Abstract
Non‐native forest pathogens can cause dramatic and long‐lasting changes to the composition of forests, and these changes may have cascading impacts on community interactions and ecosystem functioning. Phytophthora ramorum, the causal agent of the emergent forest disease sudden oak death (SOD), has a wide host range, but mortality is concentrated in a few dominant tree species of coastal forests in California and Oregon. We examined interactions between P. ramorum and its hosts in redwood and mixed evergreen forest types over an 80,000 ha area in the Big Sur ecoregion of central California, an area that constitutes the southernmost range of the pathogen and includes forest stands on the advancing front of pathogen invasion. We established a network of 280 long‐term forest monitoring plots to understand how host composition and forest structure facilitated pathogen invasion, and whether selective mortality from SOD has led to shifts in community composition. Infested and uninfested sites differed significantly in host composition due to both historical trends and disease impacts. A reconstruction of pre‐disease forest composition showed that stands that eventually became infested with the pathogen tended to be more mature with larger stems than stands that remained pathogen‐free, supporting the hypothesis of aerial dispersal by the pathogen across the landscape followed by local understory spread. The change in species composition in uninfested areas was minimal over the study period, while infested stands had large changes in composition, correlated with the loss of tanoak (Notholithocarpus densiflorus), signaling the potential for SOD to dramatically change coastal forests through selective removal of a dominant host. Forest diversity plays an important role in pathogen establishment and spread, and is in turn changed by pathogen impacts. Asymmetric competency among host species means that impacts of P. ramorum on forest diversity are shaped by the combination and dominance of hosts present in a stand.
Highlights
Non-native forest pathogens continue to establish in North American forests at a steady rate despite intense regulation and quarantine efforts to prevent their introduction and establishment (Aukema et al 2010)
Eighty-three of the 166 mixed evergreen plots were infested with P. ramorum; 70 of 114 redwood plots were infested at the time of plot establishment (Fig. 1)
P. ramorum is a disconcerting example of a nonnative, virulent pathogen causing species-specific impacts to dominant trees with the potential to substantially transform the composition of forests across a large and diverse region
Summary
Non-native forest pathogens continue to establish in North American forests at a steady rate despite intense regulation and quarantine efforts to prevent their introduction and establishment (Aukema et al 2010) These invaders lead to significant ecological and economic costs (Pimentel et al 2000, Holmes et al 2009). The pathogens with the most potential to cause such transformative changes are those that are virulent, often host-specific, and cause mortality to dominant or unique host species (Lovett et al 2006) The impacts of these pathogens may include shifts in species composition and cascading effects on ecosystem functioning and foodweb dynamics when species with key roles in the ecosystem are removed by the pathogen (Lovett et al 2006). These changes may create novel ecosystems or interactions among species
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