Rehabilitation of Semiarid Landscapes in Australia. I. Restoring Productive Soil Patches

A rehabilitation procedure designed to reestablish resource control processes in a degraded Acacia aneura woodland was successful in improving soil nitrogen and carbon content, exchange properties, and water infiltration rates. Soil respiration rates and soil fauna populations increased, and soil temperatures were moderated. The procedure comprised laying piles of branches in patches on the contour of bare, gently sloping landscapes, with the expectation that soil, water, and litter would accumulate in these branch piles, thus improving the soil habitat and its productive potential. The procedure was derived from landscape function analysis, indicating that surface water flow was the principal means of resource transfer in these landscapes. Under degradation such overland flow results in a loss of resources. This rehabilitation procedure reversed loss processes, resulting in gains in the productive potential of soils within patches. This procedure was successful despite grazing pressure being maintained throughout the experiment.     

The use of branch piles to assist in the restoration of degraded semiarid steppes

Desertification is a major environmental problem in arid and semiarid regions. Tree plantation has been commonly employed to foster the recovery of degraded areas. However, this technique is costly, and their outcomes are often uncertain. Therefore, we evaluated an alternative method for the restoration of degraded semiarid steppes that involved the construction of branch piles to attract frugivores as potential seed‐dispersing birds, promoting seed rain, and fostering the formation of woody patches. We measured the success of branch piles in terms of the number of bird visits and seed input compared to naturally occurring shrub patches. Generally, frugivorous birds visited branch piles less frequently than shrub patches. Yet, branch piles accumulated seeds of patch‐forming shrub species. Seed rain was higher under patches of the dominant shrub Rhamnus lycioides than under branch piles. In contrast, woody patches and branch piles did not differ in seed input of the less abundant Pistacia lentiscus shrub. Our study demonstrates that branch piles are used by frugivorous birds and accumulate seeds of patch‐forming shrubs. Branch piles may be a suitable method to promote the expansion of bird‐dispersed plant species and restore semiarid wooded steppes. However, their efficiency largely depends on pile persistence and economic cost.     

Recruitment potential of two perennial grasses with different growth forms at a semiarid-arid transition zone

The objective of this study was to quantify differences in recruitment potential (seed production, seed presence in the soil) for two congeneric perennial grasses (Bouteloua gracilis, Bouteloua eriopoda [Poaceae]) that dominate adjacent arid and semiarid grassland biomes. It was hypothesized that these species have different recruitment strategies at the biome transition zone that are related to differences in their growth form and longevity. Recruitment potential for each Bouteloua species was compared in patches dominated by one or both species or codominated by the invasive shrub, Larrea tridentata (Zygophyllaceae). Regional variation in recruitment was examined for B. gracilis for cases in which comparable data were available in the literature for a site located within the semiarid grassland biome. The short-lived stoloniferous species B. eriopoda produced more seeds per plant than the long-lived bunchgrass B. gracilis, yet seed viability (<60%) and presence in the soil were lower. Mean viability of B. gracilis was higher (>90%) than that of B. eriopoda, and a greater percentage of seeds produced on a square meter basis was found in the soil (10-25%). Similar patterns were found for both species in all grass-dominated patches. Bouteloua eriopoda plants growing in patches codominated by L. tridentata produced fewer seeds per plant with lower viability, and fewer seeds were found in the soil compared to grass-dominated patches. Regional comparisons found greater seed production per square meter and more seeds in the soil for B. gracilis at the transitional site compared with a cooler, wetter site located within the semiarid grassland biome. These differences in recruitment potential along with published differences in rates of seedling establishment and vegetative spread may explain, at least in part, localized patterns in species dominance.     

