Agents of Watershed Change | Watershed Academy Web

Human-Made Changes

Human-induced Change Processes: Timber Harvest
Many watershed effects of timber harvest are similar to agriculture's effects. Aquatic impacts such as altered runoff and streamflow, increased sedimentation, and addition of nutrients can result from silviculture operations. The most obvious terrestrial change associated with timber harvest is that of removal of vegetation, but other common changes involve habitat alteration, altered species diversity and stand age. There are also numerous indirect effects.

Under commercial forestry, mature, multi-aged, mixed-species forests are often replaced with young monoculture stands of commercially valuable species. In many cases, harvested lands are left denuded of live vegetation for long periods of time. These changes in forest habitat have different effects on different species. Species which rely on old-growth conditions for parts of their life cycle (e.g., northern spotted owl) may be adversely impacted, whereas species which use edge habitats (e.g., white-tailed deer) may increase in population. Intensity of disturbance generally corresponds with the spatial scale of the area cut, whereas rotation lengths control frequency. Accordingly, there is a distinction to be made between clearcutting, in which all trees harvested from a given area and replaced with an even-aged stand, and selective harvesting (i.e., some trees removed, others remain, resulting in a mixed-age stand). Whereas clearcutting results in drastic change over large areas of forest habitat, selective cutting may also produce significant impacts, as rotation lengths are typically shorter, and sites are therefore disturbed more frequently.

In addition to terrestrial impacts, timber harvesting may have significant effects on stream discharge and water quality. Loss of mature vegetative cover leads to decreased evapotranspiration and correspondingly increased peak flows. These effects are compounded by road construction, which creates permanently bare surfaces and compacts soil, resulting in decreased infiltration and increased surface runoff. Logging roads also fragment and isolate habitat patches from smaller, less mobile animals such as salamander species, and roads' stream crossings and culverts sometimes become barriers to fish passage. Surface runoff on or near logging roads effectively increases stream density in logged watersheds, resulting in more rapid drainage and higher fluctuations between peak flows and base flow. Increased surface runoff, along with fewer trees for bank stabilization, generally causes increased erosion and siltation, while road construction on steep slopes often results in slope failure and excess sediment delivery to streams. Overall effects can also include elevated and more variable water temperature, increased turbidity, and higher uniformity of substrates, which generally impair habitat for a number of fish and invertebrate species.

Riparian buffers are required in many jurisdictions, but these have not always been successful in mitigating effects on streams. In general, it should be recognized that the effects of logging on stream discharge and temperature are caused by watershed-level processes and, as such, are unlikely to be completely mitigated by riparian buffers alone. (Harr 1986, Murphy 1995, Jones and Grant 1996) A major change in federal land management policy in the 1990s has begun integrating watershed analysis into timber management and multi-purpose land management on federal lands across the US.