Multi‐decadal stability of woody cover in a mesic eucalypt savanna in the Australian monsoon tropics

Previous analyses of historical aerial photography and satellite imagery have shown thickening of woody cover in Australian tropical savannas, despite increasing fire frequency. The thickening has been attributed to increasing precipitation and atmospheric CO₂ enrichment. These analyses involved labour‐intensive, manual classification of vegetation, and hence were limited in the extent of the areas and the number of measurement times used. Object‐based, semi‐automated classification of historical sequences of aerial photography and satellite imagery has enabled the spatio‐temporal analysis of woody cover over entire landscapes, thus facilitating measurement, monitoring and attribution of drivers of change. Using this approach, we investigated woody cover change in 4000 ha of intact mesic savanna in the Ranger uranium lease and surrounding Kakadu National Park, using imagery acquired on 10 occasions between 1950 and 2016. Unlike previous studies, we detected no overall trend in woody cover through time. Some variation in cover was related to rainfall in the previous 12 months, and there were weak effects of fire in the year of image acquisition and the antecedent 4 years. Our local‐scale study showed a mesic eucalypt savanna in northern Australia has been resilient to short‐term variation in rainfall and fire activity; however, changes in canopy cover could have occurred in other settings. When applying this semi‐automated approach to similar studies of savanna dynamics, we recommend maximising the time depth and number of measurement years, standardising the time of year for image acquisition and using many plots of 1 ha in area, rather than fewer, larger plots.     

Land use,rangeland degradation and ecological changes in the southern Kalahari,Botswana

Dual‐scale analyses assessing farm‐scale patterns of ecological change and landscape‐scale patterns of change in vegetation cover and animal distribution are presented from ecological transect studies away from waterpoints, regional remotely sensed analysis of vegetation cover and animal numbers across the southern Kalahari, Botswana. Bush encroachment is prevalent in semi‐arid sites where Acacia mellifera Benth. is widespread in communal areas and private ranches, showing that land tenure changes over the last 40 years have not avoided rangeland degradation. Herbaceous cover is dominated in intensively grazed areas by the annual grass Schmidtia kalahariensis Stent and in moderately grazed areas by the perennial grass Eragrostis lehmanniana Nees. Nutritious perennial grass species including Eragrostis pallens Hack. Ex Schinz remain prevalent in Wildlife Management Areas. Other ecological changes include the invasion of the exotic Prosopis glandulosa Torr. and dense stands of Rhigozum trichotomum Kuntze. in the arid southwest. Regional patterns of wildlife species show that the expansion of cattleposts and fenced ranches has led to large areas of low wildlife conservation value even in areas where cattle production is not practiced. Findings show the need for integrated landscape‐scale planning of land use if the ecological value and biodiversity of the southern Kalahari is to be retained.     

Variable rainfall has a greater effect than fire on the demography of the dominant tree in a semi‐arid Eucalyptus savanna

Rainfall, fire and competition are emphasized as determinants of the density and basal area of woody vegetation in savanna. The semi‐arid savannas of Australia have substantial multi‐year rainfall deficits and insufficient grass fuel to carry annual fire in contrast to the mesic savannas in more northern regions. This study investigates the influence of rainfall deficit and excess, fire and woody competition on the population dynamics of a dominant tree in a semi‐arid savanna. All individuals of Eucalyptus melanophloia were mapped and monitored in three, 1‐ha plots over an 8.5 year period encompassing wet and dry periods. The plots were unburnt, burnt once and burnt twice. A competition index incorporating the size and distance of neighbours to target individuals was determined. Supplementary studies examined seedling recruitment and the transition of juvenile trees into the sapling layer. Mortality of burnt seedlings was related to lignotuber area but the majority of seedlings are fire resistant within 12 months of germination. Most of the juveniles (≤1 cm dbh) of E. melanophloia either died in the dry period or persisted as juveniles throughout 8.5 years of monitoring. Mortality of juveniles was positively related to woody competition and was higher in the dry period than the wet period. The transition of juveniles to a larger size class occurred at extremely low rates, and a subsidiary study along a clearing boundary suggests release from woody competition allows transition into the sapling layer. From three fires the highest proportion of saplings (1–10 cm dbh) reduced to juveniles was only 5.6% suggesting rates of ‘top‐kill’ of E. melanophloia as a result of fire are relatively low. Girth growth was enhanced in wet years, particularly for larger trees (>10 cm dbh), but all trees regardless of size or woody competition levels are vulnerable to drought‐induced mortality. Overall the results suggest that variations in rainfall, especially drought‐induced mortality, have a much stronger influence on the tree demographics of E. melanophloia in a semi‐arid savanna of north‐eastern Australia than fire.     

