| The Qinghai-Tibetan Plateau is extremely sensitive to global change and variation of land use pattern. As a result of long-term strongly grazing by livestock, the grassland in the region have displayed rapid degraded trend, such as decreased in grassland productivity and ratio of high-quality forage, community shorted, density and coverage decreased, ecological environment deteriorated, ecosystem services was weakened. In recent years, there were many researches on the response of plant traits to environmental changes at the individual level, but the studies on the changes of plant functional traits and its relationship with ecosystem function at different organizational levels (i.e. individual, population and community level) was rarely. However, the response pattern at individual level not may be mean to represent the response at both population and community level and the effects of response on ecosystem function. What relationships exist in plant functional traits among different organization levels, and how it affects the ecosystem function were in doubt. This study is based on a five years (from2007a to2011a) control experiments at Kobresia humilis meadow, Alpine Meadow Ecosystem Research Station of Chinese Academy of Sciences with three clipping levels (unclipped, stubbled3cm and stubbled lcm), two fertilizing levels (unfertilized,7.5g·m-2·a-1urea+1.8g·m-2·a-1ammonium phosphate) and two watering levels (unwatered,20.1kg·m-2·a-1water), measured aboveground weight (AGW), plant height (PH), leaf area (LA), leaf dry weight (LDW), specific leaf area (SLA) and specific leaf weight (SLW) of27major plants in community, studied variation of plant trait values (mean and variance value) of which was related with community aboveground net primary productivity (ANPP) at the individual, population and community level in different clipping levels, correlation between different traits, the underlying causes of variation values of traits from individual to community level, as well as effects of plant traits on aboveground net primary productivity and its stability at different organizational levels. The results showed that:(1) Primary productivity of the community decreased significantly with increasing clipping intensity (p<0.01); Fertilizing significantly improved it (p<0.01). The interaction both clipping and fertilizing has limited effect on stability of net primary productivity of the community (0.05<p<0.01), stability of the unclipped community is significantly lower than clipped ones after fertilized, and clipping have no significant effect on the stability of community productivity unfertilized treatment; interaction between clipping, fertilizing and watering has a significant effect on community productivity stability (p<0.01), after watering and fertilizing, productivity stability of clipped community was significantly higher than that of unclipped community. The rest treatments have no significant effects on community productivity and stability (p>0.05).(2) Clipping significantly reduces aboveground weight, plant height and specific leaf weight at individual, population and community level (p<0.05), and specific leaf area significantly increased with increasing clipping intensity (p<0.01), leaf area of heavy clipped community was significantly bigger than that of both moderate clipped and unclipped community (p<0.01). Fertilization significantly increased height at individual level (p<0.01), increased aboveground weight, plant height, leaf area, leaf dry weight, and specific leaf weight at population level (p<0.01), plant height and leaf area at community level (p<0.05). The interaction of clipping and fertilizing significantly affect plant height at population level (p<0.01), whether fertilized or not, plant height of unclipped community is significantly higher than clipped community, the interaction of fertilizing and watering significantly affect specific leaf weight at population level (p<0.05), when it is unwatered, specific leaf weight in fertilized treatment is a little higher than unfertilized treatment, after watering, it has no significant difference between fertilized and unfertilized. Watering has no significant effects on the mean value of plant traits at three levels (p>0.05). It shows that clipping lead to plant shorted, leaf thinned, biomass reduced, and fertilizing can add plant height.In the six traits, clipping only has a significant effect on the variance value of leaf area and leaf area at individual level (p<0.05), such as, the variance value of leaf area on moderate clipped treatment is significantly higher than that on severe clipped, clipping significantly increase the variance value of specific leaf area, clipping has no significant effect on trait variability at both population and community level (p>0.05), it is that clipping and watering doesn’t change difference among population and community level. Fertilizing only significantly increase the variance value of leaf area at the population level (p<0.01), fertilizing slightly increase variance value of the leaf area at community level (0.05<p<0.01), fertilizing had no significant effect on the variance values of traits at individual level (p>0.05). Watering does not change the relative varies of the trait among individuals, populations and communities level.(3) Six traits have15of trait combinations within individual, population and community levels. At the same level, either mean or variance value of traits, the correlation of most traits (66.7%-73.3%) are consistent, and negative correlation does not existed. From individual, population to community level, the proportion of uncorrelation between the mean value of traits increase (0.0%-33.3%); the proportion of positive correlation between variance value of traits increase (26.7%-46.7%); the correlations among the mean and variance values of trait decrease.