Sudden oak death impacts on soil and litterfall N dynamics

Background/Question/Methods

Novel pathogens can cause significant changes in ecosystem processes, but few studies have quantified pathogen effects and described the mechanisms driving altered ecosystem process. Sudden oak death (SOD), an emerging infectious disease caused by Phytophthora ramorum, causes selective mortality in coast redwood forests. SOD is likely to impact ecosystem processes through overstory tree mortality, selective species removal, and changes in forest microclimate. To assess the ecosystem impacts of overstory tree removal, we measured litterfall N, mass, and in situ soil N mineralization for three years in two California redwood forests impacted by this disease. Long-term changes in ecosystem processes are likely through shifts in overstory tree species. Tanoak (Notholithocarpus densiflorus) is an important overstory tree that dies rapidly following infection by P. ramorum. To understand the role of the dominant tree species on soil N cycling, we conducted a separate study using laboratory incubations of soils underlying tanoak, dead tanoak, redwood (Sequoia sempervirens) and California bay laurel (Umbellularia californica).

Results/Conclusions

Redwood accounted for the greatest annual contribution to litterfall followed by tanoak and bay laurel. Linear regression revealed that mortality of 1 m² ha^-1 tanoak basal area resulted in a corresponding decrease in tanoak litterfall of 45-51 kg ha yr^-1. P. ramorum infection had weak short-term influences on litterfall N concentration, tending to increase or have no effect on N content of tanoak and bay laurel litterfall. Tanoak mortality caused a low-magnitude but significant increase in soil NO₃-N availability (0.06 µg g^-1 NO₃-N per 1 m² ha^-1 tanoak killed). At the species level, healthy tanoak had the lowest NO₃-N availability at 2.56 µg g^-1 compared to 4.26 µg g^-1 in bay laurel and the highest NH₄-N availability at 5.1 µg g^-1 compared to 3.1 µg g^-1 in bay laurel. These patterns suggest short term changes in ecosystem processes are primarily a function of loss of tanoak biomass. Bay laurel effectively transmits P. ramorum to tanoak and is likely to increase in these forests through pathogen mediated competition. Long-term changes in litter chemistry and increased NO₃-N availability are likely to accompany increased dominance of bay laurel.