Community dynamics of desert grasslands: influences of climate,landforms, and soils

Abstract. Permanently marked vegetation transects in Big Bend National Park, Texas, USA were monitored to follow temporal dynamics of desert grassland communities on a variety of landforms and soil types over a 26-yr period after the removal of domestic livestock. Historic records indicate that the park area was severely overgrazed prior to its establishment, and our results show that the species present increased in both cover and density after the removal of livestock. However, the timing of recovery corresponded to multiyear periods of above-average precipitation. Little change was observed in between 1955 and 1960, a period dominated by several consecutive years of drought. The cover of two large shrubs common to the Chihuahuan Desert, Larrea tridentata and Flourensia cernua, increased from 1960 to 1967, a period dominated by summer drought and frequent wet winters. The cover and density of forbs, perennial grasses, and most shrubs increased on nearly all landforms between 1967 and 1981, when summers were wetter than average. In contrast, the cover of Larrea tridentata decreased during this period. Comparisons among the plant communities on each landform showed that they diverged through time after domestic livestock were removed. Presumably, differences in topographic position and soil texture influence water availability which was reflected in the species composition on each soil series. Unfortunately, we cannot isolate the effects of recovery from grazing from the effects of climate because the study design did not include control plots located within grazed pastures. Certainly, the directional trajectory of change and the regrowth of grasses into inter-shrub spaces, must, at least in part, be the result of recovery from grazing. However, our data also indicate that the desert grassland communities are sensitive to multi-year periods of above- or below-average precipitation. Clearly, the dynamics between shrubs and grasses cannot be explained by a simple successional paradigm that views increased shrub dominance as retrogression from a climax grassland. Many alternate hypotheses have been forwarded to explain the dynamics that control the vegetation composition in the desert and desert grassland region of North America. Experimental tests of these hypotheses are needed to indentify the interactions between biotic and abiotic factors that control dominance by shrubs or grasses.     

Stipa tenacissima aerial biomass estimated at regional scale in an Algerian steppe, using geostatistical tools

Data from systematic sampling may be independent or autocorrelated. In the latter case geostatistical tools are used to identify the spatial patterns within the universe sampled. Special formulas have been derived by Russo & Bresler (1982) to estimate the variance of a value averaged over several transect samples. We applied these formulas to the green biomass of the dominant perennial steppe grass, Stipa tenacissima or alfa, in a 400 km² region in North-West Algeria; thirty years ago, this region was considered one of the best sites for alfa. A two-level sampling design was implemented with stratification of the region and systematic sampling within each stratum; globally the study included fifteen transects, representing 713 1 m² quadrats. Autocorrelation up to 500 meters was detected in five semi-variograms or correlograms, which were fitted to linear models with a sill. Biomass averaged only 165 (±55) kg ha^-1. We discuss the processes that have lead to the rapid degradation of alfa steppes in northern Algeria and the variation in spatial patterns of alfa stands. Ignoring autocorrelation in systematic sampling leads to biased estimates of variances and standard errors.     

The effect of plant litter on ecosystem properties in a Mediterranean semi‐arid shrubland

Abstract. We studied the distribution of litter in a shrubland of the Negev with a semi‐arid Mediterranean climate of less than 200 mm of rainfall per year. Our focus was on the effects of litter on properties of landscape patches relevant to ecosystem processes (water runoff and soil erosion), annual plant community responses (seedling density, biomass production and species richness), and animal activity (soil disturbance by termites). Three 60‐m transects, extending across a pair of opposing north‐ and south‐facing slopes and their drainage channel, showed that litter accumulates not only under shrubs, but to a lesser extent also on the crusted inter‐shrub open areas. We used 35 experimental units (‘cells’, 0.5m × 1 m), each containing a crust and a shrub patch. Because runoff flows from crusted patches and is intercepted by shrub patches, the latter were in the lower third of the cells. Leaf litter was added in single and double amounts providing ca. 0.5 and 1.0 cm litter depth, to either, both, or none of the patches. Litter addition significantly decreased the amount of runoff, regardless of the location and amount of litter applied. Litter on the crust increased species number and seedling density of species with low abundance. Adding a double litter layer increased annual plant biomass production, while a single amount had no effect. Litter addition to the shrub patch affected neither biomass nor species richness. Litter addition to both patches at both quantities caused a large increase in termite activity. Termites caused disturbance by disrupting the crust, which may contribute to the reduction in runoff amounts. In the open, flat crust patches, annual plant communities are limited in their productivity and species richness, as there are few structures stopping the outflow of water, soil and seeds. Litter adds such structures, but affects the plant communities only when added to litter‐free crust. Litter accumulation and its patchy distribution have large impacts on landscape patch properties affecting resource distribution, plant productivity and diversity, and animal activity. Therefore, understanding litter distribution in relation to the patchy structure of the landscape of semi‐arid shrubland should be viewed as an important component of shrubland management.     

