The Science

Drought-stress disrupts tree function and growth and is an important factor that can lead to tree mortality. When under stress and weakened, trees are susceptible to infection by opportunistic pathogens that are able to further disrupt tree function. In the Ozark Border Region of central Missouri, there was a severe drought in 2012 that was followed by significant mortality of white oaks (Quercus alba L.; 10.0% of live stems) and black oaks (Q. velutina Lam.; 26.5% of live stems) in the year after. This was surprising because oaks are comparatively drought tolerant and implied that some other factor may be at play. A synthesis of forest inventory data, ecosystem fluxes (with supporting biological observations), tree-ring analyses, and documentation of a pathogen (Biscogniauxia spp., formerly hypoxylon) infection was therefore completed to better understand whether drought-pathogen interactions are important aspects of tree mortality and stand dynamics in this region.

The Impact

Large-scale oak mortality events have been documented in the forest-grasslands transition zone of the Central United States following intense drought conditions. Rising temperatures and changing patterns of precipitation are expected to intensify droughts and make them more lethal. It is therefore critical to better understand how droughts affect tree growth and mortality. The interactions between drought and pathogens have been understudied but are crucial toward more fully understanding how tree mortality rates may change under different environmental conditions. This research points to the significance of event-based oak mortality and that drought-Biscogniauxia interactions are important in shaping oak stand dynamics in this region and underscores the pressing need for more in-depth studies focused on drought-pathogen interactions.

Summary

Stand dynamics were consistent with expected patterns of decreasing tree density but increasing basal area. Basal area growth outpaced mortality, implying a net accumulation of live biomass, which was supported by eddy covariance ecosystem carbon flux observations. There was a threshold response in white and black oak trees to water stress in the previous year, giving rise to significantly elevated mortality in the year after. Individual white and black oaks that died in 2013 displayed historically lower growth with the majority of dead trees exhibiting Biscogniauxia cankers. Taken together, the synthesis points to the importance of drought-pathogens being important drivers of oak mortality “pulses” and thus stand dynamics in these forests.

Principal Investigator

Jeff Wood
University of Missouri
woodjd@missouri.edu

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

The Office of Biological and Environmental Research, within the U.S. Department of Energy Office of Science, and  the U.S. Department of Agriculture National Institute of Food and Agriculture’s McIntire-Stennis funds.

References