bluuebunny/crossref_metadata_2025 huggingface dataset:
Classification trees: An alternative non‐parametric approach for predicting species distributions
Authors: Vayssières Marc P. , Plant Richard E. , Allen‐Diaz Barbara H.
Published: May 2006

Abstract. The use of Generalized Linear Models (GLM) in vegetation analysis has been advocated to accommodate complex species response curves. This paper investigates the potential advantages of using classification and regression trees (CART), a recursive partitioning method that is free of distributional assumptions. We used multiple logistic regression (a form of GLM) and CART to predict the distribution of three major oak species in California. We compared two types of model: polynomial logistic regression models optimized to account for non‐linearity and factor interactions, and simple CART‐models. Each type of model was developed using learning data sets of 2085 and 410 sample cases, and assessed on test sets containing 2016 and 3691 cases respectively. The responses of the three species to environmental gradients were varied and often non‐homogeneous or context dependent. We tested the methods for predictive accuracy: CART‐models performed significantly better than our polynomial logistic regression models in four of the six cases considered, and as well in the two remaining cases. CART also showed a superior ability to detect factor interactions. Insight gained from CART‐models then helped develop improved parametric models. Although the probabilistic form of logistic regression results is more adapted to test theories about species responses to environmental gradients, we found that CART‐models are intuitive, easy to develop and interpret, and constitute a valuable tool for modeling species distributions.
The Southeast Saline Everglades revisited: 50 years of coastal vegetation change
Authors: Ross M.S. , Meeder J.F. , Sah J.P. , Ruiz P.L. , Telesnicki G.J.
Published: May 2006

Abstract. We examined the vegetation of the Southeast Saline Everglades (SESE), where water management and sea level rise have been important ecological forces during the last 50 years. Marshes within the SESE were arranged in well‐defined compositional zones parallel to the coast, with mangrove‐dominated shrub communities near the coast giving way to graminoid‐mangrove mixtures, and then Cladium marsh. The compositional gradient was accompanied by an interiorward decrease in total aboveground biomass, and increases in leaf area index and periphyton biomass. Since the mid‐1940s, the boundary of the mixed graminoid‐mangrove and Cladium communities shifted inland by 3.3 km. The interior boundary of a low‐productivity zone appearing white on both black‐and‐white and CIR photos moved inland by 1.5 km on average. A smaller shift in this ‘white zone’ was observed in an area receiving fresh water overflow through gaps in one of the SESE canals, while greater change occurred in areas cut off from upstream water sources by roads or levees. These large‐scale vegetation dynamics are apparently the combined result of sea level rise ‐ ca. 10 cm since 1940 ‐ and water management practices in the SESE.
Influence of late season fire on early successional vegetation of an Oklahoma prairie
Authors: Engle David M. , Palmer Michael W. , Crockett J. Scott , Mitchell Ronald L. , Stevens Russell
Published: May 2006

Abstract. The study of vegetation dynamics in tallgrass prairie in response to fire has focused on dormant season fire in late successional prairies. Our objective was to determine if late season fire of varying frequency results in divergent successional patterns in an early successional tallgrass prairie disturbed by grazing and cultivation. Specifically, we evaluated the influence of late‐summer fires of varying frequency on community composition and species richness. We collected vegetation and environmental data on two sites burned in the late growing‐season at varying frequencies. These communities differed in composition depending primarily on edaphic factors, time since the last burn, and year‐to‐year variation. We interpret the time effect as related to changes in species composition accompanying plant succession that followed disturbance either from cropping and heavy grazing on the loamy site or heavy grazing on the shallow site. Other unidentified factors also have a role in vegetation dynamics on this prairie. Community composition and species richness were not consistently responsive to frequency of growing‐season fires.
Short‐term and long‐term effects of mowing on the vegetation of two calcareous fens
Authors: Güsewell Sabine , Buttler Alexandre , Klötzli Frank
Published: May 2006

Abstract. Short‐term field experiments are often used to predict and evaluate long‐term management effects. Based on a mowing experiment in two calcareous fens near Lake Neuchâtel, Switzerland, we investigated whether short‐term treatment effects (i.e. during the first four years) were confirmed by long‐term results (13 ‐ 14 yr). Plots were mown in summer or in winter or left unmown. The main long‐term trends in overall species composition (based on percentage cover estimates) were already observable in the first four years: mown and unmown plots diverged, whereas summer‐cut and winter‐cut plots remained similar. At the individual species level, however, short‐term and long‐term treatment effects differed considerably: many species whose abundance seemed affected by treatments during the first four years showed no response in the long term, and vice versa. These discrepancies were similar when based on cover estimates or on counts of shoots. Species responses did actually depend on the time scale considered. Short‐term and long‐term treatment effects on species richness were similar (i.e. a decrease in unmown plots), although only long‐term effects were significant. Treatment effects on the above‐ground biomass varied considerably, and short‐term trends (lower biomass in unmown plots) differed from long‐term trends (higher biomass in unmown plots). Our sites showed little overall change in species composition during the period investigated, and treatment effects were low compared with other similar experiments. If study sites are less stable or treatment effects more drastic, a short‐term evaluation is expected to be even less reliable. Knowledge on species dynamics at a site may help to choose the adequate spatial and temporal scale of investigation, and thus increase the efficiency of management experiments.
Landscape‐scale regeneration dynamics of disturbed Mediterranean maquis
Authors: Kadmon Ronen , Harari‐Kremer Ruthie
Published: May 2006

