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

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Multi‐scale patterns and bush encroachment in an arid savanna with a shallow soil layer

Abstract. Question: Bush encroachment (i.e. an increase in density of woody plants often unpalatable to domestic livestock) is a serious problem in many savannas and threatens the livelihood of many pastoralists. Can we derive a better understanding of the factors causing bush encroachment by investigating the scale dependency of patterns and processes in savannas? Location: An arid savanna in the Khomas Hochland, Namibia. Methods: Patterns of bush, grass, and soil nutrient distribution were surveyed on several scales along a rainfall gradient, with emphasis on intraspecific interactions within the dominant woody species, Acacia reficiens. Results: Savannas can be interpreted as patch‐dynamic systems where landscapes are composed of many patches (a few ha in size) in different states of transition between grassy and woody dominance. Conclusions: In arid savannas, this patchiness is driven both by rainfall that is highly variable in space and time and by inter‐tree competition. Within the paradigm of patch‐dynamic savannas, bush encroachment is part of a cyclical succession between open savanna and woody dominance. The conversion from a patch of open savanna to a bush‐encroached area is initiated by the spatial and temporal overlap of several (localized) rainfall events sufficient for Acacia germination and establishment. With time, growth and self‐thinning will transform the bush‐encroached area into a mature Acacia stand and eventually into open savanna again. Patchiness is sustained due to the local rarity (and patchiness) of rainfall sufficient for germination of woody plants as well as by plant‐soil interactions.     

Soil phosphorus does not keep pace with soil carbon and nitrogen accumulation following woody encroachment

Soil carbon, nitrogen, and phosphorus cycles are strongly interlinked and controlled through biological processes, and the phosphorus cycle is further controlled through geochemical processes. In dryland ecosystems, woody encroachment often modifies soil carbon, nitrogen, and phosphorus stores, although it remains unknown if these three elements change proportionally in response to this vegetation change. We evaluated proportional changes and spatial patterns of soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) concentrations following woody encroachment by taking spatially explicit soil cores to a depth of 1.2 m across a subtropical savanna landscape which has undergone encroachment by Prosopis glandulosa (an N₂ fixer) and other woody species during the past century in southern Texas, USA. SOC and TN were coupled with respect to increasing magnitudes and spatial patterns throughout the soil profile following woody encroachment, while TP increased slower than SOC and TN in topmost surface soils (0–5 cm) but faster in subsurface soils (15–120 cm). Spatial patterns of TP strongly resembled those of vegetation cover throughout the soil profile, but differed from those of SOC and TN, especially in subsurface soils. The encroachment of woody species dominated by N₂‐fixing trees into this P‐limited ecosystem resulted in the accumulation of proportionally less soil P compared to C and N in surface soils; however, proportionally more P accrued in deeper portions of the soil profile beneath woody patches where alkaline soil pH and high carbonate concentrations would favor precipitation of P as relatively insoluble calcium phosphates. This imbalanced relationship highlights that the relative importance of biotic vs. abiotic mechanisms controlling C and N vs. P accumulation following vegetation change may vary with depth. Our findings suggest that efforts to incorporate effects of land cover changes into coupled climate–biogeochemical models should attempt to represent C‐N‐P imbalances that may arise following vegetation change.     

Land‐use Change Dynamics,Soil Type and Species Forming Mono‐dominant Patches: the Case of Pteridium aquilinum in a Neotropical Rain Forest Region

Deforestation and agricultural land degradation in tropical regions can create conditions for growth of perennial plant species forming mono‐dominated patches (MDP). Such species might limit forest regeneration, and their proliferation forces the abandonment of fields and subsequent deforestation to establish new fields. Therefore, identifying factors fostering MDP species is critical for biodiversity conservation in human‐modified landscapes. Here, we propose a conceptual framework to identify such factors and apply it to the case of Pteridium aquilinum (bracken fern), a light‐demanding species, tolerant of low soil fertility and fire. We hypothesize that bracken proliferation is promoted by land‐use changes that increase light availability, especially in sites with low soil fertility and land uses involving fire. We assessed this idea using agricultural fields in southeastern Mexico with different land‐use change histories and quantifying prevalence and cover of bracken. Five different land‐use change histories resulted from transitions among forest, crop, pasture, and fallow field stages. Of the 133 fields sampled, 71 percent had P. aquilinum; regression tree analysis indicated that 65 percent of inter‐field variation in prevalence and 90 percent in cover was explained by land‐use change history and soil type. Maximum prevalence, cover, and rates of increase in bracken were found on fields with low fertility sandy/clay soils, which had been used for crops and pasture, were frequently burned, and had high levels of light. Fields on fertile alluvial soil never used for pasture were bracken‐free. Agriculture promoting high light environments on less fertile soils is a major cause of bracken proliferation and likely that of other MDP species.     

