Net woody vegetation increase confined to seasonally inundated lowlands in an Australian tropical savanna,Victoria River District,Northern Territory

Abstract Georeferenced digital aerial photographs were used to assess changes in overstorey vegetation cover since 1948 in the Victoria River District, Northern Territory, Australia, across a range of lowland tropical savanna habitats and with explicit consideration of known and variable site‐specific grazing and fire management histories. Vegetation surveys at corresponding locations on the ground identified five distinct woody vegetation communities defined primarily by water drainage and secondarily by soil characteristics. Air‐photo analyses revealed that, contrary to popular perceptions and in contrast to results from other habitats, there has been no generalized net increase in overstorey woody vegetation cover across the full range of lowland savanna habitats. Rather, different habitats exhibited distinctly different vegetation change mechanisms: low‐lying seasonally inundated ‘wet’ habitats have experienced woody vegetation increase since 1948, whereas well‐drained ‘dry’ habitats have experienced overstorey vegetation stability or loss. In almost every instance woody vegetation increase could be attributed to the invasion or proliferation of a single species, Melaleuca minutifolia F.Muell. The extent of M. minutifolia increase was unrelated to historical grazing/fire regime. Demographic analyses for this species revealed that recruitment was often episodic and that synchronized recruitment events occurred uniformly across the full range of historical management treatments, most likely as a consequence of favourable climatic conditions in years with an extended wet season. Heavy grazing facilitated juvenile survival and/or recruitment, most likely by reducing grassy fuel loads and eliminating landscape fire. We conclude that while there has been no generalized net increase in overstorey woody vegetation cover in lowland environments, savanna dynamics are complex, and multiple change mechanisms have occurred simultaneously in different habitats, some of which have been significantly transformed since 1948. Where net woody vegetation increase has occurred it is primarily a natural consequence of episodic M. minutifolia establishment in climatically favourable years, but the extent and magnitude of this effect is likely mediated by fire/grazing regime.     

Long- and short-term effects of reindeer grazing on tundra wetland vegetation

We studied long-term (50 years) and short-term (4 years) effects of summer grazing of reindeer on subarctic tundra wetland vegetation. The long-term effects of summer grazing were studied by comparing vegetation on Finnish and Norwegian sides of the fence line separating reindeer grazing regimes. The Finnish side was intensively grazed and trampled throughout the year, whereas the Norwegian side was grazed in winter. Experimental fences were erected to examine short-term effects of grazing exclusion. Both in the long- and short-term, summer grazing decreased the height of Salix lapponum whereas the short-term effects on willow cover were less clear than the long-term effects. In contrast, Carex spp. benefited from grazing. Long-term grazing had little effect on total bryophyte cover. Grazing had negligible effects on the nutrient content of leaves of S. lapponum and Eriophorum angustifolium. We conclude that tundra wetlands can withstand moderately high grazing pressure sustained over several decades.     

Changes in heathland vegetation under goat grazing: effects of breed and stocking rate

Questions: How are heathland vegetation dynamics affected by different goat grazing management? Location: Cantabrian heathlands in Illano, Asturias, northern Spain. Methods: During 4 years, vegetation dynamics (structural composition, canopy height and floristic diversity) were studied under three goat grazing treatments with three replicates: high stocking rate (11.7 goats ha^?1) with a local Celtiberic breed, and high (15 goats ha^?1) and low (6.7 goats ha^?1) stocking rates with a commercial Cashmere breed. Results: The relative cover of woody plants, particularly heather species, decreased more while herbaceous cover increased more under local Celtiberic than under Cashmere breed grazing. Within Cashmere treatments, the cover and height of live shrubs decreased more and the herbaceous cover increased more under high than under low stocking rate. Redundancy analysis showed a significant effect of treatment × year interaction on floristic composition. Greater species richness was recorded under local goat grazing, but Shannon diversity index fell in the fourth year on these plots because of dominance by two grass species. Conclusions: Local Celtiberic goat grazing at such a high stocking rate (11.7 goats ha^?1) hinders the development of sustainable systems on these heathlands, both in environmental and productive terms, owing to the limitations in soil fertility. Nevertheless, Celtiberic goats could be useful for controlling excessive shrub encroachment and reducing fire hazard. Cashmere goat grazing at high stocking rate promoted the highest Shannon diversity by generating a better balance between woody and herbaceous plants, while shrub dominance was not altered under the low stocking rate.     

