bluuebunny/crossref_metadata_2025 huggingface dataset:
The role of seedbank and sown seeds in the restoration of an English flood‐meadow
Authors: McDonald A. W.
Published: May 2006

Abstract. In 1985 an opportunity arose to make use of an area (referred to as Somerford Mead) which had recently been used for intensive grass or cereal production but which originally had been a permanent hay‐meadow. Situated on circum‐ neutral alluvial soils over limestone gravel, it is in close proximity to the River Thames and to species‐rich flood meadows, including Oxey Mead (Fig. 1). This paper describes the early stages of an experiment to recreate a ‘typical’ flood‐meadow community using seed harvested from Oxey Mead. Before sowing the seed, soil samples were taken and seedling emergence suggested that little of the original flood‐meadow seed bank survived. Although Oxey Mead was known to contain at least 57 species, germination of samples of the sown seed in the glasshouse was confined to 12 species. All except Bromus commutatus and Trisetum flavescens and an additional nine Oxey Mead species were recorded in the field, together with 24 arable weed/ruderal species. Soil‐nutrient contents (N, P and K) were high; so the restoration of this flood‐meadow using only management techniques (hay cutting at the end of June and aftermath grazing) and the local seed bank would take a long time. The use of Alopecuros pratensis‐Sanguisorba officinalis seed mixture was justified by the accelerated succession on Somerford Mead towards the parent community.
Changes in the forest floor of Canadian southern boreal forest after disturbance
Authors: Paré David , Bergeron Yves , Camiré Claude
Published: May 2006

Abstract. The concentrations and contents of organic matter and nutrients in organic deposits on the forest floor were estimated along a 231‐yr chronosequence following fire at the southern limit of the boreal forest in eastern Canada. The sampling design was stratified to take into account the variability related to the presence of the principal tree species as well as to the presence of large gaps created by a recent spruce budworm (Choristoneura fumiferana) outbreak. The forest floor showed a steady accumulation of organic matter and total nutrients with time‐since‐fire and a 50 % decrease in the concentrations of available P and K, but not N (as determined by aerobic incubation). The increase in forest‐floor weight was accompanied by an increased storage of available N, Ca and Mg. The availability of N and Ca was more strongly affected by tree species and gaps than by time‐since‐fire. A high N‐availability was observed under Betula papyrifera and in gaps, while high a Ca‐availability was found near Populus tremuloides and Thuja occidentalis. In old sites, the forest floor of gaps, created by a recent spruce budworm outbreak, had a necromass similar to that of a young forest, but the low concentrations of available P and K of an old forest.
Plant strategies along mountain vegetation gradients: a test of two theories
Authors: Oksanen Lauri , Ranta Esa
Published: May 2006

Abstract. We test the predictions of two theories on plant strategies along vegetation gradients of a North Scandinavian mountain area, from herb‐rich birch forests at 250 m. a.s.l. to the summit at 1530 m. a.s.l. According to Grime's theory, stressful physical conditions select for an adaptational syndrome called S‐strategy. The theory predicts that the prevalence of S‐strategic features increases monotonically from forests to high‐alpine boulder fields. Alternatively, the traits that Grime regards as S‐strategic can be interpreted as adaptations to high natural‐grazing pressure which culminates in moderately stressful habitats: productive enough to sustain grazers but yet so barren that moderate grazer densities suffice to deplete the vegetation. The observed vegetation patterns support this alternative view. We thus suggest that Grime's triangle should be revised as to make the role of grazing as the central causal factor behind the third strategy explicit.
Germination and early establishment of lower salt‐marsh species in grazed and mown salt marsh
Authors: Bakker J. P. , de Vries Y.
Published: May 2006

Abstract. Lower salt‐marsh species occur more in the grazed than in the mown sites of a salt marsh in Schiermonnikoog in the Netherlands. This was demonstrated by a sowing experiment which determined which characteristic of the stand structure, height of the canopy or percentage of bare soil, is responsible for this difference. The total number of seedlings which emerged was negatively related to the height of the canopy and positively to the percentage of bare soil. The survival of seedlings of Suaeda maritima and Plantago maritima could be explained by the height of the canopy and for the latter species also by the percentage of bare soil. The survival of Aster tripolium and Plantago maritima was higher in grazed than in mown sites. Since the amount of bare soil was higher than seemed necessary for germination and survival, it was concluded that the taller canopy was responsible for the absence of lower salt‐marsh species in the mown sites.
Vegetation types of the deltaic Mediterranean coast of Egypt and their environment
Authors: Zahran M. A. , El‐Demerdash M. A. , Mashaly I. A.
Published: May 2006

Abstract. The delta section of the Mediterranean coast of Egypt canbe divided ecologically into four main habitats: salt marsh, sand formation, reed swamp and fertile non‐cultivated land. In each habitat vegetation types can be distinguished based on the dominance of single species. The salt marsh habitat comprises five such communities dominatedby: Zygophyllumaegyptium, inula crithmoides, Arthrocnemum macrostachyum, Juncus acutus and Ha‐locnemum strobilaceum. The vegetation of the sand formation includes 11 communities dominatedby: Elymus farctus ssp.farctus, Alhagigraecorum, Cynodon dactylon, Heliotropium curassavicum, Stipagrostis lanata, Thyme‐laea hirsuta, Moltkiopsis ciliata, Asparagus stipularis and Pancratium maritimum (sand dunes), Cressa crética and Phragmites australis (sand flats). The reed swamp habitat is dominated by Typha domingensis. In the fertile non‐cultivatedlandAlhagigraecorum, Cynodon dactylon and Heliotropium curassavicum are the dominants.

