Distinguishing the Effects of Environmental Stress and Forest Succession on Changes in the Forest Floor
Department of Forestry, University of Kentucky, Lexington, KY 40546-0073, USA
When interpreting change over time in forest ecosystems, distinguishing the effects of forest succession from the effects of environmental stress can be difficult. For example, the decline of some species of migrant birds reflects not only the loss of tropical winter habitat, but also the maturation of the forests of the summer habitat. Similarly, spruce decline at low elevations of the northern Appalachian Mountains is partly due not only to acid rain but also to the aging of an even-aged forest.
Difficulties associated with separating the two types of causes of change can result in a simplistic interpretation of a specific successional or environmental cause of forest change when both may be occurring. We present two case studies of changes in the forest floor in northern hardwoods.
First, the belief that 50% of soil organic matter is lost in the first 20 years after logging was based on a study comparing northern hardwood stands of different ages. We resampled a series of 13 such stands after an interval of 15 years, and found that the young stands were not, in fact, losing organic matter as rapidly as predicted from the original chronosequence study. The pattern of higher organic matter content in the forest floors of older stands compared to young stands could be explained equally well as a result of changes in logging practices over the last century as by the aging of the stand. Important differences in the treatment of these stands include the degree of disturbance to the forest floor and the intensity of harvest removals. The observed pattern of forest floor organic matter as a function of stand age was previously interpreted as a successional pattern, ignoring changes in treatment history. In the second case, observed losses of base cations from the forest floor were attributed to cation depletion caused by acid rain and declining calcium deposition. We found that young stands were gaining base cations in the forest floor; losses of base cations were restricted to older stands. In this case, the contribution of successional processes to cation loss were overlooked in favor of environmental stress as the dominant mechanism. Studies of environmental stress use repeated measures over time, but often don’t consider stand age as a factor. Studies of successional change often assume that environmental factors remain constant. We were able to consider both forest succession and external factors because we repeatedly sampled stands of different ages.