In nearly all forested ecosystems of Earth, drought-induced tree mortality has been documented during the last forty years. Focusing on what mechanisms are at play in determining which trees live, and which die during drought, has been an important step in understanding drought-induced tree mortality. Recent experiments have demonstrated that conifer trees are able to survive much higher levels of hydraulic failure than previously expected (sometimes exceeding 90 percent loss of conductivity and surviving). However, most extreme drought events experienced by trees are not lethal. Here, we discuss what survival looks like for trees that have experienced high levels of hydraulic failure during drought. In a series of greenhouse experiments, we imposed drought on populations of loblolly pine (Pinus taeda, n=83), eastern redcedar (Juniperus virginiana, n=42), and pinyon pine (Pinus edulis, n=38), Pistacia lentiscus (n=24), and Eucalyptus camaldulensis (n=34) re-watering trees at various levels of drought stress to see which survived, and which died. Using active xylem staining and x-ray micro-computed tomography, we investigated where in stems xylem remained functional after surviving extreme hydraulic failure.
Results/Conclusions
Here, we discuss both the lethal threshold for hydraulic failure in these species, and the physiological state of survivors. Across species, the lethal dose of water potential varied greatly, but the lethal amount of hydraulic failure (~80%) was similar. Generally, canopy dieback increased with increasing levels of hydraulic failure, with detectable canopy die-back occurring beyond 50 PLC in all species. At the most extreme levels of stress (> 90 percent loss of conductivity), some individuals underwent complete canopy die-off, followed by epicormic re-sprouting. We found that survivors of extreme hydraulic failure restored hydraulic function via radial growth of new xylem, while embolized xylem remained embolized even months after survival. We observed that survival of a tree’s existing canopy during extreme stress is dependent upon supply of water to the vascular cambium, so that growth (and conductivity) may be restored. In one instance, we observed death of more than half of a tree’s vascular cambium, eventually resulting in strip-barking of the tree. Finally, we discuss these observations in the context of global forest die-back (and die-off) during extreme droughts.