Water yield variation with elevation, tree age and density of larch plantation in the Liupan Mountains of the Loess Plateau and its forest management implications

Abstract

Large scale afforestation mainly for erosion control or timber production and a very strict logging ban policy in recent decades led to many over-dense stands and remarkable water yield reduction in the dryland region of the Loess Plateau in northwest China. To guide the integrated forest-water management at stand level, a study on the response of water yield from larch (Larix principis-rupprechtii) plantations to key stand structure and site factors was carried out in the Liupan Mountains. Models of leaf area index (LAI) of forest canopy and stand evapotranspiration (ET) in the growing season were developed and fitted. The growing season water yield was calculated based on water budget. The results showed that: (1) The LAI increases with rising tree density firstly quickly and then slowly and finally tending to its maximum; but firstly increases and then decreases with rising tree age and elevation. The LAI model coupling the effects of tree density, age, and elevation works well. (2) The ET model reflecting the coupled effects of precipitation, potential evapotranspiration, LAI and soil moisture can well predict the ET variation. (3) The water yield decreases gradually with rising tree density, but firstly decreases and then increases with rising tree age and elevation. The lowest water yield appears at the age of 30 years and at an elevation of 2420 m. (4) The implications of this study for integrated forest-water management are: defining water yield as the dominant forest service at high or low elevations, but quality timber production as the dominant service at medium elevations; arranging rational thinning for dense forests around the age of 30 years; designing a mosaic distribution of forest ages within watersheds. Applying the study outcomes can promote the integration of water yield management with traditional forest management to ensure the sustainability of water supply in dryland regions.