间伐强度对秦岭锐齿栎林冠层和枯落物层水化学效应的影响

将森林抚育间伐与森林水化学效应结合起来进行研究,探讨小强度间伐对森林水质的影响。基于固定样地的研究方法,在秦岭火地塘林区选择天然锐齿栎林,设置抚育间伐强度分别为5%、10%、15%和20%的样地和对照样地,定期采集大气降雨、林内雨和枯透水样品,测定其水化学物质浓度,采用对比分析的方法,研究间伐强度对锐齿栎林内雨和枯透水化学效应的影响。结果表明:间伐样地林内雨和枯透水的pH值均低于对照样地,呈弱酸性,在5%的间伐强度下,森林冠层和枯落物层对大气降雨pH值的调升作用较显著,随着间伐强度的增加,调升幅度逐渐减小;大气降雨对森林冠层和枯落物层中的SO_4~(2-)、NO_3~-和PO_4~(3-)均具有淋溶作用,尤其是对照样地林内雨和枯透水中SO_4~(2-)的浓度增幅最显著,NO_3~-次之,PO_4~(3-)最不显著。间伐样地,雨水对林冠层和枯落物层SO_4~(2-)、NO_3~-和PO_4~(3-)的淋溶作用均低于对照样地,20%的间伐强度最有利于净化雨水中的SO_4~(2-),其在林内雨和枯透水中的含量较对照样地降幅最大,间伐强度为5%时,林内雨中NO_3~-、NH_4~+和PO_4~(3-)的含量最低,三者较对照样地的含量分别降低了56.3%、46%和9.2%而枯透水中三者的降幅分别为64.6%、45%和60.8%;在10%的间伐强度下,大气降雨对林冠层和枯落物层中K~+、Ca~(2+)、Mg~(2+)的淋溶作用最强,3种离子中以Ca~(2+)和Mg~(2+)的含量增幅最为显著。林内雨中Ca~(2+)和Mg~(2+)的含量分别较对照样地增加了89.9%和120%,枯透水中二者较对照样地分别增加了72.4%和40%,K~+的增幅相对不明显;大气降雨中的Pb~(2+)、Zn~(2+)和Cd~(2+)经过森林冠层和枯落物层的阻减,其在林内雨和枯透水中的含量随着间伐强度的增加呈先增大后减小的趋势,当间伐强度达到20%时,三者含量明显降低。总体上,20%的间伐强度最有利于森林冠层及枯落物层对重金属Pb~(2+)、Zn~(2+)和Cd~(2+)的截留净化。

Effects of thinning intensity on rain water chemistry of canopies and litters of Quercus aliena var. acuteserrata in Qinling Mountain

Thinning intensity is an important factor affecting water quality within a forest ecosystem. However, there has been little research on the effects of forest thinning on water quality. This study investigated the relationship between forest thinning and rainwater chemistry to quantify the effects of thinning intensity on forest water quality. Five experimental field plots with different thinning intensities (0%, 5%, 10%, 15%, and 20%) were established in a natural prunus armandii forest of the Huoditang in Qinling Mountains. 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. Our results showed that thinning intensity had strong relationships with forest water chemistry. Rainwater was slightly acidic, and both the forest canopy and litter layer could reduce the pH value of rainwater, which decreased with increasing thinning intensity. The pH value was 0.48 in throughfall and 0.65 in litterthrough, which was highest in the plot with a thinning intensity of 5%. The concentrations of SO₄^2-, NO₃^-, and PO₄^3- increased as they passed through the forest ecosystem. Both the canopy and litter layers showed strong SO₄^2- purification in the plot with 20% thinning intensity. The increases of SO₄^2- concentration were statistically significant when compared with NO₃^- and PO₄^3- concentrations in the control plots, with fewer significant relationships in the plots with higher thinning intensities. The increase of SO₄^2- concentration was 4.422 mg/L in throughfall and 1.5 mg/L in litterthrough, compared to the concentration in the thinning plots. A sharp decline in NO₃^-, NH⁺₄, and PO₄^3- concentrations was observed in the 5% thinning plots, compared to concentrations within the control plot, with decreases of 56.3%, 46%, and 9.2% in throughfall and 64.6%, 45%, and 60.8% in litterthrough, respectively. The capability for K ⁺, Ca²⁺, and Mg²⁺ leaching in the plots with a thinning intensity of 10% was significantly greater than the leaching capability in the other plots. The increasing fractions of Ca²⁺ and Mg²⁺ were as high as 89.9% and 120%, respectively, in throughfall, whereas the rising fractions were only 72.4% and 40%, respectively in litterthrough. The increase of K⁺ was not significant. The rainwater in throughfall and litterthrough contained little Pb²⁺, Zn²⁺, and Cd²⁺ because of the interceptions of different forest 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 Pb²⁺, Zn²⁺, and Cd²⁺ were lower than those in the thinning plots. The plots with 20% thinning intensity showed the highest capability for intercepting and aborting heavy metals. The concentrations of Pb²⁺, Zn²⁺, and Cd²⁺ reduced by 10.6%, 22.4%, and 33.5%, respectively. Therefore, the canopy and litter layer in the plot with 20% thinning intensity had the strongest ability to intercept Pb²⁺, Zn²⁺, and Cd²⁺ from rainwater.