(4) Six traits have formed36of trait combinations among individual, population and community level, include6combinations of the same traits and30combinations of different traits. There is no negative correlation change in value of the same traits at the different organizational level, from individual-population, individual-community to population-community level, the proportion of uncorrelations between the mean value of traits increase, random variation enhance, but there are still66.7%of the traits remained positive correlation. Variance values at individual-community level are positively related, it shows that variability of traits at individual level only have an effect on the population level, and no impact on community level.For different combinations of traits, from individual-copulation, individual-com-munity to population-community level, the ratio of uncorrelations among the mean value of traits increased, random variation enhance, but63.3%of traits among different organization levels retain a consistent correlation, the variance value of20%trait combinations only showed positive correlation between individual and population level.(5) Organizational levels and environment conditions influence the correlation between the values of trait and the ground net primary productivity and stability of community. In addition to the severe clipping (H1)-fertilized (NF, F) gradient, the mean values of trait has significant impact on productivity of community at the rest environmental gradient. The mean value of traits impact on productivity (63.0%) at the individual level than that at population and community levels(52.0%、56.3%), both plant height and the specific leaf have an impact on productivity at each organizational level, but they positively related to productivity at individual level, plant height is positively associated with productivity at population and community level, specific leaf weight is more negatively correlated and productivity.From individual, population to community levels, the variance value of plant traits in a growing number of environmental gradient affect the stability of the communities, the ratio on total environment gradient was16.7%,50%and83.3%, respectively; the variance value of plant traits can explain coefficient of variation of productivity is33.4%,29.5%,53.2%, respectively. It shows that environmental conditions and organizational levels have influence on the stability of productivity, but the organization may play a major role, organization level is higher, the effects of the variance value of plant traits on the community stability is greater, and the stability of community is mainly the determined by ground weight, specific leaf weight and leaf weight.In summary, the effects of clipping and fertilizing on aboveground net primary productivity and stability of community was opposite, clipping can reduce productivity, but will not change the relative variability of productivity among the community, and fertilizing can improve productivity and increase the variability of the productivity, decreased stability. On the one hand, these changes was related that the effects of clipping and fertilizing on the value of plant traits at the individual, population and community level; the other hand, they was related to correlation of the value of plant traits within and among different organizational levels. Clipping stunted plant height, thinned leaf thickness and reduced specific leaf weight, aboveground weight per plant; fertilizing increased plant height and leaf area at individual and community level, and the main function traits which impacted on aboveground net primary productivity were plant height and specific leaf weight, the mean value of plant traits impacted on productivity at the individual level greater than that at the population and community levels. At the same time, relationship of the mean value of most traits within and among different levels were consistent, but with increasing organization level (i.e., individual, population to community), the mean value of the trait pairs tended to be no correlation within and among different levels, and the mean value of traits tends to show random variation. With organization level improving, the weighted effect of traits will lead to that community productivity is mainly determined by the dominant traits of the dominant species, the mean value of traits will relatively independent response to clipping and fertilizing at the community level.The organizational level is higher, the ratio of positive-relationship of the variance value of traits within same level is higher, the randomness was weakened, which is mainly determined by the intrinsic relationship between plant traits, the variance value of plant functional traits has greatest effect on stability at community level. Among different levels, where it comes to the relationship with plant traits of community level (i.e., individual-community and population-community), the variance value of paired traits showed unrelated and it changed completely randomly, community productivity stability was determined by variation of plant traits at different organizational levels, but coupling random variability of plant functional traits was the main driving mechanism which decided productivity stability of Kobresia humilis meadow community among different organization levels. It was related to which the relative variability of plant traits tended to be stabilized after long-term clipping at organizational level, the plant functional traits varied independently at the different organizational levels. |
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