Drought Impacts on the Mortality of Tree Species of Different Hydraulic Adaptation Strategies Revealed in a Decade-Long Study of a Central US Temperate Forest

Abstract:

Although tree mortality induced by episodic drought events has been extensively reported in the literature, investigation of drought-induced tree mortality in the context of long-term plant-water relations and in the continuum of hydraulic adaptation strategies of plant species has been rare. Yet such investigation can lead to in-depth understanding of tree mortality mechanisms and predictive models. Using decade-long continuous observations of tree mortality and plant water status at the Missouri Ozark AmeriFlux (MOFLUX) site, we studied characteristics of precipitation regimes that drove drought-induced tree mortality of half a dozen of tree species with different hydraulic adaptation strategies. We found that water availability determined inter-annual variations in tree mortality with a one-year time delay. The Predawn Leaf Water Potential Integral (PLWPI) and the Mean Non-Exceedance Interval (time periods with no daily precipitation rates exceeding a threshold) with a daily threshold precipitation at 5 mm day^-1 (MNEI5) of the growing season were strong predictors of the following year’s mortality of species as well as plant community as a whole. The mean daily precipitation rate and the Precipitation Variability Index (PVI) individually were also good predictors of tree mortality caused by severe drought but must be used jointly to predict tree mortality caused by intermediate water stress. The PLWPI-mortality relationship was monotonic for the same species and the plant community as a whole. But it was non-monotonic across species such that species with extreme (lowest or highest) values in the continuum of PLWPI suffered higher mortality than species with intermediate values of PLWPI. This finding reconciles contradictory reports in previous studies regarding the effects of hydraulic adaptation strategies on tree mortality across species.