Adjustment of xylem vulnerability to cavitation in a precipitation manipulation experiment

Background/Question/Methods

During 2000-2003, extreme drought across the Southwestern US resulted in widespread tree mortality: piñon pine (Pinus edulis) experienced up to 95% mortality, while juniper (Juniperus monosperma) mortality was 25% or less at surveyed sites.  Trees, as sessile and long-lived organisms, can acclimate to prolonged non-lethal stresses.  This project tests the hypothesis that cavitation vulnerability exhibits acclimation in piñon and juniper individuals subjected to modified precipitation regimes.  Decreasing xylem cavitation vulnerability of roots and shoots is a potentially important mechanism of drought survival because exceeding vulnerability thresholds should lead to diminished or complete loss of water transport, with concomitant reduced rates of transpiration and photosynthesis.  We examined cavitation vulnerability in a precipitation manipulation experiment in piñon-juniper woodland at the Sevilleta National Wildlife Refuge and LTER in central New Mexico, USA.  Precipitation treatments include water addition (~150%), water removal (~45%), ambient control and water removal structure control. We measured sapwood area-specific hydraulic conductivity (K_S, kg•m^-1•s^-1•MPa^-1) and used the air-injection technique to produce xylem vulnerability curves for root and shoot samples of piñon and juniper.  We also measured wood density and wood anatomical parameters (tracheid size, pit density, and wall thickness to span ratio) to provide an anatomical basis to physiological function.  

Results/Conclusions

We observed differences in xylem vulnerability and hydraulic conductivity between organs, species, and treatments.  Shoots had lower values of K_S and were less vulnerable than roots. Ψ_W at 50% loss of K_S (Ψ₅₀) was found to be 17.82% ± 8.07 s.d. and 54.66 ± 7.47 s.d. more negative for piñon and juniper, respectively.  Juniper had less vulnerable xylem than piñon at both root and shoot level (Ψ₅₀ was 86.9% ± 44.97 s.d. and 96.92% ± 18.34 s.d. more negative, respectively). Consistent with inter-specific differences in xylem cavitation vulnerability, wood density was higher in juniper than piñon, and juniper tracheids exhibited smaller lumens and higher tracheid wall thickness to span ratios.  We observed no differences in cavitation vulnerability in juniper measured across treatments.  In contrast, piñon cavitation vulnerability was lowest in individuals from the drought treatment in both root and shoot xylem.  However, air entry threshold (Ψ_e) for roots was significantly higher than -2.5 MPa, the approximate leaf water potential at which piñons typically cease transpiration.  This finding lends support to the hypothesis that piñon may sacrifice hydraulic integrity of roots in order to prevent water loss to the soil during periods of low Ψ_soil as a mechanism of drought avoidance.