Abstract. Long‐term regeneration dynamics of Mediterranean maquis was investigated by analysing historical aerial photographs of Mt. Carmel, one of the largest protected areas in the Mediterranean region of Israel. Two sets of aerial photographs were processed, one from 1960 (before the area was protected), and the second from 1992 (21 yr after the area was declared a nature reserve). The photographs of each year were classified into three vegetation states based on the percentage cover of trees: open maquis with tree cover < 33.3 %, moderately developed maquis (tree cover 33.3–66.6 %), and closed maquis (tree cover > 66.6 %). Grid maps constructed from the classified images were used to determine probabilities of transition between vegetation states. Closed maquis showed zero probability of transition to either open or moderately developed maquis. Probabilities of ‘forward’ transitions (transitions from low‐cover to high‐cover classes) were higher on north‐facing than on south‐facing slopes. On north‐facing slopes, the area of open maquis decreased from 87 % to 46 % during the period studied, while that of closed maquis increased from 3 % to 29 %. On south‐facing slopes open maquis decreased from 87 % to 69 % while closed maquis increased from 1 % to 8 %. Within a particular aspect, tree cover in 1960 was a reliable predictor of tree cover in 1992. This indicates that micro‐scale patterns of tree distribution in 1960 were important in determining the structure of the maquis 32 yr later. Simulations based on the empirically derived transition probabilities suggest that, under current climatic conditions, the process of maquis regeneration on Mt. Carmel may take 500–1000 yr.
Grazing response groups among understorey plants in arid rangelands
Authors: Landsberg Jill , Lavorel Sandra , Stol Jacqui
Published: May 2006

Abstract.We analysed attributes of the understorey flora in different plant communities along two grazing gradients in arid Australian rangelands. We aimed to determine if there were patterns among species in response to grazing, and easily recognizable ‘indicator response types’ for monitoring grazing‐induced change in community composition. Measurements were stratified by vegetation patch type, in woody groves and open patches. Trait selection and analyses followed a hierarchical approach which searched for patterns within major plant life forms. Patch type exerted a dominant influence on both life forms and species attributes, but interacted with grazing. Grazing was associated with loss in differentiation of species composition between patch types, rather than loss in numbers of species overall. Heavy grazing was variously associated with small size, prostrate habit, low meristems, small leaves, coated leaves, high regrowth potential, plasticity in response to grazing, and high fecundity; and light grazing with opposing attributes. Many attributes tended to vary independently of each other and grazing‐related attribute syndromes were recognisable only among grasses. This could be because the environmental filters acting on the communities have given rise to many different species with different natural attribute combinations, few of which are closely associated with grazing resistance. For grassy communities ‘large, erect tussocks branching above‐ground’ and ‘small, sprawling basal tussocks’ may have potential as response types indicative of light and heavy grazing respectively, but no response types could be identified for herbaceous communities.
Plant functional types and ecosystem function in Mediterranean shrubland
Authors: Díaz Barradas M.C. , Zunzunegui M. , Tirado R. , Ain‐Lhout F. , García Novo F.
Published: May 2006

Abstract. This study attempts to identify Plant Functional Types (PFTs) within the scrub vegetation of the stabilized sand dunes of the Doñana National Park (southwestern Spain) and to explore possible links with ecosystem function. Two 10‐km long transects were sampled along a gradient of water table depth from elevated and dry dunes to a border area close to the salt marsh.

A matrix of cover values for 20 woody species × 58 plots and a matrix of 24 morphological and physiological traits × 20 species were analysed by means of DCA and TWINSPAN to identify the main vegetation types and PFTs. In order to know the predominant plant traits, the matrix of 20 species × 58 plots was multiplied by the 24 traits × 20 species matrix. The resulting 24 traits × 58 plots matrix was analysed by means of DCA.

The main vegetation types coincide with the previous descriptions of the Park with Juniperus phoenicea woodland and xerophytic species dominating the top of the dunes and hygrophytic species with isolated Quercus suber trees in the depressions. Previous classifications of Mediterranean woody plants resulted in two main PFTs. The present analysis confirmed these two groups: maquis versus garrigue, but added four new groups. The analysis of the traits × plots matrix revealed that the main trend of variation is related to the water table gradient, while the traits related to the species of mature plant communities had little weight in the analysis because these communities are restricted to isolated patches.
Effects of grassland extensification on the population dynamics of the invasive species Chaerophyllum aureum
Authors: Magda D.
Published: May 2006

