Mortality and community changes drive sudden oak deathimpacts on litterfall and soil nitrogen cycling

摘要

Few studies have quantified pathogen impacts to ecosystem processes, despite the fact thatpathogens cause or contribute to regional-scale tree mortality. We measured litterfall mass, litterfall chemistry, and soil nitrogen (N) cycling associated withmultiple hosts along a gradient of mortality caused by Phytophthora ramorum, the cause ofsudden oak death. In redwood forests, the epidemiological and ecological characteristics of the major overstoryspecies determine disease patterns and the magnitude and nature of ecosystem change. Baylaurel (Umbellularia californica) has high litterfall N (0.992%), greater soil extractable NO3–N, and transmits infection without suffering mortality. Tanoak (Notholithocarpus densiflorus)has moderate litterfall N (0.723%) and transmits infection while suffering extensive mortalitythat leads to higher extractable soil NO3–N. Redwood (Sequoia sempervirens) has relativelylow litterfall N (0.519%), does not suffer mortality or transmit the pathogen, but dominatesforest biomass. The strongest impact of pathogen-caused mortality was the potential shift in species composition, which will alter litterfall chemistry, patterns and dynamics of litterfall mass, andincrease soil NO3–N availability. Patterns of P. ramorum spread and consequent mortality areclosely associated with bay laurel abundances, suggesting this species will drive both diseaseemergence and subsequent ecosystem function.