Question: Predicting the future abundance and distribution of invasive plants requires knowing how they respond to environmental conditions. In arid and semi-arid ecosystems where water is a limiting resource, environmental conditions and disturbance patterns influence invasions by altering acquisition and utilization of water over space and time. We ask: 1. How do variations in climatic and soil properties influence temporal soil water dynamics? 2. How does this variation affect the establishment of Bromus tectorum (cheatgrass), a cool-season annual grass that has successfully colonized much of the U.S. Great Basin?

Location: Short-grass Steppe in northeastern Colorado, USA; Arid Lands Ecology reserve in southeastern Washington, USA; and the Patagonian steppe of the Chubut province in Argentina.

Methods: We utilized a soil water model to simulate seasonal soil water dynamics in multiple combinations of climatic and soil properties. In addition, we utilized a gap dynamics model to simulate the impact of disturbance regime and seed availability on competition between B. tectorum and native plants.

Results: Our results suggest that climate is very important, but that soil properties do not significantly influence the probability of observing conditions suitable for B. tectorum establishment. Results of the plant competition model indicate that frequent disturbance causes more Bromus tectorum in invaded areas and higher seed availability causes faster invasion.

Conclusions: These results imply a general framework for understanding Bromus tectorum invasion in which climatic conditions dictate which areas are susceptible to invasion, disturbance regime dictates the severity of invasion and seed availability dictates the speed of invasion.

Question: What is the relative importance of low- and high-severity fires in shaping forest structure across the range of Pinus ponderosa in northern Colorado?

Location: Colorado Front Range, USA.

Methods: To assess severities of historic fires, 24 sites were sampled across an elevation range of 1800 to 2800 m for fire scars, tree establishment dates, tree mortality, and changes in tree-ring growth.

Results: Below 1950 m, the high number of fire scars, scarcity of large post-fire cohorts, and lack of synchronous tree mortality or growth releases, indicate that historic fires were of low severity. In contrast, above 2200 m, fire severity was greater but frequency of widespread fires was substantially less. At 18 sites above 1950 m, 34 to 80% of the live trees date from establishment associated with the last moderate- to high-severity fire. In these 18 sites, only 2 to 52% of the living trees pre-date these fires suggesting that fire severities prior to any effects of fire suppression were sufficient to kill many trees.

Conclusions: These findings for the P. ponderosa zone above ca. 2200 m (i.e. most of the zone) contradict the widespread perception that fire exclusion, at least at the stand scale of tens to hundreds of hectares, has resulted in unnaturally high stand densities or in an atypical abundance of shade-tolerant species. At relatively mesic sites (e.g. higher elevation, north-facing), the historic fire regime consisted of a variable-severity regime, but forest structure was shaped primarily by severe fires rather than by surface fires.

Nomenclature: Weber (1976).

Question: What is the relationship between soil fertility and plant species richness in the ‘fertile islands’ occurring beneath two species of legume (Cercidium praecox and Prosopis laevigata)?

Location: Tehuacán-Cuicatlán region, central Mexico.

Methods: Plant richness was measured in three micro-environments (below canopies of C. praecox, below canopies of P. laevigata and in areas without canopies). The concentration of soil nutrients (C, N and P), C and N in the microbiota, and processes of ecosystem functioning (net C mineralization rate and N mineralization) were measured. The relationship between soil variables and plant richness were assessed with ANCOVAs.

Results: Soil nutrients and species richness increases markedly under fertility islands. There were higher concentrations of C and N in the soil, faster rates of C mineralization, and higher species richness under P. laevigata canopies. The relationship between soil fertility and species richness was always positive except for total N, ammonium and net C mineralization rate under C. praecox, and for available P under P. laevigata.

Conclusions: The sign of the relationship between soil fertility and species richness varies according to the nutrient and the micro-environment. Positive relationships could result from between species complementarity and facilitation. Negative relationships could be explained by a specific limitation threshold for some soil resources (P and N for plants and C for the soil microbiota) which eliminate the possibilities of between species complementarity and facilitation above that threshold. As in all observational studies, these relationships should be considered only correlational.

Nomenclature: Dávila et al. (1993)

Question: Small and marginal forest populations are a focus of attention because of their high biodiversity value as well as the risk of population decline and loss. In this context, we ask to what extent a small, marginal Quercus suber (Cork oak) population located in the eastern Iberian Peninsula (Valencia, Spain) has the capacity for self-regeneration and what are the factors that determine its recruitment variability.