Quantitative cluster analysis revealed a lower number of clusters than there are dominance types, but on the whole the two systems are similar.

Ordination and subsequent analysis of correlation between vegetation gradients and soilfactors showed that the decisive soil factors for the variation in salt marsh vegetation are moisture availability and CaC03 content, and for the sand formation soil moisture, ‐ fertility and ‐ salinity.
Edge detection in vegetation: Jornada revisited
Authors: Orlóci László , Orlóci Marta
Published: May 2006

Abstract. We return to the problem of pattern detection with focus on locating vegetation edges. We assume transect sampling with contiguous quadrats and we postulate a vegetation model in which pattern confounds responses to random, serial, and possibly edge‐generating effects. Realizing that strong serial effects will enhance the appearance of vegetation continuity, we probe the residuals for evidence of edges. Two independently derived structures furnish the evidence: a deviations profile and an ordination map. The profile displays compositional changes along the transect as deviations from a theoretical continuum state. The ordination traces the transect as an irregular line in vegetation hyperspace. On this irregular line, the transect segments appear as apices and the compositional dissimilarities of the transect segments as inter‐apex distances. Sharp apices (in full dimension) isolated by long inter‐apex distances (also in full dimension) indicate possible edges. While the subtending angle to an apex reflects changes in population proportions, the criterion for measuring edge sharpness, the height of the deviations profile expresses quantitative effects, the criterion for measuring edge intensity. The deviations profile does not seem to have direct parallel in past edge detection. The inter‐apex distance has a parallel in the Ludwig‐Cornelius compositional distance. We describe the details of sampling and analysis, and present numerical results. We also include a short glossary in App. 5.
Savanna tree influence on understory vegetation and soil nutrients in northwestern Kenya
Authors: Weltzin Jake F. , Coughenour Michael B.
Published: May 2006

Abstract. Contrary to observations and models in which trees and herbaceous plants are viewed as competitors, we found that trees in an African savanna have positive impacts on herbaceous biomass production and composition, and on soil nutrient status. In the Turkana District of northwestern Kenya, we investigated vegetation and soil gradients along equi‐angular transects radiating from the boles of individual Acacia tortilis trees. Total herbaceous biomass averaged 260 ± 17(se) g/m2 at the bole and declined to 95 ± 8 g/m2 in the tree interspaces. Soil organic carbon and total nitrogen concentrations were greatest (0.72 % and 0.083 %, respectively) in shallow soils near the bole and declined rapidly toward the interspaces and with increasing depth. Transects were also established between tree pairs to assess effects of differential canopy proximities. Grass production averaged 220 ± 21 g / m2 below overlapping canopies, 150 ± 15 g / m2 under individual canopies, and 95 ± 8 g / m2 in interstitial areas. Detrended correspondence analysis revealed that shifts in species composition were correlated with distance from tree bole out to the edge of the canopy. Species response, in terms of relative cover, to increasing distance from the bole, seemed to fall into five general classes: 1) greatest at the bole, 2) increasing with distance from the bole, 3) greatest in the mid canopy zone, 4) least at the bole and 5) no response. Trees did not influence herbaceous compositionbeyondtree canopies. It is assumed that shade cast by the tree canopy with subsequent reductions of understory water stress and temperature and increased nutrient concentrations may be the most important factors affecting understory soil and vegetation.
A vegetation zonation from saltmarsh to riverbank in New Zealand
Authors: King Warren McG. , Wilson J. Bastow , Sykes Martin T.
Published: May 2006

Abstract. Brackish riverbank marshes have been little studied. Therefore, a plant community sequence was sampled from saltmarsh to near‐freshwater riverbank marsh on a number of disjunct marshes along the Taieri River, Otago, New Zealand, from near the mouth to 9 km inland. Salinity decreased steadily upstream, though the actual values were very different on two days sampled.

Ten communities are recognised. The major vegetation zonation was upshore more than upstream, though there were several interactions between the upshore and upstream gradients. Few species, if any, were restricted to the mid reaches of the length of river sampled. There was only a very slight upshore increase in species richness, and no trend upstream.

Sequences of communities occurred upshore on all marshes, but the sequence differed, even within a marsh. Species were assorted into communities in different ways from those of marine marshes in the area. Some species, native and exotic, occupied different beta‐niches from those they occupied in other countries. Individualistic community structure is inferred.
Population ecology of Acacia tortilis in the semi‐arid region of the Sudan
Authors: Kennenni Legesse , van der Maarel Eddy
Published: May 2006

Abstract. The population ecology of Acacia tortilis, one of the important woody species in the arid and semi‐arid regions of northern Africa, was studied in the Sudan: Ten sample plots were established and all individuals in the plots were mapped and their basal diameter, stem height, crown height and cover measured annually for three years. Individuals were grouped into different height‐classes, ‘life‐forms’: trees >1.65 m, shrubs > 0.65 m and < 1.65 m, saplings < 0.65 m and seedlings (saplings less than one year old). The population density was 510 ind./ ha in 1985, 432 ind. / ha in 1986 and 218 ind. / ha in 1987. The life‐form distribution of the population showed clear differences between plots and years.

