Recent increases in drought frequency cause observed multi-year drought legacies in the tree rings of semi-arid forests
Recent analyses on the length of drought recovery in forests have shown multi-year legacies, particularly in semi-arid, coniferous ecosystems. Such legacies are usually attributed to ecophysiological memory, although drought frequency itself, and its effect on overlapping recovery times, could also contribute. Here, we describe a multi-decadal study of drought legacies using tree-ring carbon-isotope ratios (δ13C) and ring-width index (RWI) in Pinus ponderosa at 13 montane sites traversing a winter–summer precipitation gradient in the Southwestern U.S. Sites and trees were selected to avoid collection biases that exist in archived tree-ring databanks. The spatial hydroclimate gradient and winter–summer seasonal patterns were well predicted by seasonal and inter-annual correlations between δ13C and atmospheric vapor pressure deficit (VPD). Using VPD, we found that the probability of extreme drought has increased up to 70% in this region during the past two decades. When the recent increase in drought frequency was not considered, multi-year legacies in both δ13C and RWI were observed at most sites. When the increase in drought frequency was detrended from tree-ring chronologies, some sites exhibited short legacies (1–2 years) in both δ13C and RWI, and there was a sight trend for longer legacies in RWI. However, when considered broadly across the region and multiple decades, no significant legacies were observed, which contrasts with past studies. Our results reveal that a contribution to observed multi-year legacies is related to shifts in the climate system itself, an exogenous factor, that must be considered along with physiological memory.
KeywordsPulse–press disturbance Xylem vulnerability Stress recovery Stress memory Hot droughts Climate change Vapor pressure deficit Modeling
This study was supported by a grant from the Macrosystems Program in the Emerging Frontiers Section of the U.S. National Science Foundation (NSF Award 1065790), the Ecosystems Program in the Division of Environmental Biology (NSF Award 1754430), and the Inter-University Training Program in Continental-scale Ecology (NSF Award 1137336). We thank Elisabeth Bergman, Miles Twitty, Fred Moreno, Leon Prescott Wells, Monica Vogel, Megan Mckey, Grace Kim, Alyssa Langford Abbey, and Seth Stephens for their technical assistance. Valuable discussions were provided by Valerie Trouet, Steve Leavitt, Ed Wright and Dave Meko from the Laboratory of Tree-Ring Research at the University of Arizona. The tree-ring data and isotope chronologies will be archived in the International Tree-Ring Databank or will be made available upon request to Paul Szejner (firstname.lastname@example.org).
Author contribution statement
PS and RKM conceived the ideas and designed methodology; PS, SB, and JRE collected the data; PS, SB, JRE and RKM analyzed the data; PS and RKM led the writing of the manuscript; PS, SB, JRE and RKM contributed critically to the drafts and gave final approval for publication.
- Hartmann H, Moura C, Anderegg W, Ruehr N, Salmon Y, Allen CD, Arndt SK, Breshears DD, Davi H, Galbraith D, Ruthrof KX, Wunder J, Adams HD, Bloemen J, Cailleret M, Cobb R, Gessler A, Grams TEE, Jansen S, Kautz M, Lloret F, O’Brien M (2018) Research frontiers for improving our understanding of drought-induced tree and forest mortality. New Phytol. https://doi.org/10.1111/nph.15048 CrossRefPubMedPubMedCentralGoogle Scholar
- Martins SCV, McAdam SAM, Deans RM, DaMatta FM, Brodribb TJ (2016) Stomatal dynamics are limited by leaf hydraulics in ferns and conifers: results from simultaneous measurements of liquid and vapour fluxes in leaves. Plant Cell Environ 39:694–705. https://doi.org/10.1111/pce.12668 CrossRefGoogle Scholar
- Nowacki GJ, Abrams MD, Building F (1997) Radial-growth averaging criteria for reconstructing disturbance histories from presettlement-origin oaks. Ecol Monogr 67(2):225–249Google Scholar
- PRISM Climate Group (2004) Oregon State University. In: PRISM. http://prism.oregonstate.edu
- Stokes MA, Smiley TL (1996) An introduction to tree-ring dating. The University of Arizona Press, Tucson, Arizona, USA. ISBN-10: 0816516804Google Scholar