Location: Quercus suber forest in Pinet (Valencia, Spain).

Methods: We performed a spatially explicit sampling both of the recruitment and of the potential parameters that could account for the recruitment variability. Using regression techniques we model the recruitment occurrence and abundance, and then we test to what extent the model obtained is still constrained by the spatial dependence.

Results: Quercus suber recruitment density ranges from 0 to 18.66 individuals/25m² (mean = 1.46, SD = 2.8), with a very skewed distribution. Recruitment is similar under Q. suber forests and under Pinus forests, but it is almost absent under shrublands. Thus the parameters that explain most of the recruitment variability in local vegetation types are: the presence and cover of shrubs (negative relationship with recruitment), the basal area of Q. suber and Pinus and the amount of bare soil (all positively related to recruitment). These parameters are strongly related to the ecological processes driving recruitment (i.e. dispersal and predation) and they remove most of the spatial dependence of recruitment. Most recruiters, however, are small, forming a seedling bank rather than growing to successfully colonize new habitats.

Conclusion: The results suggest that although recruitment densities are not very high, they do not limit potential regeneration in the Pinet Q. suber forest. However, successful regeneration is not observed. If we aim to increase the Pinet Q. suber population size, land management measures need to provide appropriate conditions for both seedling establishment in shrublands (e.g. shrub clearing) and seedling growth in woodlands (e.g. Pinus logging).

Question: How many years must elapse for freshly fallen Picea abies stems to be transformed into a substrate for P. abies recruitment?

Location: Natural sub-alpine spruce forest, 1200–1300 m a.s.l., western Carpathians, Poland.

Methods: Coarse woody debris (CWD) was measured on nine plots with a total area of 4.3 ha. All individuals of P. abies regeneration growing on dead wood were counted and their age was estimated. Decay rate of logs was determined using dendrochronological cross-dating of samples from logs in different decay stages.

Results: Although CWD covered only 4% of the forest floor, 43% of the saplings were growing on decaying logs and stumps. The highest abundance of P. abies recruitment occurs on logs 30–60 years after tree death, when wood is in decay stages no. 4-7 (on an 8 degree decay scale). However, much earlier colonization is possible. The first seedlings may germinate on a log during the second decade after tree death and survive for decades. Their slow growth is possibly due to the gradual progressive decomposition of wood.

Conclusions: This study confirms the importance of decaying wood for P. abies recruitment. The decaying logs exhibit continuous and favourable conditions for the germination of P. abies seeds throughout their decay process. Logs, irrespective of their decay stage and age, are colonized by young seedlings. This recruitment bank is constantly renewed.

Question: To what extent do surrogate approaches to vegetation description provide equivalent data and estimates of the patterns of variability in species composition to that which would be obtained using species-level abundance data?

Locations: Picos de Europa in Northern Spain and South Uist, Outer Hebrides, Scotland, UK.

Methods: Percentage cover data were collected from 117 plots in montane and sub-alpine grassland and woodland mosaics in the Picos de Europa and 217 quadrats across a gradient from temperate coastal dunes to upland grassland and heath on South Uist. From these data, 14 surrogate datasets were derived using supra-species (genus, family) levels of description, presence-absence, life form, growth form and plant functional types (PFTs; derived from seven easily measurable traits). The descriptive power of each of the 14 surrogate datasets in each area, relative to the original cover abundance dataset, was assessed by ‘second stage’ analysis, based on Spearman's rank correlation coefficient calculated between pairs of similarity matrices.

Results: In both areas, presence-absence data showed the closest similarities to the original cover abundance data, followed by data generalised at supra-species level. PFT, life form, growth form and leaf trait data were less similar.

Conclusions: Many situations exist where it may be more efficient or desirable to collect neither species nor abundance data and to use appropriate surrogates. The results demonstrate clearly that the information content of surrogate datasets in our study may be an acceptable substitute. These ideas merit further investigation in a wider range of habitats and contexts.

Nomenclature: Picos de Europa: Díaz-González et al. (1994) for phanerogams, Smith (2004) for bryophytes and liverworts; South Uist: Stace (1997) for vascular plants, Smith (2004) for bryophytes, Jahns (1983) and Gilbert (2000) for lichens.