Mortality in Acacia tortilis, expressed as loss in individuals per year, amounted to 167 ind. / ha / yr. Drought and lethal cutting were found to be the main mortality factors. The population declined by 15.3 % from 1985 to 1986 and by 49.5 % from 1986 to 1987.

Natality (recruitment), expressed as number of newly established saplings, was 13 ind. / ha / yr. Recruitment is apparently discontinuous and bound to years with a relatively high precipitation.
A hierarchical set of models for species response analysis
Authors: Huisman J. , Olff H. , Fresco L.F.M.
Published: May 2006

Variation in the abundance of species in space and/or time can be caused by a wide range of underlying processes. Before such causes can be analysed we need simple mathematical models which can describe the observed response patterns. For this purpose a hierarchical set of models is presented. These models are applicable to positive data with an upper bound, like relative frequencies and percentages. The models are fitted to the observations by means of logistic and non‐linear regression techniques. Working with models of increasing complexity allows us to choose for the simplest possible model which sufficiently explains the observed pattern. The models are particularly suited for description of responses in time or over major environmental gradients. Deviations from these temporal or spatial trends may be statistically ascribed to, for example, climatic fluctuations or small‐scale spatial heterogeneity. The applicability of this approach is illustrated by examples from recent research. A combination of simple, descriptive models like those presented in this paper and causal models as developed by several others, is advocated as a powerful tool towards a fuller understanding of the dynamics and patterns of vegetational change.
An 11400 year paleoecological history of a British chalk grassland
Authors: Bush M. B.
Published: May 2006

The first broad‐based, paleoecological analysis of a sedimentary sequence on the British chalk, dating to the terminal Pleistocene, reveals a history of climatic, vegetational and faunal change. The past co‐occurrence of currently allopatric species among molluscs, beetles and plants supports hypotheses of the impermanence of communities. Modern pollen rain data are utilized to refine the ecological interpretation of the fossil pollen data.

The presence of the Windermere Interstadial (Allerod) and the Loch Lomond re‐advance (Younger Dryas) are represented by decreased abundances of arboreal taxa and increased representation of cold grassland elements. Open grassland habitats appear to have been a continuous landscape element, at least locally, since the late‐glacial period on the northern British chal Mands, although their species composition has changed greatly in the last 11400 yr.
Vegetation establishment in post‐fire Adenostoma chaparral in relation to fine‐scale pattern in fire intensity and soil nutrients
Authors: Rice S. K.
Published: May 2006

At fine spatial scales (0.1–10m), chaparral communities have been shown to be strongly influenced by canopy‐gap patterns, leading to periodicities in vegetation at 4–5 m spatial scales. Fine‐scale variations in fire behavior and post‐fire erosion can lead to changes in the patterning of viable seeds and nutrients and may alter the spatial patterning of post‐fire chaparral communities. This study deals with the relationship among fire behavior, post‐fire nutrient availabilities and vegetation patterns in a 1‐yr old, post‐fire Adenostoma fasciculatum chaparral community in the Sierra Nevada Mountains, California, USA. Variations in mineral soil exchangeable cations (Ca, Mg) and extractable phosphorus (P04‐P) were correlated with ash distribution. Cations and measures of ammonium and nitrate were also correlated with fire intensity, measured by the diameter of the smallest remaining A. fasciculatum twigs following fire. Fire intensity was correlated with the pattern of post‐fire vegetation establishment based on first axis DC A scores. However, ash PO4‐P was more highly correlated with sample DCA scores, local species richness and total cover (p < 0.01), suggesting that small‐scale variations in PO4‐P which correlate with ash distributions may be important in structuring this community.

Two‐ and three‐term local variance analysis revealed a maximum of pattern intensity in DCA first axis scores at 4–5 m intervals that likely corresponds to pre‐fire canopy‐gap patterns. However, total cover showed pattern at spatial scales of 8–10 m, and was correlated at this scale with patterns of ash distribution and fire intensity. Microtopographic patterns also occur at similar spatial scales. Microtopographic patterns appear important in determining post‐fire plant nutrient and water distributions and, thereby, patterns of plant establishment. Thus, the scale and intensity of post‐fire vegetation pattern may differ considerably from pre‐fire conditions.
Characteristics of three spinifex alliances in Central Australia
Authors: Griffin G. F.
Published: May 2006

