西南亚高山森林植被变化对流域产水量的影响

The impact of forest vegetation change on water yield in the subalpine region of southwestern China

国内曾有许多研究认为,长江上游亚高山地区的森林具有增加流域年径流量的作用,所以多年来一直以此来指导该区域的森林恢复与经营实践。将以往的径流场和集水区的观测数据结合起来,应用加权平均插值方法,建立了森林采伐后产水量变化的时间序列;使用SEBAL模型、液流测定结果和水量平衡方程,根据以往研究结果,计算了各森林植被类型的年蒸散量和年产水量。分析结果表明:西南亚高山老龄暗针叶林蒸散低,具有较高的产水量;老龄林采伐后产水量增加,但随着次生植被的快速恢复,产水量增加的持续时间很短,6a后即恢复到原有水平;之后伴随着演替进入灌丛、次生阔叶林、针阔混交林或人工云杉林阶段,产水量会进一步下降并长期处于相对较低的水平,而且会持续百年以上。以往研究中该区森林可以增加年径流量的结论,只适用于其所研究的老龄暗针叶林,不适用于原始林采伐后恢复起来的天然次生林和人工营造的云杉林。在天然次生林和人工林占主导的区域,森林覆盖率增加导致产水量减少,人工林减少尤甚。为深入认识西南亚高山森林的生态水文功能,应采用配对集水区试验长期观测研究,才能为亚高山森林的恢复和经营管理提供决策支持。

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 6th 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.