Structure of herbaceous plant assemblages in a forested riparian landscape

We assessed patterns of herbaceous and woody species richness, plant-environment interactions, and correspondence between the herb and tree layer in a riparian landscape (the Ozark National Scenic Riverways, Missouri, USA). A total of 269 herb and 70 tree species were identified on 94 sample plots. Gradient analysis revealed that environmental variables and vegetation were influenced by a strong elevation gradient. However, high variability in environmental variables (pH, elevation, slope, sand, clay, organic matter) indicated a high level of substrate heterogeneity across the riparian landscape. We were unable to predict the composition of the herb understory from the canopy trees with any detailed accuracy and no clear characterization of herb species assemblages was found using cluster analysis or ecological land type (ELT) classifications. Canonical correspondence analysis (CCA) results for both tree and herb plots showed that elevation (height above river) and pH were the dominant environmental gradients influencing vegetation patterns on the first CCA axis while soil particle size exhibited the strongest correlation with the second CCA axis. Secondary gradients of importance included slope, soil container capacity, and organic matter. No significant linear or quadratic correlation was found between elevation and herb or woody species richness. Environmental variables alone or in combination, were weak predictors of herb and woody species richness, despite the patterns observed in the gradient analysis and the correlations observed in the CCA results. Ecotonal analysis showed that the herb layer exhibited a high species replacement rate at the lower elevations most susceptible to flooding (0–3 m). Above the flooding zone, there was more or less continuous species replacement, suggesting the presence of a gradual ecotone/ecocline. The tree layer exhibited much stronger discontinuities than the herb layer in the lower elevations along the height gradient (0–10 m). Recognizing the limitations of classification techniques for riparian herb assemblages and the importance of scale and heterogeneity in vegetation layers is especially important in light of mandates to preserve, protect, and manage for plant diversity.     

Environmental factors influencing spatial patterns of shrub diversity in chaparral,Santa Ynez Mountains,California

We examined patterns of shrub species diversity relative to landscape‐scale variability in environmental factors within two watersheds on the coastal flank of the Santa Ynez Mountains, California. Shrub species richness and dominance was sampled at a hierarchy of spatial units using a high‐powered telescope from remote vantage points. Explanatory variables included field estimates of total canopy cover and percentage rock cover, and modeled distributions of slope, elevation, photosynthetically active radiation, topographic moisture index, and local topographic variability. Correlation, multiple regression, and regression tree analyses showed consistent relationships between field‐based measurements of species richness and dominance, and topographically‐mediated environmental variables. In general, higher richness and lower dominance occurred where environmental conditions indicated greater levels of resource limitation with respect to soil moisture and substrate availability. Maximum richness in shrub species occurred on high elevation sites with low topographic moisture index, rocky substrate, and steep slopes. Maximum dominance occurred at low elevation sites with low topographic variability, high potential solar insolation, and high total shrub canopy cover. The observed patterns are evaluated with respect to studies on species‐environment relations, resource use, and regeneration of shrubs in chaparral and coastal sage scrub. The results are discussed in the context of existing species‐diversity hypotheses that hinge on reduced competitive dominance and increased resource heterogeneity under conditions of resource limitation.     

Microenvironmental and vegetational heterogeneity induced by phytogenic nebkhas in an arid coastal ecosystem

The nebkhas of woody plants represent distinct habitats in arid and semiarid ecosystems. Nebkhas are mounds composed of wind-borne sediment within or around shrub canopies. We studied the effects of widely spaced nebkhas of Retama raetam shrub on their microenvironment and associated herbaceous vegetation in the Mediterranean coast of Sinai Peninsula. Our measurements included nebkha size (height and width) and shrub size (canopy height and diameter). We identified four distinct microsites at each nebkha: crest, mid-slope, edge, and internebkha space. We measured soil temperature and moisture, photosynthetically active radiation (PAR), and soil properties. The plant species grown at each microsite were identified and their densities were measured. Average soil temperature and PAR were highest at internebkha space and lowest at nebkha crest. The maximum diurnal temperature and PAR of internebkhas exceeded that of nebkhas. Soil moisture and nutrient concentrations showed a gradient of spatial heterogeneity and were highest at the nebkha edge. Regression analysis indicated that total herbaceous plant density was significantly related to nebkha size, and to shrub canopy diameter and area. Detrended correspondence analysis indicated that patterns of species composition were correlated with the spatial variability in soil moisture and nutrient content along the gradient of increasing distance from the nebkha crest. It is assumed that shrub canopy and its nebkha interact in governing ecosystem functioning in this environment.     