Stochastic losses of fire‐dependent endemic herbs revealed by a 65‐year chronosequence of dispersal‐limited woody plant encroachment

The factors responsible for maintaining diverse groundcover plant communities of high conservation value in frequently burned wet pine savannas are poorly understood. While most management involves manipulating extrinsic factors important in maintaining species diversity (e.g., fire regimes), most ecological theory (e.g., niche theory and neutral theory) examines how traits exhibited by the species promote species coexistence. Furthermore, although many ecologists focus on processes that maintain local species diversity, conservation biologists have argued that other indices (e.g., phylogenetic diversity) are better for evaluating assemblages in terms of their conservation value. I used a null model that employed beta‐diversity calculations based on Raup–Crick distances to test for deterministic herbaceous species losses associated with a 65‐year chronosequence of woody species encroachment within each of three localities. I quantified conservation value of assemblages by measuring taxonomic distinctness, endemism, and floristic quality of plots with and without woody encroachment. Reductions in herb species richness per plot attributable to woody encroachment were largely stochastic, as indicated by a lack of change in the mean or variance in beta‐diversity caused by woody encroachment in the savannas studied here. Taxonomic distinctness, endemism, and floristic quality (when summed across all species) were all greater in areas that had not experienced woody encroachment. However, when corrected for local species richness, only average endemism and floristic quality of assemblages inclusive of herbs and woody plants were greater in areas that had not experienced woody encroachment, due to the more restricted ranges and habitat requirements of herbs. Results suggest that frequent fires maintain diverse assemblages of fire‐dependent herb species endemic to the region. The stochastic loss of plant species, irrespective of their taxonomic distinctness, to woody encroachment suggests that the relevance of niche partitioning or phylogenetic diversity to the management of biodiversity in wet pine savannas is minimal.     

A patch-dynamics approach to savanna dynamics and woody plant encroachment – Insights from an arid savanna

The coexistence of woody and grassy plants in savannas has often been attributed to a rooting-niche separation (two-layer hypothesis). Water was assumed to be the limiting resource for both growth forms and grasses were assumed to extract water from the upper soil layer and trees and bushes from the lower layers. Woody plant encroachment (i.e. an increase in density of woody plants often unpalatable to domestic livestock) is a serious problem in many savannas and is believed to be the result of overgrazing in ‘two-layer systems’. Recent research has questioned the universality of both the two-layer hypothesis and the hypothesis that overgrazing is the cause of woody plant encroachment.

We present an alternative hypothesis explaining both tree–grass coexistence and woody plant encroachment in arid savannas. We propose that woody plant encroachment is part of a cyclical succession between open savanna and woody dominance and is driven by two factors: rainfall that is highly variable in space and time, and inter-tree competition. In this case, savanna landscapes are composed of many patches (a few hectares in size) in different states of transition between grassy and woody dominance, i.e. we hypothesize that arid savannas are patch-dynamic systems. We summarize patterns of tree distribution observed in an arid savanna in Namibia and show that these patterns are in agreement with the patch-dynamic savanna hypothesis. We discuss the applicability of this hypothesis to fire-dominated savannas, in which rainfall variability is low and fire drives spatial heterogeneity.

We conclude that field studies are more likely to contribute to a general understanding of tree–grass coexistence and woody plant encroachment if they consider both primary (rain and nutrients) and secondary (fire and grazing) determinants of patch properties across different savannas.     

