Shifts in land use practices that accompanied European settlement of northern New England triggered fundamental changes in the dynamics of forests left standing. Here we show that the some of the most important impacts on upland forests were indirect effects stemming from changes in the relative importance of fire, wind, and ice storms as disturbance factors. We examined the effect of a changing disturbance regime on forest dynamics in an uncut forest in Vermont. Eastern hemlock (Tsuga candensis) and red pine (Pinus resinosa) that make up most of the canopy today were established in the 1600s and 1700s; regeneration of both species was stimulated by frequent fires. No fires have occurred since the mid-1800s, presumably because of human suppression of wildfires in the surrounding landscape. Two large wind disturbances have occurred since the mid-1900s. In 1950, a windstorm blew down almost 7% of the canopy, and the 1998 ice storm opened almost 5% of the canopy. We used a combination of tree-ring reconstructions, stand reconstruction from fallen trees, forest surveys, and re-sampling of permanent study plots to determine the pattern of succession in the gaps created by these wind and ice disturbances. We then used a modified Horn matrix model to simulate changes in forest composition over time assuming that wind and ice storms continue to be the dominant mode of disturbance.
Results/Conclusions
Our results show that wind and ice storms favor the regeneration of hardwoods (maple, beech, birch) and hemlock over pine. Pine stands blown down by the 1950 windstorm were overwhelmingly replaced by hemlock or hardwood stands. Hardwoods have established in greater densities during the first ten years of succession following the 1998 ice storm, and hardwoods are more abundant than hemlock in all gaps. We used the Horn matrix models to explore the possible consequences of a sustained shift from a fire-dominated disturbance regime (which prevailed through the mid-1800s) to one in which wind and ice events disturb 5-10% of the canopy every 50 years (which has prevailed through the 1900s). The model confirms that if the current disturbance regime continues, hemlock and hardwood will increase at the expense of the mixed hemlock/pine stands that currently dominate the forest. Disturbance regime, which has changed as an indirect consequence of land-use change, thus has a significant influence on forest dynamics and composition, and a long-term shift in composition from hemlock/pine to hemlock/hardwood seems likely.
