Variation In Vegetation Composition Research Articles

The contribution of termite mounds to landscape‐scale variation in vegetation in a West African national park

AbstractQuestionsTermite mounds are important sources of local‐scale soil heterogeneity and thereby affect savanna vegetation. Studies contrasting mound and savanna vegetation usually neglect variation between savanna types. We aim to answer the following questions: (1) is mound vegetation consistently different from the surrounding savanna vegetation at landscape scale; (2) is mound vegetation homogeneous across different savanna types; and (3) are differences between mound and savanna vegetation influenced more by nutrient accumulation or clay enrichment in mounds?LocationPendjari National Park, northern Benin.MethodsWe analysed the vegetation in 59 pairs of mound and savanna plots in four savanna types. We examined if mounds differed in community composition between savanna types, and from savanna plots using ordinations, and tested for correlations between community composition and soil parameters. We identified characteristic plant species for mounds in each savanna type using the IndVal method. Finally, we evaluated the relative importance of different soil parameters for differences in vegetation between mound and savanna plots using projection to latent structures regression (PLSR).ResultsThere were clear differences between savanna types for both savanna and mound plant communities, and in each savanna type, clear differences emerged between mound and savanna vegetation. Community differences correlated with differences in soil parameters in all cases. We identified ten characteristic plant species for mounds, with little overlap between characteristic species from different savanna types. Across savanna types, 63% of the variation in mound – savanna vegetation contrast was explained by variation in nine soil parameters, most importantly clay content.ConclusionEven at the landscape scale, the influence of mounds on vegetation depends on habitat context. Thus, local mound vegetation patterns can only be understood considering the influence of different soil parameters in each savanna type. However, enrichment of clay and macronutrients together explain the largest part of the differences between mound and savanna vegetation. Further variation in vegetation composition among mounds is likely driven by biotic interactions and spatial distribution of mounds.

Climate change and ecosystem composition across large landscapes

ContextClimate change alters the vegetation composition and functioning of ecosystems. Measuring the magnitude, direction, and rate of changes in vegetation composition induced by climate remains a serious and unmet challenge. Such information is required for a predictive capability of how individual ecosystem will respond to future climates.ObjectivesOur objectives were to identify the relationships between 20 climate variables and 39 ecosystems across the southwestern USA. We sought to understand the magnitude of relationships between variation in vegetation composition and bioclimatic variables as well as the amount of ecosystem area expected to be affected by future climate changes.MethodsBioclimatic variables best explaining the plant species composition of each ecosystem were identified. The strength of relationships between beta turnover and bioclimate gradients was calculated, the spatial concordance of ecosystem and bioclimate configurations was shown, and the area of suitable climate remaining within the boundaries of contemporary ecosystems under future climate projections was measured.ResultsAcross the southwestern USA, four climate variables account for most of the climate related variation in vegetation composition. Twelve ecosystems are highly sensitive to climate change. By 2070, two ecosystems lose about 4000 (15 %) and 7000 (31 %) km2 of suitable climate area within their current boundaries (the Western Great Plains Sandhill Steppe and Sonora-Mojave Creosotebush-White Bursage Desert Scrub ecosystems, respectively). The climatic areas of riparian ecosystems are expected to be reduced by half.ConclusionsResults provide specific climate and vegetation parameters for anticipating how, where and when ecosystem vegetation transforms with climate change. Projecting the loss of suitable climate for the vegetation composition of ecosystems is important for assessing ecosystem threats from climate change and for setting priorities for ecosystem conservation and restoration across the southwestern USA.