Vegetation pattern shift as a result of rising atmospheric CO2 in arid ecosystems

Arid ecosystems are expected to be among the ecosystems most sensitive to climate change. Here, we explore via model calculations how regular vegetation patterns, widely observed in arid ecosystems, respond to projected climatic shifts as provided by general circulation model output. In our model, the photosynthesis and respiration terms are explicitly linked to physiological attributes of the plants and are forced with the primary climatic drivers: atmospheric CO₂, air temperature, and precipitation. Under future climate scenarios, our simulations show that the system’s fate depends on whether the enhancements to photosynthesis due to elevated atmospheric CO₂ outweigh the increases in respiration due to higher air temperatures and the increases in water stress due to lower rainfall. A scalar measure is proposed to quantify this balance between the changes in the three climate drivers. Our model results suggest that knowing how the three primary climate drivers are evolving may provide hints as to whether the ecosystem is approaching desertification.     

黔中石漠化地区不同土地利用类型土壤种子库特征

在贵州省普定县喀斯特石漠化地区通过种子萌发试验,对封山育林、退耕还林2a、农耕地等3种不同土地利用类型的土壤种子库进行分析.结果表明,不同利用类型土壤中种子数量差异显著,封山育林土壤中平均种子密度为1664粒/m2,退耕还林2 a土壤中平均种子密度为8060粒/m2,农耕地土壤中平均种子密度为6239粒/m2.土壤中的种子集中分布在表层0～5cm范围内,随土层深度变化,土壤中所含种子数量和物种数呈减少的趋势.不同利用类型土壤种子库物种相似指数较低,物种数、多样性指数、均匀度及生态优势度指数以退耕还林地最大,农耕地高于封山育林地.土壤种子库所含物种数较接近,物种组成以草本植物为主,菊科、禾本科占优势.封山育林地、退耕还林地属于进展演替,封山育林地处于较高演替阶段,农耕地属于逆行演替.应引进适宜物种以促进植被恢复.     

Des ressources végétales endémiques pour optimiser durablement les opérations de réhabilitation du couvert forestier en milieu méditerranéen et tropical : exemple des plantes facilitatrices vectrices de propagation des champignons mycorhiziens

The overexploitation of natural resources, resulting in an increased need for arable lands by local populations, causes a serious dysfunction in the soil's biological functioning (mineral deficiency, salt stress, etc.). This dysfunction, worsened by the climatic conditions (drought), requires the implementation of ecological engineering strategies allowing the rehabilitation of degraded areas through the restoration of essential ecological services. The first symptoms of weathering processes of soil quality in tropical and Mediterranean environments result in an alteration of the plant cover structure with, in particular, the pauperization of plant species diversity and abundance. This degradation is accompanied by a weakening of soils and an increase of the impact of erosion on the surface layer resulting in reduced fertility of soils in terms of their physicochemical characteristics as well as their biological ones (e.g., soil microbes). Among the microbial components particularly sensitive to erosion, symbiotic microorganisms (rhizobia, Frankia, mycorrhizal fungi) are known to be key components in the main terrestrial biogeochemical cycles (C, N and P). Many studies have shown the importance of the management of these symbiotic microorganisms in rehabilitation and revegetation strategies of degraded environments, but also in improving the productivity of agrosystems. In particular, the selection of symbionts and their inoculation into the soil were strongly encouraged in recent decades. These inoculants were selected not only for their impact on the plant, but also for their ability to persist in the soil at the expense of the residual native microflora. The performance of this technique was thus evaluated on the plant cover, but its impact on soil microbial characteristics was totally ignored. The role of microbial diversity on productivity and stability (resistance, resilience, etc.) of eco- and agrosystems has been identified relatively recently and has led to a questioning of the conceptual bases of controlled inoculation in sustainable land management. It has been suggested that the environmental characteristics of the area to rehabilitate should be taken into account, and more particularly its degradation level in relation to the threshold of ecological resilience. This consideration should lead to the optimization of the cultural practices to either (i) restore the original properties of an ecosystem in case of slightly degraded environments or (ii) transform an ecosystem in case of highly degraded soils (e.g., mine soils). In this chapter, we discuss, through various examples of experiments conducted in tropical and Mediterranean areas, the performance of different strategies to manage the microbial potential in soils (inoculation of exotic vs. native species, inoculation or controlled management potential microbial stratum via aboveground vegetation, etc.) based on the level of environmental degradation.     

