不同放牧强度下川西北高寒草地植物功能群特征变化

Functional group characteristics of plant community at different grazing intensities in alpine grassland of northwestern Sichuan

放牧是川西北高寒草地最主要的生产方式,过度放牧可能造成草地生态功能的严重退化,但不同强度的放牧如何逐步引起草地植物群落演变与生态功能退化还未得到深入研究.调查当地长期放牧形成的梯度(围栏禁牧:长期围栏,NG;轻牧:1牛/hm^(2),LG;中牧:2牛/hm^(2),MG;重牧:3牛/hm^(2),HG),研究生长盛期不同放牧强度下植物群落与功能群组成、多样性、生物量及其分配的变化,以理解放牧对草地植物的潜在影响和退化过程,为高寒草地生产实践的适应性管理提供理论基础.结果表明,随着放牧强度增加,群落高度出现显著下降;HG样地群落密度显著小于其他3个处理(P<0.05).群落Margalef指数、Shannon-Wiener指数、Simpson指数、Pielou指数呈单峰变化,LG样地最大,HG样地最小.LG样地植物地上生物量显著大于NG、MG和HG样地(P<0.05);NG样地植物地下生物量显著小于LG样地,但显著大于HG样地(P<0.05);各处理间总生物量存在显著差异(P<0.05),LG样地总生物量最高,HG样地最低.NG样地禾本科生物量显著大于MG和HG样地;LG样地莎草科和杂类草生物量显著大于NG、MG和HG样地,HG样地莎草科、豆科和杂类草生物量均为最低.禾本科相对生物量大小的样地顺序为HG>NG>LG>MG,NG样地中禾本科相对生物量高达57.85%,HG样地则达到了79.54%;MG样地莎草科相对生物量最高达93.36%,其次是LG样地,为54.94%,HG样地最低;LG样地杂类草相对生物量最高,显著大于MG和HG样地.上述结果说明,过度放牧和围栏封育都会不同程度地降低植物群落多样性和生产力,进而导致草地生态系统功能降低和退化.(图5表2参44)

Grazing is the main production measure for alpine grasslands in northwestern Sichuan,China.Overgrazing may degrade the ecological functions of grassland.However,how different grazing intensities gradually cause the evolution of grassland plant communities and the degradation of ecological functions has not been thoroughly studied.This study aims to investigate the effects of grazing on the structure and functional group composition,diversity,and biomass allocation of the alpine grassland community,and provide the theoretical basis for the adaptive management practices of the alpine grasslands.Four grazing plots with different grazing intensities were set up in the alpine grassland in the northwest of Sichuan Province as follows:no grazing by fence(NG),one yak per hectare as light grazing(LG),two yaks per hectare as middle grazing(MG),and more than three yaks per hectare as heavy grazing(HG).Results showed that community height significantly decreased with the increased grazing intensity,and the intensity of HG was least among the three plots(P<0.05).The community Margalef index,Shannon-Wiener index,Simpson index,and Pielou index significantly changed with the increased grazing intensity(P<0.05).The community Margalef index was highest under LG,whereas the community Pielou index was lowest under HG.The aboveground biomass of LG was significantly higher than that of NG,MG,and HG(P<0.05).The underground biomass of NG was significantly lower than that of LG but higher than that of HG(P<0.05).There was a significant difference in total biomass among all the treatments(P<0.05);total biomass was highest under LG but lowest under HG.Biomass of NG was significantly higher than that of MG and HG.Sedge family and hybrid grass biomass of LG was significantly higher than those of NG,MD,and HG,and sedge family,legumes,and hybrid grass biomass of HG was lowest.The relative biomass of Gramineae was of the following order:HG>ND>LG>MG.The relative biomass of Gramineae was 57.85%under NG and 79.54%under HG.The relative sedge biomass of MG was 93.36%,followed by LG(54.94%),with HG having the lowest relative biomass.The relative hybrid grass biomass of LG was higher than that of MG and HG.These results suggested that both overgrazing and fencing could decrease plant community diversity and productivity and even degrade the ecological function of the alpine grasslands.