Long-term implications for streamflow of changes in vegetation cover and stand height in regenerating karri stands in south-west Western Australia

Abstract

In mountain-ash forests in Victoria, streamflow from regrowth stands was found to be less than from the mature stands they replaced. To assess the likelihood for this to occur in the karri forest of southwest Western Australia, vegetation-cover and top-height data for regrowth stands were reviewed because these two parameters affect evapotranspiration and, thus, streamflow from a stand. Vegetation cover in regrowth stands reached that of mature, unlogged stands after 5–10 years of regeneration, continued to rise for another 5–10 years and then stabilised at a value above that of mature stands. Top height increased continuously since regeneration began, bu ta decreasing rate. The top height of mature stands was approached after some 60 years of regeneration. Most of the streamflow in southwest Western Australia is generated when water becomes perched on top of clay horizon, which is typically found bellow some 30–100 cm of sandy-to-loamy surface soil, and flows downslope into the streams. Because young trees are smaller, they need to overcome less lift and less resistance to get water to the leaves. For the same amount of vegetation cover, a young tree can therefore remove water from the surface soil faster than a mature tree. Also, water use increases with vegetation cover and the vegetation cover of regrowth stands soon exceeds that of mature stands. Hence, after it has recovered from the initial increase caused by clearfelling, which is expected to happen after 10–15 years of regeneration, streamflow from karri regrowth stands is likely to become less than from the mature stands they replaced. If the smaller lift and smaller resistance are the main reasons for the faster water use by regrowth stands, lower streamflows are likely to persist until the regrowth stands are about 60 years of age. If the greater vegetation cover is the main reason, lower streamflows are likely to persist for much longer. In State Forest areas, thinning of regrowth stands would overcome a reduction in streamflow. In National Parks and other reserves adequate fire-protection measures should be maintained to prevent the regeneration of large areas due to wildfires, as this is likely to lead to significant reductions in streamflow.