Variation in species composition and vegetation structure of succulent scrub on Tenerife in relation to environmental variation

Abstract. On Tenerife, the occurrence of environmental gradients over short distances provides a unique opportunity to investigate the relationship between vegetation and environmental factors. In the semi‐arid coastal region of Tenerife, floristic composition, species richness and vegetation structure of perennial plants have been studied in 67 locations covering the existing precipitation gradient. On the island as a whole, variation in species composition could be best explained by mean annual precipitation; at coastal sites, substrate age and soil characteristics also played a significant role. On the other hand, substrate chemistry and the type of eruptive material explained little of the floristic variation. Stand biomass was strongly correlated with mean annual precipitation and was, on the youngest lava flows studied, also affected by substrate age. The native stem succulent species made up the bulk of total biomass along the whole precipitation gradient. Disturbed and undisturbed sites differed significantly in stand biomass and cover. Species richness was correlated with precipitation and substrate age. Distribution of plant functional types was also related to the precipitation gradient. The relative abundance of hemicryptophytes and shrubs with non‐hairy leaves increased with increasing precipitation whereas the ratio of shrubs with hairy/non‐hairy leaves and succulent plants decreased. Some alien plants were quite frequent at disturbed sites but, on the whole, they contributed little to the species spectrum and to the stand biomass. Undisturbed sites remained almost free of introduced species not considering annuals.     

How environmental and disturbance factors influence species composition in temperate Australian grasslands

Abstract. A distinctive feature of Australian vegetational history is the abruptness of change since European settlement, involving the influx of exotic species and the imposition of exogenous disturbances which are novel in both intensity and character. This can produce two sources of habitat variability: the natural patterns arising from environmental variation, as well as an overlying effect of disturbance. The relative importance of these two types of variables were compared in temperate herbaceous vegetation. Canonical Correspondence Analysis showed that environment and disturbance had similar contributions to floristic variability. Individually, lithology, altitude and soil disturbance were the strongest variables while slope position, grazing and water enrichment were slightly less important. Despite generally low levels of site specificity, groups of species associated with lithology, slope position, altitude and different disturbance regimes were identified. Exotic species were associated with higher levels of disturbance, but showed levels of environmental specialization similar to the native component. Through combination of this analysis with a previous analysis of species richness for the same data set, it became evident that environmental variation mostly resulted in species substitutions while disturbances led to losses of species, with partial replacement by exotics. Synthesizing these results, we identified three broad groups in relation to tolerance of levels of exogenous disturbance: (1) intolerant species - native taxa intolerant of severe disturbances and constituting the species - rich component of the vegetation; (2) tolerant species - exotic and native taxa occurring at both disturbed and undisturbed habitats and (3) disturbance specialists - predominantly exotic species, correlated with high levels of disturbance.     

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.     

Responses of exotic plant species to fires in Pinus ponderosa forests in northern Arizona

Abstract. Changes in disturbance due to fire regime in southwestern Pinus ponderosa forests over the last century have led to dense forests that are threatened by widespread fire. It has been shown in other studies that a pulse of native, early‐seral opportunistic species typically follow such disturbance events. With the growing importance of exotic plants in local flora, however, these exotics often fill this opportunistic role in recovery. We report the effects of fire severity on exotic plant species following three widespread fires of 1996 in northern Arizona P. ponderosa forests. Species richness and abundance of all vascular plant species, including exotics, were higher in burned than nearby unburned areas. Exotic species were far more important, in terms of cover, where fire severity was highest. Species present after wildfires include those of the pre‐disturbed forest and new species that could not be predicted from above‐ground flora of nearby unburned forests.     

Variation in Greek phrygana vegetation in relation to soil and climate

Abstract. Variation in species composition and community structure in nine Greek phrygana (dwarf-shrub) sites was related to soil and climate variables by using Canonical Correspondence Analysis. The distribution of the Greek phrygana communities appeared to be related to soil rather than to climate. It is postulated that human action has eliminated the original variability related to climate factors in western Greek ecosystems; as a result, phrygana communities in this part of Greece are the result of human impact.     