Effect of artificial shade and grazing removal on degraded grasslands: Implications of woody restoration for herbaceous vegetation

Extensive degraded short tussock grasslands of New Zealand's eastern South Island were dominated by woody vegetation prior to burning and livestock grazing associated with human settlement starting 800 years ago. There is increasing interest in restoring some of these grasslands back to a woody state. However, because of the long time frames involved in establishing a woody cover, it is difficult to predict the impacts that woody restoration will have on the extant herbaceous flora. Using a factorial trial with artificial shade and grazing exclusion, we assessed the potential impact of woody restoration on the structure and composition of the herbaceous flora over a six‐year period. The imposition of artificial shade resulted in significant increases in total species richness and the total cover of herbaceous vegetation, increases in cover of several individual forb and grass species and decreases in the cover of bare ground, moss and lichen in shade treatments. There were also changes in the overall community composition of shaded treatments reflecting these changes in vegetation cover and species richness. We found no statistically significant effects of grazing exclusion. We suggest that increased soil moisture resulting from shade addition plays an important role in increasing the herbaceous component of the flora. While woody restoration will have a range of effects on the herbaceous understorey, for example through competition and changes in soil conditions, our findings are important for planning future woody restoration in these degraded tussock grasslands. In particular, our results suggest that the best approach to ensure the persistence of herbaceous vegetation in woody restorations might be to ensure that restoration plantings result in a spatially heterogeneous vegetation arrangement.     

Predicting the Effects of Woody Encroachment on Mammal Communities,Grazing Biomass and Fire Frequency in African Savannas

With grasslands and savannas covering 20% of the world’s land surface, accounting for 30–35% of worldwide Net Primary Productivity and supporting hundreds of millions of people, predicting changes in tree/grass systems is priority. Inappropriate land management and rising atmospheric CO₂ levels result in increased woody cover in savannas. Although woody encroachment occurs world-wide, Africa’s tourism and livestock grazing industries may be particularly vulnerable. Forecasts of responses of African wildlife and available grazing biomass to increases in woody cover are thus urgently needed. These predictions are hard to make due to non-linear responses and poorly understood feedback mechanisms between woody cover and other ecological responders, problems further amplified by the lack of long-term and large-scale datasets. We propose that a space-for-time analysis along an existing woody cover gradient overcomes some of these forecasting problems. Here we show, using an existing woody cover gradient (0–65%) across the Kruger National Park, South Africa, that increased woody cover is associated with (i) changed herbivore assemblage composition, (ii) reduced grass biomass, and (iii) reduced fire frequency. Furthermore, although increased woody cover is associated with reduced livestock production, we found indigenous herbivore biomass (excluding elephants) remains unchanged between 20–65% woody cover. This is due to a significant reorganization in the herbivore assemblage composition, mostly as a result of meso-grazers being substituted by browsers at increasing woody cover. Our results suggest that woody encroachment will have cascading consequences for Africa’s grazing systems, fire regimes and iconic wildlife. These effects will pose challenges and require adaptation of livelihoods and industries dependent on conditions currently prevailing.     

Effects of grazing and topography on long-term vegetation changes in a Mediterranean ecosystem in Israel