Depth distribution and composition of seed‐banks in alien‐invaded and uninvaded fynbos vegetation

South African fynbos vegetation is threatened on a large scale by invasive woody plants. A major task facing nature conservation managers is to restore invaded areas. The aim of this study was to determine the restoration potential of fynbos following dense invasion by the Australian tree Acacia saligna. The impacts of dense invasion on seed‐bank composition and depth distribution were investigated to determine which fynbos guilds and species have the most persistent seed‐banks. Soil samples were excavated at three different depths for invaded and uninvaded vegetation at two sand plain and mountain fynbos sites. Seed‐banks were determined using the seedling emergence approach. Invasion caused a significant reduction in seed‐bank density and richness at all sites. There was a significant, but smaller, reduction in seed‐bank density and richness with soil depth at three sites. Seed‐bank composition and guild structure changed following invasion. Low persistence of long‐lived obligate seeders in sand plain fynbos seed‐banks indicates that this vegetation type will be difficult to restore from the seed‐bank alone following alien clearance. The dominance of short‐lived species, especially graminoids, forbs and ephemeral geophytes, suggests that regenerating vegetation will develop into a herbland rather than a shrubland. It is recommended that seed collecting and sowing form part of the restoration plan for densely invaded sand plain sites. As seed density remained higher towards the soil surface following invasion, there is no general advantage in applying a mechanical soil disturbance treatment. However, if the shallow soil seed‐bank becomes depleted, for example following a hot fire through dense alien slash, a soil disturbance treatment should be given to exhume the deeper viable seed‐bank and promote recruitment.     

Patch dieback of Colophospermum mopane in a dysfunctional semi‐arid African savanna

Abstract Patch dieback occurred in an almost monospecific Colophospermum mopane (Kirk ex Benth.) Kirk ex J. Léonard woodland in the Northern Province, South Africa, following severe droughts in 1988–1989 and 1991–1992. Discrete patches of dieback and adjacent paired areas of ‘healthy’ vegetation lost an average of 87 and 13% of basal area to mortality, respectively. Whole trees mostly died on ‘dead’ plots, while single‐stem mortality prevailed on ‘live’ plots. Tree mortality decreased with increasing stem number per tree. Patch dieback did not occur on sandy soils. On fine‐textured soils, variation in soil type, topography or slope did not affect dieback. Dieback was influenced by vegetation structure, soil surface condition and soil chemistry. Intense intertree competition, shown by self‐thinning occurring prior to dieback, was a precondition for dieback. Intertree competition had heightened during the 30 years prior to dieback because of an increase in woody cover. Dieback patches had changed from functioning as sinks of water and sediment to sources of these as a result of loss of perennial herbaceous cover, decreased water retention on bared surfaces, and accelerated erosion during 50 years of livestock ranching. Vegetation had thus become increasingly drought‐prone, exacerbated in places by soils with a high sodium concentration. Dieback had occurred because the water requirements of C. mopane could no longer be met during drought years on the dysfunctional patches.     

Vegetation dynamics in a Quercus‐Juniperus savanna: An isotopic assessment

Abstract. Woody plants are increasing in many grassland and savanna ecosystems around the world. As a case in point, the Edwards Plateau of Texas, USA, is a vast region (93 000 km²) in which rapid woody encroachment appears to be occurring. The native vegetation (prior to the Anglo‐European settlement 150–200 yr ago) and the biogeochemical consequences of woody encroachment in this region, however, are poorly understood. To assess these matters we measured plant and soil δ¹³C, soil organic C and soil N content from grasslands and two important woody patch types (mature Quercus virginiana clusters and Juniperus ashei woodlands) in this region. Soil δ¹³C values showed that relative productivity of C₃ species has increased in grassland and both woody habitats in recent times. δ¹³C of SOC in grasslands and Q. virginiana clusters increased with depth from the litter layer to 30 cm (grasslands =?21 to ?13‰Q. virginiana clusters =?27 to ?17‰) and were significantly different between habitats at all depths, indicating that Q. virginiana has been a long‐term component of the landscape. In J. ashei woodlands, soil δ¹³C values (at 20–30 cm depth) near the woodland edge (‐13‰) converged with those of an adjacent grassland (‐13‰) while those from the woodland interior (‐15‰) remained distinct, indicating that the woodland has been present for many years but has recently expanded. Concentrations and densities of SOC and total N were generally greater in woody patches than in grasslands. However, differences in the amount of SOC and N stored beneath the two woody patch types indicates that C and N sequestration potentials are species dependent.     

Bryophytes and their microhabitats in coniferous forest pastures: should they be considered in the pasture management?

