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1. Seasonally dry tropical forests (SDTFs) are one of the most threatened forests world-wide. These species-rich forests not only cope with several acute (e.g. forest loss) and chronic (e.g. overgrazing and firewood extraction) human disturbances but also with climate change (e.g. longer and more severe droughts); yet, the isolated and combined effects of climate and acute and chronic human disturbances on SDTF vegetation are poorly known. 2. Given the environmental filter imposed by drought in SDTFs, the composition and structure of vegetation is expected to be strongly associated with annual precipitation, and thus the effects of human disturbances on vegetation may also depend on precipitation (i.e. interacting effect). 3. We tested these hypotheses in the Brazilian Caatinga – a SDTF threatened by climate change and human disturbances. We evaluated the isolated and combined (both additive and multiplicative) effect of precipitation, a chronic disturbance index and acute disturbance (landscape forest cover) on the diversity, stem density, evenness, taxonomic composition and above-ground biomass of adult trees and shrubs across 19 0·1-ha plots distributed along a disturbance and precipitation gradients. 4. We recorded 5541 stems from 129 species. Precipitation showed a stronger (positive) effect on species diversity than acute and chronic disturbances and, as expected, the effect of disturbance depended on precipitation (interacting effect): that is, species diversity (especially the number of rare species) was negatively related to forest loss but positively related to chronic disturbance in wetter sites, whereas in drier sites, species diversity was weakly related to forest cover, but strongly and negatively related to chronic disturbance. Contrary to species diversity, community evenness, stem density and biomass were weakly related to all predictors. 5. Synthesis. Precipitation appears to be a strong environmental filter determining the distribution of water-demanding plant species. Chronic disturbance in wetter (high-productive) forests may favour species diversity by increasing ecosystem heterogeneity (intermediate disturbance hypothesis). Yet, the biodiversity costs of chronic disturbance are higher in drier (low-productive) forests; that is, there is a co-limitation imposed by drought and disturbance in drier forests. Overall, our findings indicate that rapid climatic changes in the region will probably have strong negative effects on this seasonally dry tropical forest.

Climate change is progressively redistributing species towards the Earth’s poles, indicating widespread potential for ecosystem collapse. Detecting early-warning-signals and enacting adaptation measures is therefore a key imperative for humanity. However, detecting early-warning signals has remained elusive and has focused on exceptionally high-frequency and/ or long-term time-series, which are generally unattainable for most ecosystems that are under-sampled and already impacted by warming. Here, we show that a catastrophic phase-shift in kelp ecosystems, caused by range-extension of an overgrazing sea urchin, also propagates poleward. Critically, we show that incipient spatial-pattern-formations of kelp overgrazing are detectable well-in-advance of collapse along temperate reefs in the ocean warming hotspot of south-eastern Australia. Demonstrating poleward progression of collapse over 15 years, these early-warning ‘incipient barrens’ are now widespread along 500 km of coast with projections indicating that half of all kelp beds within this range-extension region will collapse by ~2030. Overgrazing was positively associated with deep boulder-reefs, yet negatively associated with predatory lobsters and subordinate abalone competitors, which have both been intensively fished. Climate-driven collapse of ecosystems is occurring; however, by looking equatorward, space-for-time substitutions can enable practical detection of early-warning spatial-pattern-formations, allowing local climate adaptation measures to be enacted in advance. Climate change is redistributing species poleward, threatening widespread socio-ecological disruption as key tipping-points are exceeded. This study examines space-time dynamics of kelp ecosystem collapse over a 15-year period along the warming coastline of eastern Tasmania and shows that an early-warning signal of kelp ecosystem collapse is recognisable well-in-advance.

BackgroundGrasslands are a major part of the global ecosystem, covering 37 % of the earth's terrestrial area. For a variety of reasons, mostly related to overgrazing and the resulting problems of soil erosion and weed encroachment, many of the world's natural grasslands are in poor condition and showing signs of degradation. This review examines their contribution to global food supply and to combating climate change.ScopeGrasslands make a significant contribution to food security through providing part of the feed requirements of ruminants used for meat and milk production. Globally, this is more important in food energy terms than pig meat and poultry meat. Grasslands are considered to have the potential to play a key role in greenhouse gas mitigation, particularly in terms of global carbon storage and further carbon sequestration. It is estimated that grazing land management and pasture improvement (e.g. through managing grazing intensity, improved productivity, etc) have a global technical mitigation potential of almost 1·5 Gt CO2 equivalent in 2030, with additional mitigation possible from restoration of degraded lands. Milk and meat production from grassland systems in temperate regions has similar emissions of carbon dioxide per kilogram of product as mixed farming systems in temperate regions, and, if carbon sinks in grasslands are taken into account, grassland-based production systems can be as efficient as high-input systems from a greenhouse gas perspective.ConclusionsGrasslands are important for global food supply, contributing to ruminant milk and meat production. Extra food will need to come from the world's existing agricultural land base (including grasslands) as the total area of agricultural land has remained static since 1991. Ruminants are efficient converters of grass into humanly edible energy and protein and grassland-based food production can produce food with a comparable carbon footprint as mixed systems. Grasslands are a very important store of carbon, and they are continuing to sequester carbon with considerable potential to increase this further. Grassland adaptation to climate change will be variable, with possible increases or decreases in productivity and increases or decreases in soil carbon stores.