Effect of grazing on plant patterns in arid ecosystems of Patagonian Monte

Our objective was to assess the relationship between the spatial patterning of perennial grasses (total, grazed, and non‐grazed) and shrub patches in rangelands under different grazing pressures of the Patagonian Monte. We selected three grazed paddocks with the usual stocking rate for the area, where previous studies showed that a piosphere formation is common. At each paddock, we analysed the grain of heterogeneity at sites located at two distances from the single watering point (near, far), using high‐resolution aerial photographs. At these sites, we also assessed in the field the density, size, cover, and spatial patterning of grazed and non‐grazed perennial grasses and shrub patches. The grain of heterogeneity of shrub patches was coarser in sites near the watering point than in those distant from it, as a consequence of the increase in size of both, bare soil and shrub patches. Field sampling showed that a coarse grain of heterogeneity relative to fine‐grained sites resulted from changes in species composition, increased bare soil areas and reduced perennial grass cover. In coarse‐grained sites, lower perennial grass cover resulted from lower density and/or smaller size of grass bunches than in fine‐grained sites. We did not find significant differences among sites in the proportion of perennial grazed grasses. Since the density and cover of perennial grasses was higher in fine‐ than in coarse‐grained sites, we suggested that fine‐grained sites are more important as feeding stations than coarse‐grained sites. The consequences of this differential use could lead to degradation of fine‐grained sites and to higher homogeneity in spatial plant structure and floristic composition within paddocks with respect to the condition observed at present, increasing the size of the highly degraded zone within the piosphere. At the patch level, we found that at about one third of the sampled transects, both total and non‐grazed perennial grasses were spatially aggregated with shrub patches. However, in most transects grazed perennial grasses were indifferently distributed in relation with shrub patches, showing that grazers display high selectivity of foraging sites at macro level (i.e. high and low grazing pressure sites at the paddock level), but random occupancy of vegetation units (randomness in the distribution of grazed perennial grasses at the patch level). The intensity of the positive association between non‐grazed grasses and shrub patches was higher in fine‐grained than in coarse‐grained sites and may be attributed to higher protection against herbivores associated to denser shrub patches in fine‐ relative to coarse‐grained sites. We concluded that a feedback exists between the spatial distribution of species preferred by grazers and the spatial patterning of use of these species.     

Effects of Heterogeneous Competitor Distribution and Ramet Aggregation on the Growth and Size Structure of a Clonal Plant

Spatially heterogeneous distribution of interspecific competitors and intraspecific aggregation of offspring ramets may affect the growth and size structure of clonal plant populations, but these have been rarely studied. We conducted a greenhouse experiment in which we grew a population of eight offspring ramets (plants) of the stoloniferous clonal plant Hydrocotyle vulgaris aggregately or segregately in two homogeneous treatments with or without a competing grass Festuca elata and a heterogeneous treatment with a patchy distribution of the grass. In patchy grass treatments, H. vulgaris produced markedly more biomass, ramets and stolons in open patches (without grasses) than in grass patches, but displayed lower size variations as measured by coefficient of variation of biomass, ramets and stolons among the eight plants. In open areas, H. vulgaris produced statistically the same amounts of biomass and even more stolons and showed higher size variations in patchy grass treatments than in open (no grass) treatments. In grass areas, H. vulgaris grew much worse and displayed higher size variations in patchy grass treatments than in full grass treatments. Ramet aggregation decreased the growth of H. vulgaris in open treatments and in both open and grass patches in patchy grass treatments, but had little effect in full grass treatments. Ramet aggregation had little effect on size variations. Therefore, heterogeneous distribution of competitors can affect the growth and size structure of clonal plant populations, and ramet aggregation may decrease population growth when they grow in open environments or heterogeneous environments with a patchy distribution of interspecific competitors.     

Responses of savanna lawn and bunch grasses to water limitation

The grass layer of African savannas consists of two main vegetation types: grazing lawns, dominated by short, mostly clonally reproducing grasses, and bunch grasslands, dominated by tall bunch grasses. This patchy distribution of vegetation types is mostly created by large herbivores, which selectively feed on the more nutritious lawn grass species. Besides grazing, herbivores trample the soil, thereby causing soil compaction, with possible consequences for water infiltration. This raises two questions: (i) is water more limiting in grazing lawns than in bunch grasslands and (ii) are lawn grasses more drought tolerant than bunch grasses? To study these questions, we compared drought conditions in both lawn and bunch grasslands in a South African savanna. Additionally, in a climate room, we compared the performance of three lawn and three bunch grass species under a control and a water limitation treatment. Thirdly, we investigated whether there are differences between lawn and bunch grasses in traits related to drought tolerance. Our results show that despite large differences in water availability in the field, lawn and bunch grasses did not differ in their growth response to drought. Drought reduced growth of both growth forms equally. However, we found strong intrinsic trait differences between growth forms, with lawn grasses having higher specific root length and relative growth rate and bunch grasses having a higher root:shoot ratio. These results suggest that after drought-induced plant death, lawn grasses might be more capable of recolonizing patches of bare soil.     

