Species trait shifts in vegetation and soil seed bank during fen degradation

Fens in Central Europe are characterised by waterlogged organic substrate and low productivity. Human-induced changes due to drainage and mowing lead to changes in plant species composition from natural fen communities to fen meadows and later to over-drained, degraded meadows. Moderate drainage leads to increased vegetation productivity, and severe drainage results in frequent soil disturbances and less plant growth. In the present article, we analyse changes in plant trait combinations in the vegetation and the soil seed bank as well as changes in the seed bank types along gradient of drainage intensity. We hypothesize that an increase in productivity enhances traits related to persistence and that frequent disturbance selects for regeneration traits. We use multivariate statistics to analyse data from three disturbance levels: undisturbed fen, slightly drained fen meadow and severely drained degraded meadow. We found that the abundance of plants regenerating from seeds and accumulating persistent seed banks was increasing with degradation level, while plants reproducing vegetatively were gradually eliminated along the same trajectory. Plants with strong resprouting abilities increased during degradation. We also found that shifts in trait combinations were similar in the aboveground vegetation and in soil seed banks. We found that the density of short-term persistent seeds in the soil is highest in fen meadows and the density of long-term persistent seeds is highest in degraded meadows. The increase in abundance of species with strong regeneration traits at the cost of species with persistence-related traits has negative consequences for the restoration prospects of severely degraded sites.     

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.     

Herbivore regulation and irreversible vegetation change in semi-arid grazing systems

Models made to explain sudden and irreversible vegetation shifts in semi-arid grasslands typically assume that herbivore density is independent of the state of the vegetation, e.g., under the control of humans. We relax this assumption and investigate the mathematical implications of vegetation-regulated herbivore population dynamics. We show that irreversible vegetation change may also occur in systems where herbivore population dynamics are affected by changes in plant standing crop. Our analysis furthermore shows that irreversible vegetation change may occur for a larger set of soil and climatic conditions when herbivore numbers are independent of the vegetation, as compared to systems where vegetation density determines herbivore population size. Hence, our analysis suggests that irreversible vegetation change is less likely to occur in systems with natural herbivore population dynamics than in systems where humans control herbivore density.     

Productivity: key factor affecting grazing exclusion effects on vegetation and soil

In this study, we inquire into the effects of short-term goat grazing abandonment on plant species and functional composition, bare ground and net primary productivity (NPP) in two traditionally grazed pastures located in the Canarian Network of Natural Protected Areas and the Natura 2000 Network. In addition, we analyse soil chemical properties, biomass tannin content and energetic value to find out how grazing abandonment affects soil fertility and forage quality of these agroecosystems. Grazing exclusion effects on plant species and functional composition, as well as on soil fertility depended on the productivity of the studied pasture. Erect forbs and shrubs (endemic to Macaronesian region and native) were favoured by grazing removal in the most productive pasture, while soil fertility decreased in the driest and least productive site. An increase in NPP after exclusion was consistent among study sites. Although we consider goat grazing as necessary for maintaining traditional agroecosystems, we also suggest controlling it over time, allowing some periods of rest to give endemic shrub species time to recover from near propagule sources.     

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.     

Effect of slight vegetation degradation on soil properties in Brachypodium pinnatum grasslands

Water is the most important factor controlling plant growth, primary production, and ecosystem stability in arid and semi-arid grasslands. Here we conducted a 2-year field study to explore the contribution of winter half-year (i.e. October through April) and summer precipitation (May through September) to the growth of coexisting plant species in typical steppe ecosystems of Inner Mongolia, China. Hydrogen stable isotope ratios of soil water and stem water of dominant plant species, soil moisture, and plant water potential were measured at three steppe communities dominated by Stipa grandis, Caragana microphylla, and Leymus chinensis, respectively. The fraction of water from winter half-year precipitation was an important water source, contributing 45% to plant total water uptake in a dry summer after a wet winter period (2005) and 15% in a summer where subsoil moisture had been exploited in the previous year (2006). At species level, Caragana microphylla exhibited a complete access to deep soil water, which is recharged by winter precipitation, while Cleistogenes squarrosa completely depended on summer rains. Leymus chinensis, Agropyron cristatum, and Stipa grandis showed a resource-dependent water use strategy, utilizing deep soil water when it was well available and shifting to rain water when subsoil water had been exploited. Our findings indicate that differentiation of water sources among plants improves use of available soil water and lessens the interspecific competition for water in these semi-arid ecosystems. The niche complementarity in water sources among coexisting species is likely to be the potential mechanism for high diversity communities with both high productivity and high resilience to droughts.     

