刈割、施肥对高寒草甸物种多样性与生态系统功能关系的影响及群落稳定性机制

Effects of clipping and fertilizing on the relationships between species diversity and ecosystem functioning and mechanisms of community stability in alpine meadow

植物群落中不同"功能身份"物种的多样性与特定生态系统功能之间具有何种关系及其作用机制尚不明确。通过在高寒矮嵩草(Kobresia humilis)草甸为期5年的刈割(不刈割、留茬3cm、留茬1cm)、施肥(施肥、不施肥)和浇水(浇水、不浇水)控制实验,研究了刈割与土壤资源获得性梯度上不同"功能身份"物种(群落中所有物种、响应物种、作用物种和共有物种)的多样性变化与群落地上净初级生产力和稳定性的关系以及稳定性机制。研究结果显示:群落中响应物种、作用物种和共有物种数分别占全部物种数的36.6%、18.3%和64.8%,物种多样性对生态系统功能具有不同的效应,净初级生产力主要受响应物种和作用物种的多样性变化影响,而稳定性则主要由共有物种的多样性变化决定;群落稳定性的维持主要依赖于共有物种的多样性增加,其作用机制是投资组合效应,而超产效应和异步性效应对稳定性并无作用;刈割和施肥对物种多样性、稳定性和净初级生产力具有相反的影响,前者能增加物种多样性和稳定性,并降低净初级生产力,而后者的作用正相反。这与群落中全部物种的多样性变化受刈割影响较大,而作用物种的多样性变化受资源获得性影响较大有关。上述结果表明高寒草甸生态系统地上净初级生产力主要由少数影响生产力的作用物种的多样性决定,而稳定性则由大量共有物种的多样性所掌控。投资组合效应是物种多样性导致稳定性的机制。由于群落中不同物种的多样性效应具有分异性,对于特定的生态系统功能而言,物种的"功能身份"可能比物种多样性本身更重要,不加区别地笼统定义物种多样性与生态系统功能的关系可能欠妥。

Aims Recent theoretical and empirical work suggests that species diversity enhances the primary productivity and stability of communities. However, the relationships between the diversity of different species types （i.e., total species, response species, effect species and common species）, the special function of ecosystems, and the potential mechanism driving stability remain unclear. Our objective is to address the question by comparing the diversity effect of these different types on aboveground net primary productivity （ANPP） and community stability. Methods Our experiment was conducted in alpine meadow at the Haibei Research Station of the Chinese Acad- emy of Sciences from 2007 to 2011. We used a split-plot design with clipping treatment in the whole plot using three clipping levels （stubbled 1 cm, 3 cm and unclipped）. Subplots were treated with fertilizer （urea 7.5 g·m-2.a-1 ＋ ammonium phosphate 1.8 g.m 2.a-1 and unfertilized） and watering （20.1 kg·m-2·a-1 and unwatered）. Important findings We observed that the diversity of different species types affected ecosystem functioning differently. ANPP was mainly affected by the diversity of response species and effect species, whereas commu- nity stability was largely affected by that of common species. The maintenance of stability depended on increas- ing diversity in common species, and the potential mechanism was the portfolio effect. Both the over-yielding effect and asynchrony effect, however, had no influence on stabilizing the community. Clipping had enormous effects on the diversity of total species, whereas the changes in diversity of response species mainly connected with resource availability. Thus, clipping and fertilization had reverse effects on species diversity, ANPP and stability, i.e., the former increased both species diversity and stability and decreased ANPP, while the latter had opposite effects on them. Our results suggest that ANPP is driven by the diversity of a few effect species because they have a great influence on ANPP, while stability is driven by the diversity of a large number of common species because they can coexist stably in the community. The portfolio effect is the main mechanism of the diversity-stability relationship. The diversity effect of different species differs among each other; therefore, in terms of specific ecosystem functioning, we infer that ＂functional identity＂ of species in community is more important than diversity per se and it may be incorrect if we did not discriminate when defining the relationship between species diversity and ecosystem function in any situation. Key words