Research The Effects Of Thinning Intensities On Water Quality Of Quercus Aliena Var. Acuteserrata Forest On The South Slope Of Qinling Mountain

The southern-slope of the Qinling Mountain is the source of the Danjiangkou Reservoir, which is one of the most important forest water supply regions for the Middle Line of the South-to-North Water Transfer Project. The area is mostly covered by forests. The forest water quality directly affects the diverted water quality. Forest vegetation in this area is mainly natural forests or plantations that recovered after harvesting or vandalism in the last century. So it is difficult for most plantations to be active in water conserving as the high canopy density and poor growth. Therefore, it is necessary to tend and thin in natural forest. However, little data exists in studides of the relationship between thinning intensity and forest water quality. The effect of thinning intensity on water quality of different forest space layer is unclear. The Qinling region exists slightly acid rains, also is rich in the mineral resource of Lead and Zinc ore. The acid rain and heavy metal pollution will threaten the quality of forest water. Soil layer is the key level affecting water quality effects of forest ecological system.Therefore, thinning intensity effects on the water quality of soil layers is necessary to study under different levels of pH and heavy metal contamination in the rain. These studies not only have important theoretical scientific evaluation of the relations between forest thinning intensity and water quality, but also have practical applications for water conservation forest management.Huiditang is a significant area for concentration of water conservation forests in the southern-slope of the Qinling Mountain. One of the main forest species in the forest is the Quercus aliena var. Acuteserrata forest, which distributes widely in this area. So this study taking the natural Quercus aliena var. Acuteserrata forest in Huiditang, south slope of Qinling mountains as the object in the year 2012 and 2013, which was under close-to-natural management of different thinning intensities(5%, 10%, 15%, 20%) and the unthinned forest as the control, the effects of thinning intensites on forest water quality was evaluated comprehensively. Rainwater samples were collected twice a month from June to September in 2012 and 2013. Water chemistry was measured to analyze the chemical effects of tending and thinning intensity on throughfall and litterthrough. In order to investigate the effect of soil on water quality in Quercus aliena var. Acuteserrata by simulating the rainfall with different pH and different concentration of heavy metals, a soil column method was applied to study the release of heavy metals in the forest soil under different thinning intensities.The research conclusions were as follows:(1)The atmospheric rainfall was slightly acidic during the rainy season in Huoditang forest area. It was found that there are not remarkable interannual changes in the pH of rainfall. SO42-、NO3-、Ca2+、K+ were the more abundant ions, in the case of heavy metals, Pb、Zn、Cd present higher concentration.(2)The results showed that thinning intensity had strong relationships with forest water chemistry. Forest canopy layer could reduce the pH walue of rainfall, which increased with increasing thinning intensity at one year after a year form thinning. The pH of throughfall was higher in the plots with 15%-20% thinning intensity than other plots. The concentrations of SO42-, NO3-, PO43- and base ions increased as they passed through the forest ecosystem.The canopy showed strong SO42- purification in the plot with 20% thinning intensity. The increases of SO42- concentration were statistically significant when compared with NO3- and PO43- concentrations in the control plots, with fewer significant relationships in the plots with higher thinning intensities. A sharp decline in NO3-, NH4+, and PO43- concentrations was observed in the thinning plots with intensity above 15%, compared to concentrations within the control plot. The capability for K +, Ca2+, and Mg2+ leaching in the plots with a thinning intensity above 15% was significantly greater than the leaching capability in the other plots. The rainwater in throughfall contained little heavy metals because of the interceptions of canopy layers. The thinning intensity was also strongly related to the changing tendency of heavy metal ions. Compared with the control field plot, the concentrations of Pb, Zn, Cd, Cu, Cr, Al were lower than those in the thinning plots. The plots with 20% thinning intensity showed the highest capability for intercepting and aborting heavy metals. A year after thinning, the concentration of all ions in the throughfall water generally declined.(3)The pH value of litterthrough was lower than in the throughfall, and it rasied with increasing thinning intensity above 15%. With lower thinning intensity, the concentrations of SO42- in litterthrough were lower than throughfall, however, NO3-、NH4+、PO43-、K+、Ca2+、Mg2+ and heavy metals showed significantly higher concentrations in the litterthrough. A year after thinning, the litter layer showed strong SO42-, NO3-, PO43 and heavy metal ions purification in the plot with 15%-20% thinning intensity. A sharp decline in SO42-, and Cd concentrations was observed in the thinning plots with intensity of 20%, compared to concentrations within the other plots. These two ions were almost completely intercepted by the litter layer. While the capability for NH4+, K +, Ca2+, and Mg2+ leaching in the thinning plots was significantly greater than before thinning.(4)The integrated quantitative evaluation on water quality of throughfall and litterthrough was made by principle components analysis method. Sort the entire list of water quality by thinning intensity, the sequence of the water quality in the throughfall scored by comprehensive assessment on was 20% thinning intensity > 15% thinning intensity > 10% thinning intensity > 5% thinning intensity > controlled plot。While in the litterthrough, the result was 5% thinning intensity > 20% thinning intensity > 10% thinning intensity > controlled plot > 15% thinning intensity。With the thinning intensity of 20%, the forest water quality was better than other intensities.(5)Forest soil had some buffering capacity for regulating the pH value of rainfall. Under simulated rainfall conditions, the forest soil could increase the pH value of acidic rain water by infiltrating to deep layer; when the rain was alkaline, the pH value was reduced. In less than 15% thinning intensity plots, the soils showed weaker regulation ability on acidic rain. It was beneficial for the soil to combat the effects of acid rain with the thinning intensity of 20%, respectively.(6) The heavy metal of rainfall through the forest soil showed as follows: With different mass concentration of heavy metals in rainfall, the soil had both desorption and adsorption role on Cd, Pb and Zn in rainfall. It could stimulate the adsorption of heavy metals Cd, Pb and Zn from rainfall by improving the pH value of water-soil system. When the mass concentration of heavy metals in rainfall was low, it appeared main soil desorption effects; While at high mass concentration of heavy metals in rainfall, soil was mainly for adsorption. The heavy metal desorption rate decreased with increasing thinning intensity, however, the adsorption rate increased. The thinning intensity of 20% was most favorable with regard to desorption of heavy metals of forest soil.