Abstract
Forests are expected to become more vulnerable to drought-induced tree mortality owing to rising temperatures and changing precipitation patterns that amplify drought lethality. There is a crucial knowledge gap regarding drought–pathogen interactions and their effects on tree mortality. The objectives of this research were to examine whether stand dynamics and ‘background’ mortality rates were affected by a severe drought in 2012; and to evaluate the importance of drought–pathogen interactions within the context of a mortality event that killed 10.0% and 26.5% of white (Quercus alba L.) and black (Q. velutina Lam.) oak stems, respectively, in a single year. We synthesized (i) forest inventory data (24 years), (ii) 11 years of ecosystem flux data with supporting biological data including predawn leaf water potential and annual forest inventories, (iii) tree-ring analyses of individual white oaks that were alive and ones that died in 2013, and (iv) documentation of a pathogen infection. This forest displayed stand dynamics consistent with expected patterns of decreasing tree density and increasing basal area. Continued basal area growth outpaced mortality implying a net accumulation of live biomass, which was supported by eddy covariance ecosystem carbon flux observations. Individual white and black oaks that died in 2013 displayed historically lower growth with the majority of dead trees exhibiting Biscogniauxia cankers. Our observations point to the importance of event-based oak mortality and that drought–Biscogniauxia interactions are important in shaping oak stand dynamics in this region. Although forest function has not been significantly impaired, these drought–pathogen interactions could amplify mortality under future climate conditions and thus warrant further investigation.
Highlights
Forests account for ∼45% of global terrestrial carbon stocks (Bonan 2008) and ∼50% of gross primary production (GPP) (Beer et al 2010), and play a key role in the carbon–climate system
The number of trees that died per plot was significantly related to tree density in 1992, but the mean annual mortality rate of 2.0% across plots was independent of tree density in 1992
We found no significant relationships between plot-level mortality rate and slope, aspect, elevation, or soil series, probably due to the similarity of conditions among plots
Summary
Forests account for ∼45% of global terrestrial carbon stocks (Bonan 2008) and ∼50% of gross primary production (GPP) (Beer et al 2010), and play a key role in the carbon–climate system. Severe regional-scale drought-induced tree mortality has drawn attention to the potential vulnerability of forests—and forest carbon stocks and sink strength—to future climate change (Allen et al 2010, 2015, Anderegg et al 2015). The loss of plant hydraulic function and carbon starvation during drought are important pathways leading to tree mortality (McDowell and Allen 2015, McDowell et al 2015, 2013, Anderegg et al 2016). Much of the recent literature directed towards elucidating the coupled hydraulic impairment-carbon mechanisms of drought-induced tree mortality has examined highly xeric locations and responses to extreme long-term drought (Anderegg et al 2014, 2013, 2012, McDowell et al 2013) and do not directly address important interactions among drought and biotic agents of dieback (McDowell et al 2011, Oliva et al 2014), long-term declines in vigor (Berdanier and Clark 2016), or hydraulic deterioration (Pellizzari et al 2016). Of particular interest here are the possible effects of increased frequency of short (sub-seasonal) ‘global-change’ type drought (i.e. high VPD and temperature) (Eamus et al 2013) on tree mortality
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