Disturbance-mediated switches between forest and barrens on a Michigan sand plain

Prior to clearcutting and fire a century ago, the Kingston sand plain supported a mixed white pine-eastern hemlock-beech forest. Harvesting and fire destroyed the forest, replacing it with barrens vegetation dominated by grasses, bracken fern, blueberries, and reindeer moss. A three-year experiment shows that barrens vegetation competes strongly with tree seedlings for water, which significantly reduces tree seedling survival and growth. The spatial distribution of trees on the plain results from this water limitation; trees are closer to both other trees and to stumps than would occur if the distribution of trees and stumps were random. Near stumps, soil temperature is lower and soil moisture is greater than in areas more distant from trees and stumps. Soil temperature is also lower under tree canopies than in open areas. These data suggest that positive feedback switches from barrens vegetation on water availability are important for the persistence of both barrens and forest on the plain. Barrens vegetation has feedback switches that reduce water availability and limit tree survival, while forest vegetation has feedback switches that increase water availability and facilitate tree growth. The anthropogenic disturbances a century ago likely disrupted forest feedback switches by removing the forest canopy and forest floor organic matter, thus reducing the water available to support tree growth.; The barrens vegetation on the plain today is anthropogenic in origin. However, the effects of logging and fire are similar to those of natural disturbances, and it is possible that both barrens and forest are natural vegetation types on the Kingston Plain. A 10,500 year pollen record from the Kingston Plain shows four periods with pollen assemblages similar to those of the modern barrens, indicating the presence of barrens vegetation on the Kingston Plain in the past. A corresponding charcoal record shows that these barrens periods have concomitant peaks in the charcoal accumulation rate, suggesting that fire may have facilitated the formation of ancient barrens. The similarity between ancient and modern barrens suggests that disturbances that substantially disrupt the feedback switches between forest vegetation and water availability are likely critical for the creation and persistence of barrens.