Plant composition and diversity in a semi-natural Mediterranean island landscape: the importance of environmental factors

Abstract

Islands crucially contribute to the Mediterranean Basin’s high floristic diversity, which, however, is at risk facing climate and land-use changes. Besides the identification of highly diverse areas, the knowledge about factors favouring diversity is of great importance. We analysed plant species diversity and composition related to environmental factors over varied vegetation units on a former Italian prison island in the northwest of Sardinia. Due to a long history of land use with grazing and later abandonment the nowadays protected island features a semi-natural landscape and can serve as an example for strongly anthropogenic altered insular ecosystems. Floristic composition, soil properties, microclimate and ungulate abundance were assessed. Relationships of vegetation composition and diversity with abiotic variables were examined by Canonical Correspondence Analysis, which indicated the importance of air temperature, soil moisture, slope gradient and C/N ratio for floristic differentiation. Most important abiotic factors for plant species richness were relative air humidity and soil moisture, while floristic diversity was mainly determined by air temperature and pH. Furthermore, observation data pointed to an adverse influence of ungulate abundance for plant species diversity. Regarding nature conservation, grazing intensity thus must be critically taken into account, especially for sensitive vegetation units like the coastal garrigue.     

Diversity and floristic patterns of mediterranean grasslands: the relative influence of environmental and land management factors

Managed grasslands are normally ecosystems with high species diversity. They are associated with traditional extensive livestock grazing. Land-use changes, in particular gradual abandonment or grazing intensification, lead to major changes in floristic patterns of these grasslands, some included in priority habitats of the EU Habitats Directive. In order to analyse these patterns of change, Mediterranean grasslands of eastern areas of mainland Portugal were studied aiming to: (1) establish ecological gradients underlying their floristic patterns; (2) examine the relative importance of both environmental and land-use factors on their floristic composition; (3) assess how floristic composition and species richness are affected by land-use factors. Vegetation sampling was carried out from 2008 to 2009 following phytosociological procedures. Canonical Correspondence Analysis was applied. Variation partitioning was used to assess the relative influence of land-use variables. Richness, legumes cover, endemic species and bryophytes cover were compared in four land-management regimes: unmanaged; extensive grazing by sheep; extensive grazing by cattle and sheep; frequent soil tillage. Significant differences were found in floristic and diversity patterns, revealing the importance of grazing as a management tool for maintaining or improving floristic diversity. From our findings slight extensive grazing can contribute to both maintenance of species diversity and to the increase in legumes cover and thus to palatability and economic value of studied grasslands. Interactions between herbivores and grasslands, considering also their influences on soil properties, have an important role in the equilibrium states which promote biodiversity and conservation of the services of these ecosystems.     

Vegetation-environment relationships on a species-rich coastal mountain range in the fynbos biome (South Africa)

The species-rich fynbos of the southern Langeberg Mountains, South Africa was studied along three transects (a) to evaluate the compatibility of a floristic classification of the southern Langeberg vegetation with a fynbos biome-wide structural classification of mountain vegetation, (b) to describe the environmental gradients to which the vegetation responds and (c) to investigate the relationship between the vegetation and the abiotic environmental variables which determine the pattern of distribution of the fynbos communities on the southern Langeberg.Principal Components Analysis (PCA) was used to determine correlations between environmental variables independent of vegetation data. Similarities between the 46 communities (determined by floristics) from the three transects were determined using cluster analysis and grouped into 14 higher-level units. Detrended Correspondence Analysis (DCA) was then used for indirect gradient analysis after which Canonical Correspondence Analysis (CCA) was used in a direct gradient analysis of the vegetation with the environmental variables.Compatibility between the floristic and structural classification of the vegetation was analysed. The PCA principal gradient was defined as one from sites with high rock cover, shallow soils and north aspects to those with low rock cover, deeper soils and south aspects. The second gradient is most strongly positively correlated with percentage organic carbon and most strongly negatively correlated with soil clay content. In contrast to the PCA, the DCA showed that the principal gradient is a precipitation gradient, with the response of the vegetation dominated by the change from wet to dry conditions and from low to high winter incoming radiation. The CCA showed that the variation in the mountain habitats to which the vegetation responds can be predicted from a combination of a few environmental variables. The principal gradient was one of change from high to low mean annual precipitation with an opposite change in winter incoming radiation. The second gradient was described by percentage surface rock cover and soil clay content. A simple model using the environmental factors selected in the CCA was proposed for predicting the distribution of floristically determined community groups in the fynbos vegetation of the Langeberg and the southern Cape coastal mountains in general.     

