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Disturbance may be an important determinant of plant community composition and diversity owing to its effects on competitive interactions, resources, dominance and vigour. The effect of type, timing and frequency of disturbance on grass and forb species richness was examined using data from a long-term (> 50 yr) grassland burning and mowing experiment in KwaZulu-Natal, South Africa.
Grass species richness declined considerably (> 50%) in the absence of disturbance, whereas forb species richness was unaffected. Annual burning in sites not mown in summer tended to increase grass species richness relative to triennial burning (22% increase) with the reverse being true in sites mown twice in summer (37% decline). Forb species richness declined by 25% in sites mown twice in summer relative to sites mown in early summer only. Disturbance was necessary to achieve maximum grass species richness presumably by removing litter and increasing the availability of light. The interaction of time of mowing in summer (early versus late) and time of burning during the dormant period (spring versus winter) had the most dramatic effect on species richness. Time of burning had no effect on richness in sites mown in early summer, but winter burning resulted in a dramatic decline (27–42%) in richness in sites mown in late summer. This effect may be related to possible greater soil desiccation with this combination of disturbances.
Plant traits which may give an indication of a plant's strategy for nutrient acquisition and regeneration are known for numerous grassland species. This study aimed to establish whether there is any relationship between two plant traits: specific leaf area (SLA) and number of reproductive tillers, and sward structural characteristics which influence herbage intake by grazers (bulk density and digestibility, leaf:stem ratio). Comparison is made for nutrient-rich (Dactylis glomerata) and nutrient-poor (Festuca rubra) grass species. We hypothesized that these traits are responsive to environmental gradients and also act on the processes of the ecosystem. Both grasses were compared with two P-fertilizer rates in two localities (200 and 1300 m a.s.l.) which differed in their temperature:radiation ratios. For the vegetative phase SLA was well correlated with sward characteristics: D. glomerata, which has the higher SLA, has the lower bulk density and higher digestibility. The values of SLA and vegetation bulk density varied according to growing conditions (P-rate and temperature:radiation ratio), but the ranking of the species remained the same because the phenotypic plasticity that exists for plant traits was also observed for sward structure and composition. That suggested the possibility of grouping natural grassland species for their relevant characteristics for grazers according to SLA values. Over the reproductive phase, the proportion of stems was well correlated to the percentage of reproductive tillers. However, the percentage of reproductive tillers was a very plastic trait for both species, depending on the growing conditions, and resulting in a density-dependent effect, particularly for F. rubra. The species studied were too plastic and too similar in their regenerative strategy so that there is no unique relationship between percentage of reproductive tillers and stem proportion, regardless of the species and the growing conditions. The number of reproductive tillers is not a suitable plant trait which could be used to rank species for leaf and stem proportions in the sward.
Abbreviations: DM = Dry matter; LAR = Leaf area ratio; LWR = Leaf Weight Ratio; SLA = Specific Leaf Area.
Various attempts have been made to describe and map the vegetation of southern Africa with recent efforts having an increasingly ecologi cal context. Vegetation classification is usually based on vegetation physiognomy and floristic composition, but phenology is useful source of information which is rarely used, although it can contribute functional information on ecosystems. The objectives of this study were to identify a suite of variables derived from time-series NDVI data that best describe the phenological phenomena of vegetation in southern Africa and, secondly, to assess a classification of pixels of the study area based on NDVI variables using a pre-existing map of the biomes that was delimited on the basis of life forms and climate. A number of variables were derived from the satellite data for describing phenological phenomena, which were analysed by multivariate techniques to determine which variables best explained the variation in the satellite data. This set of variables was used to produce a phenological classification of the vegetation of southern Africa, the results of which are discussed in relation to their concordance with the existing biome boundaries.
Abbreviations: AVHRR = Advanced Very High Resolution Radiometer; NDVI = Normalized Difference Vegetation Index; NOAA = National Oceanic and Atmospheric Administration.
The flora of clear-cuttings with soil scarification in forests was compared < 1–2 yr after cutting with that in mature herb-rich forests in SW Finland. The total and mean numbers of vascular plant species both in the study areas and in the sample plots, were almost double in clear-cut areas compared to mature forests. Clear-cuttings and mature forests were distinctly separated by multivariate analyses (DCA). Several dozen species not found in forests were common in clear-cut areas. Most of them probably belong to the neglected native species pool of early boreal forest succession and are dependent on the long-term persistent seed bank or effective wind dispersal. It is emphasized that in forests many plant species are confined to the very early stages (< 2 yr) after disturbance. The storage effect of the long-term persistent seed bank is crucial for the maintenance of plant diversity in boreal forests. Probably a considerable part of the flora of agricultural areas is composed of species that were originally disturbance dependent forest plants. Scarification is beneficial to disturbance dependent plants and may be useful in restoration of populations of species of early succession.