Ecohydrological feedbacks in peatlands: an empirical test of the relationship among vegetation, microtopography and water table

AbstractCurrent models and theories of the formation and maintenance of microtopography in ombrotrophic peatlands (bogs) assume autogenic feedbacks between vegetation composition, water table depth (WTD) and microtopography. A hypothesized outcome of autogenic feedbacks is a strong association among spatial variations in vegetation composition, WTD and microtopography. We tested and corroborated this hypothesis using fine spatial scale (<2 × 2 m2) data from two 20 × 20 m2 plots at Mer Bleue, a temperate bog. Furthermore, we partitioned the spatial variation of plant communities into portions explained by WTD as well as fine‐scale and broad‐scale spatial structures using distance‐based Moran's eigenvector maps. We hypothesized that plant distributions are more strongly related to WTD than to microtopography and found that this hypothesis was supported in only one of the two sampled plots, suggesting that the feedbacks among WTD, vegetation and microtopography could be dependent on location within a bog. A plot closer to the centre (apex) of the bog showed stronger relationships among WTD–microtopography and vegetation than a plot closer to the margin. Our results support current models and theories of the development of bogs wherein plant communities, water table and microtopography are strongly associated because of underlying ecohydrological feedbacks but highlight that strength and direction of feedbacks may vary by location within a bog. Affirming the presence of these structural relationships and identifying variability in them is a key step towards better understanding peatland carbon cycling, especially in the context of increasing anthropogenic and natural disturbances to peatlands. Copyright © 2016 John Wiley & Sons, Ltd.

Variation in vegetation composition, biomass production, and carbon storage in ridge top forests of high mountains of Garhwal Himalaya

The present study was aimed to anticipate how forest composition, regeneration, biomass production, and carbon storage vary in the ridge top forests of the high mountains of Garhwal Himalaya. For this purpose five major forest types—(a) Pinus wallichiana, (b) Quercus semecarpifolia, (c) Cedrus deodara, (d) Abies spectabilis, and (e) Betula utilis mixed forests—were selected on different ridge tops in the Bhagirathi Catchment Area of the Uttarkashi District of Garhwal Himalaya. The highest species richness (10 species) and stand density (804 ± 184.5 stems ha−1) were recorded in Abies spectabilis forests, whereas lowest species richness (4 species) and species density (428 ± 144.7 stems ha−1) were found in Quercus semecarpifolia forests. The total basal cover (TBC) values were maximum (91.1 ± 24.4 m2 ha−1) in Cedrus deodara forests and minimum (26.5 ± 11.7 m2 ha−1) in Pinus wallichiana forests. The highest total biomass density (TBD) (464.2 ± 152.5 Mg ha−1) and total carbon density (TCD; 208.9 ± 68.6 Mg C ha−1) values were recorded for Cedrus deodara forests; however, lowest TBD (283.4 ± 74.8 Mg ha−1) and TCD (127.5 ± 33.7 Mg C ha−1) values for Quercus semecarpifolia forests. Our study suggests that Abies spectabilis-dominated forests should be encouraged for biodiversity enrichment and reducing carbon emissions on ridge top forests of high mountains.

High frequency floral changes at the Paleocene–Eocene boundary revealed by comparative biomarker and palynological studies

The Cap d’Ailly area (Upper Normandy, France) shows several terrestrial–lagoonal sections recording the negative carbon isotope excursion (CIE) associated with the Paleocene–Eocene Thermal Maximum (PETM; 55.8Ma). A study of the biomarkers and spore/pollen content of the Vasterival section gave complementary information on paleofloral changes that occurred around the Paleocene–Eocene (P–E) boundary. Aliphatic hydrocarbon fractions revealed a high abundance of vascular plant biomarkers, including tricyclic diterpanes derived from conifers, fernenes derived from ferns and des-A-triterpenes derived from angiosperms. Whereas the vegetation of the depositional environment surroundings seemed alternately dominated by ferns and angiosperms (revealed from biomarkers), the regional flora seemed dominated strictly by angiosperms such as Juglandaceae (revealed by pollen). Spore/pollen assemblages revealed no major turnover in the paleovegetation, but slight variation in vegetation composition. However, both biomarkers and spores/pollen exhibited two intervals of rapid floral change: the first occurred around the P–E boundary and the second at the top of the strictly terrestrial unit that coincides with a marine transgression. Comparison of the data with published paleoclimatic results, obtained from the same samples, indicated that the first interval of floral change correlates with strong paleohydrological perturbations, whereas the second is linked to an increasing marine influence.

