Effects of invasion by Tsuga canadensis on a North American forest ecosystem

Temperate forest ecosystems have gained and lost tree species throughout the last 10,000 years as tree species ranges expanded and contracted in response to changing climate. The ecosystem-level effects of these events are illustrated by the invasion of forested landscape in northern Michigan, USA 3000 yr B.P. by hemlock (Tsuga canadensis). Fossil pollen from small forest hollows, recording trees within 50 m, provides histories for individual hemlock-dominated, hardwood-dominated and mixed stands which now form a mosaic over the landscape. At the time of hemlock entry the regional climate was gradually becoming more moist. Climatic changes were apparently amplified by the effect of hemlock on microclimate and fuel type, reducing the frequency of fire. Fire-sensitive species increased throughout the forest, changing the species composition of hardwood stands as well as hemlock stands. At present sugar maple (Acer saccharurn) stands are not easily invaded by hemlock, but tend to persist for thousands of years, with minor shifts in the positions of the boundaries. Hemlock invasion slowed rates of nutrient cycling, changed microclimate, and affected deer habitat, as well as reducing fire frequency. Hemlock invasion increased diversity by producing the landscape mosaic of hemlock- and hardwood-dominated forests.