| Grazing and clipping are the mainly ways to manage and use for the grassland ecosystem, and also the most important human interference to affect the structure and function of the grassland community. Overgrazing and clipping removes the most of the growth tissues and the photosynthetic organs of plants and results in reduction in the biomass of plants. However, compensatory growth characteristics of plants can alleviate or avoid the negative effects by herbivores. Plant compensatory growth is a positive response to grazed or other destroyed, and including three response modes:1) overcompensatory growth, grazing have a positive impact on plant growth, so that the the net biomass accumulation of grazed plant is more than that of ungrazed grazing; 2) undercompensatory growth, the net accumulation of grazed plant is significantly lower than that of the plants without grazed; 3)fullcompensatory growth, plant biomass after feeding did not differ between the grazed and ungrazed plants. While, it is a open question that whether the plant populations of grassland ecosystems is prevalent overcompensat-ory growth, especially for the severely degraded alpine meadow in the east of the Tibetan Plateau. In the region, it is particularly important to study compensatory growth characteristics of the typical plant species. Hence, two experiments were carried out in Haibei Research Station in order to study the compensatory growth pattern and mechanisms of Stipa aliena from 2007 to 2010 and try to provide a theoretical basis for the optimal management of the grazing system. Experiment1: Research on the compensatory growth of Stipa aliena at three natural habitats, pen pasture (heavy grazing), pass pasture (moderate grazing) and control pasture (no grazing) in 2007. Experiment 2: Research on the compensatory growth of Stipa aliena through manual clipping (stubbled lcm, 3cm and unclipped), watering (watered vs. unwatered) and fertilizing (fertilized vs. unfertilized) from 2007 to 2010. At the same time, we analyzed the relationship between environment factors and the compensatory growth of Stipa aliena by using Canonical Correspondence Analysis (CCA). The main results showed as follows:(1) In Experiment 1, The community coverage was highest at control ungrazed pasture, lower at pen pasture, and lowest at pass pasture. At the pen pasture, Potentilla anserina and Elymus nutans were the dominant species; At the pass pasture, Kobresia humilis and Elymus nutans were the dominant species and at the control pasture, Stipa aliena and E. nutans were the dominant species. Compared with the control pasture, the defoliation ratio of community was highest at pen pasture and lowest at pass pasture. (2) The defoliation ratio of Stipa aliena was 57.96%,15.76% and 0.00% at the pen pasture, the pass pasture and the control pasture, respectively. Fullcompensation occurred for roots at pass pasture and under-compensation was found in other plant organs at pen pasture and pass pasture. The ramets density, coverage, and plant height of the species were highest at ungrazed pasture, and was lowest at pen pasture in June 2007. Stoping grazing in current year (caged treatment) had a significant effect on the ramets density and plant height. There was a significant interaction between habitats and caged treatment on the plant height, but these differences were disappeared in August. This indicated that stoping grazing hadn't notable impact on ramet density and plant height. In June, biomass allocation to growth and storage organs were no difference between pen pasture and pass pasture, but growth allocation was highest and storage allocation were lowest at ungrazed pasture. The difference in biomass allocation among habitats was also disappeared in August. The relative growth rate of Stipa aliena (beside the inflorescences) significantly differed among habitats. And that of aboveground and leaves of plant were highest at pass pasture, lower in control pasture and lowest in pen pasture. The relative growth rate of roots was lower in pen pasture and was no differences between pass pasture and control pasture.(3) Canonical Correspondence Analysis indicated that the most important factors that impacted compensatory growth, relative growth rate, ramet density, and plant height of the species was defoliation ratio, next was nitrate nitrogen in soil in August, and the impact of other factors were lower. This study also display phosphorus in soil is the necessary element to plant growth of Stipa aliena, but soil moisture was not limitative factor. This results maybe in relation to a high rainfall in the region or greater capability of tolerance to draught of the species.(4) In Experiment 2, regardless of fertilization or not, community coverage was no difference between clipping treatments, but under stubbled 3cm, the community coverage was greater in fertilization treatmeng than that in no fertilizer. In 2009, community coverage was lowest under stubbled 3cm and no differences between others. In 2010, community coverage was no difference between clipping treatments. Community coverage decreased with year under each clipping treatment. Whether the irrigation or not, community coverage decreased with year. Fertilizer can increase the community coverage, but during the study period, community coverage was decline year by year.(5) Stubbled 3cm,1cm and unclipped treatment after clipping equivalent to the defoliation ratio of Stipa aliena with 72.79%,20.32% and 0.00% respectively. Aboveground organs exhibited under-compensation under clipping treatments. The ramets density, relative growth rate, growth allocation were significantly increase, plant height, coverage and reproduction allocation were decrease. The ramets density changed from overcompensation under stubbled 3cm to undercomp- ensation under stubbled 1cm with years. Clipping increased the number of tiller, at the same time, decreased the coverage and plant height of the species. Growth allocation was increased by clipping with stubbled 1cm, biomass allocation to reproductive organs was reduced with increasing clipping intensity.(6) Fertilization has promotive effect on RGR of aboveground organs, and the effect is more remarked under stubbled 3cm. These results show that additional fertilizing could effectively stimulated aboveground regrowth of the species.(7) Generally, additional water is an important factor during plant growth period. But in our study, watering did not siginificantly effected the compensatory regrowth of the species. That may confirm that water is not a limited factor for compensatory growth of Stipa aliena.In short, aboveground organs of Stipa aliena exhibited undercompensation at pen pasture and pass pasture, and roots exhibited fullcompensation at pass pasture. The species did not show overcompensatory response to rich resources of the soil under grazed pasture, but show compensatory ability at lower resource conditions. Stipa aliena is an important dominant species of the apline meadow and have a poor tolerance ability to grazing defoliation, therefore, periodic resting or decreasing grazing pressure could be a cost-efficient management strategy to favour the species and to maintain the species diversity, ecosystem functioning and the feed value of the grassland. |
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