Question: Is there any change in seed density and species richness in Mediterranean surface soil banks during summer? Are there any relationships between these summer variations and seed traits (weight, length and shape), without and with controlling the phylogenetic effect?

Location: Central Spain.

Methods: Samples of the surface soil seed bank were collected in two Mediterranean systems, grassland and scrubland, at two points in the year: immediately after the summer production peak and immediately prior to the autumn germination peak. We used Canonical Correspondence Analysis ordination to check for changes in floristic composition and ANOVAs to check for changes in seed density and species richness between summer and autumn samples. We used multiple regression analysis to analyse the relationship between summer variations in soil seed density and form traits, with and without controlling phylogenetic relations.

Results: Soil seed density dropped significantly during the summer in the two systems (28% in grasslands, repeated measures ANOVA test; F = 58.19, P < 0.01; 72% in scrublands, repeated measures ANOVA test; F = 75.67, P < 0.001). Species richness, however, only dropped significantly in the scrubland (32%; repeated measures ANOVA test; F = 9.17, P < 0.05). Variation in the floristic composition of the shallow banks was only significant in the scrubland.

Changes in shallow bank density were related significantly to seed morphology features, with greater drops in species with heavier seeds in grasslands and species with longer seeds in scrubland.

Conclusions: Our results show a substantial loss of seeds in the uppermost soil layer during the summer period between the point of peak production and the autumn germination peak. This drop was clearer in the scrubland than in the grassland. Longer and/or heavier seeds underwent the greatest declines in density at the end of summer, indicating a more intense effect of post-dispersal predation on large-seeded species.

Abbreviations: CSA = Cross-species analysis; PIA = Phylogenetically independent analysis; PIC = Phylogenetically independent contrast.

Questions: 1. Are there competitive response strategies for light in juvenile plants? 2. If so, do plant traits (e.g. seed weight, relative growth rate, height and biomass) correlate with the strategies?

Location: Controlled greenhouse study using perennial vegetation typical of wet meadows in Northeast Ohio, USA.

Methods: We used two light manipulations in a greenhouse to screen ten replicates of 19 plant species for three proposed competitive response strategies (‘escape’, ‘forage’, ‘persist’). We measured the time it took a seedling to die and the maximum height achieved when grown in the dark to assess two strategies, persist and escape. The biomass of seedlings when grown under a controlled, low-intensity, shifting light source was measured to test a third strategy, forage.

Results: We found significant variation across species in the measurements used to assess each strategy. The species ranking for each strategy was not concordant across strategies. Traits were found that correlated with the escape strategy (seed weight, height and biomass) and persist strategy (time to reach maximum height). No traits were found that correlate with the forage strategy.

Conclusions: There appear to be trade-offs by plants in the three strategies tested in this study. Species which had the best performance on one strategy typically scored poorly on the other strategies. However, many species fall in the middle range, ranking similarly across the ‘persist’, ‘escape’, and ‘forage’ strategies.

Abbreviations: CR = Competitive response, RGR = Relative growth rate.

Nomenclature: Gleason & Cronquist (1991); Anon. (1997).

Question: The aim of the present study is to examine how a rare flowering and massive die-off event of a dominant understorey bamboo affected tree regeneration processes in a subalpine Nothofagus forest. In particular, we assess the effects of the die-off event on tree seedling demography (establishment and survival) and on the height growth of previously established juveniles, and we determine how these effects vary in relation to stand age and canopy conditions.

Location: Carirriñe Pass and Choshuenco Volcano at 1200 m a.s.l. in a mixed subalpine temperate forest in northern Patagonia (40°S, 71°W), spanning the Chilean and Argentinian border.

Methods: Regeneration processes in Nothofagus pumilio and N. dombeyi were examined under different canopy conditions (closed or gap), in both young and old stands, and in patches of flowered and non-flowered Chusquea bamboo during the first three growing seasons following the simultaneous flowering and die-off of Chusquea montana f. montana in November–December 2001. We installed permanent plots under all forest conditions and measured seedling and sapling height growth and new seedling establishment following the bamboo die-off. To assess potential variation in tree seed sources, seed fall was collected and seed viability was measured. To assess for potential variation in the understorey light conditions and infer its influence on tree regeneration, hemispherical photographs of the canopy were taken at each plot.

Results: Bamboo die-off triggered accelerated growth in height of Nothofagus seedlings and saplings in gaps in old stands. Bamboo die-off did not result in new tree seedling establishment; however, die-off did increase the survival of newly established seedlings.

Conclusion: The predominant response of Nothofagus spp. to the bamboo die-off event was the accelerated height growth of previously established juveniles (i.e. a re-organization response). Successful recruitment into the canopy of these relatively shade-intolerant tree species appears to be dependent on previously existing canopy gaps. It is likely that the release in height of previously established juveniles of Nothofagus after the bamboo die-off will eventually result in development of even-sized and relatively even-aged, patches of canopy trees.

Nomenclature: Muñoz (1966); Matthei (1997).

Question: Large variation in the similarity between seed bank and standing vegetation generally occurs along environmental gradients. We asked: 1. How is seed bank-vegetation similarity in Mediterranean semi-arid annual plant communities related to variation in primary productivity; 2. How is this productivity-similarity relationship affected by exclusion from grazing.

Location: Mediterranean, semi-arid rangeland in the Northern Negev desert, Israel.

Methods: Density of seeds and plants, and species composition of the seed bank and vegetation were compared in grazed and ungrazed subplots, in four neighbouring topographic sites differing in productivity. Seed bank samples were collected in autumn, just before the rainy season, and vegetation samples in the following spring, at peak above-ground biomass, when net primary productivity was assessed.

