Walden trees and shrubs leaf out earlier now than in Thoreau's time

Posted by Richard Primack, Amanda Gallinat and Caroline Polgar

"One attraction in coming to the woods to live was that I should have the leisure and opportunity to see the spring come in." -Thoreau, Walden

From 1852 to 1860 Henry David Thoreau recorded the leaf out times of common trees and shrubs around Walden Pond and elsewhere in Concord. For the past 5 years, we have recorded the leafing out dates of 43 of the same species in Concord. We have found that these woody plants, including red maple and blueberries, are now leafing out 18 days earlier than in Thoreau’s time. Earlier leafing out times can be added to the list of spring phenomena in Concord and elsewhere affected by the warming temperatures associated with climate change, including flowering dates, butterfly flight times, and bird arrivals. 

New leaves of Black Oak (Quercus velutina) in Concord, MA

We also carried out lab experiments to test the responsiveness of 50 tree and shrub species in Concord to warming temperatures in the late winter and early spring associated with predicted climate change. For the past two winters, we collected leafless dormant twigs from each species, and placed them in cups of water in our lab.  Over the following weeks, we observed how quickly each species produced their leaves in these unseasonably warm lab conditions. 

 We clipped and rinsed twigs weekly before returning them to the light banks in the lab

We evaluated twigs weekly for budburst or new leaves

We found that invasive non-native shrubs, such as Japanese barberry and multiflora rose, are able to leaf out within a few days once they are exposed to warm temperatures even in the middle of winter, whereas native shrubs and trees need to go through a longer winter chilling period before they can leaf out -- and even then their response is slow. These experiments, building on Thoreau’s observations, show that as spring weather continues to warm, the invasive shrubs will gain a further competitive advantage.

 One month after twigs were collected in January, most exotic shrubs had leafed out, about half of the native shrubs, and very few of the native trees had leafed out.

We describe these observations in greater detail in an article just published in the scientific journal New Phytologist.

click here for a copy

 Most of our 2013 twig clipping team, hard at work in the lab!