Invasive forest pathogens affect the characteristics, microbial colonisation, and decomposition of leaf litter in streams

Abstract

1. Invasive tree pathogens threaten forests worldwide, but their effects on streams are poorly understood. Nevertheless, tree infections that lead to changes in the characteristics of litter inputs to streams may affect stream communities and ecosystem processes. 2. We studied cross-ecosystem effects derived from Phytophthora cinnamomi, Phytophthora ×alni, and Ophiostoma novo-ulmi infection on Castanea sativa (chestnut), Alnus lusitanica (alder), and Ulmus minor (elm) trees, respectively, by assessing physical and chemical characteristics of senescent leaves from healthy, symptomatic, and highly symptomatic individuals. Leaf litter from the three health statuses per tree species was then incubated in laboratory microcosms and the effects of tree infection on microbial decomposers and leaf litter decomposition were assessed. 3. Tree infection significantly affected leaf litter characteristics, microbial decomposers and leaf litter decomposition, and the health status of trees conditioned these effects differently depending on the tree species. In C. sativa, leaf litter of highly symptomatic trees had higher toughness, higher polyphenolic concentration and slower decomposition than leaf litter of symptomatic and healthy trees. In A. lusitanica, leaf litter of highly symptomatic trees had higher phosphorus concentration, lower carbon:phosphorus ratio and faster decomposition than leaf litter of symptomatic and healthy trees. Finally, in U. minor, leaf litter of highly symptomatic trees had higher nitrogen concentration and lower carbon:nitrogen ratio than leaf litter of healthy trees, and faster decomposition than leaf litter of symptomatic and healthy trees. Effects of changes in litter characteristics on litter decomposition were mediated by changes in microbial decomposer colonisation and activity. 4. The composition of the aquatic hyphomycetes communities associated with C. sativa and U. minor litter varied depending on the tree health status. Most striking was the two-fold higher aquatic hyphomycetes species richness in litter of U. minor trees infected with O. novo-ulmi than in litter of healthy U. minor trees. 5. Tree infection alters the nutritional quality of leaf litter, potentially affecting the functioning of aquatic ecosystems strongly dependent on riparian litter inputs.