Abstract. The consequences of extensification on grassland invasion by weeds was studied in French Pyrenean hay‐meadows through a survey of the population dynamics of one very successful colonizer: Chaerophyllum aureum. Experiments established in natural permanent grasslands allow us to test the effect of different intensities of cutting on the demography of adult and seedling populations and on the production of seeds and their survival in soil. Although an early cut can control population density by drastically reducing invasion by new genotypes, the results reveal a positive effect of early cutting on adult and seedling survival probably by decreasing intra‐specific competition. Allocation of resources to sexual reproduction in response to cutting is limited, affecting especially the seed viability in soil. Nevertheless, the invasive ability of this species is due largely to a strategy whereby those adults showing a high survival rate tend to be perennial.
Fire, canopy cover and seedling dynamics in Mediterranean shrubland of northeastern Spain
Authors: Lloret Francisco
Published: May 2006

Abstract. Seedling establishment and survival of the main woody species of early‐successional shrubland in northeastern Spain were studied from 1992 to 1995 with emphasis on the importance of vegetation cover (existence of open areas and the situation beneath the plant canopy) and microhabitat (occurrence of stones, litter and bare soil). In the absence of fire, vegetation cover (measured at a scale of 30cm × 30 cm) was not correlated with seedling emergence of most species, nor with growth and survival of seedlings up to one year old. Seedlings older than one year showed a similar pattern: their density was not significantly different in both kinds of habitats. The emergence of seedlings was mainly associated with the presence of mature plants – which can provide seeds – and with the absence of unsuitable microhabitats (large stones, deep litter and bare soil).

In April 1994 a wildfire burned the study area. This enabled a study of the pattern of post‐fire establishment. After the wildfire, seedling emergence of several species increased and most species produced seedlings with higher survival and growth rates than in the period before the fire. Pre‐fire cover, however, did not show significant effects on post‐fire seedling dynamics in most cases.
Effects of environment and grazing disturbance on tree establishment in meadows of the central Cascade Range, Oregon, USA
Authors: Miller Eric A. , Halpern Charles B.
Published: May 2006

Abstract. Within the last century there has been widespread establishment of trees in mountain meadows of the Pacific Northwest. We reconstructed patterns of tree invasion at 17 meadow sites in the central Cascade Range of Oregon, USA ‐sites representing diverse physical environments and vegetation types and experiencing different histories of recent anthropogenic disturbance (sheep grazing). Spatial distributions and age structures of invasive tree populations were analysed with respect to climatic records and grazing history. Patterns of establishment varied considerably among meadows, reflecting strong differences in environment and grazing history. In montane hydric meadows, tree establishment was spatially clumped beneath large old trees and on elevated microsites; however the timing of invasion differed between sites with stable versus fluctuating water tables. In upland mesic/dry montane meadows, timing of invasion corresponded with cessation of sheep grazing (early 1940s) and the onset of wetter summers (mid 1940s). In the subalpine zone, climate and aspect interacted to produce contrasting histories of invasion on north‐ and south‐facing slopes.

Establishment on north‐facing slopes, concentrated in heath‐shrub communities, coincided with regional warming (ca. 1920–1945) when snowpacks were lighter and melted earlier. Recruitment of trees onto south‐facing slopes occurred later, when conditions were wetter (1945–1985). In many environments, the spatial distribution of recruitment suggests that once trees have established, autogenic factors become increasingly important as individual trees or groups of trees alter the physical or biotic conditions that once inhibited establishment.

Knowledge of the factors that influence invasion, and of their varying importance across gradients in environment and vegetation, is critical to predicting future changes in these dynamic systems.
Biological and environmental characteristics of three cosmopolitan mosses dominant in continental Antarctica
Authors: Lewis Smith R.I.
Published: May 2006

Abstract. Bryum argenteum, B. pseudotriquetrum and Ceratodon purpureus are the predominant mosses in Victoria Land, continental Antarctica. All have cosmopolitan distributions and are widespread throughout Antarctica with wide ecological amplitudes resulting in considerable morphological variation. They are well adapted to tolerate the physiological stresses imposed by the severe environment. This study investigates aspects of their growth, physiology and survival in response to habitat constraints, especially hydrology. Their distribution is controlled almost exclusively by moisture availability. Each species tends to predominate in a specific zone along hydrological gradients, with B. pseudotriquetrum on moist soil, C. purpureus on drier soil, and B. argenteum on unstable stream margins, fluvial deposits and the marginal capillary zone. Where conditions are optimal, each species can form a turf 6–10 cm thick. Nutrient status of the soil does not appear to be an important determinant in the distribution pattern within communities. The thermal regime of the moss turf varies according to its moisture content; for a period of ca. six weeks during the summer, with the frequent long spells of 24‐h sunshine, temperatures remain above 0 °C for much of the time even though air temperatures are frequently below the freezing point. This allows growth and metabolic activity to proceed continuously at a relatively rapid rate for quite long periods. Annual shoot incremental growth can exceed 3.5 mm in each species. Growth of B. argenteum may be inhibited by UV‐ B radiation. The optimal temperature for photosynthesis in each species is around 15 °C, but significant carbon fixation occurs at 5 °C. Photosynthetic rates at 5, 10 and 20 °C were B. argenteum > B. pseudotriquetrum > C. purpureus.
Post‐fire bryophyte dynamics in Mediterranean vegetation
Authors: Esposito A. , Mazzoleni S. , Strumia S.
Published: May 2006

