亚高寒草甸植物群落种多度分布关系及相似性对氮磷添加的响应

以青藏高原亚高寒草甸为研究对象,采用随机区组设计,通过连续4a添加N、P,研究了不同施肥(N、P、N+P)处理下群落物种丰富度、种多度分布模式以及群落相似性的变化特征。结果显示:(1)N、N+P连续添加4年后,随N素添加水平的增加,草地植物群落物种丰富度逐渐降低(P0.001);种多度分布曲线的斜率逐渐增大;N+P添加处理对植物群落物种丰富度和种多度分布(SAD)曲线的影响较单独N添加处理更显著,如N15P15处理下群落物种丰富度的降幅最大,达对照群落的65.5%;(2)单一N或N+P处理中,不同添加量间的植被组成趋异,而相同添加量的植被组成趋同(stress level=0.152);(3)N、N+P添加引起刷状根的丛生型禾本科植物逐渐在植物群落中占据优势;(4)P素添加对群落物种丰富度、种多度分布曲线、群落相似性和不同生长型组成及比例的影响不显著;(5)植物生长型特征和N/P添加处理可解释56.97%植物群落的物种多度分布特征。这些结果表明:亚高寒草甸地区N添加引起植物群落组成的重新排序、优势种的变化、SAD曲线逐渐陡峭,群落的相似性增加;N富集时,添加P素会增加N素的利用效率,且群落结构受N、P供应水平的影响。

Responses of species abundance distribution and community similarity to nitrogen and phosphorus additions in a subalpine meadow

Nitrogen (N) and phosphorus (P) addition can cause species richness loss in grassland ecosystems. However, few studies have reported the changes in species abundance distribution (SAD) patterns and the development direction of plant communities after fertilization. In the present study, for 4 years, a randomized block design experiment, using N and/or P fertilizers at different concentrations, was conducted in a subalpine meadow in Qinghai-Tibet. With data from the fourth year observation, we analyzed the effects of N, P, and N+P additions on species richness, SAD, and community similarity. We also evaluated the contributions of species richness and plant growth form on community structure and species abundance. Results indicated that within the N or N+P addition treatments, community species richness significantly decreased (P<0.001), the slope of SAD patterns tended to be steeper with higher levels of N addition after four years of treatment. The community species richness and SAD curve responses were more significant in treatments with combined N+P addition than in those with only N addition. For example, in plots with N supply, mean species richness was 44 at an N level of 5 g/m², 33 at 10 g/m², and 30 at 15 g/m². In contrast, in plots with N+P supply, mean species richness values were 37, 29, and 16 at the three N+P addition levels, respectively. Plots with 15 g/m² N+P addition showed the largest extent in species richness loss, which could be 65.5% of the mean control plot species richness. Another result was that in plots with the same resource (single N or combined N+P) addition, vegetation composition diverged among treatments with different N concentrations, whereas vegetation composition converged between treatments with the same N concentrations (stress level=0.152). Furthermore, in the N and N+P addition plots, gramineous plants with brushy roots and caespitose stems gradually became dominant. In contrast, plants with the characteristic of rosette-like leaves, and straight branches, stems, rhizomes, and roots gradually decreased in the plant community. In addition, the responses of community species richness, SAD curves, and community similarity to P addition were not significant (P>0.05). Finally, 56.97% of SAD could be explained by differences in plant growth forms and changes of species richness, induced by the different treatments. Species richness was positively related to plants with rosette-like leaves and rhizomes, but a remarkable negative correlation existed between species richness and plants with brushy roots and caespitose stems. These results suggest that N addition induces a series of changes in plant community composition. These changes include species loss and rearrangement, dominant species turnover and changes of relative abundance of community species (i.e., a change in the slope of the SAD curve). Furthermore, communities tend to be more similar at the same level of N supply, and more divergent at different levels of N addition. Under the condition of N accumulation, P addition could promote the use efficiency of N, and this effect can be amplified with increasing concentrations of N supply. Overall, the community structure was affected by multiple resources and resource supply levels in this subalpine meadow.