Comparison of biodiversity and ground cover between a commercial rotationally grazed property and an adjacent nature reserve in semi‐arid rangeland

Continuous livestock grazing can have negative effects on biodiversity and landscape function in arid and semi‐arid rangelands. Alternative grazing management practices, such as rotational grazing, may be a viable option for broad‐scale biodiversity conservation and sustainable pastoral management. This study compared ground cover, plant species composition and floristic and functional diversity along gradients of grazing intensity between a pastoral property rotationally grazed by goats and an adjacent nature reserve (ungrazed by commercial livestock) in semi‐arid south‐eastern Australia. Understorey plant species composition differed significantly between the rotationally grazed property and the nature reserve, with a greater proportion and frequency of palatable species recorded in the nature reserve. Understorey plant species richness, diversity, functional biodiversity measures and ground cover declined with increasing grazing pressure close to water points under commercial rotational grazing management. However, at a whole‐paddock scale, there were few differences in plant biodiversity and ground cover between the rotationally grazed property and the nature reserve, despite differences in overall plant species composition. Flexible, adaptive, rotational grazing should be investigated further for its potential to achieve both socio‐economic and biodiversity conservation outcomes in semi‐arid rangelands to complement existing conservation reserves.     

Impacts of a mineral lick‐centred land use system on woody vegetation cover in an East African Savannah

Evaluation of woody vegetation changes with distance from a salt crater was conducted in the semi‐arid rangelands of southern Ethiopia. Data on live woody plants were collected over three seasons at 0, 1, 4, 6, 9 and 12 km from the salt crater. The density and diversity of woody plants differed significantly (p < .01) along the distance gradient. Six woody plant families were identified of which Fabaceae and Burseraceae were the dominant families. Acacia drepanolobium, Acacia nilotica, Commiphora africana and Acacia mellifera were among the severely encroaching woody species. There were high proportions of seedlings and saplings recorded closer to the salt crater showing a vigorous recruitment by woody plants. Woody plant encroachment along the 12‐km transect ranged from a low to severe encroachment, which could be translated into poor rangeland condition. Changes in soil characteristics increased grazing pressure and sedentary settlement around the salt crater, and the breakdown of traditional institutions seems to be major contributing factors to these vegetation changes. We suggest that severely encroached areas could be improved through a combination of methods such as bush clearing, prescribed fire, browsing animals and proper grazing management.     

Groundwater drawdown drives ecophysiological adjustments of woody vegetation in a semi‐arid coastal ecosystem