Abstract. Bryophyte dynamics after fire in the Mediterranean macchia of Southern Italy was studied both by diachronic and synchronic approaches. Changes of bryophyte cover and species composition were found in relation to both age and fire intensity. During the first 2 yr after fire, bryophytes dominated the plots which had experienced the highest fire intensity while herbs were dominant in plots affected by lighter fires. Pioneer species, such as Funaria hygrometrica, Barbula convoluta and Bryum dunense, characterized recent intense fires, whereas Bryum torquescens, B. radiculosum and B. ruderale were dominant after less intense burning. Pleurochaete squarrosa, Tortula ruraliformis and Tortella flavovirens dominated intermediate successional stages. Pleurocarpous mosses were dominant only in the older closed stands. Different patterns of regeneration strategies were described: spores dominated early stages of intense fire, while vegetative propagules characterized later successional stages and less severely burned areas. Although bryophytes usually have a low abundance in Mediterranean vegetation, their role in post‐fire vegetation dynamics may be locally enhanced according to burning conditions.
Sixty years of vegetation dynamics in a south boreal coniferous forest in southern Norway
Authors: Nygaard Per Holm , Ødegaard Torbjørn
Published: May 2006

Abstract. Vegetation data from permanent plots were collected in 1931, 1961 and 1991 in a south boreal forest 20 km north of Oslo in southern Norway. Major changes were found in the vegetation composition during those 60 years. The main changes were a reduction in the frequency of species and the frequency of joint occurrences of vascular species such as Andromeda polifolia, Calluna vulgaris, Cornus suecica, Eriophorum vaginatum, Maianthemum bifolium, Melampyrum pratense, Trientalis europaea, Vaccinium uliginosum and V. oxycoccus, and mosses, e.g. Dicranum fuscescens, Hylocomium splendens, Pleurozium schreberi, Ptilidium ciliare and Ptilium crista‐castrensis.

The observed changes were interpreted as being induced by internal processes e.g. notably a long‐term change from paludified forest to mesic forest. In particular the growth of Picea abies seems to be a main driving force. The dominance of Picea abies and Vaccinium myrtillus appears to have rendered the conditions more unfavourable for other species. A doubling of the living stem biomass of P. abies during the last 67 yr shows that this old‐growth forest has not yet reached a steady state. It was demonstrated that species such as Deschampsia flexuosa and Molinia caerulea did not increase in frequency in response to nitrogen deposition, as has occurred elsewhere in northern Europe. pH in the humus layer increased with 0.2 unit from 1961 to 1991.

The results of this study indicate that protection from logging has initiated the reduction of species in the field layer and bottom layer. This study questions if monitoring of forest vegetation should be restricted to protected forests as is the practice in Scandinavia today. We recommend that also areas with some kind of selective cutting will be used for monitoring of forest vegetation.
Relationship between spatial strategies and morphological attributes in a Uruguayan grassland: a functional approach
Authors: Altesor A. , Pezzani F. , Grun S. , Rodríguez C.
Published: May 2006

Abstract. In a native grassland in Uruguay subjected to continuous grazing by cattle, four permanent plots of 40 × 40 cm were established. Plots were divided into 16 × 16 cells and the presence of species was recorded seasonally during two years. The spatial dynamic of the dominant species was defined through three parameters: persistence, short‐distance spread and long‐distance spread. The association between the spatial strategies and certain morphological attributes that were presumed to be important to the spatial behaviour of the species, was examined using Correspondence Analysis.

Four types of spatial strategies were observed: (1) persisting over the year in the same cell without showing any type of mobility; (2) continuously growing due to the solid advancing front of ramets, persisting or not persisting (3) in the same cells; and (4) multi‐strategy with high persistence and both high short‐ and long‐distance spread. Among the selected attributes, the erect/prostrate habit, the capacity of vegetative propagation by means of bulbs, rhizomes or stolons, the ratio length/width of leaves and the season of active growth were the characteristics most closely associated with the spatial strategies.
Four decades of old field vegetation development and the role of Celastrus orbiculatus in the northeastern United States
Authors: Fike Jean , Niering William A.
Published: May 2006