Vegetation in Danish beech forests: the importance of soil,microclimate and management factors,evaluated by variation partitioning

The importance of management regime on floristic variation (mosses and vascular plants) in four Danish beech forests was investigated. Sixty-four blocks were sampled, representing beech stands of different age and management types. Nineteen potential explanatory variables were recorded and tested with Monte-Carlo tests and Canonical Correspondence Analysis. In addition results were evaluated by use of Detrended Correspondence Analysis. Explanatory variables were divided into three groups; soil, microclimatic and management parameters. The amount of variation explained by each group of variables was calculated by use of variation partitioning. The group consisting of management variables explained most variation, on local as well as regional scale. Management related variables explained more variation in vegetation than any other variables together. This indicates the importance of management as determining species composition in Danish beech forests. Management related variables explained most variation on local scale. On a regional scale, soil parameters explained the major part of the variation. The results suggest that thirty years without management are sufficient to change species composition significantly, as compared to managed forests.     

Gradient analysis of dry grassland vegetation in Denmark

Abstract. We present a gradient analysis of 620 vegetation samples covering most of the floristic and environmental variation in semi‐natural grassland vegetation on well‐drained soils in Denmark. Vegetation was sampled using frequency in subplots. Explanatory variables were surface inclination, aspect, pH, geographical co‐ordinates together with indications of soil type. Detrended Correspondence Analysis revealed four floristic gradients that could be interpreted in ecological terms by measured variables supplemented with site calibrations based on weighted averaging of Ellenberg's indicator values. All four axes were interpreted using rank correlation statistics, and linear and non‐linear multiple regression of sample scores on explanatory variables. The first gradient was from dry calcareous to humid acidic grasslands; the second reflected an underlying gradient in fertility; the third reflected regional differentiation and the fourth was associated with variation in intensity of competition as indicated by association with calibrated Grime‐CSR values for the plots. We applied subset ordination to the data as a supplement to traditional permutation and correlation statistics to assess the consistency of ordination results. DCA axes 1 and 2 were consistent in space and time. This gradient analysis is discussed in a context of plant strategy theory and species diversity models. Ecocline patterns lend support to the view that grazing not only favours the ruderal strategy but also the stress‐tolerant strategy. The low rank of competition as an explanatory variable for the floristical gradients supports the notion that competitive effects play a subordinate role for species composition compared to microclimate and soil conditions in infertile semi‐natural grasslands.     

Community composition and soil properties in northern Bolivian savanna vegetation

Abstract. Direct gradient analysis (Canonical Correspondence Analysis) of northern Bolivian savanna vegetation revealed correlations between the composition of plant communities and physical and chemical soil properties. Cover/abundance values for 193 species from 27 sites were related to data on eight soil factors. A water-regime variable and the percentage of sand and silt were correlated with the first axis of the species-environment biplot and explained most of the variation in community composition. Along this axis, species and sites of flood-plain vegetation were separated from sites not affected by flooding rivers. The second axis of the biplot was correlated with soil-chemical variables, namely extract-able phosphate, base saturation, organic carbon, pH, and effective cation exchange capacity. Part of the variation in community composition can be explained by these soil nutrient variables. Grassland communities were separated from woody vegetation along the soil nutrient gradient, and floodplain communities of white-water rivers from those of clear-water rivers. The results of the gradient analysis indicate that the soil texture-moisture gradient is the prime factor determining the variation in the floristic composition of the savanna communities examined, and that, in addition, the soil nutrient gradient accounts for some of the variation.     

