Drought-influenced mortality of tree species with different predawn leaf water dynamics in a decade-long study of a central US forest

Abstract

Abstract. Using decade-long continuous observations of tree mortality and predawn leaf water potential (ψpd) at the Missouri Ozark AmeriFlux (MOFLUX) site, we studied how the mortality of important tree species varied and how such variations may be predicted. Water stress determined inter-annual variations in tree mortality with a time delay of 1 year or more, which was correlated fairly tightly with a number of quantitative predictors formulated based on ψpd and precipitation regimes. Predictors based on temperature and vapor pressure deficit anomalies worked reasonably well, particularly for moderate droughts. The exceptional drought of the year 2012 drastically increased the mortality of all species, including drought-tolerant oaks, in the subsequent year. The drought-influenced tree mortality was related to the species position along the spectrum of ψpd regulation capacity with those in either ends of the spectrum being associated with elevated risk of death. Regardless of species and drought intensity, the ψpd of all species recovered rapidly after sufficiently intense rain events in all droughts. This result, together with a lack of immediate leaf and branch desiccation, suggests an absence of catastrophic hydraulic disconnection in the xylem and that tree death was caused by significant but indirect effects. Species differences in the capacity of regulating ψpd and its temporal integral were magnified under moderate drought intensities but diminished towards wet and dry extremes. Severe droughts may overwhelm the capacity of even drought-tolerant species to maintain differential levels of water potential as the soil becomes exhausted of available water in the rooting zone, thus rendering them more susceptible to death if predisposed by other factors such as age.

Are Northeastern U.S. forests vulnerable to extreme drought?

“…Thus, leaf abscission and stomatal closure are important mechanisms that reduce transpiration and minimize the risk of cavitation for temperate deciduous species (Federer 1980;Roberts et al 1980;Lucier and Hinckley 1982;Ginter-Whitehouse et al 1983;Gu et al 2007). Consistent with these findings, Gu et al (2015) observed greater mortality of drought-tolerant Q. alba and Q. rubra compared with other drought-sensitive and intolerant species (A. saccharum, F. americana) following an extreme drought in 2012, which was primarily associated with the lack of regulation of predawn water potential of the drought-tolerant species.…”

“…Quercus spp.) experienced higher mortality following an extreme drought year in the Midwestern USA as compared with presumably more drought intolerant species, Acer saccharum and Fraxinus americana (Gu et al 2015). The inconsistency in classification across studies presents a conundrum that plagues drought research: uncertainty in how we define tolerance and what criteria should be used to classify species in a reliable and meaningful way.…”

“…However, there are also cases where species that tend to be classified as drought intolerant (A. saccharum, F. americana) experienced localized and regional dieback associated with drought events (Hibben and Silverborg 1978;Hendershot and Jones 1989;Roy et al 2004). Furthermore, species classified as drought tolerant with anisohydric behavior, such as Juniperus virginiana (Ginter-Whitehouse et al 1983;Bahari et al 1985), have been shown to experience lower mortality than other co-occurring species (Gu et al 2015).…”

On underestimation of global vulnerability to tree mortality and forest die‐off from hotter drought in the Anthropocene

“…Further, warmer temperatures in the absence of significant drought have been shown to increase tree growth rates, particularly in deciduous species (Way and Oren 2010). Overall, the direct mortality consequences of warming could be relatively small and limited when considered globally, as earth system feedbacks (e.g., greater atmospheric moisture and precipitation) combined with multiple treelevel compensatory processes can buffer future Xiong et al 2011, Kharuk et al 2013, Zhang et al 2014b, Zhou et al 2013, 2014bAustralasia, Semple et al 2010; Europe,Čater 2015; North America, Vogelmann et al 2009, Zegler et al 2012, Baguskas et al 2014, Hart et al 2014, Kane et al 2014, Twidwell et al 2014, Gu et al 2015South America, Brienen et al 2015. v www.esajournals.org tree mortality relative to hotter droughts (e.g., Klein et al 2014a).…”

Contrasting strategies of hydraulic control in two codominant temperate tree species

“…It is common for species within the same ecosystem to employ opposing hydraulic strategies (e.g., risk prone or risk adverse; McCulloh et al, ; Ford et al, ). Disparities in transpiration volume and timing, due to differences in whole‐plant hydraulic strategies employed within the same forest, have important implications for forest growth and response to drought and disturbance (Roman et al, ; McDowell et al, ; Matheny et al, ; Gao et al, ; Gu, Pallardy, Hosman, & Sun, ; Wullschleger, Meinzer, & Vertessy, ). Several water flux studies have shown that many species of ring‐porous, anisohydric oak continue to transpire after other species curtail their water use during mild to moderate drought (e.g., von Allmen, Sperry, & Bush, ; Baldocchi & Xu, ; Hernandez‐Santana, Martinez‐Fernandez, Moran, & Cano, ; Matheny et al, ).…”