Question: In relation to a single fire, do repeated wildfires in Mediterranean type ecosystems (1) reduce plant species richness or diversity; (2) modify patterns of abundance or dominance of plant species or (3) alter plant composition?

Location: Pinus halepensis dominated communities of Catalonia, northeastern Iberian Peninsula, western Mediterranean Basin.

Methods: Regional, paired design with 14 study sites, each consisting of a once burnt area (1994) and a twice burnt area (1975-1993 and 1994). Ten years after the last fire, we recorded all vascular plant species present in nested plots and quantified their relative abundances on transects. We compared species richness, diversity, dominance and relative abundance and species-area correlations between paired once and twice burnt areas and assessed their floristic composition similarity.

Results: No statistically significant differences were found in species richness or diversity. Slopes of species-area correlations were higher in once burnt areas. In twice burnt areas, dominance by one or two species was higher. P. halepensis showed lower relative abundance and nanophanerophytes showed higher relative abundance. No differences were found for resprouter, seeder or resprouter-seeder species. Floristic composition similarity between paired areas tended to be higher in less productive sites.

Conclusions: Fire recurrence had contrasting effects on species richness at different spatial scales. Repeated burning reduced the relative abundance of the dominant tree species, which resulted in a higher relative abundance of shrubs. It also promoted the dominance of herbs, particularly Brachypodium retusum. However, it did not change the relative abundance of regenerative groups. Paired areas were more similar as they were more Mediterranean in terms of climatic conditions.

Nomenclature: Bolòs et al. (1990).

Question: This paper compares published palynological studies from coastal swamps containing the same suite of species. We ask the following questions: (1) does succession follow the same pathways in different swamp systems, or (2) at different times? If not, (3) how variable are the patterns and (4) what are the likely driving factors?

Location: Great Barrier Island, Northern New Zealand.

Methods: Eighteen pollen profile diagrams were studied from four estuarine wetlands, ranging from mangroves to swamp forest. Recognition of a transition between vegetation stages was by subjective consideration of the relative abundances of pollen of key indicator species at different depths in the sedimentary sequence.

Results: A linear sequence of vegetation communities beginning with mangroves and followed by estuarine marsh communities composed of Juncus kraussii, Leptocarpus similis and Baumea juncea was recognised in almost all pollen diagrams. Further transitions, from Baumea to a terrestrial system of Leptospermum shrubland or Cordyline/Dacrycarpus swamp forest, followed two main pathways associated with autogenic accumulation of peat and terrigenous sediment input respectively. At Kaitoke and Awana the marine/freshwater transition occurred before the arrival of humans on Great Barrier Island. At Whangapoua, increased sedimentation followed anthropogenic burning of adjacent forest, and this transition was faster and is still in progress.

Conclusion: Palynology and current vegetation zonation patterns concur to demonstrate that the marine sedimentation phase of estuarine succession is predictable and linear. Baumea marks the transition to the freshwater phase, in which varied successional patterns are determined by interactions between hydrology, sediment input, and peat accumulation. Natural and human disturbances drive sedimentation rates, and interact with autogenic factors, to dictate vegetation transitions in these later stages. The intensive impact (mainly burning) during Polynesian times had a much greater effect on estuaries and swamps than the pre-Polynesian natural processes, greatly accelerating plant succession.

Question: How do spatial patterns and associations of canopy and understorey vegetation vary with spatial scale along a gradient of canopy composition in boreal mixed-wood forests, from younger Aspen stands dominated by Populus tremuloides and P. balsamifera to older Mixed and Conifer stands dominated by Picea glauca? Do canopy evergreen conifers and broad-leaved deciduous trees differ in their spatial relationships with understorey vegetation?

Location: EMEND experimental site, Alberta, Canada.

Methods: Canopy and understorey vegetation were sampled in 28 transects of 100 contiguous 0.5 m × 0.5 m quadrats in three forest stand types. Vegetation spatial patterns and relationships were analysed using wavelets.

Results: Boreal mixed-wood canopy and understorey vegetation are patchily distributed at a range of small spatial scales. The scale of canopy and understorey spatial patterns generally increased with increasing conifer presence in the canopy. Associations between canopy and understorey were highly variable among stand types, transects and spatial scales. Understorey vascular plant cover was generally positively associated with canopy deciduous tree cover and negatively associated with canopy conifer tree cover at spatial scales from 5–15 m. Understorey non-vascular plant cover and community composition were more variable in their relationships with canopy cover, showing both positive and negative associations at a range of spatial scales.