Seasonal and interannual variation in vegetation composition: Implications for survey design and data interpretation

AbstractUnderstanding how vegetation composition varies with season and interannual climate variability is important for any ecological research that uses vegetation data derived from surveys for the basis of inference. Misunderstanding this variation can influence land management and planning decisions, leading to poor implementation of biodiversity offsetting mechanisms, for example. We monitored plots (400 m2) grazed by livestock paired with adjacent ungrazed plots in derived native pastures four times a year over 2.5 years on the North‐West Slopes of New South Wales. Species density in plots varied greatly with season and interannual rainfall. Highest species density was recorded in spring, though species density in summer was not significantly lower, nor was a spring–summer peak in species density evident in the 2009 drought. Surveys in spring 2008 had the highest species density, and recorded only 60–72% of the total species recorded at each site over 2.5 years. Variation in the proportion of total site diversity represented in combinations of two or three surveys was large, though the best combinations comprised surveys from spring and summer in years of above‐average rainfall, either from the same spring‐summer, or from different years. Compositional differences among sites were much greater than within sites, showing that differences among sites related to broad environmental gradients were not overwhelmed by seasonal and interannual variability in site composition. When grazing was excluded, there was no evidence of competitive exclusion by the dominant grasses, and no directional shift in composition. The implications of these findings for ecological research depend on the question being addressed: if capturing a large proportion of site diversity is important, then surveys must be carefully timed, or repeat surveys must be conducted. Single surveys did not effectively capture site diversity for use in biodiversity offsetting, and the timing of repeat surveys was critical.

A long‐term perspective on woody plant encroachment in the desert southwest, New Mexico, USA

AbstractObjectiveTo assess potential causes of woody plant encroachment into desert grassland systems using a 5500‐yr‐old sediment record from a ciénega in New Mexico.LocationCloverdale Ciénega, southwestern New Mexico.MethodsUtilizing fossil pollen and charcoal preserved in wetland sediments, a long‐term record of variations in vegetation composition and fire activity was generated. The record was compared to published data on drought, the El Niño/Southern Oscillation (ENSO), carbon dioxide concentrations and known dates of grazing impacts to evaluate various hypotheses regarding the timing and causes of woody plant encroachment in desert grasslands.ResultsThe woody plant encroachment observed in the past 200 yrs is unprecedented in the context of the previous 5500 yrs of vegetation history. The observed increase in woody plant abundance was not related to droughts or changes in ENSO event frequency, and was contemporaneous with the rise in atmospheric CO2 and known grazing impacts. Charcoal influx increased in conjunction with historic woody plant pollen abundance and therefore the encroachment was not related to fire exclusion.ConclusionsThe long‐term sediment record from Cloverdale Ciénega provides the context for evaluation of the various hypotheses for woody plant encroachment. At Cloverdale Ciénega we demonstrate that woody plant encroachment of the last 200 yrs is unprecedented and outside of the historic natural range of variability of the previous five millennia. We also demonstrate that woody plant encroachment is not related to climatological changes such as drought or increases in ENSO frequency. Increases in woody plant pollen are associated with increases in carbon dioxide, and high‐intensity grazing during the late 19th and early 20th centuries.

