Distribution of Phytophthora species within recreational chestnut, beech and cork oak forests

Abstract

Tree decline caused by exotic Phytophthora pathogens is a serious issue affecting forests around the world. The presence of Phytophthora has been reported in heavily visited natural forests; however, whether recreational forests are at higher risk of being attacked by Phytophthora than non-recreational forests has yet to be explored. In this study, we sampled soil from 44 forests in Catalonia (NE Spain) with high or low recreational use that were dominated by chestnut (Castanea sativa), beech (Fagus sylvatica) or cork oak (Quercus suber). A systematic soil sampling of 277 plots in stands of different tree species, predominantly Pinus and Quercus spp., was also conducted across Catalonia to compare soils of recreational forests with high and low recreational use with the baseline incidence of Phytophthora in the region. The putative impacts of the presence of Phytophthora in recreational areas were assessed by measuring tree defoliation and regeneration, and by performing pathogenicity trials using Phytophthora isolates from soil and root samples to inoculate seedlings of tree species growing in the region. The incidence of native and invasive Phytophthora species was higher in chestnut, beech and cork oak forests than in forests of other tree species. For chestnut and beech forests, we found that the likelihood of finding an invasive Phytophthora in forests with high recreational use was more than three times higher than in forests with low recreational use (76% vs 21%, respectively). No differences were observed in cork oak forests. In plots with invasive Phytophthora species, beech exhibited higher defoliation levels than in plots without Phytophthora. The presence of both native and invasive Phytophthora species was associated with lower chestnut and cork oak regeneration. Our data indicate that Phytophthora is widespread in some forests used for recreation and seems to be associated with defoliation and low regeneration. However, the primary mechanism explaining the high prevalence of Phytophthora in recreational areas remains unclear. Disturbance, soil movement due to construction, or visitors moving inoculum within and across sites could be studied as possible contributing factors to establish effective management actions.