干旱半干旱区灌丛斑块与降水量响应关系的熵模型模拟

灌丛斑块分布格局是灌木在干旱缺水条件下对生存环境的自我调节和适应的具体表现。应用熵理论和Klausmier模型,解释了灌丛斑块水分聚集原理并模拟了不同年降水条件下灌丛斑块的最佳面积比值(即最佳灌丛盖度)。研究结果表明:灌丛斑块生物量与其土壤含水量呈反比例函数关系,当生态系统处于稳定状态时(即熵最大状况下),年降水量与灌丛斑块面积比值符合一定的线性关系。研究采用内蒙古草原地区的野外调查数据,获得模型所需参数,进而模拟了不同年降水量条件下灌丛斑块最佳面积比值,研究结果可为半干旱地区植被保护与恢复提供参考。     

Differential effects of lichens,mosses and grasses on respiration and nitrogen mineralization in soils of the New Jersey Pinelands

In the New Jersey Pinelands, severely disturbed areas often do not undergo a rapid succession to forest; rather, a patchy cover of lichens, mosses and grasses persists for decades. We hypothesized that these plant covers affect soil microbial processes in different ways, and that these effects may alter the successional dynamics of the patches. We predicted that the moss and grass covers stimulate soil microbial activity, whereas lichens inhibit it, which may in turn inhibit succession. We collected soil cores from beneath each type of cover plus bare soil within two types of highly disturbed areas—sites subjected to hot wildfires, and areas mined for sand. Organic matter (OM) content, soil respiration and potential N mineralization were measured in the cores. Soils under mosses were similar to those under grasses; they accumulated more OM and produced more mineral N, predominantly in the form of ammonium, than either the bare soils or the soils beneath lichens. Mineralization under lichens, like that of the bare soils but unlike the soils beneath mosses or grasses, was dominated by net nitrification. These patterns were reproduced in experimentally transplanted moss and lichen mats. Mosses appear to create high-nutrient microsites via high rates of OM accumulation and production of ammonium, whereas lichens maintain low-nutrient patches similar to bare soil via low OM accumulation rates and production of mineral N predominantly in the mobile nitrate form. These differences in soil properties may explain the lack of vascular plant invasion in lichen mats, in contrast to the moss-dominated areas.     

Temporal and micro-spatial patterning of seedling establishment. Consequences for patch dynamics in the southern Monte,Argentina

We compared the temporal and micro-spatial patterning of seedling emergence and establishment of two cohorts of perennial grasses and shrubs in the southern Monte, Argentina. Samplings were carried out in two contrasting areas (grazed and non-grazed) during four years. We found lower densities of emerged and established seedlings of perennial grasses in the grazed relative to the non-grazed area. Conversely, emerged seedlings of shrubs did not vary between the grazed and the non-grazed area and densities of established shrub seedlings were higher in the grazed than in the non-grazed area. We only found differences between cohorts in seedling emergence of perennial grasses. These differences could be associated with the amount of precipitation in the year previous to the emergence. Both in the grazed and non-grazed area, seedlings of perennial grasses were concentrated at the periphery of plant patches. We defined a plant patch as a discrete unit of the spatial pattern of vegetation surrounded by, at least, 20 cm of bare soil from the nearest neighbour patch. Emergence in perennial grasses was more frequent at the southern/western patch-periphery than at other patch-periphery locations. Established seedlings of perennial grasses, however, were homogeneously distributed throughout patch periphery. Emergence in shrubs was more frequent at the centre and periphery of patches than at inter-patch microsites. In contrast, established seedlings of shrubs were homogeneously distributed among microsites. Our results suggests that differential seedling establishment between life forms is the outcome of complex biotic and abiotic interactions and feedbacks at the patch level between seedlings and established plants. Both life forms appear to have a different role in the origin, dynamics, and maintenance of spotting vegetation. Because of the ability to establish both at inter-patch and patch microsites, shrubs could be identified as colonizers or initiators of small plant patches in bare soil or they may contribute to increase the cover and size of pre-existing plant patches. Both processes would be promoted in grazed areas. Due to the ability to establish at patch peripheries, perennial grasses would contribute to the isodiametric growth of pre-existing patches, preferentially in non-grazed areas. This revised version was published online in June 2006 with corrections to the Cover Date.     

