美国东北部森林因人类活动而改变

Forests in the northeastern US have been radically ¹ transformed over the last four centuries by human activity, and their relationship with climate factors like rainfall weakened, according to research published September 4 in the open access journal PLOS ONE by Jonathan Thompson and colleagues from the Smithsonian Institution. Though the land use history of the northeastern United States is well documented, its ecological ² consequences remain poorly understood. Based on more than 300,000 references to individual trees within colonial-era property records, the researchers found that logging, agriculture and natural reforestation have changed forest composition across 9 states, ranging from Maine to Pennsylvania. While most varieties of trees persist, modern forests are distinct from those in pre-colonial times., ,Beech ³, oak, hemlock⁴（铁杉） and spruce are less abundant, with the sharpest decrease seen in beech proportions in Vermont, western Massachusetts, and northern Pennsylvania. Once forming an average of 22 percent of pre-colonial forests, only about 7 percent of forests now are composed of beech. Fir, cherry and maple ⁵ trees increased in abundance, with maples ⁶ experiencing the highest absolute change in proportions from 11 percent in the past to 31 percent in modern forests., ,The authors observe that despite these changes and opportunities for species invasion or loss, the varieties of trees in modern forests are remarkably ⁷ similar to pre-colonial forests. The modern forest is more homogenous⁸（同类的） and less structured by changes in local climate. Thompson elaborates, "If you only looked at a list of tree species, you'd have the impression that Northeast forests haven't changed. But once you start mapping the trees and counting them up, a very different picture emerges."