Predicted droughts and anthropogenic water use will increase groundwater lowering rates and intensify groundwater limitation, particularly for Mediterranean semi‐arid ecosystems. These hydrological changes may be expected to elicit differential functional responses of vegetation either belowground or aboveground. Yet, our ability to predict the impacts of groundwater changes on these ecosystems is still poor. Thus, we sought to better understand the impact of falling water table on the physiology of woody vegetation. We specifically ask (a) how is woody vegetation ecophysiological performance affected by water table depth during the dry season? and (b) does the vegetation response to increasing depth to groundwater differ among water‐use functional types? We examined a suite of physiological parameters and water‐uptake depths of the dominant, functionally distinct woody vegetation along a water‐table depth gradient in a Mediterranean semi‐arid coastal ecosystem that is currently experiencing anthropogenic groundwater extraction pressure. We found that groundwater drawdown did negatively affect the ecophysiological performance of the woody vegetation. Across all studied environmental factors, depth to groundwater was the most important driver of ecophysiological adjustments. Plant functional types, independent of groundwater dependence, showed consistent declines in water content and generally reduced C and N acquisition with increasing depths to groundwater. Functional types showed distinct operating physiological ranges, but common physiological sensitivity to greater water table depth. Thus, although differences in water‐source use exist, a physiological convergence appeared to happen among different functional types. These results strongly suggest that hydrological drought has an important impact on fundamental physiological processes, constraining the performance of woody vegetation under semi‐arid conditions. By disentangling the functional responses and vulnerability of woody vegetation to groundwater limitation, our study establishes the basis for predicting the physiological responses of woody vegetation in semi‐arid coastal ecosystems to groundwater drawdown.     

Post-pastoral changes in composition and guilds in a semi-arid conservation area, Central Otago, New Zealand

Changes in the vegetation of Flat Top Hill, a highly modified conservation area in semi;arid Central Otago, New Zealand, are described four years after the cessation of sheep and rabbit grazing. Unusually moist weather conditions coincide with the four-year period of change in response to the cessation of grazing. Between 1993 and 1997, the average richness and diversity (H') of species increased, and the average proportion of native species decreased significantly. The vegetation was significantly richer in exotic annual and perennial grass species, exotic perennial forbs, exotic woody species and native tussock grasses in 1997 than in 1993. Eight response guilds of species are identified. Most "remnant" native shrubs and forbs were stable, in that they remained restricted to local refugia and showed little change in local frequency. However, taller native grass species increased, some locally, and others over wide environmental ranges. Rare native annual forbs and several native perennial species from "induced" xeric communities decreased, and this may be a consequence of competition from exotic perennial grasses in the absence of grazing. The invasive exotic herb Sedum acre decreased in abundance between 1993 and 1997, but several other prominent exotic species increased substantially in range and local frequency over a wide range of sites. Exotic woody species, and dense, sward-forming grasses are identified as potential threats to native vegetation recovery.     

Thresholds of potential concern as benchmarks in the management of African savannahs

In the Kruger National Park (KNP), South Africa, ecosystem managers use a series of monitoring endpoints, known as thresholds of potential concern (TPCs), to define the upper and the lower levels of accepted variation in ecosystems. For woody vegetation, the current TPC suggests that woody cover should not drop by more than 80% of its 'highest ever' value. In this paper, we explore the utility of palaeoecological data in informing TPCs. We use calibrated fossil pollen data to explore variability in vegetation at two sites over the past 5000 years, to provide a long-term record of changes in woody vegetation cover and a context for interpreting more recent vegetation change. The fossil pollen data are calibrated using studies of modern pollen and vegetation from KNP; arboreal pollen percentage was simulated using pollen-landscape modelling software for savannah landscapes of varying woody vegetation cover, and the relationship between vegetation and pollen data was quantified using nonlinear regression. This quadratic equation was then applied to fossil pollen data in order to estimate woody vegetation cover from arboreal pollen percentages. Our results suggest that the TPCs have not been exceeded during the period represented in the pollen record, because estimated woody vegetation cover has remained above 20% of its highest ever value. By comparing the fossil pollen data with TPCs, our study demonstrates how palaeoecological data can be presented in a form that is directly relevant to management objectives.     

Desertification,no‐change or alternative states: Can we trust simple models on livestock impact in dry rangelands?