Facilitative Effects of Aloe Shrubs on Grass Establishment,Growth, and Reproduction in Degraded Kenyan Rangelands: Implications for Restoration

Under the harsh environmental conditions present in severely overgrazed, semiarid rangelands, facilitator plants offer a promising tool for ecological restoration. This study investigated facilitative effects of Aloe secundiflora—a native drought‐tolerant, unpalatable, thorny shrub—on grass establishment in degraded rangelands in Kenya. We planted native perennial grass seeds adjacent to three neighbor treatments: transplanted mature aloe shrubs, piles of thorn branches that provided similar physical protection to aloes, and control treatments with no facilitator. We monitored grass performance for three growing seasons. During the first growing season, association with aloe shrubs significantly improved seedling survival and plant size of individual grasses, whereas grass survival in thorn treatments was intermediate between aloe and control treatments. At the population level, aloe neighbor treatments were associated with the greatest grass abundance and cover in all three seasons and reproductive output in the second season. Control treatments were associated with the poorest grass performance for all three variables. The findings indicate that planting aloes can improve the effectiveness of grass reseeding for rangeland restoration, exceeding the benefits gained from the more common strategy of using thorn branch piles. The utility of aloes in particular is further enhanced by the economic value of these plants; medicinal sap can be sustainably harvested from aloes planted for restoration.     

Grassland structure in native pastures: links to soil surface condition

Summary When grassland is grazed by livestock, the structure of the sward changes in a patchy manner. With continuous selective grazing there is a mosaic of short and tall patches but as grazing intensifies the area of short‐grazed patch increases until the paddock has a lawn‐like appearance. This mosaic of patch structures can be stable, as short patches tend to attract repeated grazing and tall patches tend to be avoided. Because heavy grazing can detrimentally affect soil and water functions in grassland (ultimately resulting in erosion), we aimed to assess how well the physical structure of the sward reflects soil surface condition. We described four grassland patch structures that were assumed to reflect different levels of present grazing, and to some extent, past grazing pressure. We assessed patch structure and two other grass‐related variables (basal area of a ‘large tussock’ functional group and basal area of all perennial grass) as possible indicators of soil surface condition. Three indices of condition were measured in the field. The infiltration and nutrient cycling index declined progressively across patch structures, consistent with increasing grazing pressure. The stability index was found to be reduced only for the most heavily grazed grass structure (short patches). We found the ‘large tussock’ grass functional group to be a more sensitive indicator of soil surface condition than the group consisting of all perennial grasses. We found no evidence of sudden soil surface condition decline beyond a certain level of grass basal area, that is, there was no evidence of thresholds, rather, incremental loss of condition accompanied grass decline. We are thus not able to further refine an earlier proposed management recommendation ‘Graze conservatively to maintain dominance of large and medium tussock grasses over 60–70% of the native pastures’, except to suggest the use of short patches as a more practical indicator, rephrasing the recommendation as ‘Graze conservatively to allow a maximum of 30% short‐grazed patches in native pastures’.     