山西霍山灌丛空隙草本植物物种多样性与更新

采用物种重要值和物种多样性指数,分析了山西霍山灌丛空隙内主要草本组成及其多样性随灌丛空隙面积梯度的变化规律。结果表明:在自然状况下形成的10 m2的灌丛空隙最多,占51.2%。灌丛空隙内草本植物的重要值高于灌丛下草本,且以10~20 m2的空隙为最高。主要草本可分为3类:(1)对灌丛空隙更新反应不明显的草本,(2)对灌丛空隙有正更新反应的草本,(3)对灌丛空隙有负更新反应的草本。灌丛空隙内的物种丰富度、多样性指数和均匀度指数均高于灌丛下,而生态优势度均小于灌丛下,呈现"单峰型"的变化趋势,且在10~20 m2达到峰值。因此,适宜大小的灌丛空隙是森林生态系统草本层植物多样性维持的重要途径。     

Vegetation changes along gradients of long-term soil development in the Hawaiian montane rainforest zone

The development of the Hawaiian montane rainforest was investigated along a 4.1-million-year soil age gradient at 1200 m elevation under two levels of precipitation, the mesic (c. 2500 mm annual rainfall) vs. wet (>4000 mm) age gradient. Earlier analyses suggested that soil fertility and foliar nutrient concentrations of common canopy species changed unimodally on the same gradients, with peak values at the 20,000–150,000 yr old sites, and that foliar concentrations were consistently lower under the wet than under the mesic conditions. Our objectives were to assay the influences of soil aging and moisture on forest development using the patterns and rates of species displacements. The canopies at all sites were dominated by Metrosideros polymorpha. Mean height and dbh of upper canopy Metrosideros trees increased from the youngest site to peak values at the 2100–9000 yr sites, and successively declined to older sites. A detrended correspondence analysis applied to mean species cover values revealed that significant variation among sites occurred only on one axis (axis 1), for both soil-age gradients. Sample scores along axis 1 were perfectly correlated with soil age on the mesic gradient, and significantly correlated on the wet gradient. Higher rainfall appeared to be responsible for the higher rates of species turnover on the wet gradient probably through faster rock weathering and greater leaching of soil elements. We concluded that the changes in species cover values and size of the canopy species was a reflection of the changing pattern of nutrient availability associated with soil aging.     

Plant diversity patterns and their relationships with soil and climatic factors along an altitudinal gradient in the middle Tianshan Mountain area, Xinjiang, China

Patterns of plant diversity along the altitudinal gradient of Tianshan in central Xinjiang, China were examined. Plant and environment characteristics were surveyed from higher, south of Bogeda peak, to lower, north of Guerbantonggute desert. There were a total of 341 vascular plant, 295 herbage, 41 shrub, and seven tree species in the sampled plots. The plant richness of vegetation types generally showed a unimodal pattern along altitude, with a bimodal change of plant species number at 100-m intervals of altitudinal samples. The two belts of higher plant richness were in transient areas between vegetation types, the first in areas from dry grass to forest, and the second from forest to sub-alpine grass and bush. The beta diversity varied with altitudinal changes, with herbaceous species accounting for most species, and thus had similar species turnover patterns to total species. Matching the change of richness of plant species to environmental factors along altitude and correlating these by redundancy analysis revealed that the environmental factors controlling species richness and its pattern were the combined effects of temperature, precipitation, soil water, and nutrition. Water was more important at low altitude, and temperature at high altitude, and soil chemical and physical characters at middle altitudes. This study provides insights into plant diversity conservation of Bogeda Natural Reserve Areas in Tianshan Mountain. Nomenclatures: the scientific name for plants follows Flora of China (Compiling Committee of Flora of China).     

