title: Increased heat requirement for leaf flushing in temperate woody species over 1980-2012: effects of chilling, precipitation and insolation.
abstract: Recent studies have revealed large unexplained variation in heat requirement-based phenology models, resulting in large uncertainty when predicting ecosystem carbon and water balance responses to climate variability. Improving our understanding of the heat requirement for spring phenology is thus urgently needed. In this study, we estimated the species-specific heat requirement for leaf flushing of 13 temperate woody species using long-term phenological observations from Europe and North America. The species were defined as early and late flushing species according to the mean date of leaf flushing across all sites. Partial correlation analyses were applied to determine the temporal correlations between heat requirement and chilling accumulation, precipitation and insolation sum during dormancy. We found that the heat requirement for leaf flushing increased by almost 50% over the study period 1980-2012, with an average of 30 heat units per decade. This temporal increase in heat requirement was observed in all species, but was much larger for late than for early flushing species. Consistent with previous studies, we found that the heat requirement negatively correlates with chilling accumulation. Interestingly, after removing the variation induced by chilling accumulation, a predominantly positive partial correlation exists between heat requirement and precipitation sum, and a predominantly negative correlation between heat requirement and insolation sum. This suggests that besides the well-known effect of chilling, the heat requirement for leaf flushing is also influenced by precipitation and insolation sum during dormancy. However, we hypothesize that the observed precipitation and insolation effects might be artefacts attributable to the inappropriate use of air temperature in the heat requirement quantification. Rather than air temperature, meristem temperature is probably the prominent driver of the leaf flushing process, but these data are not available. Further experimental research is thus needed to verify whether insolation and precipitation sums directly affect the heat requirement for leaf flushing.
authors.affiliation: College of Urban and Environmental Sciences, Peking University, Yiheyuan Road 5, 100871, Beijing, China.|Department of Biology, Centre of Excellence PLECO (Plant and Vegetation Ecology), University of Antwerp, Universiteitsplein 1, B-2610, Wilrijk, Belgium.
authors.forename: Yongshuo H
authors.initials: YH
authors.lastname: Fu
authors.affiliation: College of Urban and Environmental Sciences, Peking University, Yiheyuan Road 5, 100871, Beijing, China.|Key Laboratory of Alpine Ecology and Biodiversity, Institute of Tibetan Plateau Research, Center for Excellence in Tibetan Earth Science, Chinese Academy of Sciences, Beijing, 100085, China.
authors.forename: Shilong
authors.initials: S
authors.lastname: Piao
authors.affiliation: Institute of Geography, University of Neuchatel, Neuchatel, Switzerland.|Snow and Landscape Research, WSL Swiss Federal Institute for Forest, Neuchatel, Switzerland.|Group Mountain Ecosystems, WSL Institute for Snow and Avalanche Research SLF, Davos, Switzerland.
authors.forename: Yann
authors.initials: Y
authors.lastname: Vitasse
authors.affiliation: College of Urban and Environmental Sciences, Peking University, Yiheyuan Road 5, 100871, Beijing, China.
authors.forename: Hongfang
authors.initials: H
authors.lastname: Zhao
authors.affiliation: Department of Biology, Centre of Excellence PLECO (Plant and Vegetation Ecology), University of Antwerp, Universiteitsplein 1, B-2610, Wilrijk, Belgium.
authors.forename: Hans J
authors.initials: HJ
authors.lastname: De Boeck
authors.affiliation: College of Urban and Environmental Sciences, Peking University, Yiheyuan Road 5, 100871, Beijing, China.
authors.forename: Qiang
authors.initials: Q
authors.lastname: Liu
authors.affiliation: College of Urban and Environmental Sciences, Peking University, Yiheyuan Road 5, 100871, Beijing, China.
authors.forename: Hui
authors.initials: H
authors.lastname: Yang
authors.affiliation: Max Planck Institute for Biogeochemistry, Hans Knöll Strasse 10, 07745, Jena, Germany.
authors.forename: Ulrich
authors.initials: U
authors.lastname: Weber
authors.affiliation: Department of Biosciences, University of Helsinki, Biocenter 3, FIN-00014, Helsinki, Finland.
authors.forename: Heikki
authors.initials: H
authors.lastname: Hänninen
authors.affiliation: Department of Biology, Centre of Excellence PLECO (Plant and Vegetation Ecology), University of Antwerp, Universiteitsplein 1, B-2610, Wilrijk, Belgium.
authors.forename: Ivan A
authors.initials: IA
authors.lastname: Janssens
keywords: chilling requirement; climate change; heat requirement; insolation; leaf flushing phenology; precipitation