Transpiration

The influence of an intact forest on evapotranspiration is affected by ecological disturbances, such as clear-cutting and wildfire. In general, these sorts of stand-level disturbances greatly reduce the transpiration component of evapotranspiration for several years, a change that influences other hydrological processes, such as the timing and amounts of stream flow, which may then have effects on flooding and erosion. In addition, on sites that do not drain well, substantial decreases in transpiration can increase the height of the water table.

An ecological study done at Hubbard Brook, New Hampshire, involved the clear-felling of all trees on a 39.5-acre (16-hectare) watershed, which was then kept clear of regenerating plants for two additional years. The subsequent disruption of transpiration increased the amount of streamwater flow by an average of 31% over the three-year period. Many other studies of forestry have come to similar conclusions—clear-cutting increases streamflow by decreasing transpiration. However, because disturbed forests regenerate quickly through ecological succession, the amount of foliage on the site quickly recovers, and the effect of disturbance on streamflow tends to be rather short lived. In general, the pre-cutting stream flow volumes are substantially re-attained after three to five years of revegetation has occurred, because of the recovery of the transpirational surface area of plant foliage. (Of course, the regenerated foliage occurs on relatively short grasses, herbs, and shrubs, rather than on taller trees as in the initial forest.) The largest increases in stream flow usually occur in the first year after the forest is harvested, with progressively smaller effects afterwards.