Changes in soil properties and vegetation following livestock grazing exclusion in degraded arid environments of South Tunisia

In this study, characteristics of vegetation and soil properties under continued grazing and exclusion of livestock for 6 and 12 years were examined in a degraded Stipa tenacissima steppe in South Tunisia.     

Effect of grazing on vegetation and soil of the heuweltjieveld in the Succulent Karoo,South Africa

We asked how historical and recent grazing intensity affect the patchy landscape of the heuweltjieveld in the semi-arid biodiversity hotspot Succulent Karoo. The study was carried out on a communal farmland 80 km south-west of Springbok, in Namaqualand. Heuweltjies are roughly circular earth mounds that are regularly distributed in this landscape. We sampled plant species and life-form composition, diversity measures, habitat and soil variables in 100 m² plots, placed in three visually distinguishable heuweltjie zones (centre, fringe, and matrix) and distributed across grazing camps with different recent and historic grazing intensities. Differences between heuweltjie zones were assessed with ANOVAs and multiple linear regressions. The effect of past and recent grazing intensity on soil and plant variables was analysed by Generalized Linear Models for each heuweltjie zone separately. The three zones constituted clearly distinguishable units in terms of vegetation and soil characteristics. Soil pH and cover of annual plants increased from matrix to centres, while total vegetation cover, species richness and perennial plant cover decreased in the same direction. Historic (pre-2000) grazing patterns had the strongest effects on fringes, showing the strongest soil and vegetation-related signs of overutilization with increased stocking density. Centres showed signs of overutilization irrespective of the stocking density. The much shorter exposure to recent grazing pattern (post-2000), which was nearly inverse to the historic grazing pattern, showed increase of vegetation cover (centres) and species richness (matrix) with recent grazing intensity. We interpret these effects as still visible responses of the lower grazing intensity in these camps during the historic period. No recovery under recent grazing was observed at any of the zones. We conclude that irrespective of their conducive growing conditions, once transformed to a disturbed state, heuweltjie centres recover slowly, whereas the less impacted soil and vegetation of fringes are more responsive than centres and matrix.     

Arbuscular mycorrhizal fungi associated with vegetation and soil parameters under rest grazing management in a desert steppe ecosystem

The impact of rest grazing on arbuscular mycorrhizal fungi (AMF) and the interactions of AMF with vegetation and soil parameters under rest grazing condition were investigated between spring and late summer in a desert steppe ecosystem with different grazing managements (rest grazing with different lengths of resting period, banned or continuous grazing) in Inner Mongolia, China. AMF diversity and colonization, vegetation biomass, soil properties and soil phosphatase activity were examined. In rest grazing areas of 60 days, AMF spore number and diversity index at a 0–10 cm soil depth as well as vesicular and hyphal colonization rates were higher compared with other grazing treatments. In addition, soil organic matter and total N contents were highest and soil alkaline phosphatase was most active under 60-day rest grazing. In August and September, these areas also had the highest amount of aboveground vegetation. The results indicated that resting grazing for an appropriate period of time in spring has a positive effect on AMF sporulation, colonization and diversity, and that under rest grazing conditions, AMF parameters are positively correlated with some soil characteristics.     

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.     

Catastrophic disturbance and vegetation on Little Slope,Lord Howe Island

Little Slope is a series of debris avalanche deposits below basalt cliffs 700 m high at the southern end of Lord Howe Island (31°35′S 159°05′E). There are five distinct physiographic areas on the slope, each separated by sharp boundaries which mostly correspond to boundaries between features on the cliffs above. The physiography is a consequence of a series of landslides of different, but unknown, ages. A model of the physiographic history is presented. Each physiographic area supports a different vegetation community, also separated by sharp boundaries. The present structure of two communities, Melaleuca howeana scrub and Howea forsterana forest, is a consequence of damage by feral animals. Melaleuca scrub has replaced Cyperus lucidus sedgeland destroyed by goats (Capra hircus) browsing from 1914 until their extermination in 1955. Howea forest has a markedly unimodal age distribution with very few small individuals of the dominant palms. This is a result of the combined effects of browsing by goats on small palms, and seed predation by black rats (Rattus rattus) from the 1920s, preventing regeneration. Unless rat numbers are periodically reduced to reduce seed predation, regeneration may be insufficient to guarantee long-term survival of the forest.     

