Professor University of Minnesota, Minnesota, United States
The response of ecosystems to disturbances is hypothesized to be caused by interactions between environmental variables and species’ life history strategies and traits, which integrate to determine biomass, species composition, and cycling of carbon and nutrients recovering in a predictable way. However, novel disturbance types, increased disturbance frequency and severity, and compounded disturbances are hypothesized to lead to alternative biome states with different functional properties. To test this hypothesis, we present data from a 54-year fire frequency manipulation experiments at Cedar Creek Ecosystem Science Reserve that is experiencing an outbreak of a tree pathogen. We evaluate two characteristics of the ecosystem: (i) the plant community and the subsequent type of ecosystem, and (ii) carbon (C) storage and cycling.
The spread of oak wilt disease over the past 18 years has rapidly changed the effects of fire that had developed over the 46 years of treatments prior to the outbreak. Oak wilt disease increased tree mortality in unburned forests, but it caused relatively minor changes in frequently burned savannas. Carbon storage also rapidly changed as disease led to lower tree litterfall and biomass inputs into soils. Greater litter decomposition rates in forests likely facilitated the incorporation of biomass into soils, but higher respiration rates in forests point to greater potential C losses from soils when C inputs from trees decline.
We conclude that the rapid changes in the ecosystem arose because disease forced a trade-off between biomass C storage persistence versus potential: the tree species with high C storage potential also had low C persistence and soil C gains were stimulated by higher litter decomposition, but the high decomposition rates resulted in rapid soil C losses as disease reduced litterfall inputs, likely increased by the high sand content. Thus, the compound occurrence of fire and disease lead to a novel ecosystem state with altered species composition and lower C storage capacity.