森林水源涵养功能的多尺度内涵、过程及计量方法

Concepts,processes and quantification methods of the forest water conservation at the multiple scales

近年来国内森林生态系统服务功能研究较多,但应用价值不高,其科学性受到诸多质疑。从森林水源涵养功能的多尺度内涵、过程以及其计量方法出发,对国内外研究动态和发展趋势进行了总结分析,重新审视森林水源涵养功能的研究意义,探讨森林水源涵养功能的多尺度特征。从森林水源涵养功能作用的空间尺度上看,其拦蓄洪水削减洪峰的功能仅在较小尺度上有效,而调节径流的功能只有当森林土壤的入渗量超过蒸散量时,才可能有更多地下水补给河道径流,进而增加旱季河道流量。同时森林水源涵养功能也具有明显的时间尺度特征,具体表现为在次降水事件中,由于蒸散发量较小,森林水源涵养功能的物质量等于森林不同层次的截留量,在功能上表现为拦蓄降水;在长时间尺度上,由于林地蒸散要耗去大量水分,森林水源涵养功能的物质量等于森林不同层次的截留量减去林地蒸散发量,在功能上表现为净化水质和调节径流。大多数研究仅对单一林分的个别层次蓄水功能进行研究,缺乏流域尺度或者更大空间尺度的森林水源涵养功能研究。建议从区域降尺度到流域或将坡面尺度上推到流域,集中在流域尺度解决森林水源涵养空间异质性将是解决森林水源涵养功能尺度外推的有效办法。就目前国内流行的森林水源涵养功能计量方法而言,其与尺度及研究目的有较大相关性,在研究中应根据研究目的、研究尺度和可获取的数据情况选择合适的计量方法。研究突出了不同尺度作用下森林水文过程的复杂性及重要性,并结合森林与水关系的争论问题,分析目前国内对森林水源涵养功能研究的一些误区,提出森林水源涵养功能研究的一些关键科学问题及未来可能的发展方向,主要包括:1)明确界定森林水源涵养功能的边界,探索森林水源涵养功能计量的新方法;2)加强不同时空尺度关联的森林水源涵养功能研究,包括正确评价森林水源涵养功能的时空变异规律,森林生态系统水源涵养功能的尺度效应,森林水源涵养与下游水生态安全,森林水源涵养研究范式转变等核心问题。

Forest water conservation as an important ecosystem function has caused a rising interests in the scientific community in recent years.However,there remain some arguments in several aspects regarding the forest water conservation function.For the concept of the forest water conservation,there is no clear definition about its connotation.Another concern is the scaling issue.It is a challenge to scale up the estimation for the forest water conservation from the plot scale to larger scale,e.g.,watershed scale or regional scale.Many quantification methods have been developed,but their suitability,performance,advantages and disadvantages are unknown.Many of such argument issues are significant limits of the forest water conservation in science and management.On the basis of the literature review,we clarify the important role of the water conservation in forest ecosystem function.We also summarize and analyze the concepts of the forest water conservation and its multi-scale connotation,processes and quantification methods.On the spatial scale,function of the forest in mitigating peak flow is effective at a smaller scale but not necessarily at a larger scale.More groundwater recharges into stream flow when forest soil infiltration exceeds evapotranspiration,thus increasing stream flow in the dry season.During a rainfall event,forest water conservation has different features at different temporal scales.For example,the amount of the forest conservation during a rainfall event dominates by the forest canopy interception and most water is stored in the forest due to less evapotranspiration in the rainfall event scale.At the longer temporal scale,the forest water conservation can be determined by the forest canopy interception subtracting evapotranspiration.The functional roles of the forest water conservation at longer temporal scale dominate by purifying water from natural rainfall and mitigating seasonal flow peaks of the river discharges.Most previous studies of the forest water conservation focused on water storage function at the plot scale,but lack better understanding in the forest water conservation function at the watershed or regional scales.We suggest the approach that combines downscaling from the regional to watershed scales and upscaling from the hillside to the watershed scales can an effective way of the scale extrapolation.This idea can be used to address the heterogeneity issue of the forest water conservation function at different spatio-temporal scales.Among the existing quantification approaches of the forest water conservation function,each one highly relies on the temporal and spatial scales and its application goal.An appropriate calculation method should be carefully evaluated and considered based on the objectives,data availability,temporal and spatial scales of each study case.The suitability difference in different calculation methods of the forest water conservation demonstrates the complexity and importance of the multi-scale consideration in forest hydrological processes.In the context of the long-term debates about the relationship between the forest and water,we figure out a few confusing points in the current studies of the forest water conservation function.We also propose the critical scientific questions that should be addressed and a few directions in future studies of forest water conservation function including： 1） to clearly define the connotation and boundary of the forest water conservation function,and develop innovatory methods for determining the forest water conservation function;2） to pay more attentions on the forest water conservation function studies associated with the different spatial and temporal scales,including evaluation of the spatio-temporal variations in forest water conservation function,the scaling effects on estimation of the water conservation function,the trade-off relationship between the upstream forest water conservation and the downstream ecological water need,and the development of the innovatory ways of forest water conservation practices.