Vegetation ecology of phytogenic hillocks (nabkhas) in coastal habitats of Jal Az-Zor National Park,Kuwait: Role of patches and edaphic factors

Phytogenic sandy hillocks (Arab. ‘nabkha’) are very frequently occurring aeolian deposits along the coastal plain of Kuwait. We investigated the vegetation of 42 nabkhas in a coastal habitat of Jal Az-Zor National Park, Kuwait. Sixty-two species were recorded (47 annuals and 15 perennials) in the studied nabkhas. Four vegetation types were recognized after classifying the vegetation of the nabkhas by TWINSPAN. They were named after their dominating host species which are Nitraria retusa, Zygophyllum qatarense, Haloxylon salicornicum and Panicum turgidum. Using Detrended Correspondence Analysis (DCA) and Canonical Correspondence Analysis (CCA), we assessed the relationships between environmental gradients, floristic composition, species diversity, and geomorphology aspects of the studied habitats. Notable environmental variables affecting the distribution of the vegetation types in the study area were: geomorphological aspect, size of plants forming the core of the nabkha, moisture and nutrients contents, salinity, sand and silt components, and pH.     

Grazing in remnant woodland vegetation: changes in species composition and life form groups

Abstract. Grazing by domestic livestock in native woodlands can have major effects on ecosystem functioning by the removal of plant species that form important functional groups. This paper documents the changes in floristics in a large group of remnants of native woodland left after agricultural clearing in southwestern Australia. Species richness and diversity were significantly reduced in remnants and the proportion of exotic species increased. Detrended Correspondence Analysis (DCA) was used to identify floristic and environmental patterns among plots and identified two distinct groups based on grazing intensity. This indicated that the significance of the relationship between grazing effects and DCA floristic axes was greater than edaphic characteristics that normally influence floristic patterns. Floristic characteristics of sites that were influencing the position of plots on the ordination diagram included proportion of exotic species and proportion of native perennial shrubs and herbs. Numbers of species of native shrubs and perennial herbs were significantly reduced in grazed plots and numbers of exotic annual grasses and herbs were significantly higher. Other life form groups such as native perennial grasses and geophytes were not significantly affected by grazing. Reproductive strategies of perennial species showed a significant decrease in numbers of resprouters and a significant increase in numbers of facultative seeder/sprouters. Exclosure plots showed increases in number and cover of perennial shrubs and herbs after three years whereas number and cover of exotic species did not change. Time series DCA showed that the floristic composition of exclosure plots in grazed sites became closer to that of the ungrazed sites.     

Vegetation‐environment relationships in Atlantic European calcareous grasslands

Abstract. The relationship between vegetation and environment was investigated for calcareous grasslands in a region in the west of Spain, France, Britain and Ireland defined by climatic criteria. Vegetation was sampled using objective methods and data collected on soils, land cover, location and management. Climate data were obtained from an available database. Examination of the first axis of vegetation variation as defined by Detrended Correspondence Analysis (DCA) showed a gradient from the Irish and British samples to those from France. The Spanish samples formed a separate group on the second axis. The species composition along the gradients is discussed. Correlations between the vegetation gradients and environmental variables were determined. The strongest correlations with the first DCA axis were for temperature, latitude, soil organic matter, grazing and land cover. The second DCA axis was highly correlated with rainfall, altitude and land cover. The third and fourth DCA axes were more difficult to interpret but appeared to be related to land cover. The results indicate that climate factors are important at this scale, but should not be considered in isolation and that factors relating to land cover and management should also be taken into account.     

Tropical rain forest types and soil factors in a watershed area in Guyana

Abstract. An inventory was made of the vegetation and soils of a watershed area of 480 ha in the tropical rain forest of Guyana. 252 plots of 0.05 ha were sampled. In total 111 tree species > 20 cm were recorded. A TWINSPAN analysis resulted in seven groupings. Correspondence Analysis revealed that the major environmental differentiation underlying the floristic variation is according to soil type. Finally, five main forest communities were described. Most sites in the watershed area are characterized by the dominance of one or a few species. Species distribution patterns are strongly determined by soil type and drainage class.     