Abstract. It is remarkable that after many thousands of years of pastoralism in arid and semi‐arid savannas, there is still no clear answer to the basic question: Does livestock grazing have more than a marginal effect on the dynamics of arid and semi‐arid land vegetation? A small study of semi‐arid savanna vegetation along a spatial gradient in grazing pressure, repeated three times over 19 years is used as a basis for discussing the behaviour of dry land vegetation under heavy grazing. Three basic theories are compared: (1) the theory that heavy grazing causes desertification; (2) the theory that heavy grazing causes no directed change in the vegetation of arid and semi‐arid rangelands; and (3) the theory that heavy grazing leads to a switch between alternative states of vegetation. On the basis of the current data the first two theories are rejected, but the conclusion is that there is much evidence for a more complex behaviour of dry rangelands under grazing than what is accommodated in any of the three theories. Probably, site‐specific properties including interactive and indirect effects of herbivory in the system are important for vegetation development under grazing in arid and semi‐arid rangelands.     

Retrospective monitoring of rangeland vegetation change: ecohistory from deposits of sheep dung associated with shearing sheds

This paper explores the potential of a new method of reconstructing historical vegetation change in the Australian rangelands. Historical monitoring of rangeland vegetation has been so deficient that it is not possible to determine whether a long‐term trend toward degradation has occurred (as is often assumed) or, indeed, if it is continuing to occur. Because long‐term records are unavailable any attempt to monitor vegetation retrospectively must be based on proxy measures rather than direct observation. Where historical data are lacking an integration of palaeoecological, archaeological and ecological methods is required to reconstruct the past. Our research is based on a detailed analysis of sheep faeces deposited near a shearing shed in the semiarid rangelands of south‐west Queensland between the late 1930s and the mid‐1990s. The faeces in these deposits represent the diet of sheep in the days leading up to the property’s annual shearing and as such are a potentially useful index to changes in vegetation. Results indicate significant changes in the diet of sheep since the late 1940s. The potential of this method, and its limitations, are discussed. Long‐term records are critical in understanding issues of sustainability in land management and it is intended that this paper will stimulate further research into historical vegetation change in rangelands.     

Low‐cost grass restoration using erosion barriers in a degraded African rangeland

Rangeland degradation, typified by extensive bare ground and soil erosion, is a serious problem around the world. In sub‐Saharan Africa, rangeland degradation threatens the food security of millions of people who depend on livestock and the region's large mammalian wildlife diversity. We tested the ability of five simple, low‐cost erosion barriers to promote grass and forb establishment in a bare ground‐dominated rangeland in Kenya. These treatments were: (1) trenches with small berms; (2) bundles of branches; and bundles of branches with (3) elephant dung balls, (4) burlap sacking, or (5) nylon mesh sacking inside them. We also tested whether barrier performance depended on (1) supplemental seeding with the grass Cenchrus ciliaris and (2) whether a barrier was located next to existing vegetation patches versus in the open. Within months, the trench and nylon mesh barriers had accumulated 20–50% more sediment than other treatments and had greater grass and forb seedling establishment. Seeding with Cenchrus resulted in higher herbaceous cover but was not necessary for other grasses to establish. After 3 years, the trench and nylon mesh barriers had created patches of new vegetation averaging 18–63% larger than patches created by the other treatments. Barriers that were initially adjacent to existing vegetation had created new vegetation patches averaging 65% larger than those created by solitary barriers. Results suggest that all barrier types increase grass cover but that trenches—especially if placed next to existing vegetation patches—are a particularly cost‐effective way to reduce bare ground and erosion in degraded rangelands.     

Influence of fire history on small mammal distributions: insights from a 100‐year post‐fire chronosequence

