Abstract
Abstract Drought‐induced groundwater decline and warming associated with climate change are primary threats to dryland riparian woodlands. We used the extreme 2012–2019 drought in southern California as a natural experiment to assess how differences in water‐use strategies and groundwater dependence may influence the drought susceptibility of dryland riparian tree species with overlapping distributions. We analyzed tree‐ring stable carbon and oxygen isotopes collected from two cottonwood species ( Populus trichocarpa and P . fremontii ) along the semi‐arid Santa Clara River. We also modeled tree source water δ 18 O composition to compare with observed source water δ 18 O within the floodplain to infer patterns of groundwater reliance. Our results suggest that both species functioned as facultative phreatophytes that used shallow soil moisture when available but ultimately relied on groundwater to maintain physiological function during drought. We also observed apparent species differences in water‐use strategies and groundwater dependence related to their regional distributions. P . fremontii was constrained to more arid river segments and ostensibly used a greater proportion of groundwater to satisfy higher evaporative demand. P . fremontii maintained ∆ 13 C at pre‐drought levels up until the peak of the drought, when trees experienced a precipitous decline in ∆ 13 C. This response pattern suggests that trees prioritized maintaining photosynthetic processes over hydraulic safety, until a critical point. In contrast, P . trichocarpa showed a more gradual and sustained reduction in ∆ 13 C, indicating that drought conditions induced stomatal closure and higher water use efficiency. This strategy may confer drought avoidance for P . trichocarpa while increasing its susceptibility to anticipated climate warming.
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Claims 7
Claim · model-written summary
P. fremontii maintained Delta 13C at pre-drought levels up until the peak of the drought.
- Taxon
- P . fremontii
Populus fremontiie1 - Outcome
- ∆ 13 Ce1
- Qualifier
- up until the peak of the droughte1
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e1“P . fremontii maintained ∆ 13 C at pre‐drought levels up until the peak of the drought”
Claim · model-written summary
P. fremontii showed a precipitous decline in Delta 13C at the peak of the drought.
- Taxon
- P . fremontiiPopulus fremontiie1
- Outcome
- ∆ 13 Ce2
- Qualifier
- precipitous declinee2
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e1“P . fremontii maintained ∆ 13 C at pre‐drought levels up until the peak of the drought”
e2“when trees experienced a precipitous decline in ∆ 13 C”
Claim · model-written summary
P. trichocarpa showed a more gradual and sustained reduction in Delta 13C under drought.
- Taxon
- P . trichocarpa
Populus trichocarpae3 - Treatment
- drought conditionse3
- Outcome
- ∆ 13 Ce3
- Direction
- reductione3
- Qualifier
- more gradual and sustainede3
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e3“P . trichocarpa showed a more gradual and sustained reduction in ∆ 13 C, indicating that drought conditions induced stomatal closure and higher water use efficiency”
Claim · model-written summary
Drought induced stomatal closure and higher water use efficiency in P. trichocarpa.
- Taxon
- P . trichocarpaPopulus trichocarpae3
- Treatment
- drought conditionse3
- Outcome
- water use efficiencye3
- Qualifier
- stomatal closuree3
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e3“P . trichocarpa showed a more gradual and sustained reduction in ∆ 13 C, indicating that drought conditions induced stomatal closure and higher water use efficiency”
Claim · model-written summary
The P. trichocarpa strategy may confer drought avoidance while increasing susceptibility to anticipated climate warming.
- Taxon
- P . trichocarpaPopulus trichocarpae4
- Outcome
- susceptibility to anticipated climate warminge4
- Qualifier
- drought avoidancee4
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e4“This strategy may confer drought avoidance for P . trichocarpa while increasing its susceptibility to anticipated climate warming”
Claim · model-written summary
P. fremontii used a greater proportion of groundwater in more arid river segments.
- Taxon
- P . fremontiiPopulus fremontiie5
- Outcome
- proportion of groundwatere5
- Direction
- greatere5
- Qualifier
- to satisfy higher evaporative demande5
- Setting
- more arid river segmentse5
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e5“P . fremontii was constrained to more arid river segments and ostensibly used a greater proportion of groundwater to satisfy higher evaporative demand”
Claim · model-written summary
Both cottonwood species functioned as facultative phreatophytes and relied on groundwater to maintain physiological function during drought.
Verbatim
e0“collected from two cottonwood species ( Populus trichocarpa and P . fremontii )”
e6“Both species functioned as facultative phreatophytes that used shallow soil moisture when available but ultimately relied on groundwater to maintain physiological function during drought”