重庆主城区住宅小区苔藓组成与多样性

目前对于人造生活环境中的苔藓植物组成与多样性所知甚少。本文报道了城市住宅小区中的苔藓植物多样性, 以丰富重庆市苔藓植物区系资料, 为小区环境质量评估提供依据, 为认识苔藓植物对新环境的适应能力提供科学基础。通过对重庆主城九区23个住宅小区进行详细调查, 报道苔藓植物19科33属84种。种类组成以土生的丛藓科和真藓科为主。常见种类包括小凤尾藓(Fissidens bryoides)、大叶石灰藓(Hydrogonium majusculum)、细叶小羽藓(Haplocladium microphyllum)、鳞叶藓(Taxiphyllum taxirameum)和绿片苔(Aneura pinguis)共5种。各住宅小区苔藓物种数介于5-25种之间。物种相似性程度非常低, 群落间差异较为显著, 多样性较高。苔藓生活型以丛集型为主。典范对应分析排序结果显示, 住宅小区居住户数对其苔藓物种多样性影响最大, 其次是绿地面积。研究结果表明, 住宅小区是城市苔藓植物的重要分布区域之一。地理环境差异、城市化水平、小区内部小生境条件和人为干扰程度可能是造成各住宅小区苔藓组成差异的原因。     

Relations between the floristic composition of isolated young woods and their proximity to ancient woodland

Abstract. Recent studies indicate that, in the present-day agricultural landscape, the floristic composition of young woodland communities can be fully developed if the woods are situated adjacent to ancient woodlands. Four 70-yr-old deciduous woods in the Carpathian foothills were examined in relation to three adjacent ancient oak-hornbeam and oak-pine woodlands, which are the nearest source of woodland species diaspores. On the basis of data from 208 plots, the frequencies of various species groups in the field layer of the woods were analysed. The dependence of vegetation differentiation within the recent woods on (a) distance to the border with the ancient woodlands and (b) light intensity was examined by Partial Detrended Canonical Correspondence Analysis (DCCA). A significant relation between distance to ancient woodland and species composition was found for recent woods on rich brown soils. The vegetatively propagating species, myrmecochores and small autochores attained higher cover values near ancient woodland; endozoochores and anemochores were most abundant further away. Within recent, more open woods on poor podzolic and leached brown soils, colonisation is strongly inhibited by dense growth of Carex brizoides; here, vegetation regeneration is much slower than in woods on rich soils much further away from the source of diaspores.     

Effects of prescribed fire on the composition of woody plant communities in southeastern Texas

Abstract. The response to prescribed burning of plant communities ranging from dry to wet habitats was monitored using permanent plots sampled from 1989 to 1993. Temporal controls for fire effects were provided by matched sets of plots protected from fire by newly constructed fire breaks. Changes in species composition were studied by ordination of strata of trees (> 5 cm DBH), small trees (2–5 cm DBH), large saplings (1–2 cm DBH), and small saplings and seedlings (50–140 cm tall). Results show that changes occurred largely in the small tree stratum, in which xeric species increased in importance. Although there were changes in sapling and seedling strata, no clear direction of change was recognized. Fire had little effect on the tree stratum. Of the seven community types under study, three types, sandhill, upland pine, and upperslope pine-oak, were most strongly affected, as indicated by post-fire change in positions of samples representing these communities in ordination space and reduction in understory species abundance. Samples representing the other four mesic and wet communities showed little or no change in their positions. These short-term results indicate that changes in vegetation resulting from fire were small and were mostly restricted to the dry types in which possible compositional change is expected to occur. This differential effect of fire suggests that the influence of fire is secondary to that of topographical and soil gradients in determining vegetation pattern under current fire regimes. Fire seems to reinforce an overall vegetation gradient controlled by soil in southeastern Texas.     

