing log volumes led to increased substrate burn severity. These patterns help inform forest management decisions regarding fire, both in Big Sur and in other areas of California as the pathogen continues to expand throughout coastal forests

Appendix A. Fuels vary as Phytophthora ramorum becomes established and the disease progresses in a stand.

Fuels vary as Phytophthora ramorum becomes established and the disease progresses in a stand

Appendix D. Additional metrics describing burn severity following the Basin Complex fire.

Additional metrics describing burn severity following the Basin Complex fire

Appendix C. Pre-fire fuel abundance in SOD-infested and uninfested plots.

Pre-fire fuel abundance in SOD-infested and uninfested plots

Appendix E. The abundance of host species mortality does not predict burn severity.

The abundance of host species mortality does not predict burn severity