The dynamics of Mediterranean vegetation over 28 years was studied in the Northern Galilee Mountains, Israel, in order to identify and quantify the major factors affecting it at the landscape scale. Image analysis of historical and current aerial photographs was used to produce high resolution digital vegetation maps (pixel size = 30 cm) for an area of 4 km² in the Galilee Mountains, northern Israel. GIS tools were used to produce corresponding maps of grazing regime, topographic indices and other relevant environmental factors. The effects of those factors were quantified using a multiple regression analyses. Major changes in the vegetation occurred during the period studied (1964–1992); tree cover increased from 2% in 1964 to 41% in 1992, while herbaceous vegetation cover decreased from 56% in 1964 to 24% in 1992. Grazing, topography and initial vegetation cover were found to significantly affect present vegetation patterns. Both cattle grazing and goat grazing reduced the rate of increase in tree cover, yet even intensive grazing did not halt the process. Grazing affected also the woody-herbaceous vegetation dynamics, reducing the expansion of woody vegetation. Slope, aspect, and the interaction term between these two factors, significantly affected vegetation pattern. Altogether, 56% and 72% of the variability in herbaceous and tree cover, respectively, was explained by the regression models. This study indicates that spatially explicit Mediterranean vegetation dynamics can be predicted with fair accuracy using few biologically important environmental variables.     

Mapping gains and losses in woody vegetation across global tropical drylands

Woody vegetation in global tropical drylands is of significant importance for both the interannual variability of the carbon cycle and local livelihoods. Satellite observations over the past decades provide a unique way to assess the vegetation long‐term dynamics across biomes worldwide. Yet, the actual changes in the woody vegetation are always hidden by interannual fluctuations of the leaf density, because the most widely used remote sensing data are primarily related to the photosynthetically active vegetation components. Here, we quantify the temporal trends of the nonphotosynthetic woody components (i.e., stems and branches) in global tropical drylands during 2000–2012 using the vegetation optical depth (VOD), retrieved from passive microwave observations. This is achieved by a novel method focusing on the dry season period to minimize the influence of herbaceous vegetation and using MODerate resolution Imaging Spectroradiometer (MODIS) Normalized Difference Vegetation Index (NDVI) data to remove the interannual fluctuations of the woody leaf component. We revealed significant trends (P < 0.05) in the woody component (VOD_wood) in 35% of the areas characterized by a nonsignificant trend in the leaf component (VOD_leaf modeled from NDVI), indicating pronounced gradual growth/decline in woody vegetation not captured by traditional assessments. The method is validated using a unique record of ground measurements from the semiarid Sahel and shows a strong agreement between changes in VOD_wood and changes in ground observed woody cover (r² = 0.78). Reliability of the obtained woody component trends is also supported by a review of relevant literatures for eight hot spot regions of change. The proposed approach is expected to contribute to an improved assessment of, for example, changes in dryland carbon pools.     

Impact of land use and tenure changes on the Kitenden wildlife corridor,Amboseli Ecosystem,Kenya

This study assesses the ecological pressure exerted by changing land use and tenure on the Kitenden wildlife corridor, a critical cross‐border link between the Amboseli and Kilimanjaro national parks. The implications on viability of the two high‐value protected areas and their respective dispersal areas are both negative and serious. The extent of land use change and its impacts were assessed through household and vegetation surveys, while wildlife abundance was measured using transect walks. Approximately 30% of the study area has shifted from community to private land ownership over the last two decades. Except for baboon and vervet monkey, most wildlife avoided the cultivated area. Vegetation composition on the noncultivated area has been greatly altered by intense wildlife and livestock use, where mean herbaceous vegetation cover differed significantly among range‐plant categories (F_{3, 524} = 29.015, p < 0.05). The frequency of increaser I (21.4%) differs greatly from that of decreaser and forbs, at 8.3% and 7.4%, respectively Tree recruitment was low, with a significant difference in mean density among age classes (F_{2, 110} = 3.663, p < 0.05). Only through land leasing agreements between landowners and conservation organisations, and a widely supported land use plan, can the spread of cultivation be controlled and complete cessation of wildlife movement be prevented.     