Spatial heterogeneity and irreversible vegetation change in semiarid grazing systems

Recent theoretical studies have shown that spatial redistribution of surface water may explain the occurrence of patterns of alternating vegetated and degraded patches in semiarid grasslands. These results implied, however, that spatial redistribution processes cannot explain the collapse of production on coarser scales observed in these systems. We present a spatially explicit vegetation model to investigate possible mechanisms explaining irreversible vegetation collapse on coarse spatial scales. The model results indicate that the dynamics of vegetation on coarse scales are determined by the interaction of two spatial feedback processes. Loss of plant cover in a certain area results in increased availability of water in remaining vegetated patches through run-on of surface water, promoting within-patch plant production. Hence, spatial redistribution of surface water creates negative feedback between reduced plant cover and increased plant growth in remaining vegetation. Reduced plant cover, however, results in focusing of herbivore grazing in the remaining vegetation. Hence, redistribution of herbivores creates positive feedback between reduced plant cover and increased losses due to grazing in remaining vegetated patches, leading to collapse of the entire vegetation. This may explain irreversible vegetation shifts in semiarid grasslands on coarse spatial scales.     

Patch dynamics in grazed subtropical native pastures in south‐east Queensland

Abstract Patch formation is common in grazed grasslands but the mechanisms involved in the formation and maintenance of patches are not clear. To increase our knowledge on this subject we examined possible reasons for patch formation and the influence of management on changes between patch states in three experiments in native pasture communities in the Crows Nest district, south‐east Queensland. In these communities, small‐scale patches (tall grassland (dominated by large and medium tussock grasses), short swards (dominated by short tussock grasses and sedges), and lawns (dominated by stoloniferous and/or rhizomatous grasses)) are readily apparent. We hypothesized that the formation of short sward and lawn patches in areas of tall grassland was due to combinations of grazing and soil fertility effects. This was tested in Experiment 1 by applying a factorial combination of defoliation, nutrient application and transplants of short tussock and stoloniferous species to a uniform area of tall grassland. Total species density declined during the experiment, was lower with high nutrient applications, but was not affected by defoliation. There were significant changes in abundance of species that provided support for our hypotheses. With light defoliation and low nutrients, the tall grassland remained dominated by large tussock grasses and contained considerable amounts of forbs. With heavy defoliation, the pastures were dominated by medium tussock grasses and there were significant decreases in forbs and increases in sedges (mainly with low nutrients) and stoloniferous grasses (mainly with high nutrients). Total germinable seed densities and those of most species groups were significantly lower in the heavy defoliation than the light defoliation plots. Total soil seed numbers were not affected by nutrient application but there were fewer seeds of the erect forbs and more sedge seeds in plots with high nutrients. The use of resting from grazing and fire to manage transitions between patches was tested. In Experiment 2 , changes in species density and abundance were measured for 5 years in the three patch types with and without grazing. Experiment 3 examined the effects of fire, grazing and resting on short sward patches over 4 years. In Experiment 2 , total species density was lower in lawn than short sward or tall grassland patches, and there were more species of erect forbs than other plant groups in all patch types. The lawn patches were originally dominated by Cynodon spp. This dominance continued with grazing but in ungrazed patches the abundance of Cynodon spp. declined and that of forbs increased. In the short sward patches, dominance of short tussock grasses continued with grazing but in ungrazed plots their abundance declined while that of large tussock grasses increased. The tall grassland patches remained dominated by large and medium tussock species. In Experiment 3 , fire had no effect on species abundance. On the grazed plots the short tussock grasses remained dominant but where the plots were rested from grazing the small tussock grasses declined and the large tussock grasses increased in abundance. The slow and relatively small changes in these experiments over 4 or 5 years showed how stable the composition of these pastures is, and that rapid changes between patch types are unlikely.     

Plant Community Recovery Following Restoration in Semiarid Grasslands

The Conservation Reserve Program (CRP) is an extensive land use in the United States, which restores cultivated land to perennial vegetation through seeding. Low precipitation and high potential evapotranspiration are major limitations to the establishment and growth of seeded species in semiarid regions. We tested the rate of development of plant functional types across a chronosequence of restored fields using a model of plant succession. We also determined how the seeding of non‐native (introduced) relative to native perennial grasses influenced plant community recovery. In contrast to the native shortgrass steppe (SGS), recently seeded CRP fields had high cover of annuals, forbs, C₃, and introduced species. The seed mix determined which perennial grasses dominated the plant community within 18 years, but slow establishment prolonged early seral stages, allowed for the spread of colonizing perennial grasses, and limited recovery to less than half the canopy cover of undisturbed shortrass steppe. Species density declined in restored fields as seeded perennial grass cover increased and was lower in CRP fields seeded with introduced compared to native perennial grasses. Plant community composition transitioned to C₄ and native species, even if fields were not seeded with these species, and was modified by shifts in the amount and seasonality of precipitation. Thus, in semiarid CRP fields, we found that the potential for recovery depended on time since CRP enrollment, seed mix, and climatic variability. Full recovery, based on similarity to vegetation cover and composition of undisturbed SGS, requires greater than 20 years.     