Conclusions: The spatial structure and relation of boreal mixed-wood canopy and understorey vegetation varied with spatial scale. Differences in understorey spatial structure among stand types were consistent with a nucleation model of patch dynamics during succession in boreal mixed-wood forests.

Nomenclature: Anderson et al. (1990); Moss (1994).

Question: How is grazing intensity associated with species and morpho-functional traits (MFTs) composition, productivity and richness of annual dominated grasslands? Have native and exotic species similar associations to this gradient?

Location: Anthropogenic grassland in the Espinal vegetation in the sub-humid area of the mediterranean type climate region of Chile (35°58′ S, 72°17′ W).

Methods: Data were obtained from a long-term (eight years) experiment with six stocking rates (1 to 3.5 sheep/ha). Detrended Correspondence Analysis (DCA) and regression analysis were used to determinate the relationship between grazing intensity and biomass, richness, abundance and traits of the species.

Results: The first DCA axis was related to grazing intensity and explained most of the floristic variation (69.3%); the abundance of some non-native species, e.g. Vulpia megalura were highly correlated with this axis. In the DCA for MFTs the first axis explained 87% of the variance and was also related to grazing intensity; the abundance of small size plants and shallow roots increased with grazing intensity. The relative abundance of grasses and composites, but not of legumes, changed with stocking rate: as grazing intensity increased composites became the predominant species to the detriment of grasses. The above-ground biomass measured in exclusion cages declined with increasing grazing pressure. The richness of exotic species was greater compared to native ones at low stocking rates, but they converge to similar values at higher stocking rates. However, the relative abundance of exotic species was greater than 75% in all stocking rates.

Conclusions: Grazing intensification has large effects in the structure of grassland in central Chile. With grazing intensities greater than 1 sheep/ha species characteristics change; evolving in a few years (6–8) towards a similar community regardless of the stocking rate. The overgrazed community has more native than exotic species richness, possibly due to greater defence traits against herbivory of this group of species.

Nomenclature: Marticorena & Quezada (1985).

Questions: 1. Do the species composition, richness and diversity of sapling communities vary significantly in differently sized patches? 2. Do forest patches of different sizes differ in woody plant colonization patterns?

Location: São Francisco de Paula, Rio Grande do Sul, Brazil, 29°28′ S, 50°13′ W.

Methods: Three woody vegetation types, differing in structural development (patch size) and recovering for 10 years from cattle and burning disturbances, were sampled on grassland. We analysed the composition and complexity of the woody sapling communities, through relative abundance, richness and diversity patterns. We also evaluated recruitment status (residents vs. colonizers) of species in communities occurring in different forest patch size classes.

Results: 1. There is a compositional gradient in sapling communities strongly associated with forest patch area. 2. Richness and diversity are positively correlated to patch area, but only in poorly structured patches; large patches present richness and diversity values similar to small patches. 3. Resident to colonizer abundance ratio increases from nurse plants to large patches. The species number proportion between residents and colonizers is similar in small and large patches and did not differ between these patch types. 4. Large patches presented a high number of exclusive species, while nurse plants and small patches did not.

Conclusions: Woody plant communities in Araucaria forest patches are associated with patch structure development. Richness and diversity patterns are linked to patch colonization patterns. Generalist species colonize the understorey of nurse plants and small patches; resident species cannot recruit many new individuals. In large patches, sapling recruitment by resident adults precludes the immigration of new species into the patches, limiting richness and diversity.

Nomenclature: APG 2003; Anon. 2005).

Aim: Concentration of species occurrences in groups of classified sites can be quantified with statistical measures of fidelity, which can be used for the determination of diagnostic species. However, for most available measures fidelity depends on the number of sites within individual groups. As the classified data sets typically contain site groups of unequal size, such measures do not enable a comparison of numerical fidelity values of species between different site groups. We therefore propose a new method of measuring fidelity with presence/absence data after equalization of the size of the site groups. We compare the properties of this new method with other measures of statistical fidelity, in particular with the Dufrêne-Legendre Indicator Value (IndVal) index.