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

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Cattle and Weedy Shrubs as Restoration Tools of Tropical Montane Rainforest

Over the last 150 years, a large proportion of forests in Latin America have been converted to pastures. When these pastures are abandoned, grasses may slow re‐establishment of woody species and limit forest regeneration. In this study, we explored the use of cattle in facilitating the establishment of woody vegetation in Colombian montane pastures, dominated by the African grasses Pennisetum clandestinum (Kikuyo) and Melinis minutiflora (Yaraguá). First, we described woody and herbaceous vegetation in grazed and non‐grazed pastures. Second, we tested the effect of grazing and seed addition on the establishment and growth of woody species. We also determined if the effect of grazing was different in P. clandestinum and M. minutiflora pastures. We found that low stocking density of cattle greatly increased density, number of branches per individual (a measure of “shrubiness”), and basal area of woody species, but also reduced woody plant species richness and diversity. In the grazed area, the shrubs Baccharis latifolia (Chilca) and Salvia sp. (Salvia) were the most abundant. The combined effect of grazing and shading from the shrubs reduced herbaceous vegetation by 52 to 92%. In the grazing/seed addition experiment, grazing increased establishment of woody seedlings, particularly of the shrub Verbesina arborea (camargo), but the largest effect was seed addition. Where grasses are an important barrier to regeneration, grazing can facilitate the establishment of shrubs that create a microhabitat more suitable for the establishment of montane forest tree species.     

Microsite and herbaceous vegetation heterogeneity after burning Artemisia tridentata steppe

Woody vegetation can create distinct subcanopy and interspace microsites, which often result in resource islands in subcanopies compared to interspaces. This heterogeneity in soil resources contributes to herbaceous vegetation heterogeneity in plant communities. However, information detailing the impact of disturbance, such as fire, that removes the woody vegetation on microsites and herbaceous vegetation heterogeneity is limited. The purpose of this study was to determine the influence of burning on microsites and herbaceous vegetation in subcanopies and interspaces. Six study sites (blocks) were located at the Northern Great Basin Experimental Range in shrub (Artemisia tridentata ssp. wyomingensis (Beetle & A. Young) S.L. Welsh)-bunchgrass plant communities and one half of each block was burned to remove A. tridentata. Herbaceous vegetation and microsite characteristics were measured 2 years post-fire in intact and burned subcanopies and interspaces. Burning resulted in microsite and herbaceous vegetation differences between intact and burned subcanopies and intact and burned interspaces. However, burned subcanopies and burned interspaces appeared to be relatively similar. The similarity in microsite characteristics probably explains the lack of differences in herbaceous vegetation cover and biomass production between burned subcanopies and burned interspaces (P > 0.05). However, some microsite and herbaceous vegetation characteristics differed between burned subcanopies and burned interspaces. Our results suggest that disturbances that remove woody vegetation reduced microsite and herbaceous vegetation heterogeneity within plant communities, but do not completely remove the resource island effect. This suggests soil resource heterogeneity may influence post-fire community assembly and contribute to diversity maintenance. The Eastern Oregon Agricultural Research Center is jointly funded by the USDA-Agricultural Research Service and Oregon State Agricultural Experiment Station. Mention of a proprietary product does not constitute a guarantee or warranty of the product by USDA, Oregon State University, or the authors and does not imply its approval to the exclusion of other products.     

Estimation of aboveground biomass in conserved areas of Stipa tenacissima L. stands in the high steppes of western Algeria by mean of the Landsat 8 imagery‐based vegetation indices

The main aim of this paper was to evaluate the use of OLI spectral data as a tool to assess the steppe vegetation in a conservation context. The field sampling was conducted for two specific areas of treatment (a) an exclosure area and (b) a free grazing area. After testing several vegetation indices (VIs), the optimal results were obtained for the Normalised Difference Vegetation Index (NDVI)‐based aboveground biomass model with r² = 0.61 and r² = 0.72 for total and perennial biomass, respectively. No difference between observed and predicted total and perennial biomass was found (p = 0.700 and p = 0.306, respectively). The comparison between the two treatments using the field sampling revealed a significant difference on total plant cover (p = 0.016) and total biomass (p = 0.005), with a plant cover of 50.6% and a biomass of 325.771 kg dry matter per hectare (kg DM ha^?1) on average in grazed area and 66.9%, 1,407.869 kg DM ha^?1 in exclosure. Finally, a concordance is noted between the results obtained by the NDVI‐based biomass model and the field sampling‐based biomass.     