Regularity of species richness relationships to patch size and shape

This study aims to assess the degree of regularity in the effect of patch size and patch shape on plant species richness across a macroscale region, and to evaluate the implications for nature conservation.
Our study area covers south-eastern Norway and contains 16 agricultural landscapes with 2162 patches.
To analyse regularity a local linear mixed model (LLMM) was applied. This procedure estimates the richness trends due to shared effects of size and shape, and simultaneously provides the landscape-specific random effect. The latter is a direct estimate of the degree of irregularity between the landscapes, conditioned on specific values of size and shape.
The results show a positive interaction between the shape and size of patches, which is repeated for all landscapes. The shape of the patches produces more regular patterns in species richness than the size of patches. This we attribute to effects of dispersal and distance to neighbouring patches of different environmentally conditioned species pools. Large and complex patches have shorter average distance to neighbouring patches (of different types) than large simple-shaped (circular) patches have. We attribute the higher species richness of the former, given a similar area, to a higher number of species dispersed from the outside into the more complex plot. For small patches, however, the distance to the edge is short relative to normal dispersal distances, for patches of all shapes. This explains why the positive effect of shape complexity on species richness is stronger for large patches. This interpretation is supported by a strong spatial correlation conditioned on the most complex patches.
Theories of dynamics in biodiversity in patchy landscapes must consider shape as a regulator at the same level as size, and both shape and size of patches should be simultaneously taken into account for management planning.     

Scale-dependent foraging and patch choice in fractal environments

Many spatially complex environments are fractal, and consumers in these environments face scale-dependent trade-offs between encountering high densities of small resource patches versus low densities of large resource patches. I address the effects of these trade-offs on foraging by incorporating scale-dependent encounter of resources in fractal landscapes into classical optimal foraging theory. This model is then used to predict optimal scales of perception (foraging scale) and patch choice in response to spatial features of landscapes. The model predicts that, for a given density of resources, landscapes with greater extent and fractal dimension and that contain patchy (low fractal dimension) resources favour large foraging scales and specialization on a small proportion of resource patches. Fragmented (low fractal dimension) landscapes of small extent with dispersed (high fractal dimension) resources favour smaller foraging scales and generalists that use a large proportion of available resource patches. These predictions synthesize the results of other spatially explicit consumer–resource models into a simple framework and agree reasonably well with results of several empirical studies. This study thus places optimal foraging theory in a spatial context and suggests evolutionary mechanisms of consumers' responses to important spatial phenomena (e.g. habitat fragmentation, resource aggregation). This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of Soil Nutrient Heterogeneity and Earthworms on Aboveground Biomass of Experimental Plant Communities

Soil nutrients are commonly heterogeneously distributed and earthworms are one of the most common soil organisms. While effects of both soil nutrient heterogeneity and earthworms have been well studied, their interactive effect on plant community productivity has rarely been tested. In a greenhouse experiment, we constructed experimental plant communities by sowing seed mixtures of four grasses, two legumes and two forbs in either a heterogeneous soil consisting of low and high nutrient soil patches or a homogeneous soil where the low and high nutrient soil patches were evenly mixed. The earthworm Eisenia fetida was either added to these soils or not. Aboveground biomass of the whole communities, grasses and legumes did not differ between the homogeneous and heterogeneous soils or between the soils with and without earthworms. However, soil nutrient heterogeneity reduced aboveground biomass of forbs, and such an effect did not interact with earthworms. In response to soil heterogeneity and earthworms, biomass ratio of the three functional groups showed similar patterns as that of their biomass. At the patch level, aboveground biomass of the whole community, grasses and legumes were greater in the high than in the low nutrient soil patches within the heterogeneous soil. A similar pattern was found for the forbs, but this was only true in the absence of earthworms. Our results suggest that soil nutrient heterogeneity and earthworms may not influence aboveground biomass of plant communities, despite the fact that they may modify the growth of certain plant functional groups within the community.     

