Effects of plant coverage on shrub fertile islands in the Upper Minjiang River Valley

The patchy distribution of vegetation in dry land results in well-documented "fertile islands". However, the response of shrub fertile islands to plant recovery and the underlying mechanisms, such as the linkage plant and soil properties, remain unknown.We sampled soils from areas with three different plant coverages(25%, 45%, and 75%) and three of their adjacent inter-plants to investigate soil physicochemical and microbial properties in the upper Minjiang River arid valley. The results showed that these factors were influenced by the persistence of plants that contrasted with the inter-plant interspaces. We found fertile islands in under-plant soil that were enhanced with increasing plant coverage, from 25% to 45% and 75%; however, there were no significant differences between 45% and 75% plant coverage apart from the soil clay content and the fungi to bacteria ratio. The soil microbial communities in under-plant soil were strongly influenced by the total soil carbon(TC), soil organic carbon(SOC),and available nitrogen(AN), whereas the microbial communities in inter-plant soil were primarily constrained by the AN and available phosphorous(AP). Moreover, the inter-plant soil properties, including gravimetric soil water content, pH, electrical conductivity(EC), and soil C:N ratio, were also strongly influenced by adjacent vegetation, which suggested that fertile islands may be beneficial for plant recovery in this region.

Fertile island effect in the sedimentary process of Tetraena mongolica Maxim nebkhas in steppe–desert ecotones on the Inner Mongolia Plateau,China

Phytogenic mounds(nebkhas)formed by shrubs are a common phenomenon in arid and semiarid areas.The formation of nebkhas is accompanied by the appearance of the fertile island effect.Quantitative evaluation of the shrub sand compost island effect is a key link in preventing soil erosion and nutrient loss.This study took the typical shrub Tetraena mongolica in desert areas as the research object and quantified the sand-trapping capacity of the shrub.We revealed the influence of sediment texture and volumetric soil water content on fertile islands during the development of T.mongolica nebkhas.The results showed that(1)the single shrub intercepted large amounts of sediments due to the high density of branches of T.mongolica,forming nebkhas that were positively correlated with the shrub size.(2)The overall soil nutrient content below the shrub was greater than the soil nutrient content outside the shrub,forming a typical fertile island effect.The soil organic carbon(SOC),alkaline hydrolytic nitrogen(AHN),available phosphorus(AP),and available K(AK)content all increased gradually with increasing nebkha volume.Compared with the deep soil of nebkhas,the nutrient content of the surface soil was generally higher.(3)There was a positive correlation between the volumetric water content and nutrient content in nebkhas.(4)The semi-ellipsoid shape of T.mongolica enabled it to intercept large amounts of coarse-grained material.Fine sand(100–250μm)was the main particle size in the sediment aggregates inside the nebkhas.The fine sand content generally increased with increasing shrub size.Redundancy analysis(RDA)revealed that the fine sand content of the nebkha sediments had a strong positive correlation with the soil nutrient content.This paper provides an example for evaluating the fertile island effect during the deposition process of nebkhas in the desert transition zone.

Spatial heterogeneity of soil chemical properties between Haloxylon persicum and Haloxylon ammodendron populations

Spatial heterogeneity is a ubiquitous feature in natural ecosystems, especially in arid regions. Different species and their discontinuous distribution, accompanied by varied topographic characteristics, result in soil resources distributed differently in different locations, and present significant spatial heterogeneity in desert ecosystems. In this study, conventional and geostatistical methods were used to identify the heterogeneity of soil chemical properties in two desert populations, Haloxylon persicum Bunge ex Boss., which dominates on the slopes and tops of sand dunes and Haloxylon ammodendron (C. A. Mey.) Bunge, which inhabits interdunes in the Gurbantunggut Desert of Xinjiang, China. The results showed that soil pH, electrical conductivity (EC), soil organic carbon (SOC), available nitrogen (AN) and available phosphorus (AP) were significantly higher in H. ammodendron populations than that in H. persicum. The coefficient of variation (CV) indicated that (1) most parameters presented a moderate degree of variability (10% 【 CV 【 100%) except pH in both plots, (2) the variability of soil pH, EC and AP in H. ammodendron populations was higher than that in H. persicum populations, and (3) SOC and AN in H. ammodendron populations were lower than that in H. persicum populations. Geostatistical analysis revealed a strong spatial dependence (C0/(C0+C) 【 25%) within the distance of ranges for all tested parameters in both plots. The Kriging-interpolated figures showed that the soil spatial distribution was correlated with the vegetation distribution, individual size of plants, and the topographic features, especially with the plants nearest to sampling points and the topographic features. In each plot, soil EC, SOC, AN and AP presented similar distributions, and fertile islands and salt islands occurred in both plots but did not affect every individual plant, since the sampling distance was larger than the size of such fertile islands. The results of topographic effects on soil heterogeneity suggested significant differences between the interdunes and dune-tops. Different topographic characteristics (physical factors) between plots result in the differences in SOC, AN and AP, while the heterogeneity of soil pH and EC arise from plant species and their distribution (biotic factor). Such biotic and physical factors did not occur in isolation, but worked together on soil heterogeneity, and played important parts in improving the soil properties. Hence these factors were ecologically valuable in the highly resource-stressed arid study area.