太白山北坡落叶阔叶林物种多样性特征

对太白山北坡落叶阔叶林从不同类型,层次的丰富度,均匀度和物种多样性指数及其与环境梯度的关系等方面进行了分析。结果表明,处于中山带的锐齿槲栎林及辽东栎林的多样性较高,高海拔与低海拔处的群落多样性较低。水热梯度对物种多样性有很大影响。植物生长型与物种多样性的关系表现为灌木层的多样性指数（Ｄ）和均匀度大于草本层,而草本层大于乔木层,草木层物种丰富度最大,灌木层次之,乔木层最小,不同群落间灌木层各多样性测     

Small-scale environmental heterogeneity and the analysis of species distributions along gradients

Abstract. The observed distribution of a species along an environmental gradient is strongly affected by environmental variability within a quadrat. Because a quadrat does not represent a point along an environmental gradient, but rather a range of conditions, it is likely to contain species not typically associated with the mean conditions in the quadrat. Systematic relationships exist between a species' true distribution, the observed distribution as a function of mean quadrat environment, and the frequency distribution of the environment within that quadrat. The observed species habitat breadth increases and the observed maximum abundance decreases as within-quadrat environmental heterogeneity increases. If species distributions or beta diversities are to be compared among species or coenoclines, they should be correctedforintra-quadratheterogeneity.Wederive simple corrections for environmental heterogeneity. The distributions of hardwood forest understory species along a soil acidity gradient in the North Carolina piedmont are presented as an example.     

Floristics and soil relations of upland swamp vegetation near Sydney

Abstract We examined relations between vegetation and soils, using multivariate methods, in hitherto poorly-known upland swamps on the Woronora Plateau, south of Sydney. A major trend in floristic composition was related to the height and cover of the herbaceous stratum and reflected a gradient in soil moisture and nutrients. A second trend in floristic composition was related to the height and cover of the shrub stratum, and may reflect the influence of recurring fires on certain dominant shrub species. Five plant communities were recognized on the basis of floristic composition and were distinguishable by their different soil habitats and/or structural characteristics. We report some of the highest species-richness values in the world (at scales of 1–15 m₂) for shrub/sedge-dominated vegetation, with up to 70 vascular plant species in 15 m2. Variation in species richness is inversely related to the resource gradient and positively related to the penetration of light through the vegetation canopy. This pattern is consistent with resource-competition models and warrants further investigation.     

Longitudinal patterns of plant diversity in the North American boreal forest

Spatial patterns of plant diversity in the North American boreal forest were examined according to three plant life forms (woody plants, herbaceous plants, and bryophytes) and two taxonomic levels (species and genus), using sixty 9-ha plots sampled in white spruce (Picea glauca (Moench) Voss) and black spruce (Picea mariana (P. Mill.) B.S.P.) ecosystems along a transcontinental transect from the Pacific coast eastwards to the Atlantic coast. The patterns of inventory diversity (represented by alpha diversity), differentiation diversity (represented by the similarity index, habitat-heterogeneity index, similarity decay rate, and length of the first axis in detrended correspondence analysis), and pattern diversity (represented by the mosaic diversity index) were assessed along the transect in both ecosystem types. At the stand level, central North America had the highest alpha diversity in terms of the number of species or genera, and western North America had a higher alpha diversity than eastern North America. At the continental scale, herbaceous plants had the highest beta diversity in terms of floristic change from the eastern to western North America, bryophytes had the lowest beta diversity, and woody plants were in the middle, regardless of ecosystem type and taxonomic level. Central North America had the lowest mosaic diversity across the boreal transect of North America. The white spruce ecosystems had a higher alpha diversity than the black spruce ecosystems regardless of plant life form, taxonomic level and geographic location. The white spruce ecosystems tended to have more bryophytes, less woody plants, and higher species:genus ratio than the black spruce ecosystems. In general, the white spruce and black spruce ecosystems shared the same patterns in diversity changes at different spatial scales, plant life forms, and taxonomic levels across the transect studied. The existing patterns of plant diversity in the North American boreal forest area resulted from a combination of ecological processes and spatial configuration.     