Large herbivores maintain a two‐phase herbaceous vegetation mosaic in a semi‐arid savanna

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Stem demography of Prosopis glandulosa var. torreyana in vegetation arcs and associated bare areas

Abstract. Vegetation arcs are characterized by dense patches of up to 100% plant cover surrounded by bare areas where cover is around 5%. These patterns of contracted vegetation have been reported mainly in alluvial fans of arid and semiarid zones (known as bajadas) with gentle slopes (0.2–2%), summer rainfall, and heavy showers that run as sheet-flow. It has been suggested that run-off water from bare areas is stopped at the front of the arcs and, depending on the amount of rainfall, may advance superficially into the arc, reaching the downslope limit less often. These dynamics would imply a gradient of water availability inside the arc, higher at the upper edge. We termed this the moisture gradient hypothesis. We indirectly tested this hypothesis by following the demography of a cohort of stems of the honey mesquite Prosopis glandulosa var. torreyana in three vegetation arcs and their corresponding bare areas during a yearly cycle at the Bolsón de Mapimí in the Chihuahuan Desert. We selectively removed different plant life-forms (trees, shrubs and herbs) in three positions within the arcs; no manipulation was possible in the bare areas due to low plant cover. The results suggested that removal of other plant life-forms, mainly grasses, affected growth and survival of mesquite low (but not high) stems, making them accessible to medium-sized mammal browsers (jackrabbits and packrats). These results suggest that, at least for the honey mesquite, water availability in vegetation arcs was not the most strongly limiting factor for shoot performance and demography, and that browsing damage, a biotic constraint, took precedence over resource limitation.     

Monitoring ecological indicators of rangeland functional integrity and their relation to biodiversity at local to regional scales

Abstract Functional integrity is the intactness of soil and native vegetation patterns and the processes that maintain these patterns. In Australia's rangelands, the integrity of these patterns and processes have been modified by clearing, grazing and fire. Intuitively, biodiversity should be strongly related to functional integrity; that is, landscapes with high functional integrity should maintain biodiversity, and altered, less functional landscapes may lose some biodiversity, defined here as the variety and abundance of the plants, animals and microorganisms of concern. Simple indicators of biodiversity and functional integrity are needed that can be monitored at a range of scales, from fine to coarse. In the present paper, we use examples, primarily from published work on Australia's rangeland, to document that at finer patch and hillslope scales several indicators of landscape functional integrity have been identified. These indicators, based on the quantity and quality of vegetation patches and interpatch zones, are related to biodiversity. For example, a decrease in the cover and width (quantity) and condition (quality) of vegetation patches, and an increase in bare soil (quantity of interpatch) near cattle watering points in a paddock are significantly related to declines in plant and grasshopper diversity. These vegetation patch‐cover and bare‐soil indicators have been monitored traditionally by field‐based methods, but new high‐resolution, remote‐sensing imagery can be used in specific rangeland areas for this fine‐scale monitoring. At intermediate paddock and small watershed scales, indicators that can be derived from medium‐resolution remote‐sensing are also needed for efficient monitoring of rangeland condition (i.e. functional integrity) and biodiversity. For example, 30–100‐m‐pixel Landsat imagery has been used to assess the condition of rangelands along grazing gradients extending from watering‐points. The variety and abundance of key taxa have been related to these gradients (the Biograze project). At still larger region and catchment scales, indicators of rangeland functional integrity can also be monitored by coarse‐resolution remote‐sensing and related to biodiversity. For example, the extent and greenness (condition) of different regional landscapes have been monitored with 1‐km‐pixel satellite imagery. This regional information becomes more valuable when it indicates differences as a result of land management. Finally, we discuss potential future developments that could improve proposed indicators of landscape functional integrity and biodiversity, thereby improving our ability to monitor rangelands effectively.