Annual-perennial plant relationships and species selection for desert restoration

Exotic plant invasion is a growing concern in the conservation and management of indigenous arid land ecosystems. By creating areas of ameliorated microclimates and fertile soil below their canopies, perennial plants might influence exotic annual plant invasions. We conducted a quantitative literature review of studies that compared exotic annual plant abundance among native perennial plant species and interspace （open areas） microsites in North America＇s Mojave Desert, where exotic plant invasion has corresponded with increasing extent of wildfire and broad-scale ecosystem transformation. Ten studies compared exotic annual plant abundance between inter- spaces and below a total of 36 native perennial species. These studies revealed that： （1） With few exceptions, most native perennial species supported a greater abundance of exotic annuals than interspaces, indicating overall facilitation of exotic species by native perennials. （2） Exotic species abundance varied by orders of magnitude among native perennial species, with some perennial species harboring amounts of exotics similar to interspaces. （3） Distributions of dominant exotic species varied, where Bromus rubens displayed a greater affinity for below-perennial microsites than did Schismus spp. and Erodium cicutarium that often were most abundant in interspaces. Results suggest that the degree of facilitation of exotic plants warrants consideration when selecting native perennial species for revegetation and restoration projects.

Vegetation Change and Soil Nutrient Distribution along an Oasis-Desert Transitional Zone in Northwestern China

Many studies have focused on soil nutrient heterogeneity and islands of fertility in arid ecosystems. However, few have been conducted on an oasis-desert transitional zone where there is a vegetation pattern changing from shrubs to annual herbs. The goal of the present study was to understand vegetation and soil nutrient heterogenity along an oasis-desert transitional zone in northwestern China. Three replicated sampling belts were selected at 200 m intervals along the transitional zone. Twenty-one quadrats （10 x 10m） at 50m intervals were located along each sampling belt. The vegetation cover was estimated through the quadrats, where both the soil under the canopy and the open soil were sampled simultaneously. The dominated shrub was Haloxylon ammodendron in the areas close to the oasis and Nitraria tangutorum dominated the areas close to the desert. In general, along the transitional zone the vegetation cover decreased within 660 m, increased above 660 m and decreased again above 1 020 m （close to the desert）. The soil nutrients （organic matter, total N, NO3^- and NH4^＋） showed significant differences along the zone. The soil nutrients except the soil NH4^＋ under the canopy were higher than those in open soil, confirming ＂islands of fertility＂ or nutrient enrichment. Only a slight downward trend of the level of ＂islands of fertility＂ for soil organic matter appeared in the area within 900 m. Soil organic matter both under canopy and in interspace showed a positive correlation with the total vegetation cover, however, there was no significant correlation between the other soil nutrients and the total vegetation cover. We also analyzed the relationship between the shrubs and annuals and the soil nutrients along the zone. Similarly, there was no significant correlation between them, except soil organic matter with the annuals. The results implied that annual plants played an important role in soil nutrient enrichment in arid ecosystem.

Interactions of the indigenous evergreen shrub Sabina vulgariswith coexisting species in the Mu Us sandland

Aims Facilitation is widespread in plant communities and particularly common in highly stressful environments.In the semi-arid Mu Us sandland where soil water and nutrients are short and grazing is heavy,many species grow within the large patches formed by the dense individuals of the unpalatable clonal shrub Sabina vulgaris,the only natural evergreen shrub species in the Mu Us sandland.However,the interactions between S.vulgaris and these coexisting species remain unclear,and we hypothesize that S.vulgaris can facilitate at least some species because within the patches vegetation is not grazed and soil water and nutrients may also be higher.Methods We measured the maximum height,coverage and number of individuals of each vascular species,little cover and thickness of biological crust at the soil surface in 1 m×1 m plots in three types of microsites associated with 40 patches of S.vulgaris:(i)at the center,(ii)at the inner edge and(iii)outside the patches.We also took soil samples and measured soil water content and content of total N,P,K and organic matter.Important Findings Soil water,nitrogen,organic matter and litter cover were the highest at the patch center,lowest outside the patches and intermediate at the inner edge of the patches,whereas thickness of biological crust was greater outside than at the center or at the inner edge.Among the 32 species recorded,six species preferred to occur within the patches,suggesting that S.vulgaris can facilitate these species most likely by grazing exclusion,increasing water and/or nitrogen resources in soil.However,most(19)species did not show preference and seven preferred outside.Also,species richness,pooled cover and number of individuals of all species were greater outside than within the patches,and such effects did not vary with the size of the S.vulgaris patches.These results suggest that the dominant interactions between S.vulgaris and the coexisting species are competition.The findings add to our knowledge that facilitation can be shown even when the competitive effects from the potential nurse plants are very strong.