Abstract. The three most extensive spinifex alliances in arid Australia are characterised across a spectrum of post‐fire successional states by their plant species, soil chemical characteristics and rainfall. Mean annual rainfall and some nutrients are correlated with the distributions of the alliances; the Triodia pungens alliance occurs in the highest rainfall region with the highest nutrient levels, the Plectrachne schinzii alliance in the mid‐rainfall region with medium nutrient levels, and the T. basedowii alliance in the lowest rainfall region with a wide range of nutrient levels. This control on distribution can be over‐ridden by topographic position. Where the T. pungens alliance occurs in low rainfall areas it is on sites favoured by soil moisture concentrations higher than the surrounding areas. Non‐spinifex plant species are weakly associated with these alliances and their distributions relate to mean rainfall. Soil nutrient levels are generally low and, except for carbon, do not differ significantly between alliances. Soil grain sizes do not differ between alliances.
Fire and other variables as determinants of forest/fynbos boundaries in the Cape Province
Authors: Manders P. T.
Published: May 2006

Abstract.Forests in the southwestern Cape are restricted to sheltered stream banks and patches of rock scree owing to protection from recurrent fire and the more favourable soil moisture conditions in these habitats. The effects of a prescribed burn on 17 March 1987 on the forest / fynbos boundaries in the Swartboskloof catchment are described. No evidence of erosion of the forest edge was recorded, although this could be ascribed to the low intensity of the fire.

Soil properties, primarily pH and aluminium and available phosphorus levels, differed markedly between forest and fynbos. It is argued, however, that this is a result, not a cause of vegetation change. Light extinction levels and soil moisture availability after the fire were lower in the fynbos than in the forest, and are considered to preclude incursions of forest species into fynbos of a low post‐fire age.
Seed dispersal spectra: a comparison of temperate plant communities
Authors: Willson Mary F. , Rice B. L. , Westoby M.
Published: May 2006

Abstract. We compare the dispersal spectra of diaspores from varied plant communities in Australia, New Zealand, and North America, assigning dispersal mode to each diaspore type on the basis of apparent morphological adaptations. Species with ballistic and external dispersal modes were uncommon in most communities we surveyed. Ant dispersal was also rather uncommon, except in some Australian sclerophyll vegetation types. The frequency of vertebrate dispersal ranged up to 60% of the flora, the highest frequencies occurring in New Zealand forests. Wind dispersal ranged as high as 70% of the flora, with the highest values in Alaska, but usually comprised 10–30% of the flora. Many species in most communities had diaspores with no special morphological device for dispersal. Physiognomically similar vegetation types indifferentbiogeographic regions usually had somewhat dissimilar dispersal spectra. The frequency of dispersal by vertebrates often increased and the frequency of species with no special dispersal device decreased along gradients of increasing vertical diversity of vegetation structure. Elevation and moisture gradients also exhibited shifts in dispersal spectra. Within Australia, vertebrate‐ and wind‐dispersal increased in frequency along a soil‐fertility gradient, and dispersal by ants and by no special device decreased. Habitat breadths (across plant communities) and microhabitat breadths (within communities) for species of each major dispersal type did not show consistent differences, in general. Ant‐dispersed species often had lower cover‐values than other species in several Australian vegetation types.

We discuss the ecological bases of these differences in dispersal spectra in terms of the availability of dispersal agents, seed size, and other ecological constraints. Seed size is suggested to be one ecological factor that is probably of general relevance to the evolution of dispersal syndromes.
BOOK REVIEWS
Authors: Monson Russell K.
Published: May 2006

Book reviewed in this issue.

Lambers, H., Cambridge, M. L., Konigs, H. & Pons, T. L. (eds.) 1989. Causes and Consequences of Variation in Growth Rate and Productivity of Higher Plants, x + 354 pp. SPB Academic Publishing, The Hague. ISBN 90–5103–033–9. Price: USD 90.00.
Tree invasion into a mountain‐top meadow in the Oregon Coast Range, USA
Authors: Magee Teresa K. , Antos Joseph A.
Published: May 2006

Abstract. The grassy meadow on the top of Marys Peak, in the Oregon Coast Range, is being invaded, primarily along the margins, by Abies procera. To examine vegetation and environmental changes across the forest‐meadow transition and to evaluate factors affecting tree invasion, belt transects were established at 10 sites. Correlations of environmental variables with ordination axes from Detrended Correspondence Analysis suggest that the sites were distributed along a moisture gradient. Although the sites varied considerably in species composition and environment, tree invasion of the meadow was occurring at all sites. Reduction in abundance of the dense herbaceous vegetation of the meadow is required before the small seedlings of Abies procera can become established. Almost all tree invasion of the meadow occurs in the narrow forest‐meadow ecocline because trees at the edge of the forest reduce the cover of herbaceous plants in the adjacent meadow. On some sites, above average snow accumulations further reduce vigor of meadow vegetation and increase tree establishment. On the driest sites wet summers facilitate tree establishment. Infrequent fires, which remove the fire sensitive Abies procera, are probably required to reverse the slow but persistent trend of forest expansion and ultimately maintain the meadow.
Vegetation types and patterns in Senegal based on multivariate analysis of field and NOAA‐AVHRR satellite data
Authors: Frederiksen Peter , Lawesson Jonas Erik
Published: May 2006

