Similar response of canopy conductance to increasing vapor pressure deficit and decreasing soil conductivity with drought among five morphologically contrasting but co-occurring pine species

Abstract

Knowledge of plant hydraulics facilitates our understanding of the capabilities of forests to withstand droughts. This common-garden study quantified the hydraulic response to variation in sandy soil conductivity and atmospheric vapor pressure deficit (VPD) of five morphologically contrasting, wide-ranging pine species (Pinus virginiana, P. echinata, P. taeda, P. elliottii, P. palustris) of the Southeastern US, a region experiencing relatively high occurrence of hydrological droughts, which are projected to increase in frequency and severity. We employed a Bayesian hierarchical model to estimate xylem hydraulic parameters associated with drought vulnerability curves (VC) for terminal branches and shallow roots. We found that branches in all of the pine species were more resistant to cavitation-induced embolism and had greater hydraulic safety margin than roots. Among all species, P₅₀ (i.e., water potential at which 50 % conductivity is lost) and S₅₀ (i.e., the slope of VC centered on P₅₀) of roots showed an increasing trend from shorter- to longer-needle species. By contrast, hydraulic conductivity at saturation (k_sat) of either branches or roots did not exhibit any trend with needle length. We devised a simplified index for daily average canopy conductance (G_cI), computed from high-frequency sap flux measurement. Regression of G_cI showed that mean daytime VPD accounted for most of the variation in G_cI (> 60 %), followed by unsaturated soil hydraulic conductivity (k_soil; 24 %), whereas in situ root conductivity accounted for the least (< 5 %). All species exhibited a linear-log relationship between the variation in G_cI unexplained by VPD and k_soil, a pattern consistent with preventing soil water conditions from dropping to low levels where k_soil declines dramatically. We concluded that the shorter-needle species (P. virginiana and P. echinata) are likely to tolerate drought better than the other species, due to more resistant roots and a moderate-to-high sensitivity of G_cI to VPD and k_soil.