Aim Soil and vegetation degradation are among the most serious consequences of overgrazed grasslands. Changes in vegetation may affect soil seed bank size and composition. We evaluated the contribution of the seed bank to recovery of the degraded grasslands. Methods The vegetation and seed bank of a degraded grassland and in a good condition grassland of Patagonia were sampled for analysis of species richness and composition, vegetation cover, and seed density, focusing in all cases on functional groups. We also studied the influence of seasonal weather conditions (precipitation and temperature) on the vegetation and seed bank of the degraded grassland over 2 years. Results Vegetation cover and seed bank size were notably lower in the degraded grassland. Perennial grasses had been replaced by shrubs in the vegetation. Annual herb seed density was higher in the grassland in good condition than in the degraded grassland. Pappostipa speciosa were the only perennial grass seeds present in the seed banks of both grasslands. Species composition similarity of the seed banks of the two grasslands was high. Differences between years in the richness, vegetation cover and seed bank size of functional groups in the degraded grassland were related to weather conditions. Conclusions Recovery of the matrix species can improve the entire system through regeneration of microsites and an increase in soil fertility. To reduce overgrazing and halt soil degradation we suggest a decrease in the livestock stocking rate, allowing recovery of the vegetation from the soil seed bank, which represents a legacy from the previous vegetation.

1. In addition to acute transformations of ecosystems caused by deforestation, old-growth forests world-wide are being increasingly altered by low-intensity but chronic human disturbance. Overgrazing and the continuous extraction of forest products are important drivers of chronic disturbance, which can lead to the gradual local extinction of species and the alteration of vegetation structure. 2. We tested this hypothesis in the Brazilian Caatinga vegetation, one of the most speciesrich and populated semi-arid regions of the world. Using a multimodel averaging approach, we examined the impact of five recognized indicators of chronic disturbance (i.e. proximity to urban centre, houses, roads, density of people and livestock) on the diversity, abundance and evenness of 30 woody plant communities. We separately tested the response of seedlings, saplings and adults to identify the ontogenetic stages that are most susceptible to chronic disturbance. 3. We recorded over 11 000 individuals belonging to 51 plant species. As expected, most indicators of chronic disturbance were negatively related to species diversity and stem abundance, with a variable effect on community evenness. The density of people and density of livestock were the main factors driving changes in plant communities, with a stronger negative impact on seedling and sapling diversities. Species composition also varied significantly with disturbance indicators, irrespective of ontogeny. 4. Our results show the potential negative impact that chronic disturbance can have on Caatinga plant assemblages and highlight the fact that disturbance resulting from an extractivism-based and subsistence economy are probably driving old-growth forest stands towards shrub-dominated secondary stands. 5. Synthesis and applications. These findings indicate that chronic disturbance should not continue to be neglected and we argue for: (i) research and rural programmes able to support better practices in terms of land use and sustainable exploitation of forest resources, (ii) improved governance and law enforcement to shift extractivism towards sustainable standards, and (iii) expanding the coverage and effective implementation of strictly protected areas.

The legendary Mongolian Plateau has faced increasing environmental challenges associated with overgrazing, and achieving a sustainability transition for this region needs herders’ participation. However, why herders let grasslands be overgrazed even after property rights were privatized—“the tragedy of privatization”—remains unclear. We aimed to understand the causes of overgrazing in Xilingol, Inner Mongolia, and sought deep leverage points of intervention by examining livestock decision-making processes with semi-structured interviews. We found the following: (1) Herders generally recognized grassland degradation with decreased plant diversity and vegetation height. (2) Nearly half of herders were not satisfied with their current quality of life, especially in terms of income, food security, energy security, and clean water. (3) Herders prioritized economic benefits and food provisioning services of grasslands and did not think of overgrazing as an important cause for grassland degradation. (4) Herders tended to protect their own grasslands but over-exploited leased grasslands. (5) Herders tried to keep a high number of livestock without being able to anticipate climatic and economic fluctuations. (6) The government’s Forage-Livestock Balance policy was widely ignored by herders. We conclude that herders’ zeal for higher living standards, misperceptions about key drivers of grassland degradation, decoupling of herders’ income from grasslands, inability to cope with drought, and ineffective policies together constitute the underlying causes for overgrazing. Future grassland policies should focus more on the deep leverage points of intervention including reducing poverty and economic inequality, improving the grassland property system, reconnecting the long-term health of leased grasslands to herders’ livelihoods, and developing holistic livestock management strategies that integrate science with herders’ traditional ecological knowledge.

