The Influence Mechanism Of Slope Direction Factors On The Construction Of Grassland Vegetation Community In The Loess Plateau

The relationship between plants and the environment is the center content of the ecology research. Functional diversity of a plant community, which is based on plant traits, has been proposed as a key component for predicting ecosystem function. Knowledge about how plant functional traits and functional diversity change along with the different slope aspects at the community level is important for understanding the formation of plant communities under distinct slope aspects. However, to date, previous studies on plant communities in sunny and cloudy slope environments have mainly been descriptive. To our knowledge, no research has dealt with the relationship between the slope aspect and community assembly based on plant functional diversity (i.e., functional richness, evenness and divergence).In this study, we chose natural grassland community of Malan Mountain in Loess Plateau as the research object, surveyed the environmental factors and vegetation distribution of four different slope, measured plant functional traits (plant height (H)-. leaf area (LA)?specific leaf area (SLA) and leaf dry matter content (LDMC)) of four different slope, calculated species diversity indices?three types of functional diversity indices (i.e., functional richness, evenness, and divergence), which included three one-dimensional indices based on single traits (i.e., FRci, FRO, FDvar) and three multi-dimensional indices based on two traits (i.e., FRci, FEve, FDQ). Then,we conducted a comparison of the changing pattern of species diversity, plant functional traits and the three types of functional diversity along with the four slope aspects (the cloudy, sunny, semi-cloudy, and semi-sunny slope) by one-way analysis of variance, discussed the main restrictive environmental factors of the vegetation difference of the four different slopes, explained the construction of plant community and species coexistence mechanisms of the four different slopes through functional diversity. We aimed to uncover the relationship between slope aspects and plant functional traits, species diversity, three types of functional diversity indices, explain the reaction of the loess plateau grassland community to potential environmental changes or the environment filter, answer how the slope of the loess plateau work on the construction of grassland community, predict the change of grass plant development model in the future.Our results indicated that:(1) There was significant difference among the four slope aspects in community species diversity:species diversity (species richness index, Shannon-Wiener index, Simpson index and Pielou evenness index) of the cloudy slope and the semi-cloudy slope are higher than that of the sunny slope and semi-sunny slope, and thier main influencing factors are soil moisture, and there are extremely significant correlation among various index.(2) There was significant difference among the four slope aspects in plant functional traits:the value of H?LA and SLA on the cloudy slope was greater than the others, while the value of LDWC on the sunny slope was greater than the others. The cloudy slope has deeper soil layer, better soil development, stronger potential of digestion to the litter by soil microorganisms, thus, resulting in a higher nutrient use of space, more suitable for the growth of plants.(3) There was significant difference among the four slope aspects in plant functional richness:there was no significant difference among the four slope aspects in one-dimensional functional richness index of H and SLA, while the multi-dimensional functional richness index of plants on the sunny slope was lower than those of the plants on the other slopes. The main factors influencing the multivariate function richness is the soil pH, namely, the soil acidity is greater, FRci index is lower, characters distribution in multidimensional niche space is more "narrow". So the existing species of sunny slope occupy smaller niche space, not fully utilize the potentially available resources, have a less buffering capacity to the environment.(4) There was significant difference among the four slope aspects in plant functional evenness:on the cloudy slope, the one-dimensional functional evenness of H was significantly greater compared to those on the semi-sunny slope, while that of SLA was not significantly influenced by the slope aspect; the multi-dimensional functional evenness indices of plants on the cloudy slope was greater than those of the plants on semi-sunny slope, and the main factors influencing the multivariate function evenness is the soil salinity. Plants of the cloudy slope and the semi-cloudy slope distribute more regular, utilize resources more adequately. Plants of the sunny slope and the semi-sunny slope have species distribution gap, which can lead to productivity and stability decreases, and invasion of the opportunity increase.(5) There was significant difference among the four slope aspects in plant functional divergence:the one-dimensional functional divergence of H on the cloudy and semi-cloudy slope were significantly greater than those on the semi-sunny and sunny slope, while that of SLA on the semi-cloudy slope was significantly greater than those on the sunny slope; the multi-dimensional functional divergence index and Rao's quadratic entropy of plants was significantly greater on the semi-cloudy slope compared with those on the sunny slope. It can be concluded that ecological niche differentiation degree of the cloudy slope or semi-cloudy slope is higher, resource competition of those is lower, the resources use of those is more efficient; while ecological niche differentiation degree of the sunny slope or semi-sunny slope is lower, resource competition of those is higher, the resources use of those is less efficient.Our results indicated that:at the community level, slope factor is a powerful environmental filter in the process of vegetation community assembly in the loess plateau, and it has influenced the plant traits, species diversity and functional diversity. For plant traits, more niche space was occupied, more resources were sufficiently exploited, lower competition intensity was taken place, and a high level of niche differentiation existed on the cloudy and semi-cloudy slopes. By contrast, because of the limitation of all kinds of soil conditions and environmental factors, planutraits occupied less niche space on the sunny and semi-sunny slope, resources were exploited less sufficiently, and competition for resources tended to be stronger.At the community-level, the changing pattern of plant functional traits, species diversity and functional diversity along the four different slope aspects suggests that the existence of an environmental fitter effect on functional traits under the process of community assembly in the Loess region. This study not only sheds light on the community assembly mechanism, provides a theoretical basis for vegetation restoration and vegetation protection of the Loess Plateau mountain, but also has the important instruction function about predicting how small scale environmental factors work on distribution of plant functional traits, layout of the species, especially it is important for instructing mountain community building and community recovery process which is affected by environmental filter. Otherwise, once planted a species that does not adapt to the local environment, that may cause both economic and ecological losses, so following the adaptation mechanism of environment and vegetation, understanding the role of environmental filter on community assembly, has important indicator of the vegetation restoration of the area.