Lower winter chilling requirements could give woody invasive species an edge in warmer springs

Background/Question/Methods

Advancing spring temperatures have resulted in earlier onset of leaf-out in temperate ecosystems. Comparing current leaf-out dates in Concord, Massachusetts with the leaf-out records of Henry David Thoreau shows that local spring leaf-out has advanced by an average of 5 days from 1853-1860 to 2009-2012. While warmer spring temperatures have consistently advanced spring phenology over the past 150 years, warmer winter temperatures may disrupt this trend. Leaf-out phenology for many species may be controlled by winter chilling requirements that could fail to be met under changing climate conditions. We experimentally tested the winter chilling requirement of 19 species in the winter of 2011-2012. Once a month, from December to March, we clipped 20 twigs of each study species from plants in the field and placed them in jars of water in a lab at room temperature. We monitored them regularly and observed the number of days before each species leafed out. In the winter of 2012-2013, we are replicating and expanding this experiment to include 54 local native and invasive shrubs and trees. These study species were selected from a group for which we have Thoreau’s records and which we are currently monitoring for leaf-out in Concord, MA.

Results/Conclusions

In 2011-2012, we found that the invasive species in our experiment had a lower winter chilling requirement than the native species tested. Preliminary results from January 2013 support this trend. Non-native shrub species have, on average, the lowest winter chilling requirement; our 15 species tested took on average 16.7 days to leaf out once inside room temperature conditions. Under the same conditions, native shrubs took on average 26 days to leaf out, while native trees had the greatest winter chilling requirement, averaging 32 days from clipping to leaf-out. By the end of one month, 10 of the 16 native tree species collected in January did not yet have leaves fully emerged and unfolded. These results indicate that winter plays an important role in spring phenology. As spring temperatures warm, species with a high chilling requirement may reach a physiological limit that delays their leafing out times. Our results suggest that shorter winters will give non-native, invasive shrubs a lead on native species in spring primary production, and eventually may lead to increased dominance of these species in temperate New England forests.