Ponderosa pine (Pinus ponderosa) is an ecologically and economically important species occupying diverse habitats in the western United States. During the 2012-2015 California drought, mortality in ponderosa pine contributed the death of over 100 million trees. Drought-induced hydraulic failure is considered a strong contributing factor to tree mortality, where xylem blockage (i.e. embolism) results in decreased water delivery to the canopy. Though most studies focus on woody plant-tissues, needles are a critical point in the hydraulic pathway and often experience the most extreme water potentials during drought. Characterizing the thresholds vulnerability to drought-induced hydraulic failure and how these thresholds may vary among populations of widespread species is important to determining how species may respond to climate change.
To describe variation in ponderosa needle response to drought, we measured trees from four populations along a rainfall gradient in California. To determine if hydraulic failure in needles is more common in the xylem tracheids versus outside-xylem tissues, we used micro-CT imaging and hydraulics methods to estimate the loss of needle conductivity during desiccation. We measured a suite of anatomical traits associated with drought-tolerance, with the expectation that trees from xeric habitats would be more resistant to the effects of drought.
Results/Conclusions
Micro-CT images showed that needle tracheids were highly resistant to embolism, losing 12% of their functional area at -1.9 MPa. The images also showed deformation and changes in airspace tissues outside of the xylem. Our hydraulics data suggest that the needles as a whole are highly vulnerable (12% loss of conductivity at -0.15 MPa). We found no significant differences in either inside- vs. outside-xylem vulnerability across the four populations. However, the most southern and xeric population had significantly thicker waxy cuticle, smaller mesophyll area, smaller cross-sectional area, and shorter needle lengths. Additionally, this population exhibited lower minimum stomatal conductance rates.
Our results suggest that ponderosa pine needles show low plasticity for needle vulnerability across its range in California, but morphological and anatomical differences in the southernmost population may help to delay declines in water potential from reaching critical thresholds. Since ponderosa pine needles appear to be similarly vulnerable to drought across its range, it is important to identify locations where abiotic and biotic factors predispose trees to hydraulic failure and mortality.