Exposed cliffs of the Swiss Jura mountains harbour a highly diverse flora with many rare and endangered plant species. Many cliffs are frequently visited by rock climbers. We examined the impact of rock climbing on vascular plants in the lower part of four cliffs of the Gerstelflue (NW Switzerland) by comparing the vegetation of climbed and unclimbed areas. In climbed areas plant cover and species density were reduced. Similarly, the density of forbs and shrubs decreased, whereas the density of ferns tended to increase. In addition, rock climbing caused a significant shift in plant species composition and altered the proportions of different plant life forms. Rock climbing can be a threat to sensitive plants of the limestone cliff community.
To assess whether winter mowing in wetlands fulfils the aim of preventing succession towards drier communities, 34 permanent quadrats (15 m2) were surveyed annually from 1984–1985 to 2000 within large mown and unmown (control) areas (several ha) in a calcareous lake shore fen (W Switzerland). Three trends were noticed: decrease of aquatic species, spread of Cladium mariscus and establishment of woody species (especially Alnus glutinosa and Frangula alnus). None of these trends was prevented by mowing, but mowing did prevent the accumulation of C. mariscus litter and kept woody saplings small. Succession was generally slow and often occurred in the form of sudden, discrete changes. Plant species richness increased with mowing and remained constant without mowing. Soil disturbance by the mowing machine contributed more to the effects of management on species composition than the periodic removal of biomass. It is concluded that mowing every three years in winter is insufficient to preserve semi-aquatic communities against succession but sufficient to maintain the plant species richness of a low productive, regularly flooded fen.
In this study we compared the effects of fire on understorey vegetation in the Québec southern boreal forest with effects of salvage-logging (clear-cutting after fire). All 61 400-m2 sampling sites were controlled for overstorey composition (Deciduous, Mixed and Coniferous) and disturbance type, which consisted of three fire impact severity (FIS) classes (Light, Moderate and Extreme) and two harvesting techniques (Stem-only and Whole-tree Harvesting). Percent-cover data of vegetation and post-disturbance environmental characteristics were recorded in the field during the first two years after fire as well as soil texture. Ordination of fire alone demonstrated that, on Coniferous sites, fire initiates a succession whereby the understorey Coniferous sites approaches that of Deciduous-Mixed sites, due to the release of the understorey from Sphagnum spp. dominance, this pattern being a function of FIS. On Deciduous-Mixed stands, increased FIS resulted in a transition from herb to shrub dominance. Ordination of all five disturbance types showed that the impact of salvage-logging on understorey composition was within the range of fire, but marginalized to the extreme end of the FIS spectrum. Variance partitioning demonstrated that overstorey and soil texture were the most important explanatory variables of fire alone, while disturbance type explained the largest independent fraction of understorey variation when salvage-logging was introduced. Salvage-logging also results in significant reductions in understorey abundance, richness and diversity, while indicator species analysis suggests that it favours meso-xerophytic to xeric species. Results are interpreted in light of shade-tolerance dynamics, forest floor disturbance and soil moisture regimes. Implications for sustainable forest management are discussed.
In fragmented woodlands in agricultural regions of Australia, roadside environments (road verges) provide important refuges for threatened native fauna and isolated populations of plant species. However, as roads are transport corridors for humans and their vehicles, species survival is affected through destruction, fragmentation and modification of remaining habitat by human activity. Few studies have recognised the effects of anthropogenic disturbances, in the form of historical roadworks activities, on adjacent roadside plant populations. The aim of this study was to investigate the effects of soil disturbance from roadworks on the spatial patterns and structural dynamics of roadside Acacia populations in an agricultural area in southeastern Australia. Stem size and disturbance data were collected from 135 roadside populations of three species of Acacia shrubs in the Lockhart Shire area. Spatial pattern analysis using the Network K-function and Discriminant Function analyses showed that road verge width, road category, disturbance intensity and distance to nearest town were highly significant variables in relation to disturbance from roadworks and shrub structural dynamics. Colonising populations were more abundant along gravel roads where soil disturbance intensity was high, whereas stable populations were more abundant where soil disturbance intensity was low. Senescent populations were more abundant in narrow, little used road verge corridors 4–6 km from nearby towns. These results suggest that anthropogenic disturbance regimes may be critical to shrub recruitment and persistence in roadside environments, which has important consequences for conservation of threatened native flora and fauna in agricultural landscapes.