A spatially predictive baseline for monitoring multivariate species occurrences and phylogenetic shifts in mediterranean southern Australia

AbstractQuestionClimate change is driving shifts in the composition of vegetation but the lack of controls and confounding spatial factors pose challenges for detecting the climate signal in observed changes through time and space. We tested whether climate can be isolated as a driver of spatial vegetation composition at the landscape scale in mediterranean southern Australia by considering landscape factors (e.g. soil gradients) and spatial structure (relative geographic isolation). The aim was to develop principles for selecting spatial analogues for climate change and provide a spatially predictive baseline for monitoring.MethodsA landscape‐scale monitoring transect spanning 550 km was established. Whole community presence/absence of vascular plant species in plots was modelled as a multivariate response to environmental and spatial variables. Species and phylogenetic composition–environment relationships were also explored using indirect gradient approaches, partial correlations and distance decay models.ResultsA total of >2900 occurrences of >400 plant species were recorded. Relative vegetation composition was predicted by mean temperature and soil properties, such as electrical conductivity and texture. Spatial structure was critical, as decay in compositional similarity with geographic distance and spatial autocorrelation of nested plots were involved in turnover patterns. The rate of change in species composition with changes in temperature equated to complete species turnover within the habitats sampled.ConclusionsThe influence of climate on spatial variation in vegetation composition can be quantified, accounting for distance decay. Landscape gradients (particularly soil properties) tended to be orthogonal to climate and explained some turnover. Spatial analogues for climate change would need to be similar in soil properties and not too geographically distant. Composition resulting from more extreme climate change scenarios may have no spatial analogue due to the importance of neutral distance decay at larger spatial scales in determining compositional differences. We illustrate these principles with a sequence of warming, drying analogues. The spatial transect provides a framework for monitoring composition by directly incorporating temporal data and using spatial analysis to inform the expected direction of compositional shifts with climate change.

Using habitat extent and composition to predict the occurrence of woodland birds in fragmented landscapes

The removal, alteration and fragmentation of habitat are key causes of biodiversity decline worldwide. In Australia, temperate woodlands have been disproportionately cleared following European settlement. Biodiversity decline in such systems may be reversed by restoration of native vegetation on agricultural land. However, rebuilding functioning habitat will require understanding the determinants of species distributions in existing habitat. We used logistic regression of bird occurrence data from 240 sites across northern Victoria, to determine the probability of occurrence of 29 woodland-dependent bird species. We modelled occurrence as a function of habitat variables that characterise both the extent (amount) and composition of native vegetation surrounding sites. Our specific goal was to determine whether the predictive performance of models is improved by accounting for both extent and composition of native vegetation compared with models that characterise native vegetation by extent alone. For nearly all species, accounting for vegetation composition in addition to extent and weighting habitat variables by distance improved the explanatory power of models, explaining on average 5.4 % (range 0–27.6 %) of the residual uncertainty in models that accounted for extent alone. Models that incorporate variation in vegetation composition can not only provide more accurate predictions of species occurrence, but also guide more appropriate restoration. They highlight the need for restoration to incorporate sites with fertile soils that support productive vegetation types. These models of woodland birds will be used to inform a spatially-explicit optimisation model for restoring native vegetation cover on agricultural land in this region, with the goal of achieving biodiversity gains while minimizing loss to production.

Regional and local patterns of riparian flora: Comparison between insular and continental Mediterranean rivers

We studied riparian species distribution patterns along 4 rivers in 2 Mediterranean regions, an island and a continental area. Our aim was to determine the following aspects at 2 spatial scales: i) floristic patterns; ii) major gradients affecting riparian flora; iii) the role of biogeography on riparian vegetation. A stratified random sampling design with segments identified as strata was used. Univariate and multivariate statistics were applied to detect patterns of variation in the species data and main floristic gradients. At regional scale, variation in life forms and phytogeographical element composition showed strongest correlations with climatic variables. At local scale only the continental rivers had many differences in phytogeographical elements linked to their greater length with respect to insular rivers. With regard to the distributions of the riverbed flora, geography emerged as the main gradient at the regional scale. The structural gradient was important at local scale, even if the lateral gradient drove the greatest variation in vegetation composition in continental rivers. Variance partitioning underlined that the biogeographical location strongly affected riverbed flora patterns. Our results support the idea that the flora of Mediterranean riparian ecosystems is not strictly “azonal” but can be defined as “semi-azonal”. Species distribution showed a strong association with climate, which was the primary correlate of change in floristic composition between riparian sites at both scales.

