Soil pathogens and spatial patterns of seedling mortality in a temperate tree

The Janzen-Connell hypothesis(1,2) proposes that host-specific, distance- a nd/or density-dependent predators and herbivores maintain high tree diversi ty in tropical forests. Negative feedback between plant and soil communitie s could be a more effective mechanism promoting species coexistence because soil pathogens can increase rapidly in the presence of their host(3), caus ing conditions unfavourable for local conspecific recruitment(4-6). Here we show that a soil pathogen leads to patterns of seedling mortality in a tem perate tree (Prunus serotina) as predicted by the Janzen-Connell hypothesis . In the field, the mean distance to parent of seedling cohorts shifted awa y from maternal trees over a period of 3 years. Seedlings were gown in soil collected 0-5 m or 25-30 m from Prunus trees. Sterilization of soil collec ted beneath trees improved seedling survival relative to unsterilized soil, whereas sterilization of distant soil did not affect survival. Pythium spp ., isolated from roots of dying seedlings and used to inoculate healthy see dlings, decreased survival by 65% relative to controls. Our results provide the most complete evidence that native pathogens influence tree distributi ons, as predicted by the Janzen-Connell hypothesis, and suggest that simila r ecological mechanisms operate in tropical and temperate forests.