Exploration of spatial and temporal patterns of fish diversity and composition in a tropical estuarine system of Mexico

There are many studies of fish assemblages intemperate systems, but little informationexists about tropical estuarine systems,particularly in the Western Atlantic Regionincluding Mexico. We investigated the fishcommunity structure in the Pueblo Viejo lagoon,Veracruz. Biological samples were collectedmonthly for one year at six sites: three withdense stands of Ruppia maritima and threesites without submerged vegetation. For eachsample, temperature, salinity, dissolvedoxygen, turbidity and depth were analyzed. Theunbiased Simpson diversity index, CanonicalCorrespondence Analysis (CCA) and DiscriminantAnalysis (DA) were used to analyze the fishcommunity. Environmental abiotic variablestended to show significant temporal, but notspatial differences. Fish diversity showedonly a weak significant correlation with watertemperature, and relatively strong diversitypeaks, from June to August and April, wererelated to production peaks in the system. Thetwo first axes of CCA accounted for 65% of thespecies-environment biplot variance, whichsuggested that monthly changes of salinity,turbidity and precipitation, and presence orabsence of submerged vegetation, were the mostimportant environmental variables indetermining the observed variability in fishcommunity composition. Further, fish diversitywas significantly different between habitatswith and those without the presence ofsubmerged vegetation (P < 0.02). A DA showedsignificant differences (P < 0.03) in fishcommunity composition between both kinds ofhabitats, with Lagodon rhomboides,Mugil curema and Menidia beryllina(substantially more abundant in habitat withsubmerged vegetation) as the most importantspecies in the discrimination of spatial fishcomposition. Considering both habitatscombined, fewer differences were observed infish diversity and community compositionbetween rainy and dry seasons. Fish communitycomposition showed greater similarity betweenseasons than between habitats, despite the factthat environmental abiotic variables showed aninverse pattern, suggesting that site factors,such as the presence of submerged vegetation,play a more important role in the maintenanceof fish community patterns than those relatedto temporal influence.     

Effects of ecological factors on plant communities of Ziwuling Mountain, Shaanxi Province, China

This paper explores the relationships between environmental factors and community patterns. In 60 plots in the Malan Forest Region of the Loess Plateau, 17 variables were used as indicators of spatial position, soil and topography characteristics. Plant community types were analyzed by TWINSPAN (Two-way Indicator Species Analysis). Significant environmental factors were selected by forward selection and Monte Carlo tests. The relationship between communities and environmental factors was identified by DCCA (Detrended Canonical Correspondence Analysis). Partial CCA (Canonical Correspondence Analysis) partitioned observed compositional variation into components explained by environment, locality and covariation between them. The results showed that: (1) the plant communities can be divided into 13 types including 4 different successional stages. (2) Salt and alkali-hydrolyzable nitrogen are the significant factors related to plant community distribution at early stage. Elevation and humus thickness are the significant factors in the transitional stage. Elevation, slope, aspect, thickness of forest floor, humus thickness and pH are the significant factors for the sub-climax stage. Elevation, slope, aspect, pH and available phosphorus are the significant factors for the climax stage. (3) The ecological characters and the distribution patterns of plant communities in different successional stages are consistent with environmental gradients. (4) As succession proceeds, the impact of environmental conditions on plant communities increases, while the total influence of spatial location, and the interaction between the spatial location and environmental factors decrease.     

Effects of ecological factors on plant communities of Ziwuling Mountain, Shaanxi Province, China

This paper explores the relationships between environmental factors and community patterns. In 60 plots in the Malan Forest Region of the Loess Plateau, 17 variables were used as indicators of spatial position, soil and topography characteristics. Plant community types were analyzed by TWINSPAN (Two-way Indicator Species Analysis). Significant environmental factors were selected by forward selection and Monte Carlo tests. The relationship between communities and environmental factors was identified by DCCA (Detrended Canonical Correspondence Analysis). Partial CCA (Canonical Correspondence Analysis) partitioned observed compositional variation into components explained by environment, locality and covariation between them. The results showed that: (1) the plant communities can be divided into 13 types including 4 different successional stages. (2) Salt and alkali-hydrolyzable nitrogen are the significant factors related to plant community distribution at early stage. Elevation and humus thickness are the significant factors in the transitional stage. Elevation, slope, aspect, thickness of forest floor, humus thickness and pH are the significant factors for the sub-climax stage. Elevation, slope, aspect, pH and available phosphorus are the significant factors for the climax stage. (3) The ecological characters and the distribution patterns of plant communities in different successional stages are consistent with environmental gradients. (4) As succession proceeds, the impact of environmental conditions on plant communities increases, while the total influence of spatial location, and the interaction between the spatial location and environmental factors decrease.     