A model of geographical, environmental and regional variation in vegetation composition of pyrogenic grasslands of Florida

AbstractAim To develop a landscape‐level model that partitions variance in plant community composition among local environmental, regional environmental, and purely spatial predictive variables for pyrogenic grasslands (prairies, savannas and woodlands) throughout northern and central Florida.Location North and central Florida, USA.Methods We measured plant species composition and cover in 271 plots throughout the study region. A variation‐partitioning model was used to quantify components of variation in species composition associated with the main and interaction effects of soil and topographic variables, climate variables and spatial coordinates. Partial correlations of environmental variables with community variation were identified using direct gradient analysis (redundancy analysis and partial redundancy analysis) and Monte Carlo tests of significance.Results Community composition was most strongly related to edaphic variables at local scales in association with topographic gradients, although geographically structured edaphic, climatic and pure spatial effects were also evident. Edaphic variables explained the largest portion of total variation explained (TVE) as a main effect (48%) compared with the main effects of climate (9%) and pure spatial factors (9%). The remaining TVE was explained by the interaction effect of climate and spatial factors (13%) and the three‐way interaction (22%). Correlation analyses revealed that the primary compositional gradient was related to soil fertility and topographic position corresponding to soil moisture. A second gradient represented distinct geographical separation between the Florida panhandle and peninsular regions, concurrent with differences in soil characteristics. Gradients in composition corresponded to species richness, which was lower in the Florida peninsula.Main conclusions Environmental variables have the strongest influence on the species composition of Florida pyrogenic grasslands at both local and regional scales. However, the limited distributions of many plant taxa suggest historical constraints on species distributions from one physiographical region to the other (Florida panhandle and peninsula), although this pattern is partially confounded by regionally spatially structured environmental variables. Our model provides insight into the relative importance of local‐ and regional‐scale environmental effects as well as possible historical constraints on floristic variation in pine‐dominated pyrogenic grasslands of the south‐eastern USA.

Grazing as a factor structuring grasslands in the Pyrenees

Questions: What are the relative roles of abiotic and grazing management factors on plant community distribution in landscapes? How are livestock type and stocking rate related to changes in vegetation structure and composition?Location: Sub‐alpine grasslands in the central and eastern Pyrenees.Methods: Multivariate analysis and variance partitioning methods were used to evaluate the relative roles of environmental factors in structuring vegetation composition and diversity patterns in three surveys on differently managed grasslands.Results: Vegetation composition within a region was affected by environmental factors hierarchically, changing first according to abiotic factors and then to grazing management. At landscape scales, abiotic factors explained two‐fold more variation in vegetation composition than grazing factors. Within landscape units, cattle grazing increased vegetation heterogeneity at landscape and patch scales, while sheep grazing favoured the presence of a specific set of species with high conservation value. Species composition was highly responsive to management variables compared to diversity components.Conclusions: The combination of sheep and cattle grazing at various stocking rates is an effective tool to preserve the diversity of plant species and communities within a region with a long tradition of livestock management, through the scaling up of effects by local processes occurring in patches at smaller scales.

South Patagonian ombrotrophic bog vegetation reflects biogeochemical gradients at the landscape level

