Ecological Adaptability Of Artemisia Giraldii In The Loess Hill And Gully Region

Artemisia giraldii is an herbaceous subshrub of Compositae family. It constitutes steppe plant communities found in the forest-steppe subzone and the typical steppe subzone of the southern part of the warm temperate zone in China. The species plays an important role in soil and water conservation in this area. The plant is very common in Loess Plateau and has colonized its loess hill and gully region. This specific habitat is formed by a network of numerous gullies and hilly ridges. Such landform heterogeneity, characterized by high fragmentation, is responsible for redistribution of water and heat resources. We chose this loess hill and gully region, situated in the north-east of Ordos Plateau and north of Loess Plateau, as the study area. Systematic analyses of Artemisia giraldii at individual, population, and community levels were conducted to elucidate its ecological adaptability to the gully habitat and develop strategies for maintaining vegetation structure and sound soil and water conservation practices in this region. Main results of the study are as follows:1. Geographical distribution of Artemisia giraldiiArtemisia giraldii is generally found in the area between 101°and 116°of eastern longitude and 34°and 41°of northern latitude at the elevation range of 390-2800 m. The main areal of the species is centered on Loess Plateau indicating evolutionary adaptation of Artemisia giraldii to environmental character of this area.2. Community structure and habitat characteristics of Artemisia giraldii(1) We found that the optimal topographic slope for Artemisia giraldii growth was about 30°-50°. Biomass of Artemisia giraldii was substantially lower at localities above or below this value. This finding suggests that Artemisia giraldii has adapted to this gully habitat.(2) Artemisia giraldii formed two major plant communities on slopes of loess gullies. The monodominant community of sunlit slopes has lower coverage, lower species richness, and smaller average biomass of 13.88 g·m-2. Shaded slopes are occupied by a community co-dominated with Poa sphondylodes. It is characterized by higher coverage, higher species richness, and larger average biomass (36.55 g·m-2).(3) Soil properties of slopes occupied by Artemisia giraldii are quite different from soils covering ridge tops. They are characterized by smaller soil bulk density and higher soil porosity, which means they are easily eroded. Soil water under Artemisia giraldii communities is diffused faster. Slope soils are also poorer as shown by lower contents of total organic carbon (TOC), total nitrogen, and readily available nitrogen.3. Population structure of Artemisia giraldii(1) Demographic structure of Artemisia giraldii populations correlates with slope conditions. Shaded slopes have higher abundance of older individuals compared with sunlit slopes. Population densities and root-to-top ratios of Artemisia giraldii growing in gullies were higher than those on hills and ridge tops.(2) As a result of restorative succession processes after field abandonment on the ridge top, the spatial pattern of Artemisia giraldii changed from initially random to clumped at finer scale and, later, clumped at large scale. These changes were accompanied by significant overall population increase and the simultaneous decrease of the habitat area. Spatial patterns of the plant are typically random in gullies, and the area occupied by its populations is significantly smaller when compared to the ridge top. At the same time, the area occupied by Artemisia giraldii is larger than that of the ridge top.(3) Artemisia giraldii has the widest ecological niche which allowed it to utilized resources available and adapt equally well along the habitat gradient from hill ridge top to a gully. Our findings suggest that Artemisia giraldii has significant niche overlap with other species that were dominant in these communities at the earlier stages of succession and appeared earlier in the succession series. We conclude that Artemisia giraldii was a dominant species at the beginning of succession.4. Reproduction ecology of Artemisia giraldii(1) Data on reproductive allocation of Artemisia giraldii revealed that most of energy expenses of the plant were channeled to asexual reproduction and only a little was used for sexual production. (2) The weight of 1000 seeds of Artemisia giraldii was 0.0901±0.0051 g. We hypothesize this trait allows the species to achieve higher dispersal rate. Seed germination rate of Artemisia giraldii was not different from other species with similar traits. This suggests that smaller seed weight of Artemisia giraldii was not an advantage for the success of seed germination.(3) The number of ramets per genet and the asexual reproduction average diffusion area per genet of Artemisia giraldii were highest in the gully and smallest in the ridge tops.(4) Soil seed bank data showed that 75% of Artemisia giraldii seeds were concentrated in the soil at the depth of 0-5 cm. As the gully stabilizes, the seed density in the soil seed bank of Artemisia giraldii increases and its value of importance is the highest in all communities. The seed density was lowest at the ridge top and its value of importance decreased significantly.5. Physiological characteristics of Artemisia giraldiiWe compared water physical characteristics of Artemisia giraldii, Artemisia sacrorum and Stipa bungeana, and found no significant differences in their drought resistance traits. Analyses of physiological characteristics of these three species also revealed their similarity in term of drought resistance. Our findings suggest that Artemisia giraldii has developed drought resistance and adapted to dry conditions in the gully habitat.6. The overall ecological effect of Artemisia giraldiiArtemisia giraldii grows on the 40°slopes of loess gullies. It can persist there a long time and forms communities in which it plays a dominant role. Being a natural vegetation, Artemisia giraldii-formed communities self-organize and perform quite well in the area. We therefore consider it as a species useful for soil and water conservation in the hilly-gully region...