Climate and vegetation structure determine plant diversity in Quercus ilex woodlands along an aridity and human-use gradient in Northern Algeria

We studied the influence of environmental factors relating to climate, soil and vegetation cover on total species richness, species richness of different life-forms and species composition of plant communities occurring in Quercus ilex woodlands, across a 450-km long transect in Northern Algeria constituting a gradient of aridity and human use. We sampled vegetation and collected environmental data in 81 10 m × 10 m plots in five zones representing the largest Q. ilex woodlands throughout the study area, analysing them within an a priori hypothesis framework with the use of Path Analysis. Changes in plant diversity were mainly influenced by environmental factors related to precipitation and temperature regimes, as well as by total plant cover. In particular, changes in species composition were determined by factors associated with the temperature regime through their influence on both woody and annual herbaceous plant richness, and by factors related to the precipitation regime through their influence on perennial herbaceous plant richness, likely due to the differential tolerances of these functional groups to cold and water stress. Our results emphasize the importance of differences in environmental adaptability of the most important life-forms with regard to explaining compositional change (beta diversity) along aridity gradients, and the mediator role of total plant cover in relation to the effects of soil conditions on plant diversity.     

Richness and composition of herbaceous species in restored shrubland and grassland ecosystems in the northern Loess Plateau of China

Restored grasslands and shrublands are integral parts of the semi-natural landscape and are of major importance for biodiversity in the northern Loess Plateau. Determining the underlying factors that control the richness and composition of herbaceous species in restored grasslands and shrublands is urgently needed. Thus, the specific objective of this study was to evaluate the relative importance of soil, plant, and topographic explanatory variables affecting the richness and composition of herbaceous species in restored shrubland and grassland ecosystems in a typical watershed within the northern Loess Plateau. In this study, 27 restored grassland sites and 16 restored shrubland sites were sampled during September 2009. Using variation partitioning (partial canonical correspondence analysis), we determined the individual and shared effects of these three sets of explanatory variables on herbaceous biodiversity in the two restored habitats. Most of the explained variation in plant diversity was related to the pure effect of soil, plant, and topographic variables. Restored shrublands had significantly more species than grasslands, and abandoned dam farmlands had significantly more species than other grassland sites. Moreover, botanical diversity responded differently to the explanatory variables in different plant communities. The pure effects of soil properties, soil moisture in particular, accounted for the largest fractions of explained variation in species diversity in restored grasslands. Both plant and topographic variables had balancing pure effects on species diversity in restored shrublands, in particular the shrub density and slope angle. We conclude that the maintenance of a moderate density of shrubs (less than 3600 shrubs per ha), construction of check-dams, and grazing at a low stocking rate, taking conditions of soil and topographic site into account, may help to conserve biodiversity in the northern Loess Plateau.     

Forest vegetation of the Colorado Front Range: Patterns of species diversity

Summary Plant species diversity patterns of the Rocky Mountain forests were found to be at variance with patterns reported from other regions. The most centrally located forests in terms of elevation, site moisture and successional status were found to have the lowest diversity. In contrast, the peripheral and environmentally more severe sites were found to have relatively high diversity. In particular, the forest-grassland transition and the low elevation riparian forests have species diversity values as high as any yet reported from western North America.When diversity was examined in terms of variation across elevation or moisture gradients, varying results were obtained due to the interaction of these factors. The failure of previous studies to converge on generalizations about plant diversity reflects, in part, the failure of most investigators to view diversity in a regional context of variation across several interacting gradients.Diversity was seen to vary inversely with the degree of development of the forest canopy. The interaction of different components of the forest community is one reason for the failure of general patterns of plant species diversity to emerge from previous studies. A potentially rich herb community can be greatly suppressed by a single species tree stratum.Among the most successful work to date on species diversity is that on birds, a distinct albeit large and functional group. It is unlikely that similar success could have been achieved through work on all animal species simultaneously. This suggests the need to examine plant species diversity, not in terms of total diversity, but in terms of component functional groups, perhaps guilds, growing under similar microclimatic conditions and subject to similar competitive pressures.Nomenclature follows Weber (1976).     

