A recent study published through the Public Library of Science compares the modern day forests of the American Northeast to their predecessors of some 400 years ago, through reconstruction via archival records. The study examines nine states, spanning from Pennsylvania to Maine and covering 4.33 x 105 square kilometers. It compares modern data of forest composition from the 2003-2008 Forest Inventory and Analysis census to data from what were called witness trees recorded when the American colonies were still forming. These witness trees were used as markers to survey plots within towns and so they give a unique account of ecosystem composition for the time period. Similar studies have been conducted using witness tree data but never on this scale, which spans the entire region of the northeast. It is also worth noting that, as of 2010, eighty percent of this region was forested, though less than one percent was old-growth forest.
By measuring the changes in ecosystem composition, the study aims to characterize how human land use has affected natural growth. Remarkably, despite intense disturbances from agriculture and logging, the area is still comprised of largely the same native genera as it was 400 years ago. The area has remained resilient to invasion and disease, having lost only the American Chestnut which is locally extinct in the northeast due to a fungal disease which was introduced during the 1900s. Witness tree data only include genera (plural of genus, for those curious), rather than species, and so the analysis is limited by this less specific nomenclature.
While the trees may be the same, their numbers are not. What the study found is that, while the same genera occupy these forests, their relative densities have changed dramatically. In the past, areas that were closer together often had very similar compositions, whereas areas further apart would feature different types of trees. Now though, these species have spread more and are less clustered. There has also been a serious decrease in old-growth trees, which dominated four hundred years ago.
What this means is that human interaction with the environment has homogenized these ecosystems. In the past, diversity derived from local conditions and changes in the landscape. Now though, as a result of land use, species barriers before created by unique small-scale conditions have been broken down and species are distributed more evenly, so that composition is not as reliant on the environment. The past prevalence of old-growth trees, such as beech and hemlock, indicates that those were very low disturbance forests. Their loss indicates that even now, with much more stable forests following diminished land use, disturbance regimes have shifted in these environments to encourage other species to thrive.
Beyond being interesting because it spans centuries and an entire region of the United States, this study is important because it helps to characterize how heavily-used forests recover from serious disturbance. There has been massive disturbance to large forests in recent and distant history around the world, so these ideas are relevant from New York to the Amazon to Cambodia. Understanding how biodiversity is affected by disturbance in the northeastern United States could help us understand how it will be affected in places like South America or Southeast Asia, where biodiversity is often threatened but incredibly valuable. As the paper itself notes, it is very promising to see an ecosystem recover with native species intact and still thriving. The question is only how to best facilitate that in other regions.