Epsilon Open Archive

Abstract

Alder trees (Alnus spp.) are key nitrogen-fixing riparian species in the northern hemisphere. Inputs of nitrogen-rich leaf litter from alder into stream food webs can contribute significantly to nitrogen dynamics at local and landscape scales. Alder trees also provide habitats for terrestrial and aquatic organisms, and help stabilize river banks. 
Recently, substantial declines in alder stands have occurred along streams in Europe, with damages observed in some parts of North America also. A major driver has been the invasive oomycete pathogen Phytophthora alni species complex, which can spread rapidly along stream networks.
This review synthesises information on the pathogen, processes of spread and infection, and its impacts on alder. We further address the potential ecosystem-level and management consequences of the decline of alder, and highlight research needs.
The alder dieback caused by P. alni is associated with reductions in shade and quality and quantity of leaf litter. A decline in the structural integrity of branches and roots further threatens bank stability. Stream banks dominated by other tree species or no trees at all will result in ecosystem-level changes both above and below the waterline.
The P. alni taxonomic complex includes different species with varying phenotypes. An improved understanding of their environmental tolerances, virulence and evolution, along with the processes regulating the spread and impacts of the pathogen, would assist in identification of the riparian and stream systems most vulnerable not only to invasion but also to the heaviest disease outbreaks and ecosystem-level impacts.
Within the P. alni complex, the highly pathogenic hybrid species P. x alni is favoured by mild winters and warm, but not excessively hot summer temperatures suggesting possible changes in distribution and level of impact under future global climate change.