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ABSTRACT Climate extremes—e.g., drought, atmospheric rivers, heat waves—are increasing in severity and frequency across the western United States of America (USA). Tree‐ring widths reflect the concurrent and legacy effects of such climate extremes, yet our ability to predict extreme tree growth is often poor. Could tree‐ring data themselves identify the most important climate variables driving extreme low‐ and high‐growth states? How does the importance of these climate drivers differ across species and time? To address these questions, we explored the spatial synchrony of extreme low‐ and high‐growth years, the symmetry of climate effects on the probability of low‐ and high‐growth years, and how climate drivers of extreme growth vary across tree species. We compiled ring widths for seven species (four gymnosperms and three angiosperms) from 604 sites in the western USA and classified each annual ring as representing extreme low, extreme high, or nominal growth. We used classification random forest (RF) models to evaluate the importance of 30 seasonal climate variables for predicting extreme growth, including precipitation, temperature, and vapor pressure deficit (VPD) during and up to four years prior to ring formation. For four species (three gymnosperms, one angiosperm) for which climate was predictive of growth, the RF models correctly classified 89%–98% and 80%–95% of low‐ and high‐growth years, respectively. For these species, asymmetric climate responses dominated. Current‐year winter hydroclimate (precipitation and VPD) was most important for predicting low growth, but prediction of high growth required multiple years of favorable moisture conditions, and the occurrence of low‐growth years was more synchronous across space than high‐growth years. Summer climate and temperature (regardless of season) were only weakly predictive of growth extremes. Our results motivate ecologically relevant definitions of drought such that current winter moisture stress exerts a dominant role in governing growth reductions in multiple tree species broadly distributed across the western USA.
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Climate Factors Leading to Asymmetric Extreme Capture in the Tree‐Ring Record
Abstract Paleoclimate data play a critical role in contextualizing recent hydroclimate extremes, but asymmetries in tree‐ring responses to extreme climate conditions pose challenges for reconstruction and interpretation of past climate. Here we establish the extent to which existing tree‐ring records capture precipitation extremes in western North America and evaluate climate factors hypothesized to lead to asymmetric extreme capture, including timing of precipitation, seasonal temperatures, snowpack, and atmospheric river events. We find that while there is dry‐biased asymmetry in one third of western North American tree‐ring records, 45% of sites capture wet extremes as well as or better than dry extremes. Summer extremes are rarely captured at any sites. Latitude and elevation affect site‐level extreme responses, as do seasonal climate conditions, particularly in the autumn and spring. Directly addressing asymmetric extreme value capture in tree‐ring records can aid our interpretation of past climate and help identify alternative avenues for future reconstructions.
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- Award ID(s):
- 1802024
- PAR ID:
- 10460549
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 46
- Issue:
- 6
- ISSN:
- 0094-8276
- Page Range / eLocation ID:
- p. 3408-3416
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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