Spatio-temporal analysis at landscape scale of the Iberian oak decline epidemic caused by Phytophthora cinnamomi

Phytophthora cinnamomi Rands is considered a main factor behind the Iberian oak decline (IOD). This epidemic is decimating Holm oaks (Quercus ilex L.) and cork oaks (Quercus suber L.) which are the keystone trees of a multipurpose, silvo-pastoral and semi-natural ecosystem of 3.1 million hectares in the south west of Europe. Forest diseases are characterized by pronounced spatial patterns, since many of the underlying ecological processes are inherently spatial. To improve the current understanding of such processes, we carried out a complete census of diseased sites via aerial imagery at landscape scale at four different dates over a period of 35 years. We validated our photographic interpretation of P. cinnamomi presence in-situ by subsampling soil and roots of diseased sites. To analyse the role of host population heterogeneities in shaping the spread of IOD, we used a ‘self-exciting’ spatio-temporal point process model. Its so-called epidemic component represents the inoculum pressure arising from nearby foci whereas its background component allows for sporadic infections from unobserved sources or disease transmission over larger distances. The best fit was obtained with a lagged power-law for the spatial dispersal kernel, where 49% of the infections triggered by an infected site occur within a distance of 250 meters. Both risk components were found to increase over time. The rate of sporadic infections appeared to be significantly lower in silvo-pastoral systems (dehesas) than in forests and higher in mixed stands and shrub encroached oak-lands. These results may have direct implications for IOD management, for example, the estimated spatial dispersal function helps to define a suitable target area for more efficient control measures. Our results also suggest that silviculture treatments aimed at controlling the density and species composition of oak stands, as well as the abundance of shrubs, could play a key role for disease management