There is probably no more characteristic a feature of forest ecosystems than that they change over time. This change is the result of a wide variety of processes, most of which are 'natural' but some of which are human-induced (either 'unnatural' processes, or simply an increased frequency, intensity and/or extent of 'natural' change). Monitoring and predicting the effects of human activity on the condition of forest ecosystems thus involves the separation of the effects of naturally-caused disturbance on the recovery of ecosystems from comparable and/or dissimilar human-induced changes.
Factors affecting the condition of forest ecosystems include:
(1) Climate change: both natural climatic fluctuations and human-induced alterations in climate.
(2) Allogenic successional processes that involve geomorphological processes largely unrelated to biotic influences.
(3) Autogenic successional processes caused by a varietY of ecological processes.
(4) Biogenic successional processes caused by animals and plant pathogens as a result of natural fluctuations in these biota, human-induced changes in their abundance, or introductions/eliminations thereof.
(5) Changes caused by alterations of atmospheric chemistry. (6) Changes resulting from the episodic disturbance caused by wind, fire, logging, and/or forest clearing for agriculture or other land uses.
Most changes in forest ecosystems tend to be initiated by episodic events such as clearcutting, wildfire, windthrow, or insect outbreak. The response of an ecosystem to such disturbances, and its recovery thereform, will depend on the climate, soil, topography, ecosystem condition (i.e., nutrient and soil organic matter reserves), vegetation type and development, faunal characteristics of the ecosystem, and on the intensity and extent of the disturbance. Many of these are dependent on the time elapsed since the previous disturbance.
The impact of episodic disturbance and the recovery are thus under the control of a wide variety of factos and are correspondingly difficult to predict accurately if based on simple assessments of the ecosystem or a small set of 'ecological indicators'.