Landscape variables influence Phytophthora cinnamomi distribution within a forested Kentucky watershed.
Invasive pests and pathogens have contributed to widescale forest change around the world, but especially in the eastern US. Phytophthora cinnamomi, one such introduced pathogen, causes root rot in American chestnut (Castanea dentata) and shortleaf pine (Pinus echinata), among other eastern forest species of interest, and has inhibited chestnut restoration efforts in some cases. Traditionally, P. cinnamomi has been associated with low landscape positions and moister soils; however, its distribution patterns in the eastern US are poorly understood. Improved understanding of P. cinnamomi distribution may enable forest managers to prioritize sites with low risk of P. cinnamomi presence for chestnut restoration. To elucidate landscape factors associated with P. cinnamomi distribution, two sets of soil samples from an eastern Kentucky forest (representing two levels of sampling intensity) were screened for P. cinnamomi incidence, and data were analyzed for spatial distribution patterns. In general, sites in which P. cinnamomi was detected tended to be warmer (higher annual solar radiation) and drier (lower moisture indices), than sites in which P. cinnamomi were not detected. P. cinnamomi incidence was also found to be associated with oak (Quercus spp.) abundance and (weakly) negatively associated with soil microbial activity under certain conditions. Overall, P. cinnamomi was found to be distributed across a wide range of landscape variables, including both drier ridge-top sites and moister streamside sites, contrary to traditional associations. In addition, the association with oak abundance suggests that the drier upland sites preferred by oak species in eastern Kentucky are not "safe" from P. cinnamomi. Given that P. cinnamomi was found distributed across a range of environmental conditions, forest managers cannot assume that any landscape position is phytophthora-free, and soil screening should be used for site selection to inform restoration of chestnut and other susceptible species.