Examining the strength and possible causes of the relationship between fire history and Sudden Oak Death

Abstract

Fire can be a dominant process in the ecology of forest vegetation and can also affect forest disease dynamics. Little is known about the relationship between fire and an emerging disease epidemic called Sudden Oak Death, which is caused by a new pathogen, Phytophthora ramorum. This disease has spread across a large, fire-prone portion of California, killing great numbers of oaks and tanoaks and infecting most associated woody plants. Suitable hosts cover a much broader geographic range, raising concern over where the disease may spread. To understand the strength and potential sensitivities of a fire-disease relationship, we examined geographic patterns of confirmed P. ramorum infections in relation to past fire history. We found these infections to be extremely rare within the perimeter of any area burned since 1950. This finding is not caused by spatial bias in sampling for the disease, and is robust to variation in host abundance scenarios and to aggregation of closely spaced sampling locations. We therefore investigated known fire-related factors that could result in significantly lower incidence of the disease in relatively recently burned landscapes. Chemical trends in post-fire environments can influence the success of pathogens like P. ramorum, either by increasing plant nutrient stress or by reducing the occurrence of chemicals antagonistic to Phytophthoras. Succession in the absence of fire leads to greater abundance of host species, which will provide increased habitat for P. ramorum; this will also increase intraspecific competition where these trees are abundant, and other density-dependent effects (e.g. shading) can reduce resource allocation to defenses. Despite these findings about a fire-disease relationship, a much deeper understanding is necessary before fire can be actively used as a tool in slowing the epidemic.

Influence of Land-Cover Change on the Spread of an Invasive Forest Pathogen

“…This agrees with two recent studies that observed greater levels of P. ramorum infection in locations with higher abundance of the foliar host bay laurel (Maloney et al 2005, Condeso andMeentemeyer 2007). Likewise, Moritz and Odion (2005) suggested that succession toward higher host densities in the absence of fire could cause a relationship between time since fire and P. ramorum presence. Second, we found that higher densities of trees reduce maximum daily understory temperatures.…”

“…Thus, mechanical thinning of bay laurel vegetation may be an even more effective way of reducing inoculum pressure in forest understories . Ultimately, more research is needed to understand the efficacy of possible management solutions that can be applied at landscape to regional scales and the potential for unintended ecological consequences of prescribed fire or vegetation thinning in woodland ecosystems (Moritz andOdion 2005, Rizzo et al 2005). Prior to application of such management activities at broad spatial scales, small-scale manipulative experiments are needed to reveal which types of treatments may be most effective at limiting disease transmission and persistence.…”

“…For the last 100 years, forest fires in California and the western United States more generally have been actively suppressed to protect human property and wildlife (Sugihara et al 2006). This reduction in fire frequency may be causing critical changes to forest structure, including increases in the density of small, young trees, decreases in tree canopy gaps, and expansions in the perimeter of woodlands (Moritz and Odion 2005). Many areas that are currently dominated by forests may have been different types of vegetation only decades ago.…”

Time since fire and average fire interval are the best predictors of Phytophthora cinnamomi activity in heathlands of south-western Australia

“…There was little indication that changes in community composition could be reversed, owing to a reduction in juvenile survivorship and the high proportion of existing resprouters that were diseased. Because time since fire can be a strong driver of ecosystem structure and species composition, Moritz and Odion (2005) hypothesised that certain successional stages will be more prone to infection than are others. Therefore, future management of recurring fires and soil-borne disease may mediate stability, or continuing decline, of susceptible species.…”

“…These factors may also affect survival or virulence of any soil-borne pathogen; however, little is known of the effect of fire on pathogens, such as Phytophthora spp., and their interaction with the host after fire. Fire can affect disease activity by directly affecting the pathogen's survival and development (Weste 1974;Schwartz et al 1995;Moritz and Odion 2005) or, indirectly, by affecting the plant (Marks et al 1975) and/or microbial community (Treseder et al 2004) or the physical environment (Podger and Brown 1989;Shearer and Tippett 1989;Bishop et al 2010). Furthermore, plants stressed by fire, low nutrients, drought or defoliation are more likely to succumb to disease (Schoeneweiss 1975).…”