Warmest extreme year in U.S. history alters thermal requirements for tree phenology
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The frequency of extreme warm years is increasing across the majority of the planet. Shifts in plant phenology in response to extreme years can influence plant survival, productivity, and synchrony with pollinators/herbivores. Despite extensive work on plant phenological responses to climate change, little is known about responses to extreme warm years, particularly at the intraspecific level. Here we investigate 43 populations of white ash trees (Fraxinus americana) from throughout the species range that were all grown in a common garden. We compared the timing of leaf emergence during the warmest year in U.S. history (2012) with relatively non-extreme years. We show that (a) leaf emergence among white ash populations was accelerated by 21 days on average during the extreme warm year of 2012 relative to non-extreme years; (b) rank order for the timing of leaf emergence was maintained among populations across extreme and non-extreme years, with southern populations emerging earlier than northern populations; (c) greater amounts of warming units accumulated prior to leaf emergence during the extreme warm year relative to non-extreme years, and this constrained the potential for even earlier leaf emergence by an average of 9 days among populations; and (d) the extreme warm year reduced the reliability of a relevant phenological model for white ash by producing a consistent bias toward earlier predicted leaf emergence relative to observations. These results demonstrate a critical need to better understand how extreme warm years will impact tree phenology, particularly at the intraspecific level.
KeywordsBud break Climate change Extreme years Fraxinus Global change Leaf emergence Phenology Thermal models White ash
We thank K. Becklin, J. Medeiros, K. Kluthe, E. Duffy, S. M. Walker II, T. Leibbrandt, and C. Bone for field assistance, as well as E. Luedeling, S.-J. Jeong, A. Richardson, M. Migliavacca, and M. T. Holder for helpful modeling advice. We thank the University of Kansas Field Station staff for maintaining the common garden. This work was supported by NSF CAREER and other IOS awards to JKW. An NSF C-CHANGE IGERT fellowship supported JMC. MEO and JKW also acknowledge University of Kansas GRF grants. The Department of Ecology and Evolutionary Biology and the University of Kansas Research Investment Council also provided funding.
Author contribution statement
JMC, MEO, RMM and JKW conceived and designed the experiments. JMC, LMG, JHS, RMM and JKW conducted field work, and MEO, JMC and JKW developed and performed modeling and statistical analyses. JMC, MEO, JN and JKW wrote the manuscript; all authors provided editorial advice.
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