Aim Fire affects the structure and dynamics of ecosystems world‐wide, over long time periods (decades and centuries) and at large spatial scales (landscapes and regions). A pressing challenge for ecologists is to develop models that explain and predict faunal responses to fire at broad temporal and spatial scales. We used a 105‐year post‐fire chronosequence to investigate small mammal responses to fire across an extensive area of ‘tree mallee’ (i.e. vegetation characterized by small multi‐stemmed eucalypts). Location The Murray Mallee region (104,000 km²) of semi‐arid Australia. Methods First, we surveyed small mammals at 260 sites and explored the fire responses of four species using nonlinear regression models. Second, we assessed the predictive accuracy of models using cross‐validation and by testing with independent data. Third, we examined our results in relation to an influential model of animal succession, the habitat accommodation model. Results Two of four study species showed a clear response to fire history. The distribution of the Mallee Ningaui Ningaui yvonneae, a carnivorous marsupial, was strongly associated with mature vegetation characterized by its cover of hummock grass. The occurrence of breeding females was predicted to increase up to 40–105 years post‐fire, highlighting the extensive time periods over which small mammal populations may be affected by fire. Evaluation of models for N. yvonneae demonstrated that accurate predictions of species occurrence can be made from fire history and vegetation data, across large geographical areas. The introduced House Mouse Mus domesticus was the only species positively associated with recently burnt vegetation. Main conclusions Understanding the impact of fire over long time periods will benefit ecological and conservation management. In this example, tracts of long‐unburnt mallee vegetation were identified as important habitat for a fire‐sensitive native mammal. Small mammal responses to fire can be predicted accurately at broad spatial scales; however, a conceptual model of post‐fire change in community structure developed in temperate Australia is not, on its own, sufficient for small mammals in semi‐arid systems.     

The role of vegetation–microclimate feedback in promoting shrub encroachment in the northern Chihuahuan desert

Many arid and semi‐arid landscapes around the world are affected by a shift from grassland to shrubland vegetation, presumably induced by climate warming, increasing atmospheric CO₂ concentrations, and/or changing land use. This major change in vegetation cover is likely sustained by positive feedbacks with the physical environment. Recent research has focused on a feedback with microclimate, whereby cold intolerant shrubs increase the minimum nocturnal temperatures in their surroundings. Despite the rich literature on the impact of land cover change on local climate conditions, changes in microclimate resulting from shrub expansion into desert grasslands have remained poorly investigated. It is unclear to what extent such a feedback can affect the maximum extent of shrub expansion and the configuration of a stable encroachment front. Here, we focus on the case of the northern Chihuahuan desert, where creosotebush (Larrea tridentata) has been replacing grasslands over the past 100–150 years. We use a process‐based coupled atmosphere‐vegetation model to investigate the role of this feedback in sustaining shrub encroachment in the region. Simulations indicate that the feedback allows juvenile shrubs to establish in the grassland during average years and, once established, reduce their vulnerability to freeze‐induced mortality by creating a warmer microclimate. Such a feedback is crucial in extreme cold winters as it may reduce shrub mortality. We identify the existence of a critical zone in the surroundings of the encroachment front, in which vegetation dynamics are bistable: in this zone, vegetation can be stable both as grassland and as shrubland. The existence of these alternative stable states explains why in most cases the shift from grass to shrub cover is found to be abrupt and often difficult to revert.     

Passive Recovery of Vegetation after Herbivore Eradication on Santa Cruz Island,California

Understanding how insular ecosystems recover or are restructured after the eradication of an invasive species is crucial in evaluating conservation success and prioritizing island conservation efforts. Globally, herbivores have been removed from 762 islands, most with limited active restoration actions following eradication. Few studies have documented the effects of invasive herbivore removal after multiple decades of passive recovery. Here we evaluate recovery of vegetation on Santa Cruz Island, California, after the removal of feral sheep (Ovis aries) in 1984. We repeat a study conducted in 1980, and examine vegetation changes 28 years after the eradication. Before eradication, grazed areas were characterized by reduced plant cover, high exposure of bare ground, and erosion. After 28 years of passive recovery, transect data showed a 23% increase in woody overstory, whereas analysis of photographs from landscapes photographed pre‐ and post‐eradication showed a 26% increase in woody vegetation. Whole island vegetation maps similarly showed a transition from grass/bare ground (74.3% of cover) to woody plants (77.2% of cover), indicating the transition away from predominantly exotic annual grassland toward a community similar to the overstory of coastal scrubland but with an understory dominated by non‐native annual grasses. We estimate that replacement of grasses/bare ground by native woody vegetation has led to 70 and 17% increases in the stored carbon and nitrogen pools on the island, respectively. Our results demonstrate that these island ecosystems can experience significant recovery of native floral communities without intensive post‐eradication restoration, and results of recovery may take decades to be realized.     