Abstract. This paper describes four decades of in situ vegetation change in a post‐agricultural site in southern New England where the role of the introduced vine, Celastrus orbiculatus, has drastically modified the course of vegetation development. An initial perennial forb‐grassland community was replaced by two strikingly different physiognomic vegetation types‐an early hardwood forest and a vine community occupying the same site 40 yr later. This ecological phenomenon was unpredictable since Celastrus was not recorded at the site until the 1970s. Within the young hardwood forest dominated by Prunus serotina and Acer rubrum, a few old field species (five) persist, whereas 17 forest herbs have become established. Typical forest shrubs are rare or absent. The Celastrus‐dominated vine community has suppressed or eliminated pre‐existing growth to form a relatively stable community where a forest vegetation is the regional vegetation type. Celastrus also appears to be facilitating a native vine (Vitis labrusca) by forming a ‘ladder’ for its advance. Although A. rubrum and Quercus spp. could ultimately dominate the forest community, the contiguous and aggressive nature of Celastrus makes predictions questionable. This long‐term study highlights the unpredictable nature of old field succession and the role of an exotic species in threatening the natural vegetation development.
Plant functional types and ecosystem function in relation to global change
Authors: Diaz Sandra , Cabido Marcelo
Published: May 2006

Abstract. Plant functional types (PFTs) bridge the gap between plant physiology and community and ecosystem processes, thus providing a powerful tool in climate change research. We aimed at identifying PFTs within the flora of central‐western Argentina, and to explore their possible consequences for ecosystem function. We analyzed 24 vegetative and regenerative traits of the 100 most abundant species along a steep climatic gradient. Based on plant traits and standard multivariate techniques, we identified eight PFTs. Our results confirmed, over a wide range of climatic conditions, the occurrence of broad recurrent patterns of association among plant traits reported for other floras; namely trade‐offs between high investment in photosynthesis and growth on the one hand, and preferential allocation to storage and defence on the other. Regenerative traits were only partially coupled with vegetative traits. Using easily‐measured plant traits and individual species cover in 63 sites, we predicted main community‐ecosystem processes along the regional gradient. We hypothesized likely impacts of global climatic change on PFTs and ecosystems in situ, and analysed their probabilities of migrating in response to changing climatic conditions. Finally, we discuss the advantages and limitations of this kind of approach in predicting changes in plant distribution and in ecosystem processes over the next century.
Phytosociological, bioclimatic and biogeographical classification of woody climax communities of western North America
Authors: Peinado M. , Aguirre J.L. , Delgadillo J.
Published: May 2006

Abstract. In this study, combining the Braun‐Blanquet phytosociological approach with numerical methods (cluster analysis and principal coordinate ordination), woody climax associations of western North America are described, ranging from NW Baja California (Mexico) to Alaska (USA), and including some interior zones of Idaho and Montana (USA), Alberta, British Columbia and the Yukon Territory (Canada). 95 floristic associations are identified, and each is characterized by floristic combination, biogeographical range, and bioclimatic conditions.
Regeneration and coexistence of two subalpine conifer species in relation to dwarf bamboo in the understorey
Authors: Takahashi Koichi
Published: May 2006

Abstract. Microhabitats for seedling establishment and gap regeneration in subalpine forests of northern Japan were studied for two conifers, Abies sachalinensis and Picea glehnii. The abundance of understorey dwarf bamboo (Sasa spp.) was different for the four plots examined. Two types of micro‐habitats were recognized for the two conifers: ground and elevated woody substrates (fallen logs and buttresses). Picea regenerated mostly on elevated sites, while Abies regenerated on both ground and elevated sites. The densities of Picea were independent of those of Sasa, but Abies densities decreased with increasing abundance of Sasa because Sasa reduced regeneration on the ground. Density of Abies on elevated sites was higher than that of Picea, irrespective of Sasa and of the density of adult trees. There was no significant difference in growth in sapling trunk height between the two conifers, but Picea grew more slowly under the canopy than Abies and was aggregated into gaps. Thus, in forests with less Sasa, the recruitment capacity of Abies was greater than that of Picea. The long life span of Picea compensated for its low density on elevated sites. Examination of a dynamic system model showed that Picea was excluded by Abies in forests without Sasa because regeneration on the ground is more advantageous than on elevated sites, but the two conifers could coexist in forests with Sasa because of the increased relative success of regeneration on elevated sites by Picea saplings.
Differentiation diversity of Argentine cacti and its relationship to environmental factors
Authors: Mourelle Cristina , Ezcurra Exequiel
Published: May 2006

Abstract. We studied the differentiation diversity (β‐diversity or species turnover) patterns of the three main cactus growth forms (columnar, opuntioid and globose) in 318 (1° × 1°) squares covering Argentina. We analysed the degree of association between species turnover of each growth form with the spatial variation of a set of 15 environmental variables.

Species turnover was estimated in two ways: (1) by calculating species turnover along latitudinal and longitudinal gradients and (2) by evaluating the species turnover between each square and its eight surrounding neighbouring grid cells.

For the three growth forms, species turnover in latitudinal transects was mostly related to the mean within‐transect values of certain environmental variables, while in longitudinal transects it was related to the variation of some environmental variables within the transect rather than to their mean values. For columnar species, transect species turnover was mainly associated with variation in temperature, confirming the temperature‐sensivity of this growth form. For opuntioid species, turnover along transects was mainly related to topographic variables. In the case of globose cacti, transect turnover was associated with variation in temperature and rainfall.

