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.     

Catastrophic vegetation shifts and soil degradation in terrestrial grazing systems

It has long been recognized that alternative vegetation states may occur in terrestrial grazing systems. This phenomenon may be of great importance as small environmental fluctuations may lead to relatively sudden and irreversible jumps between vegetation states. Early theoretical studies emphasized saturation of herbivore feeding to explain multiple stable states and catastrophic behaviour. Recent studies on semi-arid grasslands and arctic salt marshes, however, relate catastrophic events in these systems to plant-soil interactions.     

Denitrification in a semi-arid grazing ecosystem

The effect of large herbivores on gaseous N loss from grasslands, particularly via denitrification, is poorly understood. In this study, we examined the influence of native migratory ungulates on denitrification in grasslands of Yellowstone National Park in two ways, by (1) examining the effect of artificial urine application on denitrification, and (2) comparing rates inside and outside long-term exclosures at topographically diverse locations. Artificial urine did not influence denitrification 3 and 12 days after application at hilltop, mid-slope, and slope-bottom sites. Likewise, grazers had no effect on community-level denitrification at dry exclosure sites, where rates were low. At mesic sites, however, ungulates enhanced denitrification by as much as 4 kg N ha⁻¹ year⁻¹, which was double atmospheric N inputs to this ecosystem. Denitrification enzyme activity (DEA, a measure of denitrification potential) was positively associated with soil moisture at exclosure sites, and herbivores stimulated DEA when accounting for the soil moisture effect. Glucose additons to soils increased denitrification and nitrate additions had no influence, suggesting that denitrification was limited by the amount of labile soil carbon, which previously has been shown to be enhanced by ungulates in Yellowstone. These results indicate that denitrification can be an ecologically important flux in portions of semi-arid landscapes, and that there is a previously unsuspected regulation of this process by herbivores. Received: 6 March 1998 / Accepted: 28 August 1998     

Multiscale soil and vegetation patchiness along a gradient of herbivore impact in a semi-arid grazing system in West Africa

We studied the degree and scale of patchiness of vegetation and selected soil variables along a gradient of herbivore impact. The gradient consisted of a radial pattern of `high', `intermediate' and `low' herbivore impact around a watering point in a semi-arid environment in Burkina Faso (West Africa). We hypothesised that, at a certain range of herbivore impact, vegetated patches alternating with patches of bare soil would occur as a consequence of plant-soil feedbacks and run-off-run-on patterns. Indeed, our transect data collected along the gradient showed that vegetated patches with a scale of about 5–10 m, alternating with bare soil, occurred at intermediate herbivore impact. When analysing the data from the experimental sites along the gradient, however, we also found a high degree of patchiness of vegetation and soil variables in case of low and high herbivore impact. For low herbivore impact, most variation was spatially explained, up to 100% for vegetation biomass and soil temperature, with a patch scale of about 0.50 m. This was due to the presence of perennial grass tufts of Cymbopogon schoenanthus. Patterns of soil organic matter and NH₄-N were highly correlated with these patterns of biomass and soil temperature, up to r=0.7 (P<0.05) for the in situ correlation between biomass and NH₄-N. For high herbivore impact, we also found that most variation was spatially explained, up to 100% for biomass and soil temperature, and 84% for soil moisture, with three distinct scales of patchiness (about 0.50 m, 1.80 m and 2.80 m). Here, microrelief had a corresponding patchy structure. For intermediate herbivore impact, again most variation was spatially explained, up to 100% for biomass and soil temperature, and 84% for soil moisture, with a patch scale of about 0.95 m. Here, we found evidence that vegetated patches positively affected soil moisture through less run-off and higher infiltration of rainwater that could not infiltrate into the bare soil elsewhere, which was not due to microrelief. Thus, we conclude that our findings are in line with our initial hypothesis that, at intermediate herbivore impact, vegetated patches alternating with patches of bare soil persist in time due to positive plant-soil feedbacks.     