Forest pastures, like many other semi-natural (traditional) rural biotopes, have undergone a drastic decline both in area and quality during the last century in many areas. We explored the bryophyte flora of Finnish coniferous forest pastures on acidic soil and aimed to recognize the most important microsites (rocks, coarse woody debris, tree bases, mineral soil patches and closed vegetation) for bryophyte diversity. The effects of microhabitat heterogeneity (microsite entropy) on bryophytes was also examined. We found altogether 83 bryophyte species. The only red-listed species, Tayloria tenuis, was frequently found on dung patches and a few rare ruderal species grew exclusively on bare mineral soil. Rocks comprised the most species rich microsite and many common forest floor species showed preference for this microsite. Microhabitat heterogeneity explained bryophyte species richness on both alpha (plot average) and gamma (pasture total) scales. The results suggest that certain individual bryophyte species and their microsites should be taken into account in the management of this biotope, rather than guiding the management solely on the basis of the overall bryophyte diversity.     

Trade‐offs between savanna woody plant diversity and carbon storage in the Brazilian Cerrado

Incentivizing carbon storage can be a win‐win pathway to conserving biodiversity and mitigating climate change. In savannas, however, the situation is more complex. Promoting carbon storage through woody encroachment may reduce plant diversity of savanna endemics, even as the diversity of encroaching forest species increases. This trade‐off has important implications for the management of biodiversity and carbon in savanna habitats, but has rarely been evaluated empirically. We quantified the nature of carbon‐diversity relationships in the Brazilian Cerrado by analyzing how woody plant species richness changed with carbon storage in 206 sites across the 2.2 million km² region at two spatial scales. We show that total woody plant species diversity increases with carbon storage, as expected, but that the richness of endemic savanna woody plant species declines with carbon storage both at the local scale, as woody biomass accumulates within plots, and at the landscape scale, as forest replaces savanna. The sharpest trade‐offs between carbon storage and savanna diversity occurred at the early stages of carbon accumulation at the local scale but the final stages of forest encroachment at the landscape scale. Furthermore, the loss of savanna species quickens in the final stages of forest encroachment, and beyond a point, savanna species losses outpace forest species gains with increasing carbon accumulation. Our results suggest that although woody encroachment in savanna ecosystems may provide substantial carbon benefits, it comes at the rapidly accruing cost of woody plant species adapted to the open savanna environment. Moreover, the dependence of carbon‐diversity trade‐offs on the amount of savanna area remaining requires land managers to carefully consider local conditions. Widespread woody encroachment in both Australian and African savannas and grasslands may present similar threats to biodiversity.     

Woody overstorey effects on soil carbon and nitrogen pools in South African savanna

Abstract Woody plant encroachment in savannas may alter carbon (C) and nitrogen (N) pools over the long‐term, which could have regional or global biogeochemical implications given the widespread encroachment observed in the vast savanna biome. Soil and litter %C and %N were surveyed across four soil types in two encroached, semi‐arid savanna landscapes in northern South Africa. Litter at sampling points with a woody component had a higher %N and lower C : N ratio than litter at solely herbaceous points. Severely encroached areas had lower C : N ratios throughout the soil profile than less encroached areas. Soil %C and %N were highly influenced by soil texture but were also influenced by the presence of a woody overstorey, which increased surface soil %C on three soil types but decreased it on the most heavily encroached soil type. Soil C sequestration may initially increase with bush encroachment but then decline if bush densities become so high as to inhibit understorey grass growth.     

Relationships between seed banks and spatial heterogeneity of North American alvar vegetation

Abstract. This is the first quantitative study of seed bank characteristics in North American alvar habitats. We assessed seed bank density, species richness, and species composition in 75 plots distributed among five alvar sites in Bruce Peninsula National Park, Ontario, Canada, each of which displayed areas of high and low vegetation cover within the alvar and a fully forested perimeter area. Forested habitats immediately adjacent to alvar patches contained minimal seed banks for species restricted to the alvar patches. Open alvars contained less than 1% seeds from woody forest species. This suggests that forest is not invading adjacent alvar habitat via seeds and that adjacent forest does not contain a reservoir of alvar seeds. When compared to areas on the alvar with high vascular plant cover, areas with low cover contained a slightly smaller viable seed bank, but seed banks from high and low vegetation cover plots had similar species composition and species richness. High vegetation cover plots had slightly higher mean and maximum soil depths compared with low cover plots, but no differences in other physical and chemical parameters. Thus, spatial heterogeneity in plant cover is associated only weakly with heterogeneity in below‐ground factors. Despite the availability of seed and soil resources, vegetation dynamics are constrained in areas with low plant cover, and thus alvar community development seems to respond non‐linearly to resource availability.     