The steppes of Tierra del Fuego: floristic and growthform patterns controlled by soil fertility and moisture

Northern Tierra del Fuego is the insular extreme of the Magellanic steppe, a unique example of a cold-temperate oceanic grassland in South America. We surveyed this poorly known region with a landscape-based procedure to classify plant communities and investigate environmental gradients. Sampling sites were selected with the aid of aerial photos, and floristic composition, soil, topography and landform type were recorded in each site. Classification was achieved with Two-Way Indicator Species Analysis and gradients were analyzed with Detrended Canonical Correspondence Analysis. Two broad groups of communities were separated by the presence of typical acidophilous dwarf shrubs: acidophilous/mesotrophic and neutrophilous vegetation. Along the main floristic gradient, ericoid dwarf shrubs were replaced by forbs and grasses as soil acidity decreased. The acidic habitats were on Pleistocene glacial and alluvial deposits, and had coarse texture, poorly decomposed organic matter, and low base status. The eutrophic habitats were on Tertiary rocks of medium to fine texture and had well-decomposed organic matter and high base status. A second axis of variation was associated to latitudinal position and slope aspect, revealing a precipitation/evaporation gradient. Scrubs dominated by midheight shrubs, with species from the southern forests, were at the humid extreme (higher latitudes/south aspects); and short grass steppes, with taxa commonly found further North in the continent, at the xeric end (lower latitudes/north aspects). Although fertility and moisture seem to control the overall floristic gradient, the extreme vegetation types of the fertility gradient, both having a very simple structure, could be attributed to grazing disturbance. Floristics and ecology of the Fuegian steppe resemble to a certain degree some boreal oceanic systems.     

Spatial and temporal variation of soil moisture in dependence of multiple environmental parameters in semi-arid grasslands

Grazing of grasslands changes soil physical and chemical properties as well as vegetation characteristics, such as vegetation cover, species composition and biomass production. In consequence, nutrient allocation and water storage in the top soil are affected. Land use and management changes alter these processes. Knowledge on the impacts of grazing management on nutrient and water fluxes is necessary because of the global importance of grasslands for carbon sequestration. Soil water in semi-arid areas is a limiting factor for matter fluxes and the intrinsic interaction between soil, vegetation and atmosphere. It is therefore desirable to understand the effects of grazing management and stocking rate on the spatial and temporal distribution of soil moisture. In the present study, we address the question how spatio-temporal soil moisture distribution on grazed and ungrazed grassland sites is affected by soil and vegetation properties. The study took place in the Xilin river catchment in Inner Mongolia (PR China). It is a semi-arid steppe environment, which is characterized by still moderate grazing compared to other regions in central Inner Mongolia. However, stocking rates have locally increased and resulted in a degradation of soils and vegetation also in the upper Xilin River basin. We used a multivariate geostatistical approach to reveal spatial dependencies between soil moisture distribution and soil or vegetation parameters. Overall, 7 soil and vegetation parameters (bulk density, sand, silt and clay content, mean weight diameter, mean carbon content of the soil, vegetation cover) and 57 soil moisture data sets were recorded on 100 gridded points on four sites subject to different grazing intensities. Increasing stocking rates accelerated the influence of soil and vegetation parameters on soil moisture. However, the correlation was rather weak, except for a site with high stocking rate where higher correlations were found. Low nugget ratios indicate spatial dependency between soil or plant parameters and soil moisture on a long-term ungrazed site. However, the effect was not found for a second ungrazed site that had been excluded from grazing for a shorter period. Furthermore the most important soil and vegetation parameters for predicting soil moisture distribution varied between different grazing intensities. Therefore, predicting soil moisture by using secondary variables requires a careful selection of the soil or vegetation parameters.     

