Influence of land-cover change on the spread of an invasive forest pathogen

Human-caused changes in land use and land cover have dramatically altered ecosystems worldwide and may facilitate the spread of infectious diseases.  We examined the possibility that land-cover changes between 1942 and 2000 in coastal woodlands in northern California facilitated the establishment of an invasive pathogen, Phytophthora ramorum, which causes the devastating forest disease known as Sudden Oak Death.  We assessed forest structure, understory microclimate, and symptoms of infection on P. ramorum hosts over two years (2005 and 2006) in 102 15 x 15 m² plots within a 275 km² heterogeneous forest region.  Within a 150 m radius area around each plot, we mapped types of land cover (oak woodland, chaparral, grassland, vineyard, and development) in both 1942 and 2000 using detailed aerial photos.  During this 58-year period, oak woodlands significantly increased in area by 25%, while grassland and chaparral decreased by 34% and 51%, respectively.  Woodland expansion was a significant predictor of disease severity, expressed as numbers of symptomatic stems and leaves of bay laurel (Umbellularia californica), the primary inoculum-producing host in mixed evergreen forests.  Path analysis showed that woodland expansion resulted in larger forests with higher densities of the primary host trees (U. californica, Quercus agrifolia, Q. kellogii) and cooler understory temperatures.  Together, the positive effects of woodland size and host stem density and the negative effects of understory temperature explained 65% of the variation in the number of symptomatic bay laurel trees and 40% of the variation in the number of symptomatic leaves per plot.  We conclude that enlargement of woodlands and closure of canopy gaps facilitated the establishment of P. ramorum by increasing contagion of hosts and enhancing forest microclimate conditions.  Epidemiological studies that incorporate land-use change are rare but may increase understanding of disease dynamics and improve our ability to manage invasive forest pathogens.