Feedback dynamics of grazing lawns: coupling vegetation change with animal growth

We studied the effects of grazing by Black Brant (Branta bernicla nigricans) geese (hereafter Brant) on plant community zonation and gosling growth between 1987 and 2000 at a nesting colony in southwestern Alaska. The preferred forage of Brant, Carex subspathacea, is only found as a grazing lawn. An alternate forage species, C. ramenskii, exists primarily as meadow but also forms grazing lawns when heavily grazed. We mowed plots of ungrazed C. ramenskii meadows to create swards that Brant could select and maintain as grazing lawns. Fecal counts were higher on mowed plots than on control plots in the year after plots were mowed. Both nutritional quality and aboveground biomass of C. ramenskii in mowed plots were similar to that of C. subspathacea grazing lawns. The areal extent of grazing lawns depends in part on the population size of Brant. High Brant populations can increase the areal extent of grazing lawns, which favors the growth of goslings. Grazing lawns increased from 3% to 8% of surface area as the areal extent of C. ramenskii meadows declined between 1991 and 1999. Gosling mass was lower early in this time period due to density dependent effects. As the goose population stabilized, and area of grazing lawns increased, gosling mass increased between 1993 and 1999. Because larger goslings have increased survival, higher probability of breeding, and higher fecundity, herbivore-mediated changes in the distribution grazing lawn extent may result in a numerical increase of the population within the next two decades.     

Cattle foraging habits shape vegetation patterns of alluvial year-round grazing systems

Year-round grazing with robust cattle is increasingly used as a near-natural tool for the restoration of structurally diverse grassland ecosystems in Western and Central Europe. The aim of this study was to evaluate the general success of year-round grazing and to analyze the interplay between emerging vegetation structures, grazing patterns and abiotic environmental conditions. In summer 2010 vegetation composition, aboveground biomass and soil properties were sampled at 44 quadrats of 4 × 4 m² within two year-round grazed floodplain sites in Northwestern Germany. For plot selection, we predefined structural vegetation types and later statistically determined indicative plant species for each structural type. Our results showed that year-round grazing resulted in the successful creation of eutrophic grassland communities on former agricultural land after 15 years. Soil parameters like phosphorous and potassium concentration and the flooding duration did not or only slightly differ between different structure types. In summer, cattle preferably fed at short-growing patches which were of better digestible biomass than taller patches. Hence, our data clearly demonstrate a positive feedback between grazing intensity and fodder quality leading to a patchy vegetation structure of intensively grazed swards and less frequented areas dominated by high-growing grasses and tall forbs, almost independently from primary differences in soil parameters and other site factors such as flooding duration. The remarkable structural and floristic diversity of year-round grazing systems clearly is a result of these spatially contrasting feeding patterns.     

Land quality changes following the conversion of the natural vegetation into silvo-pastoral systems in semi-arid NE Brazil

Plant and Soil - The presence of trees in grasslands frequently results in resource islands of higher soil quality. Therefore, some native trees are often preserved or agroforestry species are...     

气候和放牧对锡林郭勒地区植被覆盖变化的影响

基于锡林郭勒盟15个气象站点1981-2007年的逐月气温、降水量数据及各旗县的牲畜头数,在ArcGIS软件的支持下,分析气候干燥度和牲畜密度的空间分布,结合1981-2007年的逐旬归一化植被指数(NDVI)数据,对研究区植被覆盖变化的驱动因素进行分析.结果表明:研究期间,锡林郭勒盟气候干燥度与植被覆盖状况之间存在良好的线性回归关系;NDVI与牲畜密度之间存在良好的二项式回归关系,随着NDVI值的升高,牲畜密度先增加后降低;植被覆盖状况与干燥度和牲畜密度呈复线性相关关系,其中,NDVI与干燥度呈正相关,与牲畜密度呈负相关,且干燥度对NDVI的影响远大于牲畜密度对NDVI的影响.     

Changes in semi-arid plant species associations along a livestock grazing gradient

In semi-arid ecosystems, vegetation is heterogeneously distributed, with plant species often associating in patches. These associations between species are not constant, but depend on the particular response of each species to environmental factors. Here, we investigated how plant species associations change in response to livestock grazing in a semi-arid ecosystem, Cabo de Gata-Níjar Natural Park in South East Spain. We established linear point-intercept transects at four sites with different grazing intensity, and recorded all species at each point. We investigated plant associations by comparing the number of times that each pair of species occurred at the same spatial point (co-occurrences), with the expected number of times based on species abundances. We also assessed associations for each shrub and grass species by considering all their pairs of associations and for the whole plant community by considering all pairs of associations on each site. At all sites, the plant community had a negative pattern of association, with fewer co-occurrences than expected. Negative association in the plant community increased at maximum grazing intensity. Most species associated as expected, particularly grass species, and positive associations were most important at intermediate grazing intensities. No species changed its type of association along the grazing gradient. We conclude that in the present plant community, grazing-resistant species compete among themselves and segregate in space. Some shrub species act as refuges for grazing-sensitive species that benefit from being spatially associated with shrub species, particularly at intermediate grazing intensities where positive associations were highest. At high grazing intensity, these shrubs can no longer persist and positive associations decrease due to the disappearance of refuges. Spatial associations between plant species and their response to grazing help identify the factors that organize plant communities, and may contribute to improving management of semi-arid ecosystems.     