Local facilitation, bistability and transitions in arid ecosystems

Arid ecosystems are liable to undergo sudden discontinuous transitions from a vegetated to a desert state as a result of human pressure and climate change. A predictive framework about the conditions under which such transitions occur is lacking. Here, we derive and analyze a general model describing the spatial dynamics of vegetation in arid ecosystems considering local facilitation as an essential process. We investigate the conditions under which continuous or discontinuous transitions from a vegetated to a desert state are likely to occur. We focus on arid ecosystems but our approach is sufficiently general to be applied to other ecosystems with severe environmental conditions. The model exhibits bistability and vegetation patchiness. High local facilitation decreases the risk of discontinuous transitions. Moreover, for arid ecosystems where local facilitation is a driving process, vegetation patchiness indicates proximity to a transition point, but does not allow distinguishing between continuous and discontinuous transitions.     

Auditing reforested watersheds on the loess plateau: Fangshan Shanxi

Mapping watershed ecosystems, evaluating their ecological status and modelling land use futures are the aims of a project undertaken by an interdisciplinary team from Shanxi Forestry Academy and Watershed Systems Living Water Foundation. The project introduces geospatial methodologies and iGiS technologies for (a) mapping and modelling watersheds and (b) monitoring and evaluating rangeland restoration after reassigning collective forest lands to local farmers in accordance with land reform policies.Two contemporary geospatial technologies were instrumental in the Fangshan project. These technologies are driving a paradigm shift in the way primary industries like mining, farming and forestry utilize GIS, engage in land evaluations, resource mapping, environmental assessments and product certification.

• •Firstly, high resolution, true image 3D orthophoto mapping was produced as the iGiS map platform for the Fangshan project. The true colour orthophoto maps produced by the team proved very suitable, with the high resolution imagery achieving cartographic standards allowing draft mapping at 1:2000. Because unique x,y,z geocentroid coordinates are generated for each and every pixel in the orthophoto mapping process, detailed iGiS data bases with multiple attributes ranked parametrically were readily captured and recorded for every habitat and regolith.
• •Secondly, the Shanxi Forest Academy team were trained in geospatial methodologies for mapping watershed ecosystems and modelling their habitat/regolith/energy relationships. Using GiS imaging technologies, these cartographic simulation methodologies enable ecological modelling of watersheds and their subterranean water systems, while providing a framework for monitoring and evaluating the environmental health of watersheds using permanent benchmarks and ecological indicators.

Habitat mapping and modelling of Fangshan watersheds revealed how ecological restoration is gradually occurring through strategic combinations of planned reforestation, traditional terrace farming systems and natural regeneration. These ecological strategies are shown to be beneficial land use partners in restoring the mountain rangelands, riparian ecostructures and ecosystem functions of degraded loess plateau watersheds.     

Modeling and formal analysis of meta-ecosystems with dynamic structure using graph transformation

The dynamics of meta-ecosystems are of crucial importance and, therefore, in theoretical ecology, various model-based approaches are employed to analyze their internal effects and to derive predictions on their future behavior and interactions. However, existing model-based approaches are limited in their support for the modeling of space structure dynamics of meta-ecosystems growing and shrinking in size over time due to intrinsic and extrinsic process as well as neighboring opportunities.In this paper, we employ the formal technique of Graph Transformation (GT) to qualitatively model the space structure dynamics and ecological dynamics of meta-ecosystems. In such GTSs, states of meta-ecosystems are given by graphs and evolutions from one state to the next are derived by applying GT rules describing local modifications of graphs. As a case study, we introduce GTSs capturing the space structure dynamics and ecological dynamics of three different savanna meta-ecosystems, which vary in their space structure dynamics. We then consider several qualitative analysis technique for GTSs such as for the existence of structural stabilities and apply them in our case study. We conclude that GTSs provide a complementary avenue to that of existing approaches to rigorously model and analyze qualitative ecological phenomena with adequate support for capturing not only ecological effects but also various forms of space structure dynamics.