Hydraulic failure and tree dieback are associated with high wood density in a temperate forest under extreme drought

Abstract

Catastrophic hydraulic failure will likely be an important mechanism contributing to large-scale tree dieback caused by increased frequency and intensity of droughts under global climate change. To compare the susceptibility of 22 temperate deciduous tree and shrub species to hydraulic failure during a record drought in the southeastern USA, we quantified leaf desiccation, native embolism, wood density, stomatal conductance and predawn and midday leaf water potential at four sites with varying drought intensities. At the two driest sites, there was widespread leaf wilting and desiccation, and most species exhibited predawn leaf water potentials of 3 MPa and 460% loss of xylem conductivity in branches. Although species with high wood density were more resistant to cavitation, they had higher levels of native embolism and greater canopy dieback than species with low wood density. This unexpected result can be explained by the failure of species with dense wood to avert a decline in water potential to dangerous levels during the drought. Leaf water potential was negatively correlated with wood density, and the relationship was strongest under conditions of severe water deficit. Species with low wood density avoided catastrophic embolism by relying on an avoidance strategy that involves partial drought deciduousness, higher sensitivity of stomata to leaf water potential and perhaps greater rooting depth. These species therefore maintained water potential at levels that ensured a greater margin of safety against embolism. These differences among species may mediate rapid shifts in species composition of temperate forests if droughts intensify due to climate change.

Going Back to Basics: Importance of Ecophysiology when Choosing Functional Traits for Studying Communities and Ecosystems

“…This is because WD is weakly related to other xylem anatomical traits that better describe water use strategies (Zanne et al 2010). Even hydraulic failure and tree dieback, which are usually associated with low WD, occurred in high WD species which did not avoid the catastrophic embolism, due to low sensitivity of stomata (Hoffmann et al 2011). In conclusion, variation in wood density must also be understood based on vessel composition, vessel lumen fraction and non-lumen wood density, which are subject to different selective pressures that affect properties such as hydraulic conductivity and mechanical strength (Zanne et al 2010).…”

Tree mortality predicted from drought-induced vascular damage

“…Hydraulic failure through xylem cavitation has been shown to be a major mortality mechanism across a number of angiosperm species [18][19][20] . Using a combination of field physiological measurements, a plant hydraulic model 21 , a hydrologic model 22 , climate projections and multiple mortality data sets ( Supplementary Fig.…”

“…Our analysis moves beyond empirical relationships, however, because we have forged a link between hydroclimatic stress and physiological stress to generate cross-scale (tree-to-region) predictions of widespread tree mortality based on a plant's vulnerability to cavitation and test these predictions against three mortality data sets. Tree hydraulic traits have been associated with degree of mortality in multiple ecosystems, primarily in angiosperms 13,18 . Carbon metabolism and biotic agent attack (for example, bark beetles) are also probably important in some cases 4 , notably in more isohydric gymnosperms that dominate some boreal and temperate regions, and plant carbon status and vascular health are probably interdependent 4 .…”

“…Carbon metabolism and biotic agent attack (for example, bark beetles) are also probably important in some cases 4 , notably in more isohydric gymnosperms that dominate some boreal and temperate regions, and plant carbon status and vascular health are probably interdependent 4 . Given the ability to model vascular capacities as a function of climate and the dependence of plant health on vascular health 4,18 , the hydraulic threshold concept may prove generally useful in capturing the vulnerability of angiosperm forests to drought in many ecosystems 28 . The accumulated deficit between plant water demand and supply provides an attractive link between drought, species physiology and tree mortality.…”

“…Furthermore, both PET and AET are readily calculated within current land-surface models, and thus estimating CWD in these models is straightforward. Differences in drought vulnerability between species or between plant functional types could be incorporated both through modifications of a hydraulic threshold by plant functional traits 18 and through modifications in simulated AET across species and biomes. For example, deep rooting depths in tropical tree species would allow sustained AET during drought, avoiding lethal CWD values until later in drought.…”