Phylogenetic structure and phylogenetic diversity of angiosperm assemblages in forests along an elevational gradient in Changbaishan,China

Aims Understanding what drives the variation in species composition and diversity among local communities can provide insights into the mechanisms of community assembly. Because ecological traits are often thought to be phylogenetically conserved, there should be patterns in phylogenetic structure and phylogenetic diversity in local communities along ecological gradients. We investigate potential patterns in angiosperm assemblages along an elevational gradient with a steep ecological gradient in Changbaishan, China.Methods We used 13 angiosperm assemblages in forest plots (32×32 m) distributed along an elevational gradient from 720 to 1900 m above sea level. We used Faith's phylogenetic diversity metric to quantify the phylogenetic alpha diversity of each forest plot, used the net relatedness index to quantify the degree of phylogenetic relatedness among angiosperm species within each forest plot and used a phylogenetic dissimilarity index to quantify phylogenetic beta diversity among forest plots. We related the measures of phylogenetic structure and phylogenetic diversity to environmental (climatic and edaphic) factors.Important findings Our study showed that angiosperm assemblages tended to be more phylogenetically clustered at higher elevations in Changbaishan. This finding is consistent with the prediction of the phylogenetic niche conservatism hypothesis, which highlights the role of niche constraints in governing the phylogenetic structure of assemblages. Our study also showed that woody assemblages differ from herbaceous assemblages in several major aspects. First, phylogenetic clustering dominated in woody assemblages, whereas phylogenetic overdispersion dominated in herbaceous assemblages; second, patterns in phylogenetic relatedness along the elevational and temperature gradients of Changbaishan were stronger for woody assemblages than for herbaceous assemblages; third, environmental variables explained much more variations in phylogenetic relatedness, phylogenetic alpha diversity and phylogenetic beta diversity for woody assemblages than for herbaceous assemblages.     

Linking hydroperiod and vegetation response in Carolina bay wetlands

Hydrology filters propagule bank expression in herbaceous Carolina bays, but the strength of this filter’s effects on community composition at different points along the hydrologic gradient of these southeastern U.S. depressional wetlands is unknown. We used an experimental approach to determine the pattern of vegetation expression from propagule banks of Carolina bays exposed to different hydrologic conditions and gradients. Propagule banks of sediment cores collected from six Carolina bays were placed in bins, each of which was allocated to one of three hydrologic treatments: moist soil (MS), mid-summer drawdown (DD), or flooded (FL). After one season of vegetation development (1995) in the hydrologic treatments, half of the bins were left flat and the remaining were sloped to produce a finer moisture gradient within each bin. We compared taxa richness, community composition based on cover, and cover patterns of eight abundant species that developed in bins over the season (1996) after sloping. Species richness was significantly higher in the moist soil treatment and in sloped bins. Community composition, however, was affected by the hydrologic treatment only and not the finer-scale flooding gradient produced by sloping. Under flooded conditions, floating-leaved and submerged aquatics had higher cover; vegetation converged on simpler, less variable communities dominated by obligate wetland species, with species exhibiting different patterns of abundance over small changes in water depth. Emergent species typically had higher cover in moist soil and drawdown treatments. These results confirm a tight mechanistic link between hydrology and vegetation patterns within Carolina bays, but suggest that the strength of this link is not uniform across the gradient. The linkage weakens with drier conditions as both facultative wetland and upland species recruit into the standing vegetation.     

