Department of Plant Sciences in the Conservation Research Institute, University of Cambridge, Cambridge, UK.
Glob Chang Biol. 2020 Mar;26(3):1458-1473. doi: 10.1111/gcb.14954. Epub 2020 Jan 19.
Forest regeneration and expansion are occurring in many countries, with 80 million ha established from 2000 to 2012 under the Bonn accord and 17.5 million ha established from 1990 to 2005 according to the Food and Agriculture Organisation. Multiple reviews have linked increasing forest cover with reduced river flow and potentially detrimental effects downstream. Previous reviews have investigated trends in river flow response over time, but the influence of forest age remains uncertain. Partial river flow recovery (towards non-forested conditions) has been reported in decades following forest establishment, but the role of climate in driving these trends has not been explored. Here, we evaluate river flow trends in 43 studies following forest establishment, which provide sufficient information to distinguish the effects of ageing forests from variable climate. Our meta-analysis supports previous findings showing that forestation reduces annual river flow (by 23% after 5 years and 38% after 25 years) with greater reductions in catchments with higher mean annual precipitation, larger increases in forest cover, and which were idle, rather than agricultural land, prior to forestation. The impact of forests on river flow is sensitive to annual precipitation and potential evapotranspiration, but responses are highly variable. Forests affect river flow less when annual precipitation is low, and sensitivity to precipitation decreases as catchment aridity increases. The majority of catchments demonstrated persistent river flow declines after forest establishment. However, nine catchments showed partial flow recovery after an initial decrease, with peak flow reductions at an average age of 15 and across a range of tree species. The mean rate of recovery was 34 mm/year over 5 years. Partial flow recovery with forest age cannot be commonly expected, however, and forestation programmes should take into account that changes to annual river flow are likely to persist for up to five decades.
森林在许多国家都在再生和扩张，根据波恩协议，2000 年至 2012 年期间建立了 8000 万公顷森林，根据联合国粮食及农业组织的数据，1990 年至 2005 年期间建立了 1750 万公顷森林。多项评论将森林覆盖率的增加与河流流量的减少以及下游潜在的不利影响联系起来。以前的评论已经调查了河流流量随时间的变化趋势，但森林年龄的影响仍然不确定。在森林建立后的几十年里，已经有报道称河流流量出现部分恢复（向非森林状态恢复），但气候在推动这些趋势方面的作用尚未得到探索。在这里，我们评估了 43 项森林建立后的河流流量研究趋势，这些研究提供了足够的信息，可以区分老化森林的影响和多变的气候。我们的荟萃分析支持了以前的发现，即森林化减少了年径流量（建立后 5 年减少 23%，25 年后减少 38%），在年平均降水量较高、森林覆盖率增加较大且在森林化之前是闲置地而不是农业用地的集水区中，减少幅度更大。森林对河流流量的影响对年降水量和潜在蒸散量敏感，但响应变化很大。当年降水量较低时，森林对河流流量的影响较小，而随着集水区干旱度的增加，对降水的敏感性降低。大多数集水区在森林建立后表现出持续的河流流量下降。然而，有九个集水区在最初下降后表现出部分流量恢复，最大流量减少的平均年龄为 15 岁，涉及多种树种。在 5 年内，平均恢复率为 34 毫米/年。然而，不能普遍期望随着森林年龄的增长而出现部分流量恢复，因此森林化计划应考虑到年河流流量的变化可能会持续长达五十年。