This paper describes the use of supervised methods for the classification of vegetation. The difference between supervised classification and clustering is outlined, with reference to their current use in vegetation science. In the paper we describe the classification of Danish grasslands according to the Habitats Directive of the European Union, and demonstrate how a supervised classification can be used to achieve a standardized and statistical interpretation within a local flora. We thereby offer a statistical solution to the legal problem of protection of certain selected habitat types. The Habitats Directive protects three types of Danish grassland habitats, whereas two remaining types fall outside protection. A classification model is developed, using available Danish grassland data, for the discrimination of these five types based on their species composition. This new Habitats Directive classification is compared to a previously published unsupervised classification of Danish grassland vegetation. An indicator species analysis is used to find significant indicator species for the three protected habitat types in Denmark, and these are compared to the characteristic species mentioned in the interpretation manual of the Habitats Directive. Eventually, we discuss the pros and cons of supervised and unsupervised classification and conclude that supervised methods deserve more attention in vegetation science.
Abbreviations: DCA = Detrended Correspondence Analysis; CCA = Canonical Correspondence Analysis; CVA = Canonical Variates Analysis; HD = Habitats Directive; TWINSPAN = Two-way indicator species analysis.
We assessed survival and growth of transplanted saplings of understory broad-leaved trees (Oreopanax xalapensis, Rapanea juergensenii, Rhamnus sharpii and Ternstroemia lineata) and canopy conifers (Abies guatemalensis, Pinus ayacahuite and P. pseudostrobus) into Grassland (GR), Mid-Successional Oak Forest (MS), and Old-Growth Oak Forest (OF) stands in the central highlands of Chiapas, Mexico. A total of 727 plants were monitored over eight years. The results suggest habitat preferences of the studied species that should be considered for their conservation oriented management. Conifers had highest survival and growth in GR, while broad-leaved trees survived better under sparse canopy in MS, but grew tallest in full light. Saplings of all species in dense canopy sites in OF were smaller, and the survival of conifers was lower. An integrated response index (IRI) calculated for each species with survival and relative growth rates showed similar trends. Re-introduction of the endangered A. guatemalensis is possible in open habitats if fire, trampling and grazing are suppressed. Open habitats in current man-made landscapes in the highlands of Chiapas may limit the establishment of understory trees requiring partial shade. Inclusion of such species in restoration programs has not been usual so far; yet, local and global interest in native species for such programs is increasing. Information on the response of these kind of species in contrasting habitats may help to incorporate them in the management of high-diversity forests that may follow pine plantations.
Abbreviations: EBH = Estación Biológica Huitepec; GR = Grassland; IRI = Integrated Response Index; MS = Mid-Successional Oak Forest; OF = Old-Growth Oak Forest; PPF = photosynthetic photon flux; RGR = relative growth rate.
Shrub encroachment, i.e. the increase in woody plant cover, is a major concern for livestock farming in southern Kalahari savannas. We developed a grid-based computer model simulating the population dynamics of Grewia flava, a common, fleshy-fruited encroaching shrub. In the absence of large herbivores, seeds of Grewia are largely deposited in the sub-canopy of Acacia erioloba. Cattle negate this dispersal limitation by browsing on the foliage of Grewia and dispersing seeds into the grassland matrix. In this study we first show that model predictions of Grewia cover dynamics are realistic by comparing model output with shrub cover estimates obtained from a time series of aerial photographs. Subsequently, we apply a realistic range of intensity of cattle-induced seed dispersal combined with potential precipitation and fire scenarios. Based on the simulation results we suggest that cattle may facilitate shrub encroachment of Grewia. The results show that the severity of shrub encroachment is governed by the intensity of seed dispersal. For a high seed dispersal intensity without fire (equivalent to a high stocking rate) the model predicts 56% shrub cover and 85% cell cover after 100 yr. With fire both recruitment and shrub cover are reduced, which may, under moderate intensities, prevent shrub encroachment. Climate change scenarios with two-fold higher frequencies of drought and wet years intensified shrub encroachment rates, although long-term mean of precipitation remained constant. As a management recommendation we suggest that shrub encroachment on rangelands may be counteracted by frequent fires and controlling cattle movements to areas with a high proportion of fruiting Grewia shrubs.
In recent decades, cattle ranchers of the Pantanal of Mato Grosso, Brazil, have pointed to the accelerated spread of several herbaceous and woody plant species that invade natural and artificial pastures (campos). It has been speculated that overgrazing by an increasing number of cattle, lack of grazing in abandoned areas, or large-scale changes in environmental conditions may be the reason for this invasion. This study focuses on ecological and ecophysiological aspects of Vochysia divergens (cambará), a flood-tolerant tree that began spreading in the Pantanal during the last 30 years and is considered a very aggressive invasive plant. The study shows that the spread of cambará can be related to natural multi-years wet periods. During multi-years dry periods the species is reduced by the increasing impact of fires in the Pantanal. This points to the great importance of multi-years climatic events on the vegetation cover of the Pantanal and indicates a very dynamic development in plant communities.