Some effects of animal grazing and browsing on vegetation

Summary

This is a review of some problems posed by research on tree biology. First, phase change and in vitro culture are discussed as they affect vegetative propagation of woody plants. Then the breeding of timber trees, with an evident and important trend to clonal propagation is considered. Tropical timber tree breeding has enormous potential but has only just started (on very few species). The trend to making tropical woodlands into planted pure stands, of converting production forestry into, ‘tree farming’, an aspect of agriculture, so to speak, is apparent. ‘Ideotypes’ have fallen out of favour but some important ideas as to biomass and its allocation remain and well-defined economic objectives must become paramount. A broad review of the tropical agricultural context with regard to trees, suggests an acute need for far better understanding of them and for much introduction and breeding to generate new woody food-crops.     

Impact of grazing on vegetation dynamics in former ricefields

Abstract. In the Rhône delta, Juncus gerardi and Scirpus maritimus are often the dominant species in abandoned rice fields which are artificially flooded in early spring to improve forage production. Under these conditions they occur either in mixed communities, or form monospecific stands. Monitoring the vegetation dynamics in quadrats located in six abandoned rice fields artificially flooded from November to April confirmed the important role of grazing. In ungrazed plots, communities dominated by Scirpus maritimus mixed with Juncus gerardi developed fast. After 42 months of management Scirpus maritimus had established in nearly all quadrats and continued to expand, whereas Juncus gerardi had started to decline. In grazed plots Juncus gerardi alone dominated and continued to increase in cover up to the 42nd month. Scirpus maritimus established at low densities mainly in quadrats where Juncus gerardi was initially absent. Introduction of seeds of Scirpus maritimus in communities of Juncus gerardi under controlled conditions demonstrated the existence of the phenomenon of preemption. The increase in cover of Juncus gerardi suggests that the preemption of Juncus gerardi over Scirpus maritimus plays a more pronounced role in the field in the presence of grazing.     

Rates of vegetation change associated with prairie dog (Cynomys ludovicianus) grazing in North American mixed-grass prairie

A prairie dog (Cynomys ludovicianus) colony with a known history of habitation was studied to quantify the effects of herbivory on plant species composition, dominance, stature and diversity in a North American mixedgrass prairie. Gradient analysis was used to quantify the relationship between plant community structure, prairie dog density, burrow density and habitation history and to document community-level responses of plants subjected to heavy grazing pressure. The results quantify the type, rate and extent of change which plant populations and communities may undergo in response to the differential grazing of plants variously tolerant of defoliation.Detrended correspondence analysis indicated that 69% of the between-sample floristic variance on the site was attributable to prairie dog habitation. Perennial grasses were rapidly displaced from the site within 3 yr of colonization and were replaced by annual forbs. The net result was an increase in species richness and diversity on the prairie dog colony. Within the colony, however, the number of species was more a function of stand size than colonization history.Significant decreases in canopy stature after 2 yr of habitation resulted from replacement of mid-height grass species by shortgrass species and forbs. In addition, there was a shift from tall growth forms of off-colony species to dwarf growth forms of the same species on the colony. Decreases in litter and increases in bare soil cover were substantial during the first 2 yr of habitation but changed little thereafter.Abbreviations WCC Wind Cave Canyon - DCA Detrended Correspondence Analysis     

Climate change and vegetation response

This study, as many other current investigations in palaeoecology is focused on the long-term dynamics of vegetation and the extent to which they are controlled by climate change. Climate and classes of climate change are defined and reviewed, and examples cited of vegetation response. The concepts of vegetation, plant community and equilibrium are examined, with particular emphasis on theories on short term dynamics developed by ecologists working with temperate and boreal forests. Vegetation response to climate change can be modified by anthropogenic factors, topographic diversity and soils, life-cycle characteristics and hysteresis.I am grateful for comments on an earlier version of this paper by Keith Bennett, Les Cwynar and Glen MacDonald, and I particularly appreciate the useful remarks by Colin Prentice.I am grateful for comments on an earlier version of this paper by Keith Bennett, Les Cwynar and Glen MacDonald, and I particularly appreciate the useful remarks by Colin Prentice.     

