干旱缺水地区森林植被蒸散耗水研究

 系统总结近年来在宁夏固原六盘山、北京延庆等干旱缺水地区进行的森林植被蒸散耗水研究结果。乔、灌、草作为土壤水分限制型生态系统的坡面植被,蒸散都是水分平衡的最大分项,其中植被蒸腾又是蒸散的最大分项。植被蒸散量一般表现为高大乔木林>亚乔木林>灌木林>自然草地,但人工草地>自然草地。可依坡面产水功能将不同植被分类,自然草地和灌丛为水源生产型,亚乔木林为水源平衡型,高大乔木林和人工草地为水源消耗型。植株密度不是坡面植被蒸散大小的决定性因子,其作用更多的是调控蒸腾量及其占蒸散比例。虽然降低植株密度一般会减小蒸散,但并不是相同比例地线性下降,不同植被类型反应也不一样,表现出降低密度减少蒸散的作用从乔木、亚乔木到灌木而变弱的趋势,降低密度减少蒸散的作用是有限的。对降低密度减少蒸散的作用大小,作为调控措施的有效性,有效的密度调控范围等,还需严格的对比实验和理论研究。估计和评价植被蒸散耗水时,用叶面积指数或叶生长量指标可能比密度更符合生物学逻辑。从在干旱缺水地区建立节水、稳定、高效、多功能的坡面植被的角度而言,草地和灌丛的蒸散低于乔木林,建立稀树草原或稀树灌丛式的植被可能更利于流域产水和植被稳定。从小流域管理的角度而言,还需考虑在土壤水分承载力空间差异的基础上,探讨能兼顾产水功能、水土保持、水源涵养、植被稳定的植被空间优化配置的理论和技术。

Study on the evapotranspiration of forest and vegetation in dryland

The studies on evapotranspiration (ET) of forest and vegetation carried out in recent years in the dry areas in Liupan Mountain of Ningxia and Yanqing of Beijing were summarized. As soil water limited ecosystem, the ET of slope vegetation, regardless trees or shrubs or grasses, is always the biggest component in water budget, and transpiration (T) is the biggest component in ET. The order of ET is generally as high- arbor- forest > small-arbor-forest > shrub > natural grassland, but artificial grassland > natural grassland. According to water-yield function, slope vegetation can be classified: natural grassland and shrub as water-yielding vegetation; small-arbor-forest as water-balancing vegetation; high-arbor forest and artificial grassland as water-consuming vegetation. Density is not a determinative factor for slope ET, its function is more on regulating the value of T and the ratio of T/ET. Although ET will be generally reduced with decreasing density, but not in the same magnitude and not in linear, and different vegetations react differently. It shows a tendency that the ET-reducing effect by decreasing density tends to be weaker from igh-arbor, small-arbor to shrub. The function of ET-reducing by decreasing density is limited. More strict comparisons and theoretical researches are required for evaluating the ET-reduction effect by decreasing density, the availability of decreasing density as a measure of reducing ET, and for searching the effective range of density regulation. The LAI and leaves biomass may be better indicators than density for estimating and evaluating water consumption of vegetation, since it is more biological based. From the viewpoint of establishing slope vegetation with characters of water-saving, more stability, high efficiency and multifunction in dry regions, it is sure to say that the ET of grassland and shrub is lower then forest. The savanna-like forest or sparse forest with shrubs is more suitable for water-yielding and for vegetation stability. For a better small watershed management, the theoretical and technical research on an optimal spatial allocation of different vegetation based on the spatial heterogeneity of soil water carrying capacity should be further carried out, in order to meet the needs of water-yield, soil protection, water retention, stable vegetation simultaneously.