Changes in the canopy structure of the Mediterranean shrub Lavandula stoechas after disturbance

Abstract. This study attempts to show the dynamics of the canopy structure of the Mediterranean pioneer shrub Lavandula stoechas after man-made perturbation (i.e. grazing). The development of the vertical structure of the shrub was studied by harvesting the canopy of plants of 2–6 yr old in horizontal layers. The supportive biomass of the canopy was concentrated near the base at all ages. Leaf biomass was evenly distributed all over the vertical profile in 2- and 3-yr old plants. In 4-yr old plants it presented a maximum near the top of the canopy. For 5-yr old plants a structural transition started with leaf profiles showing a bimodal distribution. Leaf biomass predominated near the base in 6-yr old plants, suggesting that the transition was completed. Three canopy stages in the growth processes of the plant were recognized after the first year of growth: in the first one (from 2 to 3 yr old) both leaf and supportive biomass increased; in the second one (from 3 to 4 yr) leaf biomass remained stable and there was an increase in supportive biomass until the plants reached a ‘mature stage’, in 4-yr old plants; finally, in 5- and 6-yr old plants there was a decrease both in leaf and supportive biomass and plant structure showed evidence of senescence. Early transitions from seedling to 1-yr old plant and from this to 2- to 3-yr old plants were less obvious. The leaf/supportive biomass ratio always decreased with plant age, from 1.88 in seedlings to 0.01 in 6-yr old plants. Biomass density followed the pattern of supportive biomass, with an increase from 1.7 g/dm³ (2-yr old plants) to 2.4 g/dm³ (4-yr old plants). Thereafter, biomass density decreased to 0.6 g/dm³ (6-yr old plants).     

长白山地表藓类分布格局与环境因子的关系

对长白山自然保护区41个样地中的42种主要地表藓类分布与6种重要环境因子间的关系进行了定量研究。应用双向指标种分析法（TWINSPAN）发现调查的41个样地可分成4个样地组,42种地面藓 类可分成4种藓类群,应用除趋势典范对应分析（DCCA）的研究结果表明,海拨高度、土壤含砂量、土壤酸度、林冠层郁闭度、土壤含水量是5个影响42种地表藓类分布格局的主要环境因子,在DCCA排序图上,显示出藓类群与样地类型良好的对应性,藓类群1－4分别在落叶松（Larix olgensis）沼泽地、高山苔原、暗针叶林、亚高山岳松（Betula ernanni）林和岳松－落叶松林中占优势。     

Tropical forest structure and diversity in relation to altitude and disturbance in a Biosphere Reserve in central India

Questions: Is species diversity affected in protected areas where human activities are permitted or tolerated? On plots of a fixed size, does stem density alone predict number of species? Are differences in density related to disturbance and altitude? Location: Achanakmar‐Amarkantak Biosphere Reserve, central India. Methods: 42 sites, each with three replicate 10‐m radius plots, were examined. All trees (≥ 30 cm GBH) in each plot were measured for girth at breast height. α‐diversity, species richness and evenness were calculated for each site. The sites were ordinated by Nonmetric Multidimensional Scaling (NMS) using relative importance values of component species. Correspondence Analysis was used to broadly delineate communities. Anthropogenic disturbances were recorded in terms of percentage of trees lopped, scale of lopping, number of domestic livestock dung piles and foot trails (both livestock and people) for each plot. Results: The NMS analysis exhibited a near linear arrangement of sites with no evidence of discrete vegetation zones. NMS axes were significantly related to altitude and disturbance scores. With increasing elevation, basal area increased but number of species, α‐diversity and its components declined monotonically. The number of species and indices of species diversity were positively associated with tree lopping and also with total disturbance. Number of species was controlled by stem density only in plots not dominated by Shorea robusta. Conclusions: Recent levels of human disturbance are associated with higher species diversity in this biosphere reserve. There is some evidence that stands at all altitudes follow the same successional pattern to dominance by Shorea, a successional pattern that also results in decreased diversity without disturbance.     

