Abstract
Abstract Drought‐ and insect‐associated tree mortality at low‐elevation ecotones is a widespread phenomenon but the underlying mechanisms are uncertain. Enhanced growth sensitivity to climate is widely observed among trees that die, indicating that a predisposing physiological mechanism(s) underlies tree mortality. We tested three, linked hypotheses regarding mortality using a ponderosa pine ( Pinus ponderosa ) elevation transect that experienced low‐elevation mortality following prolonged drought. The hypotheses were: (1) mortality was associated with greater growth sensitivity to climate, (2) mortality was associated with greater sensitivity of gas exchange to climate, and (3) growth and gas exchange were correlated. Support for all three hypotheses would indicate that mortality results at least in part from gas exchange constraints. We assessed growth using basal area increment normalized by tree basal area [basal area increment (BAI)/basal area (BA)] to account for differences in tree size. Whole‐crown gas exchange was indexed via estimates of the CO 2 partial pressure difference between leaf and atmosphere ( p a − p c ) derived from tree ring carbon isotope ratios ( δ 13 C), corrected for temporal trends in atmospheric CO 2 and δ 13 C and elevation trends in pressure. Trees that survived the drought exhibited strong correlations among and between BAI, BAI/BA, p a − p c , and climate. In contrast, trees that died exhibited greater growth sensitivity to climate than trees that survived, no sensitivity of p a − p c to climate, and a steep relationship between p a − p c and BAI/BA. The p a − p c results are consistent with predictions from a theoretical hydraulic model, suggesting trees that died had a limited buffer between mean water availability during their lifespan and water availability during drought – i.e., chronic water stress. It appears that chronic water stress predisposed low‐elevation trees to mortality during drought via constrained gas exchange. Continued intensification of drought in mid‐latitude regions may drive increased mortality and ecotone shifts in temperate forests and woodlands.
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Claims 6
Claim · model-written summary
Trees that survived the drought showed strong correlations among and between basal area increment, BAI/BA, pa-pc and climate.
- Taxon
- Pinus ponderosa
Pinus ponderosae1 - Outcome
- correlations among and between BAIe2
- Qualifier
- Trees that survived the droughte2
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e1“using a ponderosa pine ( Pinus ponderosa ) elevation transect”
e2“Trees that survived the drought exhibited strong correlations among and between BAI, BAI/BA, p a − p c , and climate”
Claim · model-written summary
Trees that died during the drought had greater growth sensitivity to climate than trees that survived.
- Taxon
- Pinus ponderosaPinus ponderosae1
- Material
- trees that diede3
- Outcome
- growth sensitivity to climatee3
- Direction
- greatere3
- Comparator
- trees that survivede3
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e1“using a ponderosa pine ( Pinus ponderosa ) elevation transect”
e3“In contrast, trees that died exhibited greater growth sensitivity to climate than trees that survived, no sensitivity of p a − p c to climate, and a steep relationship between p a − p c and BAI/BA”
Claim · model-written summary
In trees that died, pa-pc showed no sensitivity to climate.
- Taxon
- Pinus ponderosaPinus ponderosae1
- Outcome
- sensitivity of p a − p c to climatee3
- Qualifier
- trees that diede3
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e1“using a ponderosa pine ( Pinus ponderosa ) elevation transect”
e3“In contrast, trees that died exhibited greater growth sensitivity to climate than trees that survived, no sensitivity of p a − p c to climate, and a steep relationship between p a − p c and BAI/BA”
Claim · model-written summary
In trees that died, the relationship between pa-pc and BAI/BA was steep.
- Taxon
- Pinus ponderosaPinus ponderosae1
- Material
- trees that diede3
- Outcome
- relationship between p a − p c and BAI/BAe3
- Qualifier
- steepe3
Not reportedDirection
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e1“using a ponderosa pine ( Pinus ponderosa ) elevation transect”
e3“In contrast, trees that died exhibited greater growth sensitivity to climate than trees that survived, no sensitivity of p a − p c to climate, and a steep relationship between p a − p c and BAI/BA”
Claim · model-written summary
Chronic water stress predisposed low-elevation trees to drought mortality through constrained gas exchange.
- Taxon
- Pinus ponderosaPinus ponderosae1
- Treatment
- chronic water stresse5
- Outcome
- mortality during droughte5
- Qualifier
- via constrained gas exchangee5
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e1“using a ponderosa pine ( Pinus ponderosa ) elevation transect”
e5“It appears that chronic water stress predisposed low‐elevation trees to mortality during drought via constrained gas exchange”
Claim · model-written summary
The pa-pc results are consistent with predictions from a theoretical hydraulic model.
- Taxon
- Pinus ponderosaPinus ponderosae1
- Outcome
- p a − p c resultse4
- Qualifier
- consistent withe4
- Comparator
- predictions from a theoretical hydraulic modele4
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e1“using a ponderosa pine ( Pinus ponderosa ) elevation transect”
e4“The p a − p c results are consistent with predictions from a theoretical hydraulic model”