Decadal decline in herbaceous species richness in wetland forests: Effects of an introduced pathogen and environmental change

Abstract

In recent decades, temperate forests have been significantly disturbed by introduced pests and pathogens, which are likely to trigger cascading environmental changes. This study investigated the interactive effects of the pathogen Phytophthora alni and different environmental variables on understorey vegetation changes in wetland forests. We expected differential effects of P. alni based on moisture regimes and spatial isolation of forest sites, taking into account changes in climate, soil and canopy structure. After almost two decades, we re-surveyed vegetation on 214 forest sites and monitored P. alni that has spread rapidly and is now present in almost half of the sites. We observed a general decline in herb diversity, particularly in streamside forests, where it decreased by 23 %. These changes were largely explained by other environmental factors and to a lesser extent by the invasion of Phytophthora alni. The greatest decline in understorey diversity was observed in streamside forests, likely due to successional changes and their location as fragments in an agricultural landscape more vulnerable to drought. In contrast, the decline in species richness was less pronounced in alder carrs and spring forests, possibly because these habitats are located within larger forested areas and are less exposed to drought. The effect of the pathogen was strongest in alder carrs, suggesting that waterlogged sites with stagnant water are more vulnerable to the spread of P. alni.