Tree‐ring carbon and oxygen isotope ratios have been used to understand past dynamics in forest carbon and water cycling. Recently, this has been possible for different parts of single growing seasons by isolating anatomical sections within individual annual rings. Uncertainties in this approach are associated with correlated climate legacies that can occur at a higher frequency, such as across successive seasons, or a lower frequency, such as across years. The objective of this study was to gain insight into how legacies affect cross‐correlation in the δ13C and δ18O isotope ratios in the earlywood (EW) and latewood (LW) fractions of
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The US Southwest has been entrenched in a two‐decade‐long megadrought (MD), the most severe since 800 CE, which threatens the long‐term vitality and persistence of regional montane forests. Here, we report that in the face of record low winter precipitation and increasing atmospheric aridity, seasonal activity of the North American Monsoon (NAM) climate system brings sufficient precipitation during the height of the summer to alleviate extreme tree water stress. We studied seasonally resolved, tree‐ring stable carbon isotope ratios across a 57‐year time series (1960–2017) in 17 Ponderosa pine forests distributed across the NAM geographic domain. Our study focused on the isotope dynamics of latewood (LW), which is produced in association with NAM rains. During the MD, populations growing within the core region of the NAM operated at lower intrinsic and higher evaporative water‐use efficiencies (WUEiand WUEE, respectively), compared to populations growing in the periphery of the NAM domain, indicating less physiological water stress in those populations with access to NAM moisture. The disparities in water‐use efficiencies in periphery populations are due to a higher atmospheric vapor pressure deficit (VPD) and reduced access to summer soil moisture. The buffering advantage of the NAM, however, is weakening. We observed that since the MD, the relationship between WUEiand WUEEin forests within the core NAM domain is shifting toward a drought response similar to forests on the periphery of the NAM. After correcting for past increases in the atmospheric CO2concentration, we were able to isolate the LW time‐series responses to climate alone. This showed that the shift in the relation between WUEiand WUEEwas driven by the extreme increases in MD‐associated VPD, with little advantageous influence on stomatal conductance from increases in atmospheric CO2concentration.
more » « less- Award ID(s):
- 1754430
- NSF-PAR ID:
- 10421021
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Global Change Biology
- Volume:
- 29
- Issue:
- 15
- ISSN:
- 1354-1013
- Format(s):
- Medium: X Size: p. 4354-4367
- Size(s):
- ["p. 4354-4367"]
- Sponsoring Org:
- National Science Foundation
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