Methods: The size of site groups in the data set is equalized, while relative frequencies of species occurrence within and outside of these groups are kept constant. Then fidelity is calculated using the phi coefficient of association.

Results: Fidelity values after equalization are independent of site group size, but their numerical values vary independently of the statistical significance of fidelity. By changing the size of the target site group relative to the size of the entire data set, the fidelity measure can be made more sensitive to either common or rare species. We show that there are two modifications of the IndVal index for presence/absence data, one of which is also independent of the size of site groups.

Conclusion: The phi coefficient applied to site groups of equalized size has advantages over other statistical measures of fidelity based on presence/absence data. Its properties are close to an intuitive understanding of fidelity and diagnostic species in vegetation science. Statistical significance can be checked by calculation of another fidelity measure that is a function of statistical significance, or by direct calculation of the probability of observed species concentrations by Fisher's exact test. An advantage of the new method over IndVal is its ability to distinguish between positive and negative fidelity. One can also weight the relative importance of common and rare species by changing the equalized size of the site groups.

Nomenclature: Ehrendorfer (1973).

Questions: Can a statistical model be designed to represent more directly the nature of organismal response to multiple interacting factors? Can multiplicative kernel smoothers be used for this purpose? What advantages does this approach have over more traditional habitat modelling methods?

Methods: Non-parametric multiplicative regression (NPMR) was developed from the premises that: the response variable has a minimum of zero and a physiologically-determined maximum, species respond simultaneously to multiple ecological factors, the response to any one factor is conditioned by the values of other factors, and that if any of the factors is intolerable then the response is zero. Key features of NPMR are interactive effects of predictors, no need to specify an overall model form in advance, and built-in controls on overfitting. The effectiveness of the method is demonstrated with simulated and real data sets.

Results: Empirical and theoretical relationships of species response to multiple interacting predictors can be represented effectively by multiplicative kernel smoothers. NPMR allows us to abandon simplistic assumptions about overall model form, while embracing the ecological truism that habitat factors interact.

Question: How do environmental variables in a hyper-arid fog desert influence the distribution patterns of terricolous lichens on both macro- and micro-scales?

Location: Namib Desert, Namibia.

Methods: Sites with varying lichen species cover were sampled for environmental variables on a macro-scale (elevation, slope degree, aspect, proximity to river channels, and fog deposition) and on a micro-scale (soil structure and chemistry). Macro-scale and micro-scale variables were analysed separately for associations with lichen species cover using constrained ordination (DCCA) and unconstrained ordination (DCA). Explanatory variables that dominated the first two axes of the constrained ordinations were tested against a lichen cover gradient.

Results: Elevation and proximity to river channels were the most significant drivers of lichen species cover in the macro-scale DCCA, but results of the DCA suggest that a considerable percentage of variation in lichen species cover is unexplained by these variables. On a micro-scale, sediment particle size explained a majority of lichen community variations, followed by soil pH. When both macro and micro-scale variables were tested along a lichen cover gradient, soil pH was the only variable to show a significant relationship to lichen cover.

Conclusion: The findings suggest that landscape variables contribute to variations in lichen species cover, but that stronger links occur between lichen growth and small-scale variations in soil characteristics, supporting the need for multi-scale approaches in the management of threatened biological soil crust communities and related ecosystem functions.

In a recent Forum paper, it is argued that, in most studies, ordinal data such as the Braun-Blanquet abundance/dominance scale are not properly treated by multivariate methods. This is because conventional multivariate methods are generally adequate for ratio-scale variables only, while for ordinal variables differences between states and their ratios are not interpreted. Conversely, in this paper it is shown that using conventional multivariate procedures for evaluating ordinal data should imply a shift from a metric space to a topological data space; as such the use of ordinal data does not represent a serious methodological error, provided that results are interpreted accordingly.

Recently, version 5 of PC-ORD, one of the major commercial software packages for multivariate ecological community data analyses, was released. The new version offers a whole range of techniques and methods for analyses of ecological data. It includes modules for different types of ordination and classification, as well as other exploratory techniques such as species-area curve analysis and indicator species analysis. Data are stored in spreadsheets and can be easily manipulated in various ways. In essence, version 5 of PC-ORD offers the user a full toolbox for exploration and analysis of ecological data, packed in a user-friendly environment.