The long term effects of thinning treatments on vegetation structure and water yield

Forest thinning has the potential to be used as a water supply augmentation strategy through the reduction of sapwood and leaf areas that also decreases stand evapotranspiration. This study investigated the long term effects of a range of thinning treatments (patch-cutting, uniform thinning, strip-thinning, understorey removal) on the vegetation structure and water yield of mountain ash (Eucalyptus regnans) forests in south eastern Australia. The paired catchment method and Light Detection and Ranging (LiDAR) data were utilised to assess the post-thinning changes in water yield and vegetation structure respectively. The first post-treatment phase started at the end of the treatments in the late 1970s/early 1980s until the suspension of measurements in 1997, the second post-treatment phase started when the measurements resumed in 2007/2008 until a wildfire in February 2008. Up to 36% increase in annual water yield was detected during the first post-treatment phase. Strip-thinning produced the largest cumulative increase in water yield (up to 1813mm), followed by uniform thinning and patch-cutting. The magnitude of the treatment effect appeared to be amplified during a very wet/dry period, which may be due to post-thinning changes in the runoff generation processes. During the second post-treatment phase, a trend of decreased water yield was detected at several treated catchments. Canopy height profiles (CHPs) and mean effective leaf area index (LAI) that were derived from LiDAR data, along with recent forest inventory data, indicated a recovery of vegetation cover. This may partially explain the decrease in water yield. Meanwhile, the decade-long drought (1997–2008) that coincided with this period of decreased water yield might have exacerbated its magnitude. Thinning may not be a net water gain over the long term if the recent trend of decreasing water yield continues.