Forest Biology Lab.

Spatiotemporal dynamics of forest is a long-term process. Plant ecologists often assume that spatial mosaic of environmental factors, such as soils and topography, affects the spatial structure of plant communities at the local- to landscape-scales (<105 ? 106 m2) in temperate forests. However, this assumption has not been evaluated properly in the palaeoecological context. This talk presents two case studies ? one from southeastern Estonia and the other from northern Michigan in the US ? that discuss the reconstructed changes in the spatial patterns of local vegetation using fossil pollen from a network of small hollows in each area and with a little help from modeling approaches. Sediments from small hollows are invaluable for vegetation reconstruction within 130-300 m radius, making it possible to evaluate the extent to which the physical factors and biological phenomena, such as tree invasion, affect the local vegetation development. In SE Estonia, the “spatial grain” of the local vegetation mosaic changed stepwise and became larger ca. 9500 years ago, when spruce (Picea abies) and other late-successional trees became dominant locally. In northern Michigan, invasion of forest by hemlock (Tsuga canadensis) ca. 3500 years ago appears critical for the formation and maintenance of the hemlock-hardwood mosaic currently observed in the nature reserve we studied. The local physical factors are mostly uncorrelated to the local vegetation composition through time in both case studies. Those results demonstrate that a combination of observational, experimental and modeling approaches in palaeoecology promotes a better understanding of plausible mechanisms and factors affecting the long-term dynamics of vegetation in space and time.