PS 61-36
Xylem vulnerability to drought greater in post fire resprout oaks than in adults in a Chihuahuan desert sky-island
Water availability is a deciding factor of tree distribution. Past work has demonstrated that oaks inhabiting “sky island” forests of the northern Sierra Madre Oriental have differing susceptibilities to drought-induced xylem failure (cavitation), and heavy stem dieback likely due to embolism has been observed over the past several years of drought. These oak species are all post-fire resprouters: they can resprout from underground storage organs when fire kills above ground tissue. Post-fire resprouts should have increased root:shoot ratios relative to adults and therefore have access to more water relative to leaf demand. We expected that if resprouts exhibit plasticity, they should favor water transport efficiency over safety: showing higher maximum xylem conductances, but greater susceptibility to drought-induced cavitation indicated by higher PLC50 values (the water potential at which conductance is reduced to 50% of maximum). We examined five species of oaks common in the Davis Mountains in west Texas and compared adult physiology with that of resprouts following the 2012 Livermore Complex Fire. We measured the xylem hydraulic conductivity of an average of 7 individuals per species for adult trees and first-year post-fire resprouts before and after spinning stems in a centrifuge to generate negative xylem pressure, mimicking drought.
Results/Conclusions
For each individual plant, we calculated PLC curves and stem specific conductivity. Stem specific conductivity among resprout species was between 0.1 and 0.5 g s-1m-1MPa-1 while adults had significantly lower conductivity. Resprouts had xylem more susceptible to drought induced embolism than adults with significantly lower PLC50 values, ranging between -1 to -3 MPa. The data support the hypothesis that resprouts favor water efficiency over safety, and have xylem tissue more vulnerable to drought than adults. Although resprouts may have better water supply relative to adults, this may come at the cost of increased vulnerability to low water potentials and may explain the dieback in newly resprouting tissue that we witnessed following the 2011 and 2012 fire and drought episodes.