title: From observations to experiments in phenology research: investigating climate change impacts on trees and shrubs using dormant twigs.
abstract: OBJECTIVE
  Climate change is advancing the leaf-out times of many plant species and mostly extending the growing season in temperate ecosystems. Laboratory experiments using twig cuttings from woody plant species present an affordable, easily replicated approach to investigate the relative importance of factors such as winter chilling, photoperiod, spring warming and frost tolerance on the leafing-out times of plant communities. This Viewpoint article demonstrates how the results of these experiments deepen our understanding beyond what is possible via analyses of remote sensing and field observation data, and can be used to improve climate change forecasts of shifts in phenology, ecosystem processes and ecological interactions.
  
  
  METHODS
  The twig method involves cutting dormant twigs from trees, shrubs and vines on a single date or at intervals over the course of the winter and early spring, placing them in containers of water in controlled environments, and regularly recording leaf-out, flowering or other phenomena. Prior to or following leaf-out or flowering, twigs may be assigned to treatment groups for experiments involving temperature, photoperiod, frost, humidity and more. Recent studies using these methods have shown that winter chilling requirements and spring warming strongly affect leaf-out and flowering times of temperate trees and shrubs, whereas photoperiod requirements are less important than previously thought for most species. Invasive plant species have weaker winter chilling requirements than native species in temperate ecosystems, and species that leaf-out early in the season have greater frost tolerance than later leafing species.
  
  
  CONCLUSIONS
  This methodology could be extended to investigate additional drivers of leaf-out phenology, leaf senescence in the autumn, and other phenomena, and could be a useful tool for education and outreach. Additional ecosystems, such as boreal, southern hemisphere and sub-tropical forests, could also be investigated using dormant twigs to determine the drivers of leaf-out times and how these ecosystems will be affected by climate change.
authors: Biology Department, Boston University, 5 Cummington Mall, Boston, MA 02215, USA, primack@bu.edu.
authors: Richard B
authors: RB
authors: Primack
authors: Ecoclimatology, Department of Ecology and Ecosystem Management, Technische Universität München, Hans-Carl-von-Carlowitz-Platz 2, D-85354 Freising, Germany and.
authors: Julia
authors: J
authors: Laube
authors: Biology Department, Boston University, 5 Cummington Mall, Boston, MA 02215, USA.
authors: Amanda S
authors: AS
authors: Gallinat
authors: Ecoclimatology, Department of Ecology and Ecosystem Management, Technische Universität München, Hans-Carl-von-Carlowitz-Platz 2, D-85354 Freising, Germany and Institute for Advanced Study, Technische Universität München, Lichtenbergstrasse 2a, D-85748 Garching, Germany.
authors: Annette
authors: A
authors: Menzel
mesh_terms: D057231:Climate Change; D017753:Ecosystem; D065928:Forests; D058865:Introduced Species; D010641:Phenotype; D017440:Photoperiod; D018515:Plant Leaves / Q000502:physiology* / Q000528:radiation effects; D012621:Seasons; D013696:Temperature; D013997:Time Factors; D014197:Trees / Q000502:physiology* / Q000528:radiation effects
keywords: Dormant twigs; climate change; flowering time; frost tolerance; humidity; invasive species; leaf-out; phenology; photoperiod; shrubs; trees; winter chilling; woody plants