Limited long‐term effectiveness of roller‐chopping for managing woody encroachment

The encroachment of woody plants into grasslands, woodland, and savanna has increased markedly over the past century, prompting the use of different physical methods to remove woody plants and restore grasses. Roller‐chopping is used extensively in the Americas, but little is known about its long‐term effectiveness for restoration, and whether its effectiveness varies with the intensity of encroachment. We compared the effects of roller‐chopping, under three treatment intensities (control, single treatment, double treatment), on woody plant density, ground cover, and groundstorey plants at sites of low, moderate, and high woody plant density in a semi‐arid eastern Australian woodland over 10 years. Both single and double treatment significantly altered the size distribution of Dodonaea viscosa, which comprised more than 85% of woody plants at all sites. Thus, roller‐chopping changed the size distribution of the community from an even‐size distribution to one dominated by shorter plants, irrespective of initial encroachment level. The effectiveness of roller‐chopping was strongly site‐specific, with significant reductions in density at low‐ and high‐density sites, but no clear trend in relation to the intensity of treatment (i.e. single cf. double treatment). The effectiveness of roller‐chopping was unsustained over the long term, with the suppressive effect on woody density diminishing over time. Grass cover increased with increasing intensity of woody removal, but only at the low‐density site and with some ill‐defined, variable, and short‐term effects on plant composition. Managers should consider that the short‐term effects may not adequately reflect the long‐term results of woody plant removal using the roller‐chopper.     

Spatial and Temporal Dynamics of a Restored Population of Oryza rufipogon in Huli Marsh, South China

Studying the process of population restoration is helpful for managing and preserving endangered species. A population of Oryza rufipogon (wild rice), an endangered species, was reintroduced in 1993 into Huli Marsh. We conducted a detailed survey over a 5‐year period (1997–2001) to evaluate the present status of the population and to further our understanding of its habitat requirements and the population model. The population was surveyed using 2 × 2–m quadrats in mid‐September of each year. In total, 2,683 quadrats were surveyed covering the whole O. rufipogon reserve during each survey. The population's spatial distribution was mapped, and the maps were used to examine the relationship between patch replacement and water depth. The individual number of O. rufipogon increased steadily from 1993 to 2001. The patch number, patch area, mean patch size, and largest patch size increased over this time period, and Korcak patchiness exponents decreased. On average, 83% of the patches persisted from one year to the next. There was a significant positive correlation between the initial patch size and the size the following year. The probability of patch disappearance decreased as patch size increased. Fifty‐eight percent of the patches were located at water depths between 20 and 30 cm. Water depth had no significant effect on the patch transition from O. rufipogon to other species. The loss and gain of O. rufipogon patches were statistically correlated with the patch areas in different water depths. Our results show that the population of O. rufipogon can successfully be reintroduced to the original habitat after appropriate environmental conditions have been restored. We recommend the following transplantation practices: transplant many smaller patches rather than a few larger patches, use transplant patch sizes of at least 20 m², and transplant into sites vegetated with species with different regeneration niches from the transplanted species.     

Restoration prospects of abandoned species‐rich sandy grassland in Hungary

Abstract. Secondary succession and seed bank formation was studied in a formerly grazed, abandoned, eastern Hungarian sandy steppe‐meadow (Pulsatillo‐Festucetum). The vegetation was sampled at different elevations of a sand dune which became partly invaded by the tree Robinia pseudo‐acacia ca. 10 yr ago. Pre‐abandonment vegetation records were used as historic references. Though composition of the non‐invaded grassland only changed moderately, dominance of tall grasses (Elymus hispidus, Poa angustifolia) increased significantly at the cost of annuals and low stature perennials. In the stand invaded by Robinia most grassland species were lost and replaced by nitrophytes. Vertical position influenced species abundance, but affected the composition only moderately. Fine‐scale zonation of the vegetation also changed with time. Species richness of the above‐ground vegetation and the seed density of soil samples at the lower elevation were slightly greater than at the higher sites. Seed banks of sensitive grassland specialists (e.g. Pulsatilla pratensis subsp. hungarica) disappeared during grass encroachment. Following extinction from above‐ground vegetation, restoration must rely on dispersal from adjacent areas. In contrast, several annuals and perennials, which survived this degradation stage in the above‐ground vegetation, possessed seed banks. Many of these species became extinct from the vegetation during the Robinia invasion but left viable persistent seeds. This fact is promising for restoration of the Potentillo‐Festucetum sandy pasture. Competitive weedy species and sprouting Robinia can, however, limit seedling establishment.     