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.     

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.     

Origin and evolution of the grazing guild in new world terrestrial mammals

Although vertebrate herbivory has existed on land for about 300 million years, the grazingadaptation, principally developed in mammals, did not appear until the middle Cenozoic about 30 million years ago. Paleontological evidence indicates that grazing mammals diversified at the time of the spread of grasslands. Recently revised fossil calibrations reveal that the grazing mammal guild originated during the early Miocene in South America about 10-15 million years earlier than it did during the late Miocene in the northern hemisphere. Carbon isotopic analyses of extinct grazers' teeth reveal that this guild originated predominantly in C(3) terrestrial ecosystems. The present-day distribution of C(3) and C(4) grasslands evolved on the global ecological landscape since the late Miocene, after about 7 million years ago.     

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。     

Catastrophic shifts in vertical distributions of phytoplankton The existence of a bifurcation set

A model of phytoplankton dynamics within a water column was analyzed with special consideration on the existence of a bifurcation set in the parameter space. We considered two resources, light and a limiting nutrient, for phytoplankton growth and assumed that the water column is separated into two layers by thermal and/or density stratification. It was shown that there exists a bifurcation set in the parameter space when the growth function meets several conditions that are general for growth functions of two essential resources. Specifically, these conditions include that a less abundant of the two resources limits the growth while the effect of the other is sufficiently small. Folded structure with two stable states separated by one unstable state appears in the catastrophe manifold when parameters move to a certain direction with a certain curvature from a point in the bifurcation set. These results suggest that occurrence of discontinuous transition between two alternative vertical patterns is possible nature of phytoplankton dynamics within a stratified water column.     

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.     

Importance of recent shifts in soil thermal dynamics on growing season length, productivity, and carbon sequestration in terrestrial high-latitude ecosystems

In terrestrial high‐latitude regions, observations indicate recent changes in snow cover, permafrost, and soil freeze–thaw transitions due to climate change. These modifications may result in temporal shifts in the growing season and the associated rates of terrestrial productivity. Changes in productivity will influence the ability of these ecosystems to sequester atmospheric CO₂. We use the terrestrial ecosystem model (TEM), which simulates the soil thermal regime, in addition to terrestrial carbon (C), nitrogen and water dynamics, to explore these issues over the years 1960–2100 in extratropical regions (30–90°N). Our model simulations show decreases in snow cover and permafrost stability from 1960 to 2100. Decreases in snow cover agree well with National Oceanic and Atmospheric Administration satellite observations collected between the years 1972 and 2000, with Pearson rank correlation coefficients between 0.58 and 0.65. Model analyses also indicate a trend towards an earlier thaw date of frozen soils and the onset of the growing season in the spring by approximately 2–4 days from 1988 to 2000. Between 1988 and 2000, satellite records yield a slightly stronger trend in thaw and the onset of the growing season, averaging between 5 and 8 days earlier. In both, the TEM simulations and satellite records, trends in day of freeze in the autumn are weaker, such that overall increases in growing season length are due primarily to earlier thaw. Although regions with the longest snow cover duration displayed the greatest increase in growing season length, these regions maintained smaller increases in productivity and heterotrophic respiration than those regions with shorter duration of snow cover and less of an increase in growing season length. Concurrent with increases in growing season length, we found a reduction in soil C and increases in vegetation C, with greatest losses of soil C occurring in those areas with more vegetation, but simulations also suggest that this trend could reverse in the future. Our results reveal noteworthy changes in snow, permafrost, growing season length, productivity, and net C uptake, indicating that prediction of terrestrial C dynamics from one decade to the next will require that large‐scale models adequately take into account the corresponding changes in soil thermal regimes.