Long‐term changes in semi‐arid vegetation: Invasion of an exotic perennial grass has larger effects than rainfall variability

Abstract. Questions: This paper examines the long‐term change in the herbaceous layer of semi‐arid vegetation since grazing ceased. We asked whether (1) there were differences in the temporal trends of abundance among growth forms of plants; (2) season of rainfall affected the growth form response; (3) the presence of an invasive species influenced the abundance and species richness of native plants relative to non‐invaded plots, and (4) abundance of native plants and/or species richness was related to the time it took for an invasive species to invade a plot. Location: Alice Springs, Central Australia. Methods: Long‐term changes in the semi‐arid vegetation of Central Australia were measured over 28 years (1976–2004) to partition the effects of rainfall and an invasive perennial grass. The relative abundance (biomass) of all species was assessed 25 times in each of 24 plots (8 m × 1 m) across two sites that traversed floodplains and adjacent foot slopes. Photo‐points, starting in 1972, were also used to provide a broader overview of a landscape that had been intensively grazed by cattle and rabbits prior to the 1970s. Species’abundance data were amalgamated into growth forms to examine their relationship with environmental variation in space and time. Environmental variables included season and amount of rainfall, fire history, soil variability and the colonization of the plots by the exotic perennial grass Cenchrus ciliaris (Buffel grass). Results: Constrained ordination showed that season of rainfall and landscape variables relating to soil depth strongly influenced vegetation composition when Cenchrus was used as a covariate. When Cenchrus was included in constrained ordination, it was strongly related to the decline of all native growth forms over time. Univariate comparisons of non‐invaded vs impacted plots over time revealed unequivocal evidence that Cenchrus had caused the decline of all native growth form groups and species richness. They also revealed a contrasting response of native plants to season of rainfall, with a strong response of native grasses to summer rainfall and forbs to winter rainfall. In the presence of Cenchrus these responses were strongly attenuated. Discussion: Pronounced changes in the composition of vegetation were interpreted as a response to removal of grazing pressure, fluctuations in rainfall and, most importantly, invasion of an exotic grass. Declines in herbaceous species abundance and richness in the presence of Cenchrus appear to be directly related to competition for resources. Indirect effects may also be causing the declines of some woody species from changed fire regimes as a result of increased fuel loads. We predict that Cenchrus will begin to alter landscape level processes as a result of the direct and indirect effects of Cenchrus on the demography of native plants when there is a switch from resource limited (rainfall) establishment of native plants to seed limited recruitment.     

Estimating woody and herbaceous vegetation cover from time series satellite observations

In this paper we test a method to estimate the tree and grass vegetation cover over Australia from satellite-derived normalized difference vegetation index (NDVI) time series (monthly 1981–91, ≈5 km pixels) observations. The evergreen cover is assumed to track along the base of the NDVI time series, which is assumed to be equivalent to the woody vegetation cover. The base of the NDVI time series is estimated using modifications to a classical econometric model (i.e. time series is the sum of trend, seasonal and random components). Estimates of the average evergreen component during 1982–85 and 1986–89 were generally consistent with known vegetation distributions. Changes in evergreen cover were largely restricted to the south-west and south-east of Australia. Those changes were largely the result of differences in rainfall between the two periods. The proposed method for estimating woody vegetation cover is found to be generally robust. However, there are some regions where the grass (or pasture) is mostly evergreen. Some possible refinements are proposed to handle such cases.     