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.     

Behaviour of male and female parasitoids in the field: influence of patch size,host density,and habitat complexity

1. Two field experiments were carried out to examine the role of patch size, host density, and complexity of the surrounding habitat, on the foraging behaviour of the parasitoid wasp Cotesia glomerata in the field. 2. First, released parasitoids were recaptured on patches of one or four Brassica nigra plants, each containing 10 hosts that were placed in a mown grassland area. Recaptures of females were higher than males, and males and females aggregated at patches with four plants. 3. In experiment 2, plants containing 0, 5 or 10 hosts were placed in unmown grassland plots that differed in plant species composition, on bare soil, and on mown grassland. Very low numbers of parasitoids were recaptured in the vegetated plots, while high numbers of parasitoids were recaptured on plants placed on bare soil or in mown grassland. Recaptures were higher on plants on bare soil than on mown grassland, and highest on plants containing 10 hosts. The host density effect was significantly more apparent in mown grassland than on bare soil. 4. Cotesia glomerata responds in an aggregative way to host density in the field. However, host location success is determined mostly by habitat characteristics, and stronger host or host‐plant cues are required when habitat complexity increases.     

Influence of Grazing on Soil Seed Banks Determines the Restoration Potential of Aboveground Vegetation in a Semi‐arid Savanna of Ethiopia

Species composition, number of emerging seedlings, species diversity and functional group of the soil seed banks, and the influence of grazing on the similarity between the soil seed banks and aboveground vegetation, were studied in 2008 and 2009 in a semi‐arid savanna of Ethiopia. We tested whether the availability of persistent seeds in the soil could drive the transition from a degraded system under heavy grazing to healthy vegetation with ample perennial grasses. A total of 77 species emerged from the soil seed bank samples: 21 annual grasses, 12 perennial grasses, 4 herbaceous legumes, 39 forbs, and 1 woody species. Perennial grass species dominated the lightly grazed sites, whereas the heavily grazed sites were dominated by annual forbs. Heavy grazing reduced the number of seeds that can germinate in the seed bank. Species richness in the seed bank was, however, not affected by grazing. With increasing soil depth, the seed density and its species richness declined. There was a higher similarity in species composition between the soil seed bank and aboveground vegetation at the lightly grazed sites compared with the heavily grazed sites. The mean similarity between the seed banks and aboveground vegetation was relatively low, indicating the effect of heavy grazing. Moreover, seeds of perennial grasses were less abundant in the soil seed banks under heavy grazing. We concluded that restoration of grass and woody species from the soil seed banks in the heavily grazed areas could not be successful in semi‐arid savannas of Ethiopia.     

Edaphic patchiness influences grassland regeneration from the soil seed‐bank in mountain grasslands of central Argentina

The soil seed‐banks in the main natural vegetation patches that make up mountain grasslands on granite substrates in central Argentina were studied. The main natural vegetation types are moist swards, tall‐tussock grasslands and stony grasslands. Ten compound soil samples from each community at two soil depths (0–5 and 5–10 cm) were taken. The density of soil seed‐banks was highest in moist swards, intermediate in stony grasslands and lowest in tall‐tussock grasslands. Low levels of similarity were found between the established vegetation and total soil seed‐bank in tall‐tussock grasslands and stony grasslands, but the similarity was higher in swards. In all three communities the seed‐bank was most frequently transient in nature. Short‐term persistent and long‐term persistent seed‐banks were less frequent. Regeneration from the seed‐banks after disturbance is expected to differ among communities on different edaphic patches. On the basis of the density and longevity of the soil seed‐banks and the similarity to the established vegetation, potential for in situ regeneration should be lower in tall‐tussock grasslands, intermediate in stony grasslands and higher in moist swards.