Abundance of trees and grasses in a woodland savanna in relation to environmental factors

The relationships of plant species associations and underlying environmental factors in a woodland savanna in South Africa were investigated. 40 plots were included with 25 tree and 17 grass species dominating the arboreal and ground cover. Correspondence Analysis described the relationships between soil moisture retention, soil nutrients and the abundance of trees and grasses. Dry matter indices represented the accumulated effects of rainfall, fire and grazing of the herbaceous layer. Variations in the abundance of plants corresponded to well-defined gradients of soil nutrients. The distribution of grass and tree species along the ordination axes indicated that soils with high water retention capacity and high nutrient contents provided a suitable substrate for many of the tree species sampled. However, grass species abundance was high in plots with porous soils and poor nutrient availability.     

Ecohydrological characterization of a groundwater-fed alluvial floodplain mire

Abstract. In a small alluvial floodplain depression (21 ha) of the river Dijle, a selection of 56 characteristic, mainly groundwater-depending plant species was mapped using a regular 20 by 20 m square cell mapping grid. In order to understand the spatial distribution of the plant species and their vegetation types, several environmental variables, including groundwater regime, groundwater chemistry, soil texture, soil chemical composition and management type were measured in detail. Using Detrended Correspondence Analysis and Canonical Detrended Correspondence Analysis, the relation between these environmental variables and the plant species and vegetation types, respectively, is described in detail. Management type and groundwater fluctuations are recognized as the two main sets of environmental variables causing the specific spatial distribution of plant species and vegetation types for the mire-ecosystem of the silty alluvial floodplain.     

Species-environment relationship in the herb-subshrub layer of a moist Savanna site, Federal District, Brazil.

The soils are seasonally or permanently saturated in the moist grassland savanna, locally known as Campo Limpo Umido. Soil moisture variation seems to determine spatial distribution of communities. The objective of this study is to analyse the relationship between environmental variables and the patterns of spatial distribution of species in the herbaceous-subshrub layer of an area of moist grassland at the Agua Limpa Farm, Brasília, DF (15 degrees 56' to 15 degrees 59' S and 47 degrees 55' to 47 degrees 58' W Gr.). An area of 400 x 400 m was divided into four sections of 200 x 200 m where four transects were randomly sampled. A line intercept method was adopted for the phytossociological study. Superficial soils samples (0-20 cm) were collected for chemical and textural analyses. Gravimetric soil moisture was measured quarterly during the study-year. A total of 85 species in 67 genera and 24 families were found. The diversity was high, Shannon's index, H', was 2.60 nats.cover(-1). Floristic composition of the transects in soils with a high gravimetric soil moisture and high content of organic matter and sand differed from those transects in soils with a lower gravimetric soil moisture indicating seasonal variation. A Canonical Correspondence Analysis (CCA) showed significant correlations between soil texture and soil moisture features and species distribution. Gravimetric soil moisture, organic matter, clay, silt and sand were significantly correlated to species distribution in the moist grassland determining mosaics in the vegetation.     

Combined effects of environment and grazing on vegetation structure in Argentine granite grasslands

Abstract. Effects of grazing and environment on vegetation structure have been widely acknowledged, but few studies have related both factors. We made 57 floristic samples in a highly variable landscape of mountain grasslands in central Argentina; 26 sample were in fence‐lines with contrasting vegetation. For each sample, we recorded topographic and edaphic parameters, as well as grazing intensity indicators. Floristic gradients were analysed with DCA and relations with abiotic and grazing‐related variables were detected with DCCA. Floristic axis 1 was explained by edaphic parameters associated to topography, ranging from communities in well drained soils on upper topographic positions to hydromorphic vegetation in poorly drained soils on lower topographic positions. Species richness decreased as soil moisture increased. Floristic axis 2 was associated with present and long‐term grazing indicators, and reflected shifts in vegetation physiognomy and species evenness. Tall tussock grasslands, with low species evenness and evidences of low or null grazing intensity were located at one extreme. Tussocks were gradually replaced by short graminoids and forbs towards the centre of the gradient, as grazing increased, and evenness reached a maximum. In degraded sites with heavy long‐term grazing intensities, short perennial species were replaced by an annual species, and evenness decreased. The magnitude of changes in floristic composition produced by grazing decreased with increasing soil moisture, and vegetation‐environment relationships were stronger in moderate to highly grazed situations than in lightly or non grazed situations.     