Abstract. Woody vegetation was studied in 44 sites in Senegal. Species composition and density were analysed with TWINSPAN and Canonical Correspondence Analysis; 16 vegetation types were defined. A supervised, multispectral and multitemporal classification of day and night, dry season NOAA‐AVHRR imagery was used to identify their distribution with a classification accuracy ranging from 60–100%.
On the dynamics, floristic subdivision and geographical distribution of várzea forests in Central Amazonia
Authors: Worbes Martin , Klinge Hans , Revilla Juan D. , Martius Christopher
Published: May 2006

Abstract. Within different stands of the white‐water inundation forest (várzea forest) in the Central Amazon region, composition, abundance, frequency and basal area of tree species were recorded. Determinations of age and radial growth rates were conducted using dendrochronological methods. Results show significant differences in age, history and species composition between stands as well as different growth strategies among dominant species. Assignment of tree species to growth strategies by means of anatomical and morphological features together with quantitative aspects of vegetational analysis permit the further differentiation of successional stages of várzea forests. General features of successional stages were quantitatively described and compared with forest types from outside the várzea. Many tree species of the várzea forests are widespread in South America, and not limited to floodplains. Their occurrence on sites with distinct dry seasons suggests that they are not specifically adapted to flooding but are tolerant to seasonality in general.
Importance of the soil seed bank of disturbed sites in Chilean matorral in early secondary succession
Authors: Jiménez Héctor E. , Armesto Juan J.
Published: May 2006

Abstract. The dynamics of the seed bank may provide clues to the process of recovery of the vegetation of disturbed sites. The role of the seed bank may be more important in areas with a seasonal climate than in areas where seedling recruitment is not limited to one season. We studied the seed bank and the seed rain in three sites of the Chilean mediterranean‐climate region (33° 48'S) which differed in the degree of anthropic disturbance: a closed‐canopy, second‐growth forest; an open matorral; and an old‐field. Additionally, we tested the germination of seeds from the soil and from the current‐year seed crop. The seed bank varied considerably between the two years of study, although no change in the vegetation could be observed. Seed density and species richness were lower in 1989 than in 1988. The seed bank of the second‐growth forest changed less between years, while the old‐field showed the largest change. The highest seed rain occurred under shrub patches in the open matorral, while few seeds fell in the spaces between shrub clumps or in the old‐field. In the forest, seed rain was low and correlated with species cover. Germination was low (0 ‐ 15%) in tests using either soil samples or fresh seeds.

These results indicate that matorral succession is a very slow process, limited mainly by low germination and low arrival of propagules to open areas. Most woody species have animal‐disseminated fleshy propagules. The presence of established shrubs which may serve as perches or refuges for animals increases species richness in the seed rain and the seed bank.
Changes in prairie vegetation under elevated carbon dioxide levels and two soil moisture regimes
Authors: Nie D. , Kirkham M. B. , Ballou L. K. , Lawlor D. J. , Kanemasu E. T.
Published: May 2006

Abstract. It is important to know how increasing levels of atmospheric CO2 will affect native vegetation. The objective of this study was to determine the effect of elevated CO2 concentrations on species composition in a tallgrass prairie kept at a high water level (730 mm of water in a 2000 mm soil profile) and a low water level (660 mm of water in 2000 mm). 16 cylindrical plastic chambers were placed on the prairie to maintain two levels of CO2 (ambient or twice ambient) during two growing seasons in 1989 and 1990. Frequency of species was determined on 25 July 1989 and on 5 and 10 October 1990.

At the beginning of the study, Poa pratensis (Kentucky bluegrass), the dominant C3 species, had the highest frequency of 43.3%, but decreased with time. However, at the end of the experiment and under the high soil‐water level, there were more P. pratensis plants in the elevated CO2 treatment (frequency: 13.5%) than in the ambient CO2 treatment (1.0%). Under the low soil water regime, the reverse occurred (frequencies: 3.6% and 11.0% for high and low CO2, respectively).

The frequency of major C4 plants, Andropogon gerardii (big bluestem), A. scoparius (little bluestem) and Sorghastrum nutans (Indian grass) was not affected by CO2. However, water did affect their frequency. Under low water, the frequency of A. gerardii decreased between 1989 and 1990. Under both soil moisture levels, the frequencies of S. nutans and A. scoparius increased. At the end of the study, Indian grass grown with high water had the highest frequency of all species on the prairie (frequency at the end of the study in October, 1990, of 44.4% and 47.4% for the high and low CO2 levels, respectively). Unlike Indian grass, little bluestem grew better under low water conditions than under high water conditions.