Dry grassland plant diversity conservation using low-intensity sheep and goat grazing management: case study in Prague (Czech Republic)

After abandonment, dry grassland (Festuco-Brometea) areas decline due to gradual overgrowing by woody species and the expansion of perennial tall grass species. Dry grassland vegetation was formed by extensive livestock grazing, thus grazing is considered one of the most natural methods for managing this type of vegetation. Six years after introducing low-intensity sheep and goat grazing in seven nature reserves in Prague (Czech Republic), the following impact of this management on dry grassland vegetation was observed: The cover of expansive woody species, particularly Ligustrum vulgare, and to a smaller extent Cornus sanguinea and Prunus spinosa declined. In addition, a significant, long-term declining trend of the expansive species Arrhenatherum elatius was also observed. Also the cover of Pimpinella saxifraga and Allium senescens declined significantly with regard to statistical evaluation. On the contrary, the cover of Achillea millefolium, Centaurea stoebe, Securigera varia, Elytrigia repens, Erysimum crepidifolium, Falcaria vulgaris, Fallopia convolvulus and Verbascum lychnitis increased. The cover of species characteristic of dry grasslands (Festuco-Brometea) increased significantly. No changes were observed in the number and cover of the Red List species. In addition, the presence of nitrophilous and ruderal species increased. Species diversity also significantly increased. From our findings we can conclude that managing dry grasslands with low-intensity grazing can help to keep dry grassland vegetation in good condition and conserve its plant diversity. Nomenclature: Kubát et al. (2002) for taxa and Moravec et al. (1995) for syntaxa.     

Plant diversity patterns in the Aljibe Mountains (S. Spain): a comprehensive account

The Aljibe Mountains are located in the southern tip of the Iberian Peninsula and have a remarkable biogeographical interest. The complete plant species list (trees, climbers, shrubs, perennial and annual herbs, ferns, lichens, bryophytes and macroscopic algae) was recorded in four 0.1 ha plots from each of the most representative community types (Quercus suber woodland, Q. canariensis forest, open heathland and Q. coccifera shrubland). Up to 119 plant species were found in total in the Q. suber woodland plot. The diversity of woody plants was analysed from 44 samples of cover (100 m line), and the herbaceous layer was explored in 200 quadrats (of 0.5 × 0.5 m). Three biodiversity components (species richness, endemism, and taxonomic singularity) were evaluated in both shrub and herbaceous layers. Open heathlands showed the highest richness of endemic species, both woody and herbaceous. The highest number of woody species was found in the evergreen Q. suber woodland, and of herbaceous species in the semi-deciduous Q. canariensis woodland. Taxonomic singularity was higher in Q. canariensis woodlands and Q. coccifera shrublands for woody species, but there were no significant differences in the herbaceous layer. Local species diversity of heathlands in this region resembles that of South African heathlands (fynbos), despite the obvious geographic and floristic distance, and contrasts with the low diversity of biogeographically closer, European temperate heathlands. The Aljibe Mountains show high diversity values for different life forms (from trees to mosses) and spatial scales (from community to region), and are rich in endemic species. Thus, this area should be recognised as a relevant unit within the Mediterranean plant diversity hot spots.     

Ungulate browsers promote herbaceous layer diversity in logged temperate forests

Ungulates are leading drivers of plant communities worldwide, with impacts linked to animal density, disturbance and vegetation structure, and site productivity. Many ecosystems have more than one ungulate species; however, few studies have specifically examined the combined effects of two or more species on plant communities. We examined the extent to which two ungulate browsers (moose [Alces americanus]) and white‐tailed deer [Odocoileus virginianus]) have additive (compounding) or compensatory (opposing) effects on herbaceous layer composition and diversity, 5–6 years after timber harvest in Massachusetts, USA. We established three combinations of ungulates using two types of fenced exclosures – none (full exclosure), deer (partial exclosure), and deer + moose (control) in six replicated blocks. Species composition diverged among browser treatments, and changes were generally additive. Plant assemblages characteristic of closed canopy forests were less abundant and assemblages characteristic of open/disturbed habitats were more abundant in deer + moose plots compared with ungulate excluded areas. Browsing by deer + moose resulted in greater herbaceous species richness at the plot scale (169 m²) and greater woody species richness at the subplot scale (1 m²) than ungulate exclusion and deer alone. Browsing by deer + moose resulted in strong changes to the composition, structure, and diversity of forest herbaceous layers, relative to areas free of ungulates and areas browed by white‐tailed deer alone. Our results provide evidence that moderate browsing in forest openings can promote both herbaceous and woody plant diversity. These results are consistent with the classic grazing‐species richness curve, but have rarely been documented in forests.     

