华北落叶松人工林蒸散及产流对叶面积指数变化的响应

Simulated responses of evapotranspiration and runoff to changes in the leaf area index of a Larix principis-rupprechtii plantation

定量评价林地蒸散和产流等水文过程对冠层叶面积指数(LAI)的响应,对于深入认识森林植被的生态水文过程及其发生机制,实现半干旱区林水综合管理和区域可持续发展是非常必要的。应用集总式生态水文模型BROOK90,模拟分析了不同降水年型(丰水年、平水年、枯水年)下,位于半干旱区的宁夏六盘山叠叠沟小流域内华北落叶松(Larix principis-rupprechtii)人工林的水文过程对冠层LAI变化的响应关系。结果发现:林分总蒸散量、冠层截留量、蒸腾量与LAI都呈显著的正相关关系(R^2>0.99,P<0.01),而土壤蒸发量、产流量则与LAI均呈显著的负相关关系(R^2>0.99,P<0.01);在不同的降水年型下,各水文过程变量与LAI的关系都可以很好地用指数函数来表达,且都存在着一个LAI阈值。当LAI低于阈值时,各水文过程变量随LAI的变化幅度较大;但高于阈值时,各变量的变化十分缓慢并趋于稳定。在不同降水年型下,各变量LAI阈值之间存在着一定的差异。一般地,丰水年各变量的LAI阈值要大于枯水年,尤其是冠层截留和土壤蒸发。在丰水年,各水文过程变量随LAI增加而变化的速率要比在平水年、枯水年更快,说明在水分充足年份中各变量的波动更多取决于LAI变化,而在水分亏缺的年份中则可能更多地受到水分条件的限制。模拟结果表明,通过减少冠层LAI(如间伐)导致的林分的降低蒸散耗水和增加产流的作用是有限的,这是由于林分蒸散降低的幅度要比LAI降低的幅度小。例如,在平水年,当LAI从4.2变为2.0(减少幅度52.4%)时,林分年蒸散仅从357.2 mm减少至333.9 mm(减少幅度6.5%)。

A quantitative evaluation of the effects of the leaf area index （LAI） of the forest canopy on evapotranspiration and runoff from a forest is necessary for a deeper understanding of eco-hydrological processes and their mechanisms, with the aim to achieve integrated management of forests regions. In this study, the lumped eco-hydrological processes to varying canopy LAI of a Larix principis- and water resources and regional sustainable development in semi-arid BROOK90 model was used to simulate the responses of hydrological rupprechtii plantation in the small semi-arid watershed of Diediegou, in the northern region of the Liupan Mountains in Ningxia, under various conditions of annual precipitation （ wet, normal, and dry years）. Annual evapotranspiration, annual canopy interception, and annum forest transpiration were all significantly positively correlated with the canopy LAI of the plantation （R^2〉 0.99, P〈0.01 ）, while both annual soil evaporation and annual runoff were significantly negatively correlated with canopy LAI （R^2〉 0.99, P〈0.01 ）. All hydrological processes that we studied varied with canopy LAI as exponential functions regardless of the annual precipitation in the year of study. The response of each variable to canopy LAI was associated with a threshold canopy LAI. When the canopy LAI was below this threshold, the change in each variable with respect to LAI was rapid; however, when the canopy LAI was above the threshold, the change in each variable was measurably slow, and gradually stabilized. Additionally, the thresholds for canopy LAI differed with differences in annum precipitation. In general, the thresholds of canopy LAI for the variables studied were higher in wet years than in dry years, especially for canopy interception and soil evaporation. For example, the threshold of canopy LAI for annual evapotranspiration was 1.6, 1.5, and 1.3 in wet, normal, and dry years, respectively; the threshold of canopy LAI for canopy interception in dry years （4.3） was significantly lower than that in wet （6.2） and normal years （6.4）, and the threshold of canopy LAI for soil evaporation was distinctly higher in wet years （4.3） than in dry （3.1） and normal years （3.0）. Moreover, the rates of change of each variable with increasing LAI were higher in wet years than in normal and dry years, indicating that change in each variable was more dependent on canopy LAI in wetter years, but more limited by insufficient precipitation and soil water in drier years. Meanwhile, our results indicated that the effects of the decrease in evapotranspiration and increase in water yield in a given forest stand caused by a reduction in canopy LAI （such as by thinning of the leaf canopy） are less than expected, because the decrease in the rate of annual evapotranspiration was smaller than the rate of decrease of canopy LAI. For example, when canopy LAI reduced from 4.2 to 2.0 （a relative reduction of 52.4%） in a normal year with an annual precipitation of 418.5 mm, the annual evapotranspiration decreased from 357.2 mm to 333.9 mm （ an absolute reduction of 23.3 mm and a relative reduction of 6.5%）. The results of this study can provide guidance to scientific forest management in semi-arid regions.