AbstractQuestion: Which environmental variables affect the floristic composition of south Patagonian bog vegetation along a gradient of climate and biogeochemical changes with increasing distance from the Pacific ocean?Location: Trans‐Andean transect (53° S), southern PatagoniaMaterial and Methods: Floristic composition, peat characteristics (water level, decomposition, pH, total nitrogen, total carbon, ash content and plant available P, K, Na, Ca, Mg, Fe, Mn, Zn, and Al) and climatic constraints of ombrotrophic peat‐lands were measured at 82 plots along a gradient of increasing distance from the Pacific Ocean.Results: Climatic constraints and biogeochemical peat characteristics significantly change with increasing distance from the Pacific. Peatland vegetation shifted from hyperoceanic blanket bogs dominated by cushion forming vascular plants to the west to Sphagnum bogs to the east. Climatic and biogeochemical variables explained a large proportion of the floristic variation along the first DCA axis. The second axis represented a water level gradient. When ‘distance to the Pacific’ was defined as a covariable in partial CCA, the proportion of variance explained declined for most other variables, especially in the case of annual precipitation and exchangeable base cations and related traits. The differences in biogeochemical characteristics related to peat were mainly attributed to the input of sea‐borne cations.Conclusions: While variation in vegetation composition along a longitudinal gradient crossing the southern Andes was attributed to climatic constraints as expected, vegetation composition was also strongly affected by the biogeochemical characteristics of peat. Sea spray was of high ecological importance to peat chemistry and, consequently, to floristic composition. Presumably, south Patagonian peat bogs represent a glimpse of pre‐industrial environments, so that these peat bogs may act as reference systems with respect to atmospheric inputs in mire ecology research.

Floristic composition, environmental variation and species distribution patterns in burned grassland in southern Brazil

In regularly burned grassland on Morro Santana, Porto Alegre, RS, Brazil, we investigated differences in the floristic composition and their relation to soil properties, aspect and distance from the forest border. In 48 plots of 0.75 m2, we identified a total of 201 species from a local species pool of approximately 450 to 500 species. Most species occurred in low frequencies, showing clumpy distribution patterns in the studied area. Multivariate analysis showed that plots close to the forest edge clearly differed from plots in the open grassland concerning composition and structure. Plots exposed to the north differed from plots on the top of the hill both in the composition of species as well as in soil variables, mainly due to shallower soil in the former. No strong relation between soil properties and variation in vegetation composition could be detected at a finer scale. The studied grassland, as all grassland vegetation in southern Brazil, is very rich in species compared to other grassland formations worldwide. However, this high biodiversity and conservational value of Campos vegetation in general has so far not been recognized properly. Disturbance is essential to maintain this open vegetation type and its species richness. Fire should be considered as a management option in the absence of grazing.

Three hundred years of forest and land‐use change in Massachusetts, USA

Abstract Aim The aim of this study was to document changes in forest composition, structure and distribution across Massachusetts, USA, from the time of European settlement (seventeenth century) to the present, and to investigate environmental and historical influences on regional patterns of variation.Location The study area encompasses the State of Massachusetts (69.9–73.5°E, 41.3–42.9°N), a 21,000‐km2 area in the north‐eastern United States.Methods A wide range of historical sources was used to document changes in land use and land cover for the historical period. Witness trees from early land surveys enabled us to evaluate vegetation patterns prior to widespread European settlement, and to compare historical and modern species composition. Nineteenth century maps of forest cover and contemporary agricultural censuses documented forest patterns during the peak agricultural period. Geographic Information System analyses were used to relate variation in climate, geology and land‐use history to historical and modern forest composition.Results Massachusetts has a complex east‐to‐west environmental gradient involving changes in physiography, climate, geology and natural disturbance. Until the middle of the twentieth century, agriculture was the most important land‐use across the region; although the percentage of land in agriculture and the timing of major land‐use changes were remarkably consistent across the state, historical forest patch sizes varied locally and regionally in relation to physiography. Forest composition of both early historical and modern forests is most strongly related to environmental conditions, especially variation in climate. Historical land‐use resulted in a state‐wide increase in early successional tree species and a dramatic, although recovering, change in forest structure.Main conclusions At a regional scale, environmental conditions apparently control broad patterns of variation in vegetation composition. Historical land‐use practices were relatively homogenous across Massachusetts and local variation was reduced through data averaging at broad spatial scales. At finer spatial scales, historical land‐use has strong and persistent impacts on vegetation composition and structure.

