| The biodiversity formation and maintenance mechanism of plant community isone of the most important key issues in the contemporary ecological research. Nichetheory had solved this problem to a certain degree. In a simple word, stablecoexistence between competing species requires them to occupy different niches.However, because all plants acquire water, CO2, light, nitrogen, phosphorus, potassiumand a common set of minor mineral nutrients in a limited number of ways, nichecannot explain the mechanism of community construction.Neutral theory developed by Hubbell supposes that: species are ecologicallyequivalent, biodiversity in community relies on the randomness of births, deaths,migration, and immigration; at metacommunity level, regional biodiversity is thefunction of dispersal rate and speciation rate. Neutral theory includes many detailswhich are neglected by niche theory and resolves many problems which are difficult toexplain by niche theory. However, its assumption of species equivalence and theneglect of environmental effects have also raised considerable disagreement. Eachtheory has limitations that have been widely discussed. In accordance with thesediscusses in international ecological research, in this study, we analysed thecommunity construction mechanism at local community and metacommunity levelrespectively in an alpine meadow in Gannan.We measured the photosynthesis of Roegneria kamoj, Polygonumsphaerostachyum, Koeleria cristata, Kobresia myosuroides, Ligularia sagitata and Anaphalis lacteal under different soil status and investigated the variations ofcommunity composition after fertilizer to find out the mechanism ofαdiversityformation at local community level. At metacommunity level, we compared the effectsof distance and environmental gradient on the community similarity indexes of fourdifferent locations in two hills to seek the mechanism of D diversity formation. Theresults as follows:1. R. kamoji had relatively higher PWUE, but lower PNUE, making it a waterconservation species. In contrast, P. sphaerostachyum had very low PWUE, and higherPNUE, making it a nitrogen conservation species. The net photosynthetic rate of R.kamoji positive related to soil N content, and responded to the variations in soil watercontent insensitively but responded to the changes in soil N content sensitively; Thenet photosynthetic rate of P. sphaerostachyum positive related to soil water content,and responded to the variations in soil water content sensitively but responded to thechanges in soil N content insensitively. The trade-off of R. kamoji and P.sphaerostachyum between resource utilization promoted their coexisted in thecommunity under fluctuant environment.2. The photosynthesis of K. cristata, K. myosuroides, P. sphaerostachyum, L.sagitata and A. lacteal respond to the increase of soil N : P ratios differently: Naddition increased the photosynthetic assimilation rates of K. myosuroides, K. cristataand L. sagitata significantly; For A. lacteal, the photosynthetic assimilation rates onlyincreased at relatively lower nitrogen levels but significantly declined at highernitrogen levels; All additional N treated P. sphaerostachyum samples remainedconstant photosynthetic assimilation rates, which suggested that the additional Ntreatments had no significant effects on the photosynthesis of P. sphaerostachyum.Compared with their average cover in control plots, P. sphaerostachyum was reduced.A. lacteal was increased with 20 g Nm-2 addition and reduced with 140 g Nm-2addition. L. sagitata, K. myosuroides and K. cristata was increased. The results showed that species with the N : P ratios more close to soil supply ratios were predicted todominate in local communities. Species with inherently low N : P ratios (P.sphaerostachyum) is predicted to dominate in N-limited vegetation, and species withinherently high N : P ratios (K. myosuroides and K. cristata) are predicted to dominatein N-rich vegetation. Species are different in responding to nutrient enrichment, whichresults in the change of community structure finally.3. By comparing community composition of four positions in two neighbor hills,we found that community similarity indexes between plots decreased significantlyfrom South to North hillside in each hill; whereas no significant difference was foundbetween plots within the same topographic positions in two hills which were difficultto explain by dispersal limitation. Because there had a high community similarity inthe plots with same soil water content, it was possible to express the communitysimilarity as the liner function of the soil water content. As the distance between plotsin one hill was significantly shorter than it between hills, a consistent decrease incommunity similarity with distance which neutral theory predicts was not observed inthis experiment. The influence of environmental gradient should be assessed anddispersal limitation effects are not clear yet even at the scale of 8-180 m.From this study we have drawn the following conclusions:1. The trade-off between resource utilization is the foundation of nichedifferentiation, which is important in promoting species coexist and constructingcommunity. It is incomplete that explainsαdiversity only by randomness of births,deaths, migration, and immigration.2. The interspecific differences in responding to environmental disturbance arethe main factors lead to the change of community composition, which contraries to theassumption of neutral theory about species are ecological equivalent.3. Dispersal limitation cannot explain the formation ofβdiversity completely.Environment gradient, in the region involving in this study, is the main factor which results in the shift in community structure. |
