Vulnerability of Ponderosa pine needles to hydraulic failure across California
Presentation Type
Poster Presentation
Abstract/Artist Statement
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 blockages in the tree’s water transportation system (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 dangerous pulling forces on water 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 the result of blockages in the water transport system, or the xylem tracheids, versus the tissues surrounding the xylem, we used micro-CT imaging and hydraulics methods to estimate the loss of needles ability to transport water during desiccation. We also measured a suite of anatomical traits associated with drought-tolerance, with the expectation that trees from drier habitats would be more resistant to the effects of drought.
This study is the first of its kind to access the damages of drought using x-ray microtomography across a population’s range. Not only does this method of analysis allow for a more accurate estimation of embolism formation, but it also gives insight into the whole-needle desiccation process.
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 ability to move water 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 driest population had significantly thicker waxy cuticle, smaller mesophyll area, smaller cross-sectional area, and shorter needle lengths.
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.
Mentor Name
Anna Sala
Vulnerability of Ponderosa pine needles to hydraulic failure across California
UC North Ballroom
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 blockages in the tree’s water transportation system (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 dangerous pulling forces on water 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 the result of blockages in the water transport system, or the xylem tracheids, versus the tissues surrounding the xylem, we used micro-CT imaging and hydraulics methods to estimate the loss of needles ability to transport water during desiccation. We also measured a suite of anatomical traits associated with drought-tolerance, with the expectation that trees from drier habitats would be more resistant to the effects of drought.
This study is the first of its kind to access the damages of drought using x-ray microtomography across a population’s range. Not only does this method of analysis allow for a more accurate estimation of embolism formation, but it also gives insight into the whole-needle desiccation process.
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 ability to move water 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 driest population had significantly thicker waxy cuticle, smaller mesophyll area, smaller cross-sectional area, and shorter needle lengths.
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.