Forest vegetation of the Colorado Front Range

The forest vegetation of the northern Colorado Front Range was studied using a combination of gradient analysis and classification methods. A graphical model of forest composition based on elevation and topographic-moisture gradients was constructed using 305 0.1 ha samples. To derive the topographic moisture gradient, stands were stratified into eight 200 m elevation belts, and then ordinated by correspondence analysis using understory (<1 m) data. Each of the resultant gradients was scaled against a standard site moisture scalar derived from incident solar radiation and topographic position. Except for krummholz sites, the vegetation defined gradients fit the moisture scalar closely. Once scaled, these gradients were stacked vertically, sandwich-style, to create the graphical representation shown in Figure 5.Gradient analysis and ordination (direct and indirect gradient analysis of Whittaker, 1967) are frequently viewed as alternative approaches for analysis of vegetation. With gradient analysis the axes are readily interpretable, but stand placement is often difficult and at times questionable. Ordination defines an optimal arrangement for species and/or stands, but axis interpretation is often impossible. With the present combination of methods, the interpretability of gradient analysis complements the precision of placement obtained with ordination.Forest vegetation was classified by dividing the gradient model into eight series and 29 types on the basis of similar successional trends in canopy dominants. On dry, low-elevation sites above 1 700 m Pinus ponderosa woodlands dominate. With increasing elevation or site moisture, tree density increases and Pinus ponderosa, Pseudotsuga forests prevail. At middle elevations on mesic sites forests of mixed composition occur. Pinus contorta forests dominate at middle elevations over much of the central position of the moisture gradient, though these are primarily post-fire forests. With protection from fire only a small percentage of sites retain dominance by Pinus contorta. Over the lower portion of its range Pinus contorta is succeeded by Pseudotsuga, while at higher elevations Abies lasiocarpa and Picea engelmannii can eventually achieve dominance. At high elevations on all except the driest sites Picea engelmannii and Abies lasiocarpa are exclusive dominants, both after disturbance and in climax forests. Pinus flexilis dominates on the driest high-elevation sites. Above 3 500 m forests are replaced by alpine tundra, often with a transitional krummholz zone.Structure and post-fire development were examined in the context of the gradient-based classification scheme. Three generalized types of forest development were recognized as reference points in a continuum of developmental patterns varying with both elevation and soil moisture.On favorable, middle-elevation sites, trees become established rapidly after disturbance. Rapid growth results in severe overcrowding and competitive elimination of reproduction. As a consequence bell-shaped diameter distributions develop. Diversity and productivity appear to drop while biomass remains roughly constant. Following decades or even centuries of stagnation, the forests eventually breakup through mortality of the canopy trees, thereby allowing regeneration to resume. During this period of renewed regeneration, biomass, diversity, and productivity all show dramatic changes in response to the changing population structure (Fig. 9). This type of forest development can be found in forests dominated by Picea engelmannii and Abies lasiocarpa, Pinus contorta, Pseudotsuga menzeisii, Pinus flexilis or Populus tremuloides. On highest elevation forest sites or at middle elevations on the very driest sites reestablishment rates are greatly reduced. These forests dominated by Picea and Abies or Pinus flexilis gradually approach predisturbance levels of biomass, diversity and productivity, while regeneration remains at a roughly constant level. At lower elevations in the Pinus ponderosa woodlands, regeneration appears episodic, reflecting variation in seed rain and favorable conditions for seedling growth. Here, inter-tree competition is relatively unimportant and diameter distributions show irregular humps resulting from periodic recruitment.A few species pairs presented consistent problems and their treatment as single species was necessary. Garex rossii and C. brevipes were lumped as Carex rossii. Rosa woodsii and R. acicularis were lumped as Rosa sp. Cirsium scopulorum and C. coloradense were lumped as Cirsium coloradense. Extreme forms of Arnica cordifolia and A. latifolia are easily distinguishable, but as these species intergrade and hybridize extensively, they have been lumped as Arnica cordifolia. The native bluegrass, Poa agassizensis, was lumped with Poa paratensis. Solidago missouriensis includes some S. canadensis.Nomenclature follows Weber (1972) for most species. In some cases where Weber's narrow generic concept deviates from the main thrust of present-day North American systematic botany, names were changed to conform with Harrington (1954) and Hitchcock & Cronquist (1973). Voucher specimens have been deposited in the herbarium of Rocky Mountain National Park, with a few unusual species being deposited in the herbarium of the University of Colorado, Boulder.Numerous individuals have contributed generously to this project. Among those to whom I am particularly indebted are B. Chabot, R. T. Clausen, C. V. Cogbill, J. Douglas, H. G. Gauch. Jr., D. C. Glenn-Lewin, D. Hamilton, K. H. Hildebrandt, D. Mueller-Dombois, R. L. Peet, D. Stevens, E. L. Stone, J. Vleck, W. A. Weber, T. R. Wentworth, and P. L. Whittaker. I especially thank R. H. Whittaker for advice and encouragement. Financial support was provided by grants from the National Science Foundation, the DuPont Foundation, Cornell University and the University of North Carolina Research Council. The cooperation and support of the National Park Service is gratefully acknowledged.     

