Study: Global warming has U.S. trees bringing spring leaves earlier PRINCETON, N.J., Jan. 30 (UPI) -- Trees in the continental United States could send out spring leaves many days earlier in the future than they did before global warming, researchers say. Princeton University geoscientists report these climate-driven differences could lead to changes in the composition of northeastern forests and give a boost to their ability to take up carbon dioxide. Those northern parts of the United States will have more pronounced changes than the southern parts, they said, with the biggest changes occurring in Maine, New York, Michigan and Wisconsin. Compared to the late 20th century, by the year 2100 some trees could be putting out spring leaves from 8 to 40 days earlier depending on the part of the country, the researchers said. Global warming could also trigger a speed-up of the spring "green-wave," or budburst, that moves from south to north across the continent during the spring, they said. Earlier leaf growth could also bring changes in springtime weather, researcher David Medvigy said, because it causes an abrupt change in how quickly energy, water and pollutants are exchanged between the land and the atmosphere. Once leaves come out, energy from the sun is increasingly used to evaporate water from the leaves rather than to heat up the surface, which can lead to changes in daily temperature ranges, he said. The study has been published in the journal Geophysical Research Letters.