划区轮牧中不同放牧利用时间对草地植被的影响

对比研究了划区轮牧中不同时间的放牧利用对各小区的植被状况的影响。结果表明 ,在固定放牧时间长度的情况下 ,早放牧小区的草群结构比晚放牧的小区受到的影响大。家畜对早放牧小区的牧草利用率高于晚放牧的小区 ,草地生产力则表现出相反的趋势。对于可利用牧草营养 ,早放牧的小区可提供较多的粗蛋白。在生产实践中 ,要灵活应用划区轮牧制度 ,对不同时间放牧利用的各小区要根据草地的实际情况来确定其具体的放牧时间长度 ,在不同的年度也要按不同的顺序来轮换放牧 ,兼顾到草地的可持续利用与家畜生产。     

Spatial and temporal signatures of fragility and threshold proximity in modelled semi-arid vegetation

Understanding the behaviour of complex environmental systems, particularly as critical thresholds are approached, is vitally important in many contexts. Among these are the moisture-limited vegetation systems in semi-arid (SA) regions of the World, which support approximately 36 per cent of the human population, maintain considerable biodiversity and which are susceptible to rapid stress-induced collapse. Change in spatially self-organized vegetation patterning has previously been proposed as a means of identifying approaching thresholds in these systems. In this paper, a newly developed cellular automata model is used to explore spatial patterning and also the temporal dynamics of SA vegetation cover. Results show, for the first time, to my knowledge, in a cellular automata model, that 'critical slowdown' (a pronounced reduction in post-perturbation recovery rates) provides clear signals of system fragility as major thresholds are approached. A consequence of slowing recovery rates is the appearance of quasi-stable population states and increased potential for perturbation-induced multi-staged population collapse. The model also predicts a non-patterned cover where environmental stress levels are high, or where more moderate stress levels are accompanied by frequent perturbations. In the context of changing climatic and environmental pressures, these results provide observable indicators of fragility and threshold proximity in SA vegetation systems that have direct relevance to management policies.     

Micropatterns in Festuca rubra-dominated salt-marsh vegetation induced by sheep grazing

Micropatterns induced by sheep grazing, were studied in three consecutive years in a Festuca rubra-dominated salt marsh in a grazing trial with five different stocking rates (0, 1.5, 3, 4.5 and 10 sheep ha^-1). The micropatterns were formed by a mosaic of short and tall F. rubra stands on a scale of square decimeters. Permanent transects of 2 m × 10 m were used to study the stability of these patterns, and to analyze interactions between the vegetation, the marsh elevation and the sheep. Micropatterns occurred only in the lightly to moderately grazed paddocks (1.5–4.5 sheep ha^-1) with the highest spatial diversity in the 3 sheep ha^-1 transect. When grazing was excluded, micropatterns did not develop; nor did they develop in the traditionally and most intensively grazed paddock (10 sheep ha^-1). Detailed observations in one year showed that crude-protein content did not differ between green leaves from the short and tall stands, whereas in vitro digestibility was slightly higher in the short stands. In the same year, tiller density and length of full-grown leaves increased substantially in both stands from May to September. At the same time, sheep preference shifted from tall to short stands, which suggests an interplay between intake rate and digestibility in the sheep selectivity.Seven years after establishment of the grazing trial, the 10 sheep ha^-1 transect still showed a smooth relief typical of the starting point of the other transects. These transects developed a more hummocky topography, with the highest spatial diversity occurring on the 1.5 sheep ha^-1 transect. Marsh elevations were on average up to 3 cm lower in the short than in the tall stands, which indicates that the somewhat lower-elevated patches were grazed more intensively than the higher-elevated patches.In most cases, micropatterns changed from one year to the other, probably due to weather fluctuations. The incidence of tall stands was influenced by the rainfall balance. If the incidences of both the short and the tall stands were around 50%, however, the micropatterns showed a clear correlation with the marsh elevation. The rainfall balance seemed therefore a decisive factor for a possible correspondence between micropatterns in two consecutive years. Elevation differences were so subtle that greater than average sedimentation during a winter season could change the elevation pattern. Hence both rainfall balance and winter sedimentation counteracted the stability of the micropatterns. During our three-year study period, micropatterns were only stable in one out of six possible paired comparisons. This low micropattern stability contrasts with other studies in inland environments, which shows that in more dynamic environments, abiotic processes are likely to overrule summer grazing in determining vegetation patterns.