1. Overgrazing has resulted in widespread decline in biodiversity and ecosystem functioning in grasslands world-wide in recent decades. However, few studies have examined the patterns and thresholds of grazing-induced changes in community structure and ecosystem functioning along a grazing gradient and based on species-level responses and plant functional traits. 2. To identify the thresholds of grazing intensity (GI) at both species and community levels, we conducted a grazing manipulation experiment with seven levels of GI (0-9 sheep ha⁻¹) and two topographies (flat vs. slope) in a typical steppe. Four plant functional traits were measured, including specific leaf area (SLA), plant height, leaf nitrogen content (LNC) and stem: leaf ratio. 3. The threshold of GI that significantly altered community composition was at 3-75 sheep ha⁻¹ for the flat system and 3.0 sheep ha⁻¹ for the slope system. For both flat and slope systems, the threshold GI for changes in primary productivity was at 3.0 sheep ha⁻¹, beyond which the productivity decreased substantially. 4. At the species level, the abundances of common species, most of which are perennial grasses, declined at moderate grazing intensities (3.0-4.5 sheep ha⁻¹). The abundances of most rare species, which are perennial forbs, declined at low grazing intensities (1.5-3.0 sheep ha⁻¹). SLA and LNC are good predictors of species-level responses to grazing. Low SLA and high LNC species are negatively affected by high GI, while high SLA and low LNC species are little affected by grazing. The negative effect of GI on species' abundance was greater in the slope system than in the flat system. 5. Synthesis and applications. Our results indicate that the structural and functioning thresholds of grazing intensity depend on plant traits and species composition, which is mediated by topographic location. These findings, integrating plant functional traits and threshold approaches, have important implications for determining sustainable grazing intensity in grassland management and biodiversity conservation in semi-arid regions.

To understand effects of soil microbes on soil biochemistry in alpine grassland ecosystems under environmental changes, we explored relationships between soil microbial diversity and soil total nitrogen, organic carbon, available nitrogen and phosphorus, soil microbial biomass and soil enzyme activities in alpine meadow, alpine steppe and cultivated grassland on the Qinghai-Tibetan plateau under three-year warming, enhanced precipitation and yak overgrazing. Soil total nitrogen, organic carbon and NH 4 -N were little affected by overgrazing, warming or enhanced precipitation in three types of alpine grasslands. Soil microbial biomass carbon and phosphorus along with the sucrase and phosphatase activities were generally stable under different treatments. Soil NO 3 -N, available phosphorus, urease activity and microbial biomass nitrogen were increased by overgrazing in the cultivated grassland. Soil bacterial diversity was positively correlated with, while soil fungal diversity negatively with soil microbial biomass and enzyme activities. Soil bacterial diversity was negatively correlated with, while soil fungal diversity positively with soil available nutrients. Our findings indicated soil bacteria and fungi played different roles in affecting soil nutrients and microbiological activities that might provide an important implication to understand why soil biochemistry was generally stable under environmental changes in alpine grassland ecosystems.

This paper examines the theory and supporting evidence for links between desertification, drought and dust storms with a particular focus on studies undertaken in and around the Gobi Desert. Overgrazing of rangeland by pastoralists has been the most commonly cited cause of desertification in global drylands for more than 30 years, but the evidence supporting this link is not always convincing. Nonetheless, overgrazing, desertification and dust storms are frequently connected, regardless. Drought is another well-known and important driver of vegetation cover change. Distinguishing between vegetation cover adversely affected by drought and that reduced by grazing is imperative for policy makers because identifying the incorrect driver of vegetation change risks the development of inappropriate policy.

The Leymus chinensis grassland is one of the most widely distributed associations in the warm temperate grassland and due to overgrazing in recent years, it has experienced varying degrees of degradation. Vegetative regeneration via bud banks serves as the primary way of vegetation reproduction in the L. chinensis grassland ecosystem. However, the role of the bud bank in the vegetation regeneration of grazing grassland remains unclear. Based on the relationship between the under-ground bud bank and above-ground vegetation of L. chinensis grassland under different grazing stages, this study aimed to explore whether the grazing grassland could self-recover through the existing bud bank. The findings revealed that the bud density initially increased and then decreased with increasing grazing intensity, indicating that appropriate grazing promoted vegetation renewal. Moreover, grazing significantly influenced the composition of the bud bank: during the early grazing stage, the rhizome buds accounted for the main part, and tiller buds dominated during the mid-stage grazing; while during the late-stage grazing, root-sprouting buds prevailed. The meristem restriction index for light, moderate, and heavy grazing grasslands was close to one; conversely, overgrazing and extreme overgrazing grasslands exhibited the higher meristem restriction index (2.00, 3.19), suggesting that plant regeneration was constrained by bud banks under light-grazing conditions where regenerate rates failed to meet above-ground modular’s recovery requirements following overgrazing and extreme overgrazing events. Consequently, moderate grazing grasslands could achieve natural community recovery by continuously adjusting their vegetative regeneration strategies. Understanding the role of bud banks in vegetative regeneration in grazing grassland will not only supply theoretical support for the ecological succession process of degraded grassland but also provide practical experience for the sustainable management of the L. chinensis grassland ecosystem.