Effects of fire,mowing and nitrogen addition on root characteristics in tall-grass prairie

Abstract. Root harvests and root windows were used to study the influence of fire, mowing and nitrogen additions on root lengths, biomass, and nitrogen content in tall-grass prairie. Four years of nitrogen additions (10 g m² yr^?1) increased below-ground mass by 15 % and nitrogen concentration in that mass by 77 %. In general, live roots and rhizomes exhibited greater increases in nitrogen concentrations than detrital roots and rhizomes. After four years of treatment, live roots and rhizomes immobilized an additional 1.5 to 5 g/m² of nitrogen, depending upon specific treatment, while dead roots and rhizomes immobilized an additional 3 to 3.5 g/m². Average root growth parameters, as measured with root windows, were positively correlated with above-ground peak foliage biomass; however, the only significant correlation was between average new root growth and above-ground peak foliage biomass (r = 0.73, p ≤ 0.04). Root growth and decay, as measured by annual mean values for eight root windows over a four year interval, were insensitive to climatic and treatment effects.     

Changes in aquatic vegetation in Rhine floodplain streams in Alsace in relation to disturbance

Abstract. Changes are described in aquatic vegetation in oligotrophic, groundwater-fed Rhine floodplain streams in Alsace (eastern France), resulting from disturbance. Disturbance factors include changes in nutrients, either permanent ones - effluent from a waste water treatment plant or trout hatcheries - or periodic ones: flooding. Regular inputs of high levels of phosphate and ammonia modified the macrophyte vegetation in these streams. The floristic composition, which was characteristic of oligotrophic waters upstream of the eutrophicated sector, changed to that of a eutrophic situation as originally found downstream. Periodic disturbance by floods which normally occur once a year, irregularly eutrophicates the small streams, causing the development of a mixture of eutrophic and oligotrophic species. Six macrophyte communities are distinguished, indicating different trophic levels. The aquatic vegetation is adapted to the variations of phosphate and ammonia levels. Hence, aquatic macrophytes can be used as bio-indicators of fluctuations in water nutrient levels in relation to the type of disturbance.     

The vertical distribution of below-ground biomass in grassland communities in relation to grazing regime and habitat characteristics

Abstract. 40 sites, representing different pasture types in Northwest Spain, were sampled in respect of their floristic composition, distribution of above and below-ground biomass and environmental and physical variables. Five plant community types were identified by classification techniques of plant species composition. These communities were then characterized in terms of the percentage of ground covered by herbaceous and shrub vegetation, stones, rocks and gaps as well as their topographic location and characteristics of the shallow soil (pH, organic matter, nitrogen and calcium content). Bio-mass was assessed in terms of above-ground structures, surface crowns and three below-ground layers to a depth of 10 cm. Three types of grazing regime were distinguished: Concentrated Intense Grazing in early spring (CIG), Extended Intense Grazing throughout the spring (EIG), and Non-Intense Grazing (NIG). Grazing regime showed the highest association with plant community type and three broad categories were identified: xeric stressed pastures, which nevertheless received CIG, mesic pastures with EIG, and three kinds of NIG mesic pastures. The xeric communities had the highest proportion of aboveground biomass, as a consequence of their greater proportion of woody perennials. These xeric communities displayed a more gradual reduction in below-ground biomass with depth than mesic pastures, a likely consequence of the low water content in the upper soil layers. The mesic communities had a high concentration of below-ground biomass in the upper layers when they were intensely grazed. However, when grazing was low (i.e. NIG situations), these communities had greater variability in biomass profiles than any of the other pasture types. Possible causes of the patterns in biomass distribution of the intensely grazed pastures are discussed.     

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

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

The importance of environment vs. disturbance in the vegetation mosaic of Central Arizona

Abstract. The vegetation of central Arizona is a mosaic of four vegetation types: chaparral, chaparral grassland, woodland, and woodland grassland. We analysed ten environmental variables, three disturbance variables, and five disturbance indicators to answer the question: What is the relative importance of environment and disturbance in explaining the vegetation pattern of our study area? We found that chaparral, chaparral grassland, and woodland are differentiated primarily by environmental factors and have high stability in the landscape. In contrast, woodland grassland is differentiated primarily by disturbance and is likely an early‐successional stage of woodlands. Although other researchers have indicated that semi‐arid vegetation is generally unstable, the vegetation of central Arizona is composed of two systems: those with a more stable landscape position determined primarily by environmental factors and those with a less stable landscape position determined primarily by disturbance factors.