Plant Functional Traits And Community Assembly Mechanism Along The South-to North-facing Slope Gradient In A Subalpine Meadow

Although located only a few hundred meters apart and sharing the same macroclimatic zone, the microclimatic conditions on different slopes vary dramatically affecting species composition, biodiversity and productivity. Recently most studies along the south- to north-facing slope are focused on descriptions of the different vegetation; however, few studies investigated the mechanisms of the community assembly and strategies of plant resource use based on functional traits. The use of methods based on functional traits is growing in the recent ecology studies. Functional traits are defined as morpho-physio-phenological traits that impact fitness indirectly via their effects on growth, reproduction and survival, three components of individual performance.In ecology studies based on functional traits, adaptation of vegetation to environment is one of the most important aspects. Adaptation to environment matters the survival strategy of plants. In the process of evolution and development, plants formed many physiological and morphological attributes adopting environment conditions in order to diminish the adverse effect. Especially, leaf functional traits, such as specific leaf area and leaf nitrogen content, are widely considered to associate with resource use strategies. Besides, leaf functional traits have also been suggested to respond the plant investment-return strategy.Another important aspect in ecological studies based on functional traits is exploring the community assembly mechanism. The approach based on traits enables ecologists to distract from the population demography studying pairwise species and to rebuild community ecology studying more co-existing species whose abundance will be decreased or increased by their different traits. The mechanism of species co-existing associates with the niche space partitioning which also associates functional diversity. Some researchers have suggested that functional diversity may be what is important for ecosystem function. Functional diversity is the range and distribution of functional trait values in a community.On the basis of the above-mentioned background, in our study we mainly do the researches on the vegetation adaptations and community assembly and obtain important results from the three aspects as following:firstly, in this study we measured soil factors and vegetation composition along this gradient to explore the limiting resource factors affecting vegetation distribution. Secondly, we explored the resource use strategies on south- and north- facing slopes. In this study, leaf functional traits [specific leaf area (SLA), leaf nitrogen content (LNC), leaf phosphorus content (LPC) and leaf N:P ratio] and vegetation composition were investigated on south- and north- facing slopes in a sub-alpine meadow. Thirdly,beginning with studying functional diversity, we explored the niche space partitioning in plant communities to investigate the community assembly and species coexistence on different slope aspects; we chose two functional traits (specific leaf area and plant height) to study the patterns of species diversity and functional diversity along the slope gradient.Our results showed:1) Soil water content was the primary limiting factors in this gradient; species diversity increased but primary productivity declined or comparable from south- to north-facing slope; species diversity and productivity positively and negatively relates to soil water content, respectively; species diversity and productivity were negatively linearly related.2) both SLA and LPC of each functional group were significantly higher and N:P ratio was lower on north- than on south- facing slopes; furthermore, grasses dominated south-facing slopes, while north-facing slopes were dominated by forbs. In addition, soil water content was significantly related to the shift in functional groups along the south- to north- facing slope gradient; soil water availability significantly related vegetation composition and leaf functional traits.3) the pattern of species diversity and functional diversity are not similar along this gradient. Both species richness and Shannon-wiener index were increased, but oppositely functional diversity was slightly decreased or comparable from south- to north- facing slope; soil water content (SWC) was the limiting resource in this region and determined species diversity. While functional diversity was determined by both species diversity and niche differentiation with the influence of soil water content; species diversity and functional diversity formed a significant quadratic regression relationship.The main conclusions we got are as followings:1) Soil water content was the primary limiting factors in this gradient determining vegetation distribution.2) The pattern of species diversity and functional diversity are not similar along this gradient. Species diversity increased, but functional diversity decreased from south- to north- facing slope; Soil water content (SWC) was the limiting resource in this region and determined species diversity. While functional diversity was determined by both species diversity and niche differentiation with the influence of soil water content; Species diversity and functional diversity formed a significant quadratic regression relationship.3) The change of functional traits and dominant functional group implied a difference in resource-use strategy between the two slope aspects, reflecting a transition in investment-return strategy from slow on south-facing slopes to fast on north-facing slopes. Moreover, we concluded that soil water availability affected vegetation composition and leaf functional traits, resulting in the shift in resource-use strategy.