title: Declining global warming effects on the phenology of spring leaf unfolding.
abstract: Earlier spring leaf unfolding is a frequently observed response of plants to climate warming. Many deciduous tree species require chilling for dormancy release, and warming-related reductions in chilling may counteract the advance of leaf unfolding in response to warming. Empirical evidence for this, however, is limited to saplings or twigs in climate-controlled chambers. Using long-term in situ observations of leaf unfolding for seven dominant European tree species at 1,245 sites, here we show that the apparent response of leaf unfolding to climate warming (ST, expressed in days advance of leaf unfolding per °C warming) has significantly decreased from 1980 to 2013 in all monitored tree species. Averaged across all species and sites, ST decreased by 40% from 4.0 ± 1.8 days °C(-1) during 1980-1994 to 2.3 ± 1.6 days °C(-1) during 1999-2013. The declining ST was also simulated by chilling-based phenology models, albeit with a weaker decline (24-30%) than observed in situ. The reduction in ST is likely to be partly attributable to reduced chilling. Nonetheless, other mechanisms may also have a role, such as 'photoperiod limitation' mechanisms that may become ultimately limiting when leaf unfolding dates occur too early in the season. Our results provide empirical evidence for a declining ST, but also suggest that the predicted strong winter warming in the future may further reduce ST and therefore result in a slowdown in the advance of tree spring phenology.
authors.affiliation: Sino-French Institute for Earth System Science, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China.|Centre of Excellence PLECO (Plant and Vegetation Ecology), Department of Biology, University of Antwerp, Universiteitsplein 1, B-2610 Wilrijk, Belgium.
authors.forename: Yongshuo H
authors.initials: YH
authors.lastname: Fu
authors.affiliation: Sino-French Institute for Earth System Science, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China.
authors.forename: Hongfang
authors.initials: H
authors.lastname: Zhao
authors.affiliation: Sino-French Institute for Earth System Science, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China.|Key Laboratory of Alpine Ecology and Biodiversity, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, China.|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: Laboratoire des Sciences du Climat et de l'Environnement, CEA CNRS UVSQ, Gif-sur-Yvette 91190, France.
authors.forename: Marc
authors.initials: M
authors.lastname: Peaucelle
authors.affiliation: Sino-French Institute for Earth System Science, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China.|Laboratoire des Sciences du Climat et de l'Environnement, CEA CNRS UVSQ, Gif-sur-Yvette 91190, France.
authors.forename: Shushi
authors.initials: S
authors.lastname: Peng
authors.affiliation: School of Resources and Environment, University of Electronic Science and Technology of China, Chengdu 611731, China.
authors.forename: Guiyun
authors.initials: G
authors.lastname: Zhou
authors.affiliation: Sino-French Institute for Earth System Science, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China.|Laboratoire des Sciences du Climat et de l'Environnement, CEA CNRS UVSQ, Gif-sur-Yvette 91190, France.
authors.forename: Philippe
authors.initials: P
authors.lastname: Ciais
authors.affiliation: Sino-French Institute for Earth System Science, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China.
authors.forename: Mengtian
authors.initials: M
authors.lastname: Huang
authors.affiliation: Ecoclimatology, Technische Universität München, Freising 85354, Germany.|Technische Universität München, Institute for Advanced Study, Lichtenbergstraße 2a, 85748 Garching, Germany.
authors.forename: Annette
authors.initials: A
authors.lastname: Menzel
authors.affiliation: CREAF, Cerdanyola del Vallès, Barcelona 08193, Catalonia, Spain.|CSIC, Global Ecology Unit CREAF-CSIC-UAB, Cerdanyola del Vallès, Barcelona 08193, Catalonia, Spain.
authors.forename: Josep
authors.initials: J
authors.lastname: Peñuelas
authors.affiliation: Department of Atmospheric Sciences, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: Yang
authors.initials: Y
authors.lastname: Song
authors.affiliation: University of Neuchatel, Institute of Geography, Neuchatel 2000, Switzerland.|WSL Swiss Federal Institute for Forest, Snow and Landscape Research, Neuchatel 2000, Switzerland.|WSL Institute for Snow and Avalanche Research SLF, Group Mountain Ecosystems, Davos 7260, Switzerland.
authors.forename: Yann
authors.initials: Y
authors.lastname: Vitasse
authors.affiliation: Sino-French Institute for Earth System Science, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China.
authors.forename: Zhenzhong
authors.initials: Z
authors.lastname: Zeng
authors.affiliation: Centre of Excellence PLECO (Plant and Vegetation Ecology), Department of Biology, University of Antwerp, Universiteitsplein 1, B-2610 Wilrijk, Belgium.
authors.forename: Ivan A
authors.initials: IA
authors.lastname: Janssens
mesh_terms: D003080:Cold Temperature; D005060:Europe; D057232:Global Warming; D008954:Models, Biological; D017440:Photoperiod; D018515:Plant Leaves / Q000254:growth & development*; D012621:Seasons; D013997:Time Factors; D014197:Trees / Q000254:growth & development*