For the three growth forms, areas of high turnover coincided with marked transitions between different biogeographic provinces, while the areas with lowest species turnover coincide with topographically and climatically uniform plains. Species turnover between individual squares was positively associated with the proportion of summer rainfall in globose cacti, the variation of mean annual temperature in columnar cacti and was negatively related to mean annual temperature in opuntioid cacti. Compared to the other growth forms, globose cacti presented a much larger proportion of squares with a high species turnover.

In general, differentiation diversity was lower for the opuntioid and the columnar species, two growth forms with higher dispersal ability and was highest for the globose cacti, which have the lowest dispersal capacity. Environmentally heterogeneous areas, where large‐scale transitions between biomes occur, have exceptionally high species turnover, and are important target areas for the conservation of biodiversity.
Variation in the riparian landscape of the Segura River Basin, SE Spain
Authors: Alcaraz F. , Ríos S. , Inocencio C. , Robledo A.
Published: May 2006

Abstract. The woody and shrubby riparian vegetation of the Mediterranean Region of Spain shows a progressive displacement from the Querco‐Fagetea, typical of Central Europe, towards the Nerio‐Tamaricetea, which has its maximum expression in North Africa. We studied the occurrence of 107 riparian plant communities on 52 locations in the Segura River Basin (SE Spain) and their presumed relation to variation in rainfall and temperature. On the basis of agglomerative clustering and Redundancy Analysis (RDA) three riparian vegetation complexes were identified and characterized by (a) Central European‐related temperate communities, (b) predominantly North African riparian communities and (c) a combination of both community types, respectively. North African communities predominated in the lower stretches of the Segura Basin, probably due to the prevailing semi‐arid bioclimate.
Determining alternative models for vegetation response analysis: a non‐parametric approach
Authors: Bio A.M.F. , Alkemade R. , Barendregt A.
Published: May 2006

Abstract. Vegetation models based on multiple logistic regression are of growing interest in environmental studies and decision making. The relatively simple sigmoid Gaussian optimum curves are most common in current vegetation models, although several different other response shapes are known. However, improvements in the technical means for handling statistical data now facilitate fast and interactive calculation of alternative complex, more data‐related, non‐parametric models. The aim in this study was to determine whether, and if so how often, a complex response shape could be more adequate than a linear or quadratic one. Using the framework of Generalized Additive Models, both parametric (linear and quadratic) and non‐parametric (smoothed) stepwise multiple logistic regression techniques were applied to a large data set on wetlands and water plants and to six environmental variables: pH, chloride, orthophosphate, inorganic nitrogen, thickness of the sapropelium layer and depth of the water‐body. All models were tested for their goodness‐of‐fit and significance. Of all 156 generalized additive models calculated, 77 % were found to contain at least one smoothed predictor variable, i.e. an environmental variable with a response better fitted by a complex, non‐parametric, than by a linear or quadratic parametric curve. Chloride was the variable with the highest incidence of smoothed responses (48 %). Generally, a smoothed curve was preferable in 23 % of all species‐variable correlations calculated, compared to 25 % and 18 % for sigmoid and Gaussian shaped curves, respectively. Regression models of two plant species are presented in detail to illustrate the potential of smoothers to produce good fitting and biologically sound response models in comparison to linear and polynomial regression models. We found Generalized Additive Modelling a useful and practical technique for improving current regression‐based vegetation models by allowing for alternative, complex response shapes.
Vegetation of trampled soil dominated by C4 plants in Europe
Authors: Carni Andraž , Mucina Ladislav
Published: May 2006

Abstract. Plant communities of trampled soil dominated by plants characterized by the C4‐assimilation syndrome were investigated in Europe. These species, belonging to genera such as Chamaesyce, Amaranthus, Eleusine, Eragrostis and Setaria, are thermophilous, late‐germinating, prostrate herbs or grasses. The centre of their distribution is in the (Sub)Tropics. A syntaxonomic revision of the phytosociological material from Europe (incl. the Macaronesian Archipelago) revealed three alliances: the Euphorbion prostratae from Spain, the Polycarpo‐Eleusinion indicae from Italy, and Slovenian and Croatian Istria, and the Eragrostio‐Polygonion arenastri from temperate regions of Europe. The latter two syntaxa are described as new. All three alliances belong to the order Eragrostietalia (class Stellarietea mediae). Vicarious (ecologically analogous) communities occur also in southern Africa, eastern Asia and North America. The communities studied in the present paper are considered to be an impoverished form of highly diversified trampled plant communities typical of (sub)tropical areas.
Plant functional traits and environmental filters at a regional scale
Authors: Diaz Sandra , Cabido Marcelo , Casanoves Fernando
Published: May 2006

Abstract. Links between plant traits and the environment, i.e. sets of plant attributes consistently associated with certain environmental conditions, are the consequence of the filtering effect of climatic, disturbance and biotic conditions. These filters determine which components of a species pool are assembled into local communities. We aimed at testing for consistent association between plant traits and climatic conditions along a steep regional gradient, divided into 13 climatically homogeneous sectors, in central‐western Argentina. We analyzed 19 vegetative and regeneration traits of the 100 most abundant species along the gradient. For each trait, we tested for homogeneity of frequencies of different categories between sectors and the regional species pool, using the χ2 statistic. We rejected H0 in 71% of the pair‐wise comparisons, which strongly suggests a ‘filtering effect’ of climatic factors on key plant functions. Vegetative traits were filtered more often than regeneration traits. Specific leaf area, life span, ramification, canopy height, leaf weight ratio, carbon investment into support tissue and pollination mode were the traits showing differences in the largest number of pair‐wise comparisons.