Remotely sensed vegetation phenology and productivity along a climatic gradient: on the value of incorporating the dimension of woody plant cover

Aim Woody plants affect vegetation–environment interactions by modifying microclimate, soil moisture dynamics and carbon cycling. In examining broad‐scale patterns in terrestrial vegetation dynamics, explicit consideration of variation in the amount of woody plant cover could provide additional explanatory power that might not be available when only considering landscape‐scale climate patterns or specific vegetation assemblages. Here we evaluate the interactive influence of woody plant cover on remotely sensed vegetation dynamics across a climatic gradient along a sky island. Location The Santa Rita Mountains, Arizona, USA. Methods Using a satellite‐measured normalized difference vegetation index (NDVI) from 2000 to 2008, we conducted time‐series and regression analyses to explain the variation in functional attributes of vegetation (productivity, seasonality and phenology) related to: (1) vegetation community, (2) elevation as a proxy for climate, and (3) woody plant cover, given the effects of the other environmental variables, as an additional ecological dimension that reflects potential vegetation–environment feedbacks at the local scale. Results NDVI metrics were well explained by interactions among elevation, vegetation community and woody plant cover. After accounting for elevation and vegetation community, woody plant cover explained up to 67% of variation in NDVI metrics and, notably, clarified elevation‐ and community‐specific patterns of vegetation dynamics across the gradient. Main conclusions In addition to the environmental factors usually considered – climate, reflecting resources and constraints, and vegetation community, reflecting species composition and relative dominance – woody plant cover, a broad‐scale proxy of many vegetation–environment interactions, represents an ecological dimension that provides additional process‐related understanding of landscape‐scale patterns of vegetation function.     

Eco-Floristic studies of native plants of the Beer Hills along the Indus River in the districts Haripur and Abbottabad,Pakistan

The present study was conducted to elaborate vegetation composition structure to analyze role of edaphic and topographic factors on plant species distribution and community formation during 2013–14. A mixture of quadrat and transect methods were used. The size of quadrat for trees shrubs and herbs were 10 × 5, 5 × 2, 1 × 1 meter square respectively. Different phytosociological attribute were measured at each station. Primary results reported 123 plant species belong to 46 families. Asteraceae and Lamiaceae were dominant families with 8 species each. PCORD version 5 were used for Cluster and Two Way Cluster Analyses that initiated 4 plant communities within elevation range of 529–700 m from sea level. Indicator species analyses (ISA) were used to identify indicator species of each community. CANOCO Software (version 4.5) was used to measure the influence of edaphic and topographic variables on species composition, diversity and community formation. Whereas Canonical Correspondence Analysis (CCA) was used to measure the effect of environmental variables which showed elevation and aspect were the stronger environmental variable among topographic and CaCO₃ contents, electric conductivity, soil pH were the stronger edaphic factors in determination of vegetation and communities of the Bheer Hills. Grazing pressure was one of the main anthropogenic factors in this regard.     

Vegetation heterogeneity and diversity in flat and mountain landscapes of Patagonia (Argentina)

Abstract. We studied floristic and diversity patterns and their environmental controls in two landscapes of contrasting topography in the Patagonian steppe. The analyses were focused on the effects of water availability gradients and landscape configuration on plant species distribution and coexistence. Floristic variation was investigated using Correspondence Analysis. The relationship between floristic and environmental variation was analyzed using Canonical Correspondence Analysis and correlation tests. We explored diversity patterns by relating spatial distance to floristic dissimilarities. The floristic gradient was determined by shrub and grass species and was related to precipitation in the flat area, and to precipitation, elevation and potential radiation in the mountain area. Site species richness increased with water availability in both areas. Mean site species richness and species turnover in space was higher in the mountain than in the flat area. Landscape species richness and floristic gradients were more concentrated in the mountain than in the flat area. In contrast to shrubs and grasses, forb species distributions were uncoordinated and probably independent of any environmental gradient. Our results suggest (1) that landscape configuration affects species composition and diversity through its direct effect on abiotic environmental heterogeneity, and (2) that the environmental controls of the community composition vary depending on the plant functional type considered.