Constructed shallow zones along navigation canals: vegetation establishment and change in relation to environmental characteristics

AbstractAn Erratum has been published for this article in Aquatic Conservation: Marine and Freshwater Ecosystems 12(2), 2002, 251 In recent decades shallow zones have been constructed along navigation canals in the Netherlands which form a potential new habitat for aquatic macrophytes and helophytes absent from traditional canals. The aim of this study was to examine the relationships between the aquatic plant vegetation that becomes established and the physical and chemical characteristics of water and sediment, in order to find the determinants of species composition and the changes therein. Data were collected in 1998 and 1999 from 80 plots in bank zones at varying stages of development since construction along two navigation canals. Plots 3–5 years old were partly dominated by rooting submerged macrophytes such as Potamogeton pectinatus, Elodea nuttallii and Potamogeton pusillus; locally non‐rooting species occurred such as Ceratophyllum demersum, Lemna minor and Spirodela polyrhiza. Older plots contained Phragmites australis, locally mixed with free‐floating species. Hydrological isolation from the eutrophic canals was indicated by the presence of Chara vulgaris. Characteristics of both water layer and sediment could explain the variation in vegetation composition. Rooting submerged macrophytes predominantly occurred in sites with a thin (<2 cm) layer of sediment with relatively low concentrations of organic matter; moreover, ammonium concentrations in the water layer and sediment pore water were relatively low. Stands of non‐rooting macrophytes and of Phragmites australis were characterized by a relatively thick sediment layer and high ammonium levels in the pore water. Light limitation in turbid water, associated with navigation and eutrophication, may also play a role. Although submerged aquatic macrophytes persist for a relatively short time, shallow zones nevertheless function as a habitat for helophyte communities and contribute to a higher aquatic biodiversity than is associated with traditional banks along navigation canals. Copyright © 2001 John Wiley & Sons, Ltd.

Unifying the relationships of species richness to productivity and disturbance.

Although species richness has been hypothesized to be highest at 'intermediate' levels of disturbance, empirical studies have demonstrated that the disturbance-diversity relationship can be either negative or positive depending on productivity On the other hand, hypothesized productivity diversity relationships can be positive, negative or unimodal, as confirmed by empirical studies. However, it has remained unclear under what conditions each pattern is realized, and there is little agreement about the mechanisms that generate these diverse patterns. In this study, I present a model that synthesizes these separately developed hypotheses and shows that the interactive effects of disturbance and productivity on the competitive outcome of multispecies dynamics can result in these diverse relationships of species richness to disturbance and productivity The predicted productivity diversity relationship is unimodal but the productivity level that maximizes species richness increases with increasing disturbance. Similarly, the predicted disturbance diversity relationship is unimodal but the peak moves to higher disturbance levels with increasing productivity Further, these patterns are well explained by the opposite effects of productivity and disturbance on competitive outcome that are suggested by the change in community composition along these two environmental gradients: higher productivity favours superior competitors while higher disturbance levels favour inferior competitors.

Habitat structure and vegetation relationships in central Argentina salt marsh landscapes