This is probably the first attempt to detect, on a quantitative, statistically conservative basis, consistent linkages between climatic factors and numerous plant traits, over a broad spectrum of environmental conditions and plant growth forms. We discuss the advantages and limitations of this approach in predicting vegetation structure and functioning under present environmental conditions, and those expected for the next century as a consequence of global change.
How season of grazing and herbivore selectivity influence monsoon tall‐grass communities of northern Australia
Authors: Ash Andrew J. , McIvor John G.
Published: May 2006

Abstract. The hypothesis that season of defoliation and herbivore selectivity may be as important as level of use in determining plant community response to grazing was tested in a monsoon grassland in northern Australia. Plots, dominated by the tussock grasses Themeda triandra and Chrysopogon fallax, were grazed by cattle at low, medium and high rates of utilization in either the early wet, late wet or dry seasons. Effects of grazing on species composition were greatest in the early wet season when high rates of utilization significantly reduced the proportion and occurrence of Themeda and increased the proportion of forbs. Grazing in the dry season had no significant effect on composition.

At medium and high levels of utilization in the early wet season, the pasture responded negatively to defoliation, only partially compensating for plant tissue lost to herbivory. The negative response to defoliation carried over to the next wet season when these same medium and high‐grazing treatments produced only 80 % and 60 % growth, respectively, of that in treatments grazed at low levels of utilization or those grazed during the dry season. The frequency of Themeda was still lower, and that of annual grasses and non‐leguminous forbs higher, in plots that had been grazed at a high rate of utilization for just eight weeks in the early wet season two years previously. Species richness and diversity were also significantly affected by this grazing disturbance. If species composition is to be maintained in these grasslands then stocking rates must be set at low levels to cope with the combined effect of undercompensation in response to defoliation in the wet season and strong dietary preferences for grazing sensitive species.
Effects of late growing‐season and late dormant‐season prescribed fire on herbaceous vegetation in restored pine‐grassland communities
Authors: Sparks Jeffrey C. , Masters Ronald E. , Engle David M. , Palmer Michael W. , Bukenhofer George A.
Published: May 2006

Abstract. We compared the effects of late dormant‐season and late growing‐season prescribed fires on herbaceous species in restored shortleaf pine‐ (Pinus echinata) grassland communities in the Ouachita Highlands of western Arkansas. Herbaceous species richness, diversity, and total forb and legume abundance increased following fire. Late growing‐season burns reduced distribution and abundance of panicums (primarily Panicum boscii, P. dichotomum, and P. linearifolium) while late dormant‐season burns increased Panicum distribution and abundance. Density of legumes (such as Stylosanthes biflora) increased following frequent or annual dormant‐season fires. However, season of fire influenced the distribution and abundance of fewer than 10 % of the species. Fire plays an essential role in pine‐grassland communities by creating and maintaining open canopy conditions that perpetuate understory herbaceous plant communities.
Flood drift and propagule bank of aquatic macrophytes in a riverine wetland
Authors: Cellot Bernard , Mouillot Florent , Henry Christophe P.
Published: May 2006

Abstract. Drift of aquatic macrophyte propagules was investigated in a wetland along the River Rhône, during the first flood after the growing season (i.e. in the winter of 1995–1996). Input and output drift were studied at the beginning, around the top, and at the end of the river overflow in the upper reach of a cut‐off channel. The soil propagule bank was sampled along the study area before and after the flood. The amount and composition of viable propagule drift and bank were determined, analysed and compared. Drift densities and richness were on average higher at the outlet of the channel than at the inlet (respectively: 23.2 vs 13.1 viable propagules/100 m3 of water and 8.7 vs 2.6 taxa per sample). Immigrating taxa were mostly in the form of helophyte seeds, whereas numerous resident hydrophyte species left the disturbed area rather as vegetative propagules.

Temporal variability in propagule bank structure was weak, and mean bank densities did not change before and after the flood (respectively: 33 047 ± 10 510 vs 35 653 ± 15 070 viable propagules/m2 of ground, including Chara). However, the density of Elodea canadensis significantly increased after the flood while that of Eleocharis acicularis decreased. This contrast suggests that flood responses vary among species. Despite a broad overlap in the taxa (18 out of 25 were common both to drift and bank collections), no significant relationship occurred in composition or structural changes between flood drift and propagule bank. Flood acted as a means of distribution of existing propagules and also as a provider of new vegetative dispersal units.
Soil seed bank composition along a forest chronosequence in seasonally moist tropical forest, Panama
Authors: Dalling J.W. , Denslow J.S.
Published: May 2006

