Northern US Forests Have Lost Biodiversity

Researchers conclude that a commonly-held belief, that forests of northern Minnesota, Wisconsin and Michigan have recovered from rapid, wide-spread logging of over a century ago, is not entirely true.

Where trees have grown back, the forests are now quite different from the original ones that were cut down.

Forests throughout the Laurentian Mixed Forest Province are less diverse than their predecessors. In the early 1800s, conifers dominated 59 percent of an ecoregion's forests, on average, but now form the canopy in only 32 percent of forests. Eastern hemlock, pine and tamarack have particularly declined in abundance. Significant reductions have also occurred in forests with large amounts of beech or yellow birch.

Taking the place of these species across the 257,000 km² Great Lakes region are aspen and maple. Forests dominated by aspen have expanded to five times their area before logging, while maples have doubled their cover. These deciduous trees benefit, in part, from current timber harvesting practices.

Consequently, forests throughout northern portions of the three states have become more uniform. Fewer species of trees commonly make up forest canopies, resulting in a loss of biodiversity and structural complexity. The forests have not recovered to what they were in the days before European settlement, but are now entirely new, simplified ecosystems.

In addition, only some of the cleared land has become reforested. On average, 59 percent of the land area is growing trees now, compared with 88 percent before clearing began 150 years ago. Agricultural crops and pasturelands have especially replaced forests in Wisconsin and in central portions of Minnesota and Michigan, where unforested land covers 85 percent of some ecoregions.