Eco-hydrological effects of Qinghai spruce (Picea crassifolia) canopy and its influence factors in the Qilian Mountains

Qinghai spruce(Picea crassifolia) is a main constructive species of water conservation forest in the Qilian Mountains,and it plays an important role in regulating the hydrological processes of this mountainous region.Therefore,rationally analyzing and evaluating eco-hydrological effects of Qinghai spruce canopy is important for understanding the water cycle and water balance of Qilian Mountains.Based on canopy interception data observed at Guantan Forest Station(100°15′0.8″ E,38°32′1.3″ N) within the Qilian Mountains,this study aims at analyzing characteristics of rainfall redistribution of the Qinghai spruce forest and its influence factors.In order to study characteristics of canopy interception of the Qinghai spruce forest,home-made rain gauges,rainfall containers and stemflow devices were carried out in the 3rd sub-plot at Guantan Forest Station in 2008 by Cold and Arid Regions Environmental and Engineering Research Institute of CAS.During the whole observation period(from June 12 to October 8 in 2008),throughfall,stemflow,and canopy interception of 34 rain events were observed.Meanwhile,we extracted the mean canopy closure and leaf area index(LAI) of the 3rd sub-plot from hemispherical images taken by Nikon digital camera fish eye(D80). The mean canopy closure and leaf area index(LAI) of the 3rd sub-plot were 77.07% and 3.41 m2/m2,respectively.Then,we analyzed the characteristics of throughfall,stemflow,and canopy interception of 34 rain events and their influence factors.The results showed that the total throughfall,rainfall interception and stemflow of the Qinghai spruce forest during the observed period were 212.6 mm,64.5 mm and 3.4 mm,and accounted for 75.8%,23.0% and 1.2% of the total atmospheric precipitation,respectively.Throughfall in the forest had great spatial variability and the variation decreased with the increase of rainfall.When rainfall was 0.6 mm,the coefficient of variation of throughfall was up to 100%. With the increase of rainfall,the spatial variation of throughfall decreased sharply and the coefficient of variation of throughfall basically fluctuated around 20% when rainfall was greater than 10 mm.Furthermore,tree canopy structure(e.g.canopy closure and leaf area index) also affected the spatial distribution of throughfall,and its impacts were more significant under the smaller rainfall conditions than under the higher rainfall conditions.When it had no rains before a rain event,the stemflow in the Qinghai spruce forest began to yield only when the rainfall reached 5.6 mm and the stemflow increased with the increase of rainfall.During the whole study period,the total stemflow and average stemflow rate of the Qinghai spruce forest were 3.4 mm and 0.58%,respectively.The moisture degree of tree canopy before a rain event greatly influenced the stemflow yielding as a result almost the same stemflow was yielded under different rainfall amount conditions(e.g.11.5 mm and 3.7 mm). Canopy interception rate mainly depended on rainfall,and it gradually decreased and finally stabilized with the increase of rainfall.Generally speaking,the capacity of canopy interception linearly correlated with canopy closure and leaf area index.However,the negative canopy interception was observed when the devices were installed under the edge of tree crown or canopy overlaps of a few trees.Therefore,we concluded that for a specific stand,canopy structure has an important role in the canopy eco-hydrological effects.