The irreversible cattle-driven transformation of a seasonally flooded Australian savanna

Aim Anecdotal historical and photographic evidence suggests that woody vegetation is increasing dramatically in some northern Australian savanna habitats. Vegetation change in savannas has important implications for pastoral land‐use, conservation management, and landscape‐scale carbon storage, and informs theoretical debates about ecosystem function. This study seeks to determine the nature, extent and cause(s) of woody vegetation change in a seasonally flooded alluvial savanna habitat. Location The study area is located within the seasonally inundated alluvial zone of the tidal portion of the Victoria River, Northern Territory, Australia. The study area has been grazed by domestic stock since c. 1900, prior to which the area was inhabited and more likely regularly burnt by Aboriginal people for thousands of years. Methods Digital georeferenced aerial photographic coverages were used to examine and quantify woody vegetation change between 1948 and 1993. Transect surveys of woody and herbaceous vegetation were carried out to ground‐truth air‐photo results and determine the nature and causes of observed vegetation changes. Results There has been a dramatic increase in woody vegetation cover throughout the study area. Vegetation change patterns are roughly uniform across the full range of edaphic habitat variation and are unrelated to the depositional age of fluvial sediments. Two woody species, Eucalyptus microtheca and Excoecaria parvifolia, are predominantly responsible for observed increases. Demographic analyses reveal that woody invasions have been episodic and indicate that in most locations peak woody species establishment occurred in the mid‐1970s. Grasses are almost absent in a majority of habitats within the study area. Instead, large areas are covered by scalded soil, dense invasive weed populations, and unpalatable forbs and sedges. What grasses do occur are predominantly of very low value for grazing. The condition of the herbaceous layer renders most of the study area almost completely non‐flammable; what fires do burn are small and of low intensity. Main conclusions Multiple working hypotheses explaining observed patterns of woody vegetation increase were considered and rejected in turn. The only hypothesis consistent with the evidence is as follows: (1) observed changes are a direct consequence of extreme overgrazing by cattle, most likely when stocking rates peaked in the mid‐1970s; (2) prolonged heavy grazing effected the complete transformation of much of the herbaceous vegetation to a new state that is not flammable; and (3) in the absence of regular fire mortality, woody vegetation increased rapidly. The relatively treeless system that existed in 1948 was apparently stable and resilient to moderate grazing levels, and perhaps also to episodic heavy grazing events. However, grazing intensity in excess of a sustainable threshold has forced a transition that is irreversible in the foreseeable future. Stable‐state transitions such as this one inform debates at the heart of ecological theory, such as the nature of stability, resilience, equilibrium and carrying capacity in dynamic savanna ecosystems.     

Effects of human–livestock–wildlife interactions on habitat in an eastern Kenya rangeland

Human–livestock–wildlife interactions have increased in Kenyan rangelands in recent years, but few attempts have been made to evaluate their impact on the rangeland habitat. This study identified drivers of increased human–livestock–wildlife interactions in the Meru Conservation Area between 1980 and 2000 and their effects on the vegetation community structure. The drivers were habitat fragmentation, decline in pastoral grazing range, loss of wildlife dispersal areas and increase in livestock population density. Agricultural encroachment increased by over 76% in the western zone adjoining Nyambene ranges and the southern Tharaka area, substantially reducing the pastoral grazing range and wildlife dispersal areas. Livestock population increased by 41%, subjecting areas left for pastoral grazing in the northern dispersal area to prolonged heavy grazing that gave woody plant species a competitive edge over herbaceous life‐forms. Consequently, open wooded grassland, which was the dominant vegetation community in 1980, decreased by c. 40% as bushland vegetation increased by 42%. A substantial proportion of agro pastoralists were encountered around Kinna and Rapsu, areas that were predominantly occupied by pastoralists three decades ago, indicating a possible shift in land use in order to spread risks associated with habitat alterations.     