Abstract. We used a forest chronosequence at the Barro Colorado Nature Monument (BCNM) to examine changes in the abundance and species composition of seeds in the soil during forest succession. At each of eight sites varying from 20 yr to 100 yr since abandonment, and at two old‐growth (> 500 yr) forest stands, we established two 160‐m transects and sampled the surface 0–3 cm of soil in cores collected at each 5 m interval. Seed densities were estimated from the number of seedlings germinated from the soil over a six‐week period. Contrary to expectation, neither the density of the soil seed bank, nor species richness or diversity were directly related to age since abandonment, but the density of the soil seed bank was correlated with the abundance of seed‐bank‐forming species in the standing vegetation. In marked contrast to published studies, herbaceous taxa were rare even in the youngest stands, and the common tree species, which accounted for most seeds in the soil, were present in all stands. The pioneer tree Miconia argentea (Melastomataceae) was the single most common species in the seed bank, accounting for 62% of seeds and present in 92% of soil samples. Rapid recovery of the vegetation of young regrowth stands on BCNM, when compared to sites elsewhere may be partly due to allochthonous seed rain from nearby mature forest stands and the lack of seed inputs of weeds and grasses from agricultural and pasture lands which may inhibit forest succession.
Are New Zealand's Nothofagus species in equilibrium with their environment?
Authors: Leathwick John R.
Published: May 2006

Abstract. Past explanations of the large disjunctions in the distribution of New Zealand's four Nothofagus species have emphasized displacement during glacial cycles followed by slow re‐occupation of suitable sites, or the effects of plate tectonics coupled with ecological and/or environmental limitations to further spread. In this study the degree of equilibrium between Nothofagus distribution and environment was compared with that of other widespread tree species by statistical analysis. Generalized additive regression models were used to relate species distribution data to estimates of temperature, solar radiation, soil water deficit, atmospheric humidity, lithology and drainage. For each species, the amount of spatial patterning remaining unexplained by environment was assessed by adding a variable describing species presence/absence on adjacent plots. Results indicate that Nothofagus species occur more frequently in environments suboptimal for tree growth, i.e. having various combinations of cool temperatures, low winter solar radiation, high root‐zone water deficit, low humidity, and infertile granitic substrates. Despite these demonstrated preferences, they exhibit substantially more spatial clustering which is unexplained by environment, than most other widespread tree species. Predictions formed from regressions using environment alone confirm that several major Nothofagus disjunctions are not explicable in terms of the environmental factors used in this analysis, but more likely reflect the effects of historic displacement coupled with slowness to invade forest dominated by more rapidly dispersing endomycorrhizal species. The technique used in this study for detecting residual spatial autocorrelation after fitting explanatory variables has potentially wide application in other studies where either regression or ordination techniques are used for analysis of compositional data.
Predicting the distribution of shrub species in southern California from climate and terrain‐derived variables
Authors: Franklin Janet
Published: May 2006

Abstract. Generalized additive, generalized linear, and classification tree models were developed to predict the distribution of 20 species of chaparral and coastal sage shrubs within the southwest ecoregion of California. Mapped explanatory variables included bioclimatic attributes related to primary environmental regimes: averages of annual precipitation, minimum temperature of the coldest month, maximum temperature of the warmest month, and topographically‐distributed potential solar insolation of the wettest quarter (winter) and of the growing season (spring). Also tested for significance were slope angle (related to soil depth) and the geographic coordinates of each observation. Models were parameterized and evaluated based on species presence/absence data from 906 plots surveyed on National Forest lands. Although all variables were significant in at least one of the species’ models, those models based only on the bioclimatic variables predicted species presence with 3–26% error. While error would undoubtedly be greater if the models were evaluated using independent data, results indicate that these models are useful for predictive mapping – for interpolating species distribution data within the ecoregion. All three methods produced models with similar accuracy for a given species; GAMs were useful for exploring the shape of the response functions, GLMs allowed those response functions to be parameterized and their significance tested, and classification trees, while some‐times difficult to interpret, yielded the lowest prediction errors (lower by 3–5%).
Tree species richness and turnover throughout New Zealand forests
Authors: Bellingham P.J. , Stewart G.H. , Allen R.B.
Published: May 2006

Abstract. Patterns of mortality, recruitment, and forest turnover were investigated using permanent plot data from temperate forests in 14 localities throughout New Zealand. Tree mortality and recruitment rates were calculated from tagged trees ≥ 10 cm diameter at 1.4 m on individual 400 m2 plots, and turnover rates were calculated as the mean of mortality and recruitment rates. Turnover rates (1.4% per year) were very similar to those recorded for tropical forests (i.e. 1.5% per year). As was shown in tropical forests, we also found significant relationships between forest turnover and species richness. In New Zealand forests there was also a decrease in species richness and turnover rates with increasing latitude. Although species richness is well known to decline with latitude, our study provides support for a possible link between seasonality and disturbance with tree turnover and species diversity. While tree mortality and recruitment rates were approximately in balance at some localities, in others there were imbalances between mortality and recruitment rates.