title: Determining past leaf-out times of New England's deciduous forests from herbarium specimens.
abstract: •
  
  
  OBJECTIVE
  There is great interest in studying leaf-out times of temperate forests because of the importance of leaf-out in controlling ecosystem processes, especially in the face of a changing climate. Remote sensing and modeling, combined with weather records and field observations, are increasing our knowledge of factors affecting variation in leaf-out times. Herbarium specimens represent a potential new source of information to determine whether the variation in leaf-out times observed in recent decades is comparable to longer time frames over past centuries.•
  
  
  METHODS
  Here we introduce the use of herbarium specimens as a method for studying long-term changes in leaf-out times of deciduous trees. We collected historical leaf-out data for the years 1834-2008 from common deciduous trees in New England using 1599 dated herbarium specimens with young leaves.•
  
  
  RESULTS
  We found that leaf-out dates are strongly affected by spring temperature, with trees leafing out 2.70 d earlier for each degree C increase in mean April temperature. For each degree C increase in local temperature, trees leafed out 2.06 d earlier. Additionally, the mean response of leaf-out dates across all species and sites over time was 0.4 d earlier per decade. Our results are of comparable magnitude to results from studies using remote sensing and direct field observations.•
  
  
  CONCLUSIONS
  Across New England, mean leaf-out dates varied geographically in close correspondence with those observed in studies using satellite data. This study demonstrates that herbarium specimens can be a valuable source of data on past leaf-out times of deciduous trees.
authors.affiliation: Boston University Department of Biology, 5 Cummington Mall, Boston, Massachusetts 02215 USA.
authors.forename: Peter H
authors.initials: PH
authors.lastname: Everill
authors.affiliation: Boston University Department of Biology, 5 Cummington Mall, Boston, Massachusetts 02215 USA.
authors.forename: Richard B
authors.initials: RB
authors.lastname: Primack
authors.affiliation: Department of Biological Science, Florida State University, Tallahassee, Florida 32306 USA.
authors.forename: Elizabeth R
authors.initials: ER
authors.lastname: Ellwood
authors.affiliation: Boston University Department of Earth & Environment, 685 Commonwealth Avenue, Room 130, Boston, Massachusetts 02215.
authors.forename: Eli K
authors.initials: EK
authors.lastname: Melaas
mesh_terms: D002980:Climate; D057231:Climate Change; D065928:Forests; D009511:New England; D018515:Plant Leaves / Q000502:physiology*; D012621:Seasons; D013696:Temperature; D014197:Trees / Q000502:physiology*
keywords: New England; climate change; forests; herbarium specimens; leaf-out; phenology; remote sensing; trees