Spring's Ever-Earlier Arrival Is a Boon for Invasive Species

That spring is happening earlier each year as our climate changes has been well documented by how both plants and animals are changing their habits. New research shows just how dramatically this is playing out in the trees of eastern Massachusetts, and that, with continued warming, invasive species may be better positioned than natives to adapt.

According to a recent study examining when leaves first appear on trees and shrubs at Walden Pond, conducted by biologists from Boston University and published in New Phytologist, spring is happening two and a half weeks earlier now than it did in the 1850s.

By comparing observations made by American naturalist Henry David Thoreau with modern observations of over 40 species, made over the past five springs, they found that all species in the area, native and non-native, were leafing out on average 18 days earlier than they did before carbon emissions began precipitously rising in the latter part of the twentieth century.

The really interesting part, which hints at the challenges native species may face in the future, came when the scientists began examining how the trees and shrubs of Walden reacted in the laboratory to simulated spring conditions.

Paper co-author Amanda Gallinat explained, “We found compelling evidence that invasive shrubs, such as Japanese barberry [pictured above], are ready to leaf out quickly once they are exposed to warm temperatures in the lab, even in the middle of winter, whereas native shrubs, like highbush blueberry, and native trees, like red maple, need to go through a longer winter chilling period before they can leaf out, and even then their response is slow.”

Native species are slower to leaf out in warmer temperatures due to the often unpredictable swings in temperature that can occur in a typical New England winter. Adapted to these conditions, the plants react slowly so as not to risk leafing out too soon and risk having the new leaves hit by freezing temperatures.

Non-native species, in contrast, have adapted to different conditions, quickly taking advantage of spring temperatures. This means that as the regional climate warms, further non-native species that leaf out quickly “will be best able to take advantage of the changing conditions,” Gallinat noted.

Across the United States spring is arriving anywhere from one to five days earlier now than it did half a century ago, with the first leaves appearing three days earlier now than in the middle part of the 20th century. For the New England states, the date of first leafing is now five days earlier in Vermont, Maine, Massachusetts, and Rhode Island; four days earlier in Connecticut; and three days earlier in New Hampshire.