Despite the increasing emergence of invasive species and their impacts on ecosystem processes, surprisingly little is known about how landscape heterogeneity affects their establishment and spread. For the sake of simplicity, studies of invasive pathogens have also generally ignored the influence of spatial heterogeneity, instead focusing on relatively homogeneous environments. In reality, most landscapes are fragmented or patchy to varying degrees, whether naturally or by human modification, and the spatial arrangement and composition of host vegetation may play a crucial role in mediating spread. We hypothesize that landscape heterogeneity plays a critical role in the dispersal and establishment of Phytophthora ramorum, the causal agent of Sudden Oak Death.

To evaluate the importance of landscape heterogeneity we examined two questions:

1) does the spatial pattern of forested habitat predict P. ramorum disease severity, and is this relationship scale-dependent;

2) what influence does spatial pattern have on the optimal microclimate conditions for P. ramorum reproduction?

We mapped the spatial distribution of suitable forest habitat for P. ramorum and established 86 randomly located field plots within a 20-km2 region of northern California. For each plot, we quantified P. ramorum disease severity and  measured the abundance of woody species. Disease severity in each plot was examined in relation to the surrounding landscape structure measured for nested landscapes of increasing scale.

P. ramorum disease severity was greatest in plots surrounded by a high proportion of contiguous forest, after accounting for plot-level variables of host abundance, elevation, canopy cover and microclimate. The explanatory power of the model increased with increasing scale up to 200 m, but was not significant at scales less than 50 m.

High disease severity was associated with lower temperatures in the field than the laboratory-determined optimal range for pathogen reproduction. Variation in microclimate conditions was explained by elevation, not the pattern of host vegetation, indicating that spatially varying disease severity was not a function of microclimate-related edge effects on pathogen growth and survival.

Both landscape-scale configuration and local composition of host habitat are related to the severity of this destructive forest disease. Increased disease severity within contiguous woodlands may have a considerable impact on the composition of such woodlands, with cascading effects on the population dynamics of both host and pathogen.

This work is a first step toward determining if landscape structure is related to patterns of Phytophtora ramorum infection, information that may be critical for predicting and halting the spread this and other invasive species.

Condeso, T.E. and Meentemeyer, R.K. 2007. Effects of landscape heterogeneity on the emerging forest disease Sudden Oak Death. Journal of Ecology 95: 364-375. (PDF)