Relationships between habitat structure and spatial variations in vegetation composition were determined in catenas of central Argentina salt marsh landscapes. Vegetation was classified following a multi-technique strategy. An analysis of species distributions along an environmental gradient was made and a redundancy analysis was used to relate the environmental variables to vegetation data. The spatial covariation was evaluated through fractal analysis. The vegetation can be divided into four discrete noda that correspond to different topographic positions (summit, backslope, footslope and toeslope, respectively): nodum of Stipa trichotoma + St. tenuissima, nodum of Distichlis scoparia, nodum of Distichlis spicata and nodum of Spartina densiflora. Each of these nodum is characterized by a definite combination of floristic groups. Symmetric aggregation of vegetation borders was observed in all three sites. The existence of vegetation discontinuities along the catenas depended largely on water table depth and salinity which in turn controlled the edaphic salinity. The arrangement of sites by multivariate analysis reflected the influence of a complex gradient representing halomorphic and hydromorphic factors. β-diversity was associated with abrupt changes in the physical structure of the catena for a reduced spatial dimension (300-m scale). Absolute diversity and evenness were higher on the summit and declined progressively toward the toeslope. The rate of change was higher on the backslope and the dominant species have different ecological amplitudes overlapping along the gradient. The main operational factors associated with the floristic differences are: (i) the variations in the chemical composition and the seasonal dynamics of the soil solution in the aerated layer of soils, (ii) the salinity and dynamics of groundwater, and (iii) the length of time that the soil is flooded during the rainy season (summer). The fractal dimension was close to 2, implying weak spatial dependence. Fractal dimension varied as a function of scale. The fractogram only revealed a significant spatial dependence on the summit. This spatial dependence was associated with short distances of gradient showing that the organizational pattern of Distichlis spp.- Spartina was related to combinations of underlying environmental factors rather than to a specific position in the catena. The catenas are highly structured spatially, floristic compositions are inextricably linked to this structure. Habitat complexity may directly affect associated vegetation by regulating the hydrohalomorphic conditions in the aerated layer of the soils. The relationships of the habitat-floristic groups are not simple, hydromorphism interacts in a complex way with halomorphism.

Revegetation Dynamics of Cliff Faces in Abandoned Limestone Quarries

1. Revegetation dynamics on vertical cliff faces in abandoned limestone quarries were examined at 18 sites in southern Ontario, Canada. 2. A wide range of biotic and abiotic variables were measured in randomly positioned plots at each site. 3. Quarry walls ranged in age from 17 to 92 years since abandonment and this known chronosequence was used to investigate successional patterns in the vegetation. 4. Site age and the density of trees adjacent to the quarry walls had the strongest influence on vegetation community composition and abundance. 5. During the first 70 years of abandonment, species richness was high and variable. Community composition was also extremely variable during this period. 6. After 70 years, species richness on quarry walls dropped suddenly and the variation in vegetation composition was reduced. 7. This period of sudden change correlates with the growth of neighbouring trees on the quarry floor that start to shade the quarry walls at about 60 years since abandonment. 8. The final form of the vegetation community on the vertical cliffs in long-abandoned quarries is similar to the naturally occurring community on cliffs of the Niagara Escarpment. 9. The results of this study suggest that natural successional processes can be used to rehabilitate the vertical cliff faces that are produced in limestone quarry operations. 10. The benefits of allowing natural processes to rehabilitate the walls of abandoned limestone quarries are: the recruitment of an uncommon, but native component of biodiversity; a reduced initial cost; and a reduced long-term maintenance cost. 11. It is recommended that abandoned limestone quarries should not be rehabilitated by backfilling of exposed quarry walls.

Vegetation change following exclusion of grazing animals in depleted grassland, Central Otago, New Zealand

Abstract. Models of semiarid vegetation dynamics were evaluated to explain changes in the grassland of interior South Island, New Zealand. Annual records were taken for six years of plant species height frequency and percentage ground cover in five plots established in 1986. One subplot at each site was fenced to exclude sheep, one to exclude rabbits and sheep, and one remained unfenced as a control.Records from 1986–1992 were analysed by ordination. The overall pattern of vegetation change shows considerable year‐to‐year variation. At some sites, variation in vegetation composition between years was as great as, or greater than, that between grazed and ungrazed subplots. Such variation is particularly evident in grazed vegetation, perhaps because it is under greater stress than ungrazed vegetation. At one site changes in vegetation total cover and species composition could be statistically related to rainfall during the first half of the growing season.The only general trends following cessation of grazing were for perennials to increase in frequency, and for year‐to‐year changes to become smaller with time. Total vegetation cover values seldom changed as a result of cessation of grazing, but tended to follow year‐to‐year changes in species frequency.The results do not in general support switch/state‐and‐transition models of semi‐arid vegetation dynamics. Vegetation change follows changes in grazing and climate with little lag. This most closely conforms with the Pulse‐phase dynamic model.