Forest harvest increases annual water yield, which is a general conclusion from paired catchment experiments in forest hydrology. However, there have been many studies suggesting that the subalpine forests harvest decreased annual water yield in Yangtze River upper reaches, these conclusions was used to guide forest restoration and management practices in the region for many years.
In this study, meta-analysis for compiling data from many former studies was used to analyze annual water yield changes resulting from change in vegetation in the subalpine region of southwestern China. Data from previous runoff plots and catchment runoff studies were combined to setup the time series of annual water yield after forest harvest, the absent value were interpolated with weighted average method. The annual evapotranspiration (ET) and annual water yield of all vegetation types in the Miyaluo sub-alpine forest region were calculated by using the Surface Energy Balance Algorithm for Land (SEBAL) model, sap flow measurements, and water balance equation.
Our results showed that old dark coniferous forests have a lower annual ET and a higher annual water yield, compared with shrubs, naturally regenerated forests and spruce plantations after harvests. In naturally regenerated areas, water yield increased rapidly in the first three years after the harvest of old growth forests, and returned to the original level at the 6^th year, and then declined continuously to a relatively lower level over a long-period that was corresponding to the vegetation succession into the shrubs, secondary broad-leaved forests, mixed forests with broad-leaved and conifer or spruce plantations. The period with a relatively lower annual water yield could sustain more than a century. Our results suggested that the conclusion that forest increased the annual water yield might only apply to old growth forests dominant by firs, and might not apply to the naturally regenerated forests and spruce plantations in the harvested areas. In the catchment covered with the naturally regenerated forests and spruce plantations, annual runoff decreased with increasing forest coverage, especially for spruce plantations. We thus recommend that the paired catchments experiment should be carried out to improve our understanding on the hydrological functions of the sub-alpine forests, and thus provide effective decision support for forest restoration and management practices in the region.