Floral composition and diversity of the lagos coastal environment,South-Western,Nigeria

The present study investigates the floral diversity of the Lagos coastal environments by sampling 21 points using the quadrat method. The study revealed two major aquatic ecosystem types (mangrove and freshwater swamp) and recorded 125 species belonging to 108 genera and 49 families. While Alchornea cordifolia and Raphia hookeri were found as the most frequent, Acalypha crenata and Acrostichum aureum were the least abundant species. The highest and lowest species-rich locations were Unilag (n = 125 species) and Abule Osun (n = 2 species), respectively. The vegetation structure of the LCE is dominated by herbaceous species. Our findings provide insights on the flora diversity of the LCE, and a guideline for the effective conservation of near-threatened species such as Nauclea diderrichii and Mitragyna stipulosa in this coastal ecosystem is presented.     

Exotic herbivores and fire energy drive standing herbaceous biomass but do not alter compositional patterns in a semiarid savanna ecosystem

Questions

Fire regime alterations are pushing open ecosystems worldwide past tipping points where alternative steady states characterized by woody dominance prevail. This reduces the frequency and intensity of surface fires, further limiting their effectiveness for controlling cover of woody plants. In addition, grazing pressure (exotic or native grazers) can reinforce woody encroachment by potentially reducing fine-fuel loads. We investigated the effects of different fire energies on the herbaceous plant community, together with mammalian wildlife herbivory (exotic and native combined) exclusion, to inform best management practices.

Location

Texas semi-arid savanna, southern Great Plains, USA.

Methods

We conducted an experiment in which we manipulated fire intensity and herbivore access to herbaceous biomass in a split-plot design. We altered fire energy via fuel addition rather than applying fire under different environmental conditions to control for differences in standing biomass and composition attributable to differential plant physiological status and fire season.

Results

High-energy fire did not reduce herbaceous biomass or alter plant community composition, although it did increase among-plot variability in composition and forb biomass relative to low-energy fire and non-burned controls. Grazing pressure from native and non-native mammalian herbivores reduced above-ground herbaceous biomass regardless of fire treatments, but did not alter community composition.

Conclusions

Managers seeking to apply high-intensity prescribed fire to reduce woody encroachment will not negatively impact herbaceous plant productivity or alter community composition. However, they should be cognizant that repeated fires necessary for greatly reducing woody plants in heavily invaded areas might be difficult to accomplish due to fine-fuel reduction from wild herbivores. High fencing to restrict access by wildlife herbivores or culling might be necessary to build fuels sufficient to conduct high-intensity burns for woody-plant reduction.     

Effects of fire on the vegetation of a lowland heathland in North-western Italy

This study focuses on the effect of fire on lowland heathlands at the extreme southern edge of their European distribution (Vauda Nature Reserve, NW Italy). Forty-nine plots (50 m radius) were surveyed between 1999 and 2006. Each year, fire occurrences were recorded and per cent cover of four vegetation types (grassland, heath, low shrubland, and tall shrubland) was estimated in each plot. Vascular plant species richness was also recorded in 255, 1 m² quadrats. After a fire, grassland vegetation expanded, but then declined rapidly as heath and shrubland recovered: 7 years after a fire, tall shrubland encroached on to more than 40% of the plots, and grassland declined from 50% to 20% cover. Between 1999 and 2006, Betula pendula shrubland greatly expanded, while grassland decreased over most of the Reserve, even where fire frequency was high. Tall shrubland had low plant diversity and was dominated by widespread species of lower conservation value. By contrast, early successional vegetation (grassland and low shrubland) had higher richness and more narrowly distributed species, indication that the development of tall shrubland causes significant species loss in the heathland. Italian lowland heathlands are characterized by high rates of shrubland encroachment that threatens both habitat and species diversity. Burning frequencies of once in 3–6 years seem appropriate in this habitat, but burning alone might not suffice without actions to increase herbivore grazing.