These results suggest that, if the climate becomes drier, A. scoparius will flourish more than S. nutans or A. gerardii, and P. pratensis may die out. Elevated CO2 might not increase survival of C3 plants under dry conditions, if temperatures are too high for them.
Facilitation of tree seedling establishment in a sand dune succession
Authors: Kellman Martin , Kading Michele
Published: May 2006

Abstract. A nucleated pattern of establishment by pine seedlings of Pinus strobus and P. resinosa around pre‐established oak trees of Quercus rubra in a sand dune succession in Ontario, Canada was examined using field observations, seed planting and habitat manipulation. Densities of young pines beneath oak canopies were approximately six times greater than in treeless areas, and densities on the north sides of the trees were significantly greater than on the south sides. However, oaks younger than 35 yr showed no preferential establishment beneath them, while the pine population structures beneath older oaks indicated single periods of successful recruitment. Pine seed planted beneath and beyond oak canopies of three sizes germinated primarily beneath the canopies of medium and large‐sized oaks, but subsequent survivorship of seedlings over two growing seasons was poor. Several micro‐environmental conditions were changed by oak canopies, but only shade showed a pattern closely corresponding to that of seedling establishment. A habitat manipulation experiment confirmed the primary role of shade in facilitating pine seedling establishment beneath oaks. Failure of pine to continue recruiting successfully beneath facilitating trees is tentatively attributed to intraspecific competition among pine individuals.
Morphological analysis of herbaceous communities under different grazing regimes
Authors: Díaz S. , Acosta A. , Cabido M.
Published: May 2006

Abstract. A methodology for the morphological analysis of herbaceous communities is presented, together with an example of its application in montane grasslands in the province of Córdoba (Argentina) subject to grazing and burning.

The method, based on multivariate ordination and classification techniques, enabled the detection of morphological changes at three levels in response to disturbance: (a) characterization of the spatial structure of the vegetation; (b) identification of morphological plant groups; and (c) quantification of morphological shifts among different individuals of a single species.

The architecture of the vegetation changed toward a progressive miniaturization of photosynthetic structures and concentration of biomass close to the ground, as disturbance intensity increased. Six morphological plant groups (modes of response) showing different behaviour in relation to competition for light and pressure from large herbivores were identified. Some species, highly preferred by ungulate grazers, showed high morphological variability among morphs growing in different grazing situations, whereas some others were morphologically uniform.
Vegetation development on moorland after control of Pteridium aquilinum with asulam
Authors: Pakeman Robin J. , Marrs Rob H.
Published: May 2006

Abstract. A range of moorland sites in the North York Moors National Park were surveyed where Pteridium aquilinum has been sprayed once with the herbicide asulam between 1 and 7 yr before. The regeneration of both P. aquilinum and the moorland vegetation were studied. The latter was variable and much slower than reported for lowland grassland or heathland, with a large proportion of the developing plant cover consisting of mosses, especially Campylopus introflexus. This slow rate of revegetation was primarily due to the paucity of micro‐sites offered for germination in dense P. aquilinum litter, the poor establishment of seedlings in this litter and its slow breakdown. The consequences of this slow and variable development of vegetation to moorland restoration are discussed.
Fire and competition in Australian heath: a conceptual model and field investigations
Authors: Keith David A. , Bradstock Ross A.
Published: May 2006

Abstract. We describe a model of heath vegetation, in which species were classified into five functional groups based on characteristics of their propagule pools, post‐fire growth, timing and mode of reproduction and competitive status. The model assumes no recruitment without fire and a simple competitive hierarchy based on vertical stature. A critical feature of the model is an initial post‐fire window of 5–6 yr in which competition from overstorey species on understorey species is reduced. Understorey functional groups differ in their ability to exploit this window.

In the field, we tested five predictions derived from the model: (a) overall species richness of understorey varies inversely with overstorey density as a result of a trend in richness of woody species, but not in herbaceous species; (b) where an overstorey was present in the previous fire interval, post‐fire population density is reduced in a functional group of understorey serotinous resprouting shrubs, but not in a group of understorey obligate‐seeding shrubs with soil seed banks; (c) in understorey serotinous resprouting shrubs, post‐fire regrowth in resprouting individuals is adversely affected by the presence of an overstorey in the preceding fire interval; (d) in understorey serotinous resprouting shrubs, levels of pre‐fire propagules are lower in the presence of an overstorey, reducing the density of post‐fire recruits; and (e) in understorey serotinous resprouting shrubs, recruitment relative to the pre‐fire population is unaffected by overstorey species within the window of reduced competition. Of these, three tests (a,b,d) supported the model, one (e) may support the model, but the results were inconclusive and one (c) did not support the model.

Limitations and further applications of the model are discussed. Our results suggest that maintenance of high densities of overstorey populations is in conflict with conservation of some understorey species. Models of the type we propose will help identify and resolve such conflicts and promote the judicious use of fire to maintain full species diversity of plant communities.
Influence of snow patterns and snow avalanches on the alpine treeline ecotone
Authors: Walsh Stephen J. , Butler David R. , Allen Thomas R. , Malanson George P.
Published: May 2006

Abstract. Snow avalanches, snow accumulation, and snow ablation patterns were mapped and analyzed to assess their impact on the three‐dimensional position, composition (closed canopy forest, open canopy forest, meadow, krummholz, and non‐vegetated surfaces), and spatial structure of the Alpine Treeline Ecotone (ATE) in a portion of Glacier National Park, Montana, USA. Multitemporal Landsat Multispectral Scanner data were processed to derive snow accumulation and ablation patterns throughout a snow season. Landsat Thematic Mapper data were processed and combined with aerial photo interpretations for discerning and characterizing snow avalanche paths. Transition matrices were used to assess the change in the state of snow cover conditions, whereas multiple regression analyses were used to examine the position and character of snow avalanche paths. The analyses were framed and implemented within a geographic information system (GIS) approach. Results indicate a snowmelt pattern progressing from zonal to azonal; influence of local site and situation factors in snow accumulation and ablation patterns; and the importance of topography, geologic structure, and lithology in defining the starting elevation and source area of snow avalanche paths. Finally, a conceptual process is presented where sites affected by stresses and disturbances are analyzed at a local spatial scale for analysis through a deterministic model, whereas regional stresses and disturbances are assessed through remote sensing and GIS approaches for analysis through empirical models.
A global perspective of regional vegetation and hydrologic sensitivities from climatic change
Authors: Neilson Ronald P. , Marks Danny
Published: May 2006