Over the past decades outdoor recreation has become progressively more important and as a result human-induced potential damage has increased. In this study, short-term effects – a 2-yr period – of human trampling on some common forest and heath communities in Central Belgium were studied experimentally. Vulnerability to disturbance was compared among plant communities in terms of resistance, resilience and tolerance, which are based on cover measurements. The herb layer of the examined mesophilous forest communities appeared to be more sensitive than the heath and dry forest community, which were dominated by more resistant graminoid (Molinia caerulea, Deschampsia flexuosa) and dwarf-shrub species (Vaccinium myrtillus, Calluna vulgaris, Erica tetralix). The analysis showed that site structure and vegetation were already affected by low intensities of trampling, while vegetation recovery during the first year after trampling was limited in most plant communities. Recovery during the second year in vegetation cover as well as height was most pronounced in mesophilous forest communities. Occasional trampling clearly can lead to increased visual evidence of previous use and continued recreational disturbance. Therefore management plans should discourage hiking activity off paths and restrict recreation activities to the least vulnerable communities.
Rich-fen vegetation influenced by hay-making in the Sølendet Nature Reserve, Central Norway, was fertilized with N, P and K in a full-factorial fertilization experiment to investigate the nutrient limitation of plant growth at both community and species levels. Above-ground biomass, shoot density and nutrient concentration were measured in several species and groups of species at three sites after two years of fertilization. At the community level, the results indicate multiple limitation by N and P in the two least productive rich-fen communities: one characterized by small sedges and herbs, and the other by high abundance of Menyanthes trifoliata and tall sedges. Increased nutrient availability had no effect on a more highly productive, tall-growing, spring-influenced community, indicating no nutrient limitation. The results at the species level correspond well with those at the community level, indicating multiple limitation by N and P in most of the dominant and sub-dominant species. However, P seems to limit growth more than N in Succisa pratensis, and N seems to limit growth more than P in Carex panicea. Furthermore, Eriophorum angustifolium seems to be limited by K. The results did not show which nutrient limits the growth of Carex dioica, C. lasiocarpa and Trichophorum cespitosum. Indications that growth in low-productive, boreal rich-fen communities is generally limited by P was not confirmed.
A method is discussed for describing the intensity of disturbance, based on the floristic composition of vegetation types. It includes the development of a hemerobiotic scale (from hemeros = cultivated). We attempted to relate this scale based on floristic patterns with functional parameters.
Following a large phytosociological survey of the Roman area, we selected 19 species of herbs and grasses, widely differing in ‘hemeroby’, and measured their Relative Growth Rate (RGR). RGR varies between 0.13 and 0.21 d−1. A significant correlation between mean hemerobiotic value, resulting from the phytosociological study, and measured RGR of juvenile plants emerges. Although Specific Leaf Area (SLA) is highly correlated with RGR, and thus explains most of its variations, as previously found in other studies, it is only weakly related to hemerobiotic state. An additional data set with 78 species from the literature supports the conclusions. This approach allows a functional quantification of hemeroby which enables useful applications, for instance in cartography. A biotope map showing a steep gradient of disturbance impact in the city of Rome is presented as a preliminary test.
A southern ridge sandhill site in central Florida, USA, was burned in 1989, 1991, and 1995 after 63 years of fire-suppression to simulate a pre-settlement fire regime. Fire changed species abundance and vegetation structure but caused only minimal changes in species turnover and diversity. There was a general trend for an increase in the cover of herbs following fire but this was a statistically significant effect for only one species, Liatris tenuifolia var. tenuifolia. Aristida beyrichiana increased, litter cover and litter depth were significantly reduced, and ground lichens were eliminated in response to burning. Scrub oaks and palmettos in the ground cover and small shrub layers (height ≤ 1 m) either increased or did not respond to burning, reflecting strong post-fire resprouting. Diversity in the ground cover and small shrub layers were not affected by fire. Scrub oaks and palmettos in the large shrub and overstorey layers (height > 1 m) were reduced in density, basal area, and longest canopy measurements in response to fire. Species diversity also decreased within these layers following fire. Some Pinus elliottii var. densa survived fire, but their density was reduced. All Pinus clausa were eliminated by fire. Periodic burning can suppress the dominance of shrubs (Quercus spp.) while increasing the cover of grasses and herbs in southern ridge sandhill vegetation.
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