title: Warmest extreme year in U.S. history alters thermal requirements for tree phenology.
abstract: 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.
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Kansas, 1200 Sunnyside Avenue, Lawrence, KS, 66045, USA.
authors.forename: Jacob M
authors.initials: JM
authors.lastname: Carter
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Kansas, 1200 Sunnyside Avenue, Lawrence, KS, 66045, USA.
authors.forename: Maria E
authors.initials: ME
authors.lastname: Orive
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Kansas, 1200 Sunnyside Avenue, Lawrence, KS, 66045, USA.|Deparment of Geography, Kansas State University, 118 Seaton Hall, Manhattan, KS, 66505, USA.
authors.forename: Laci M
authors.initials: LM
authors.lastname: Gerhart
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Kansas, 1200 Sunnyside Avenue, Lawrence, KS, 66045, USA.
authors.forename: Jennifer H
authors.initials: JH
authors.lastname: Stern
authors.affiliation: Centre for Carbon, Water and Food, University of Sydney, 380 Werombi Rd, Camden, NSW, 2570, Australia.
authors.forename: Renée M
authors.initials: RM
authors.lastname: Marchin
authors.affiliation: Department of Sociology, University of Kansas, 1415 Jayhawk Boulevard, Lawrence, KS, 66045, USA.
authors.forename: Joane
authors.initials: J
authors.lastname: Nagel
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Kansas, 1200 Sunnyside Avenue, Lawrence, KS, 66045, USA. joyward@ku.edu.
authors.forename: Joy K
authors.initials: JK
authors.lastname: Ward
mesh_terms: D057231:Climate Change; D018515:Plant Leaves; D015203:Reproducibility of Results; D012621:Seasons; D013696:Temperature; D014197:Trees
keywords: Bud break; Climate change; Extreme years; Fraxinus; Global change; Leaf emergence; Phenology; Thermal models; White ash