Abstract. A biogeographic model, MAPSS (Mapped Atmosphere‐Plant‐Soil System), predicts changes in vegetation leaf area index (LAI), site water balance and runoff, as well as changes in biome boundaries. Potential scenarios of global and regional equilibrium changes in LAI and terrestrial water balance under 2 x CO2 climate from five different general circulation models (GCMs) are presented. Regional patterns of vegetation change and annual runoff are surprisingly consistent among the five GCM scenarios, given the general lack of consistency in predicted changes in regional precipitation patterns. Two factors contribute to the consistency among the GCMs of the regional ecological impacts of climatic change: (1) regional, temperature‐induced increases in potential evapo‐transpiration (PET) tend to more than offset regional increases in precipitation; and (2) the interplay between the general circulation and the continental margins and mountain ranges produces a fairly stable pattern of regionally specific sensitivity to climatic change. Two areas exhibiting among the greatest sensitivity to drought‐induced forest decline are eastern North America and eastern Europe to western Russia. Regional runoff patterns exhibit much greater spatial variation in the sign of the response than do the LAI changes, even though they are deterministically linked in the model. Uncertainties with respect to PET or vegetation water use efficiency calculations can alter the simulated sign of regional responses, but the relative responses of adjacent regions appear to be largely a function of the background climate, rather than the vagaries of the GCMs, and are intrinsic to the landscape. Thus, spatial uncertainty maps can be drawn even under the current generation of GCMs.
The distribution of vascular plant species and guilds in space and time along a desert gradient
Authors: Cornelius Joe M. , Kemp Paul R. , Ludwig John A. , Cunningham Gary L.
Published: May 2006

Abstract. We studied distribution patterns of vascular plant species and environmental variables for three years along a permanent transect traversing a closed‐drainage watershed in the northern Chihuahuan Desert of south‐central New Mexico, USA. The transect extended for 2.7 km from a basin floor playa (1310 m elevation, fine‐textured soil), across a piedmont slope, and onto the base of a granitic mountain (1410 m elevation, coarse‐textured soil). The gradients in elevation and soils across our transect, along with variable seasonal rainfall, downslope redistribution of water and organic matter, and soil texture‐related variation in infiltration, water holding capacity, and moisture release characteristics, interact to generate a complex spatial and temporal gradient of available soil water and nitrogen. We grouped plant species into guilds according to growth form and photo synthetic pathway type. These guilds are spatially and temporally differentiated along the transect such that particular groups utilize particular seasonal phases or spatial regions of the gradient. We identified six distinct plant communities along the transect. C4 perennial grasses dominated the mesic/high nitrogen portion of the gradient, which occurred at the highest (upper piedmont grassland, dominated by Bouteloua eriopoda) and lowest (playa, dominated by Panicum obtusum) elevations along the transect. C3 shrubs were dominant in the xeric/low nitrogen portion of the gradient located near the middle of the transect (bajada shrubland, dominated by Larrea tridentata). C3 shrubs also dominated a narrow zone of vegetation located adjacent to the playa (playa fringe, dominated by Prosopisglandulosa). C4 perennial grasses, C3 subshrubs, and C3 and C4 perennial forbs and annuals were co‐dominant in the intermediate locations along the gradient, which occurred below (mixed basin slopes) and above (lower piedmont grassland) the bajada shrubland. Life‐form distribution patterns at the small scale of our study reflect some of the patterning that occurs at larger scales in response to climate gradients. The distributions of some species and guilds along the transect are apparently modified by competitive interactions.
Suppression of annuals by Tribulus terrestris in an abandoned field in the sandy desert of Kuwait
Authors: El‐Ghareeb R. M.
Published: May 2006

In an abandoned field in the sandy desert of Kuwait annual plants were less numerous in stands dominated by Tribulus terrestris than in adjacent stands dominated by other species.

In the Tribulus stands, annuals were smaller in phytomass and stature. Possible physical and biotic factors were investigated in both types of stands in order to determine the causative agent(s) for the lesser density and development of annuals in the Tribulus stands. Physical conditions of moisture, light and soil characteristics did not appear to be limiting to annuals in the Tribulus stands.