Issues affecting the measurement of disturbance response patterns in herbaceous vegetation – A test of the intermediate disturbance hypothesis

The 'intermediate disturbance hypothesis' (IDH) predicts maximum species diversity at intermediate levels of disturbance. Disturbance levels are measured by intensity or frequency of disturbance or by the time since disturbance. The IDH has been tested over a wide range of studies and communities with results either accepting or rejecting the hypothesis. The objectives of this study were to determine how observed disturbance response patterns for the same herbaceous plant community are influenced by modelling techniques, particularly in relation to the expression of disturbance gradients, choice of species diversity indices, and time of assessment since disturbance. Response patterns were examined using a multi-factorial disturbance experiment involving grazing (absent and present); fire (absent and present); soil cultivation (none, 5 and 20 cm); and amendment (none, fertiliser, and fertiliser plus clover seeds). Generalized linear models with a b-spline function were used to define response patterns for five different disturbance gradients over a 24-month period using three indices of diversity. Seven basic disturbance response models were recognized ranging from the classical IDH bell-shaped pattern through increasing or decreasing trends, to a no-change model with increasing levels of disturbance. Only 6.7% of all models were consistent with the IDH. The no-change model was found in nearly half the cases investigated and increased in occurrence with time since disturbance. The choice of the disturbance intensity gradient, the species diversity index used and the time of assessment after disturbance significantly influenced the frequency of occurrence of the disturbance response models observed. Consequently, the responses of vegetation to disturbance gradients show many patterns depending on how they are defined and modelled rather than the simple bell-shaped curve as predicted by the IDH, but the ecological mechanisms supporting some of these patterns need to be further investigated.     

Landscape patterns of understory composition and richness across a moisture and nitrogen mineralization gradient in Ohio (U.S.A.) Quercus forests

This study quantified relationships of understory vascular plant species composition and richness along environmental gradients over a broad spatial scale in second-growth oak forests in eastern North America. Species frequencies were recorded in 108 25 × 25 m plots in four study sites extending over 70 km in southern Ohio, U.S.A.. The plots were stratified into three long-term soil moisture classes with a GIS-derived integrated moisture index (IMI). In addition to the IMI, the environmental data matrix included eight soil and three overstory variables. Canonical correspondence analysis (CCA) indicated that variations in understory species composition were most strongly related to topographic variations in predicted moisture (IMI), N mineralization rate, nitrification rate, and soil pH. In addition, floristic variation at the regional scale was correlated with variations in soil texture, nitrification, pH, and PO₄ ^–, resulting from differences in the soil parent material complexes among sites. Species richness averaged 65 species/plot, and increased with moisture and fertility. Stepwise regression indicated that richness was positively correlated with N mineralization rate and nitrification rate, and inversely correlated with tree basal area. Greater richness on fertile plots was the largely the result of increasing forb richness. Forb richness per quadrat (2 m²) was most strongly and positively related to N mineralization rate. Conversely, richness of understory individuals of tree species was greatest on xeric, less-fertile plots. Our results describe general, broad-scale species-environment relationships that occurred at both the topographic scale (long-term moisture status and fertility) and the regional scale (geomorphological differences among the sites). Strong species richness-N mineralization correlations indicate an important link between below-ground processes and above-ground biodiversity. Because N availability was a strong correlate to vegetation patterns at a broad-scale, our results suggest that the increasing rates of atmospheric N deposition in the region could have a major impact on understory vegetation dynamics.