Net changes in regional woody vegetation cover and carbon storage in Texas Drylands, 1937–1999

Although local increases in woody plant cover have been documented in arid and semiarid ecosystems worldwide, there have been few long‐term, large‐scale analyses of changes in woody plant cover and aboveground carbon (C) stocks. We used historical aerial photography, contemporary Landsat satellite data, field observations, and image analysis techniques to assess spatially specific changes in woody vegetation cover and aboveground C stocks between 1937 and 1999 in a 400‐km² region of northern Texas, USA. Changes in land cover were then related to topo‐edaphic setting and historical land‐use practices. Mechanical or chemical brush management occurred over much of the region in the 1940–1950s. Rangelands not targeted for brush management experienced woody cover increases of up to 500% in 63 years. Areas managed with herbicides, mechanical treatments or fire exhibited a wide range of woody cover changes relative to 1937 (?75% to + 280%), depending on soil type and time since last management action. At the integrated regional scale, there was a net 30% increase in woody plant cover over the 63‐year period. Regional increases were greatest in riparian corridors (33%) and shallow clay uplands (26%) and least on upland clay loams (15%). Allometric relationships between canopy cover and aboveground biomass were used to estimate net aboveground C storage changes in upland (nonriparian) portions of regional landscapes. Carbon stocks increased from 380 g C m^?2 in 1937 to 500 g C m^?2 in 1999, a 32% net increase across the 400 km² region over the 63‐year period. These plant C storage change estimates are highly conservative in that they did not include the substantial increases in woody plant cover observed within riparian landscape elements. Results are discussed in terms of implications for ‘carbon accounting’ and the global C cycle.     

Regional productivity mediates the effects of grazing disturbance on plant cover and patch‐size distribution in arid and semi‐arid communities

Patch‐size distribution and plant cover are strongly associated to arid ecosystem functioning and may be a warning signal for the onset of desertification under changes in disturbance regimes. However, the interaction between regional productivity level and human‐induced disturbance regime as drivers for vegetation structure and dynamics remain poorly studied. We studied grazing disturbance effects on plant cover and patchiness in three plant communities located along a regional productivity gradient in Patagonia (Argentina): a semi‐desert (low‐productivity community), a shrub‐grass steppe (intermediate‐productivity community) and a grass steppe (high‐productivity community). We sampled paddocks with different sheep grazing pressure (continuous disturbance gradients) in all three communities. In each paddock, the presence or absence of perennial vegetation was recorded every 10 cm along a 50 m transect. Grazing effects on vegetation structure depended on the community and its association to the regional productivity. Grazing decreased total plant cover while increasing both the frequency of small patches and the inter‐patch distance in all communities. However, the size of these effects was the greatest in the high‐productivity community. Dominant species responses to grazing explained vegetation patch‐ and inter‐patch‐size distribution patterns. As productivity decreases, dominant species showed a higher degree of grazing resistance, probably because traits of species adapted to high aridity allow them to resist herbivore disturbance. In conclusion, our findings suggest that regional productivity mediates grazing disturbance impacts on vegetation mosaic. The changes within the same range of grazing pressure have higher effects on communities found in environments with higher productivity, markedly promoting their desertification. Understanding the complex interactions between environmental aridity and human‐induced disturbances is a key aspect for maintaining patchiness structure and dynamics, which has important implications for drylands management.     

Savanna woody encroachment is widespread across three continents

Tropical savannas are a globally extensive biome prone to rapid vegetation change in response to changing environmental conditions. Via a meta‐analysis, we quantified savanna woody vegetation change spanning the last century. We found a global trend of woody encroachment that was established prior the 1980s. However, there is critical regional variation in the magnitude of encroachment. Woody cover is increasing most rapidly in the remaining uncleared savannas of South America, most likely due to fire suppression and land fragmentation. In contrast, Australia has experienced low rates of encroachment. When accounting for land use, African savannas have a mean rate annual woody cover increase two and a half times that of Australian savannas. In Africa, encroachment occurs across multiple land uses and is accelerating over time. In Africa and Australia, rising atmospheric CO₂, changing land management and rainfall are likely causes. We argue that the functional traits of each woody flora, specifically the N‐fixing ability and architecture of woody plants, are critical to predicting encroachment over the next century and that African savannas are at high risk of widespread vegetation change.