Experimental evidence is presented for the leaching from T. terrestris shoots of water‐soluble substances which strongly inhibit the germination and radicle elongation of most of the associated annual species. The effect of these substances is fairly specific. Preliminary analysis indicates the presence of phenolic compounds in the leachate of T. terrestris, which are believed to play a significant role in the growth inhibition. The possible role of chemical inhibition on certain annuals in stands of T. terrestris is discussed.
Species richness patterns in the understorey of Pyrenean Pinus sylvestris forest
Authors: Pausas Juli G.
Published: May 2006

Abstract.Species richness was studied in the understorey of naturalPinus sylvestrisforest in the eastern Pyrenees. Understorey plant species were grouped in three structural groups as woody species, herbs and mosses. The response curves of total species richness and species richness of each structural group were fitted against environmental and stand‐structural parameters, using Generalized Linear Models. The results suggested that, to predict species richness, environmental parameters were more important than tree‐canopy structural parameters, in particular incoming radiation and soil nutrient concentration. The species richness response curve was often humped in relation to soil nutrient concentration. Different patterns of species richness were found for each structural group.
Effects of stand age and litter removal on the regeneration of Populus tremuloides
Authors: Lavertu Denis , Mauffette Yves , Bergeron Yves
Published: May 2006

Abstract. The objective of this study was to demonstrate the regeneration by root suckering of Populus tremuloides (quaking aspen) along a successional sere at Lake Duparquet in NW Québec with six sites of different ages ‐ 46, 74, 120, 143, 167 and 230 yr since last fire. In 1990, in each of the sites, one 20 m x 20 m plot was cut. In each plot, 4‐m2 plots were selected, both control and with litter removed. The number of suckers and their height were estimated for each plot in 1991, 1992 and 1993. Results showed that sucker production was not related to the number of P. tremuloides before the clearcut, but it was related to litter whereby increasing cover lowered the production. The number of suckers produced in the control plots did not show any significant difference along the successional sere for any of the three years. Scarified plots had more suckers than control plots, and this increase was more pronounced and significantly different for the younger stage. Regression analysis for the 1991 and 1992 data showed that sucker number in scarified plots decreased with increasing seral stage age. Mortality, after three growing seasons, was about 60% for all plots and was not affected by seral stage and treatment. We suggest that removal of litter in scarified plots exposed the roots to higher temperatures and caused wounds, both of which may have stimulated the production of suckers in young seral stages.
Stand dynamics of Pinus flexilis‐dominated subalpine forests in the Colorado Front Range
Authors: Rebertus A. J. , Burns B. R. , Veblen T. T.
Published: May 2006

Population age structure and succession were investigated in subalpine forests in the Colorado Front Range dominated by Pinus flexilis (limber pine). Age, size, and spatial data were collected from three recent burns (<100 yr old), six ca. 240 year‐old post‐fire stands, and two old‐growth stands (individuals > 400 yr old). The sequence of colonization of now extant trees on these post‐fire sites appeared to be consistent: first Pinus flexilis, then Picea engelmannii (Engelmann spruce), and later Abies lasiocarpa (subalpine fir) with a delay between the first Pinus flexilis and Abies of as long as 140 yr. The advantage of Pinus flexilis on post‐burn sites can be attributed to avian seed dispersal and the exceptional drought tolerance of its seedlings. The three recent burns were not extensive, and the delay in establishment of Picea and Abies appeared to be limited by harsh site conditions rather than lack of seed dispersal.

Spatial analysis indicated a consistent, although sometimes weak, attraction between Pinus flexilis and Picea and Pinus flexilis and Abies at a scale of 1–4 m, suggesting that Pinus flexilis may facilitate establishment of Picea and Abies seedlings by providing shade or protection from wind. On xeric to slightly xeric sites, Pinus flexilis appeared to form broadly even‐aged, non‐regenerating populations that were gradually being replaced by Picea and Abies. Replacement is proceeding at a faster rate on the least xeric sites (north aspects, valley bottoms) compared to the most xeric sites (south aspects). On the most extreme sites, Pinus flexilis formed all‐aged, self‐maintaining populations with no evidence of replacement by Picea and Abies. In these old‐growth forests with occasional trees aged at > 1300 yr, recruitment is continuous or episodic.
Effects of defoliation on the saplings of a gap‐colonizing neotropical tree
Authors: Núñez‐Farfán Juan , Dirzo Rodolfo
Published: May 2006

This paper describes a defoliation experiment on saplings of the gap‐colonizing pioneer tree Heliocarpus appendiculatus in the tropical rain forest of Los Tuxtlas (Southeast Mexico). Four levels of defoliation (0, 25, 50, and 75% of the leaf area removed) were applied to naturally established plants in a medium‐sized forest gap. Records were made of growth (height, diameter, and leaf production) and of whole‐plant and leaf survival.

Statistical comparisons for all variables showed that plants that had not been defoliated grew better than defoliated ones, but there were no statistically significant differences between the defoliation levels. The risk of death was significantly lower for control plants than for defoliated plants at any level of damage. Leaf survivorship was highly irregular. The most consistent pattern was that the leaves of intact plants always showed higher survivorship, while the most heavily defoliated ones always had the poorest survival. The survivorship pattern of leaves at intermediate defoliation levels was irregular.

The results illustrate the lack of a monotonic response to a wide range of defoliation levels, and suggest the potential effect of herbivores as reducers of vegetative growth and survival in pioneer tropical species, and as limiting agents of plant establishment in regenerating forest gaps.