Historical development of alternate communities in a hemlock- hardwood forest in northern Michigan, USA

Hemlock-hardwood forests of Sylvania are composed of 1-20 ha patches dominated either by hemlock Tsuga canadensis or by sugar maple and basswood (Acer saccharum and Tilia americana). The mosaic pattern began 3200 yr ago when hemlock extended its geographical range into the region. Local stand histories reconstructed from fossil pollen show that hemlock invasion of the preceding white pine-red maple-red oak (Pinus strobus-Acer rubrum-Quercus rubra) forest was patchy. Where hemlock invaded, hemlock-dominated stands were established which have since persisted. In the intervening patches sugar maple and basswood became dominant. Hemlock and hardwood patches each have distinctive soil humus, nutrient availability, microclimate and ground flora. Local dominance by either hemlock or sugar maple creates a local environment in which recruitment by the competing species is reduced, a positive feedback that encourages the persistence of patches. -from Authors