The time when leaves start turning green is one sign that winter is finally over, and spring is here to stay.
But the date that trees turn green changes every year, and depends a lot on temperature. A warm spring can cause leaves to bloom earlier than normal, while a cold spring can let winter cling to the forests for few more drab weeks.
However, the story gets more complicated than that. A spring that gets too warm too quickly can actually delay the process, since the mild temperatures can leave the trees vulnerable to a late-winter freeze. Similarly, if the entire winter is warmer than average, then trees that require a certain amount of “winter chilling” to bloom on time, may also be delayed in the spring.
The map above shows trends in the date when entire forests starting turning green, over the 26-year period from 1982-2008. The data comes from satellite observations of the overall color of the forest. This is a rough measurement, but it allows scientists to make judgments about ecosystems and climate at a broad scale.
The map shows that leaves are coming earlier in some parts of the country like the Rocky Mountains and Appalachians, but that spring leaf-out has been delayed over time in parts of the Southeast, Northeast and Midwest. This is consistent with climate research that shows that many types of plants have not been blooming uniformly earlier over time.
Rising temperatures may actually be behind the way this date shifts in both directions. New research suggests that the response of spring leaves to warming temperatures may not be linear, due to the effects of reduced winter chilling, and increased vulnerability to late-winter freezes.
A continuing trend of warming winters may lead to further changes in the familiar timing of the seasons.