Data from: Tree phenology responses to winter chilling, spring warming, at north and south range limits

Increases in primary production may occur if plants respond to climate
warming with prolonged growing seasons, but not if local adaptation, cued
by photoperiod, limits phenological advance. It has been hypothesized that
trees with diffuse porous xylem anatomy and early successional species may
respond most to warming. Within species, northern populations may respond
most due to the fact that growing seasons are relatively short. Species
most sensitive to spring temperature may show little overall response to
warming if reduced chilling in fall/winter offsets accelerated
winter/spring development. Because current thermal models consider only
highly aggregated variables, e.g., degree days or chilling units
(temperature sums for a season or year) they may not accurately represent
warming effects. We show that assumptions contained in current thermal
(degree-day) models are unrealistic for climate change analysis. Critical
threshold parameters are not identifiable, and they do not actually have
much to do with thresholds for development. Traditional models further
overlook the discrete nature of observations, observation error, and the
continuous response of phenological development to temperature variation.
An alternative continuous development model (CDM) that addresses these
problems is applied to a large experimental warming study near northern
and southern boundaries of 15 species in the eastern deciduous forest of
the US, in North Carolina and Massachusetts. Results provide a detailed
time course of phenological development, including vernalization during
winter and warming in spring, and challenge the basic assumptions of
thermal models. Where traditional models find little evidence of a
chilling effect (most are insignificant or have the wrong sign), the
continuous development model finds evidence of chilling effects in most
species. Contrary to the hypothesis that northern populations respond
most, we find southern populations are most responsive. Because northern
populations already have a compressed period for spring development they
may lack flexibility to further advance development. A stronger response
in the southern range could allow residents to resist northward migration
of immigrants as climate warms. If potential invaders fail to exploit a
prolonged growing season to the same degree as residents, then there is a
resident advantage. Hypothesized effects of warming for xylem anatomy and
successional status are not supported by the 15 species in this study.