Results: Quantitative and qualitative similarities between seed bank and the vegetation (Sørensen index values) varied between 0.14–0.61 and 0.40–0.68. Quantitative similarity and productivity were positively and linearly related within the low productivity range (up to 160 g.m⁻²), in both grazed and ungrazed subplots. In contrast, at higher productivity levels (up to 500 g.m⁻²), similarity decreased slightly with productivity in the ungrazed subplots, but no trend occurred in the grazed subplots. At low productivity, grazing did not affect similarity, while at higher productivity grazing prevented the reduction in similarity with increasing productivity.

Conclusion: seed bank - vegetation similarity, with and without grazing, is positively related to productivity up to a threshold range, above which soil resource availability is no longer the factor limiting plant density. Above this range grazing prevents a reduction in similarity with increasing productivity, by diminishing vegetation cover and litter accumulation that constrain germination, seedling emergence and plant survival.

Nomenclature: Feinbrun-Dothan & Danin 1998.

Question: What is the relationship between species richness of vascular plants, bryophytes and macrolichens, and two important gradients in the alpine environment, altitude and local topography?

Location: Northernmost Fennoscandia, 250–1525 m a.s.l. corresponding to the range between timberline and mountain top.

Methods: The vegetation was sampled in six mountain areas. For each 25 vertical metres, the local topographic gradient from wind-blown ridge to snowbed was sampled in quadrats of 0.8 m × 0.8 m. Patterns in species richness were explored using Poisson regression (Generalized Linear Models). Functional groups of species, i.e. evergreen and deciduous dwarf-shrubs, forbs, graminoids, mosses, hepatics and lichens were investigated separately.

Results: Functional groups showed markedly different patterns with respect to both altitude and topography. Species richness of all vascular plants showed a unimodal relationship with altitude. The same was true for graminoids, forbs and lichens analysed separately, but forb richness peaked at much higher altitudes than total richness. The richness of dwarf-shrubs decreased monotonically with altitude, whereas richness of mosses and liverworts showed an increasing trend. Significant interactions between altitude and local topography were present for several groups. The unimodal pattern for total plant species richness was interpreted in terms of local productivity, physical disturbance, trophic interactions, and in terms of species pool effects.

Conclusions: Patterns in local species richness result from the action of two opposing forces: declining species pool and decreasing intensity of competition with altitude.

Nomenclature: Nilsson (1986) and Hallingbäck (1995, 1996) for vascular plants, lichens and bryophytes, respectively.

Question: Do stressful environments facilitate plant invasion by providing refuges from intense above-ground competition associated with productive areas, or prevent it by favouring locally adapted native species?

Location: An invaded and fragmented oak savanna ecosystem structured along a landscape-level stress gradient associated with soil depth, elevation, and canopy openness.

Methods: Vegetation and environmental data were collected from 184 plots in seven savanna remnants along the gradient. Using multivariate (CCA) and post-hoc regression analyses, we determined the relationship between environment and the richness and abundance of invasives.

Results: 46 of 119 species were naturalized exotics. CCA indicated the importance of environmental variation (mostly soil depth) for community structure but not for invasion; invasive species richness was similar in all areas. However, the abundance of invasives and their impacts on native diversity appear to increase significantly in less stressful habitats. Deeper soils had lower evenness and significantly fewer native species. This result was associated with dominance by exotic perennial grasses and large increases in vegetation height, suggesting strong above-ground competition.

Conclusions: Low-stress environments were not more invasible per se but appear to be more susceptible to invasion by species with strong competitive impacts. The causes of decreasing exotic impact with decreasing soil depth may reflect shifts in competitive intensity or an increased importance of stress tolerance, both of which may favour natives. Alternatively, this ecosystem may simply lack high-impact invaders capable of dominating shallow soils. Conservation challenges are twofold for this endangered plant community: controlling invasives that currently dominate deeper-soils and accounting for a diverse pool of invaders that proliferate when the current dominants are removed.

Nomenclature: Douglas et al. 2001.

Question: How do wind and snow disturbances affect the successional replacement of Picea glehnii by Abies sachalinensis in a sub-alpine coniferous forest?

Location: Cool temperate, northern Japan.

Methods: Tree demography (growth, mortality and recruitment rates) was determined by repeated measurements of stem diameter and height, and multiple censuses in four Picea-Abies stands undergoing succession. Above-ground stand biomass, residence time and tree growth trajectories of the component species were estimated to examine successional changes in structure and dynamics. Individual based simulations were used to examine the effects of disturbances that slowed down succession. Multiple regression analyses were used to determine the relative importance of disturbance frequency and intensity on species composition during succession.

Results: Above-ground biomass was larger in P. glehnii than in A. sachalinensis stands, whereas residence time, a proxy of productivity, was much shorter for A. sachalinensis than for P. glehnii populations. During successional replacement, both species increased in initial growth rate and decreased in size-dependency of growth in canopy gaps. These plastic growth responses were more prominent in P. glehnii than in A. sachalinensis. Disturbance frequency was the most important predictor of species composition in the simulations, and wind-storms were more important than snowfall in terms of disturbance intensity.

Conclusions: The frequency of natural disturbances does not have the potential to initiate stand dynamics but it does, however, slow down succession. When disturbance is locally frequent because of the direction and pitch of the topography, early-successional P. glehnii stands may persist for thousands of years.

Abbreviations: AS = Abies sachalinensis; PG = Picea glehnii; RGR = Relative growth rate.

Nomenclature: Hayashi et al. (1985).

Question: What are the drivers of bryophyte succession in paludifying boreal Picea mariana forests?

Location: The Clay-Belt of Québec and Ontario, Canada.

Methods: The bryophyte community and habitat variables (forest floor thickness, water table, stand density, canopy openness micro-climate and presence of ericaceous species) were analysed in a chronosequence of 13 stands from 50 to more than 350 years since fire.

Results: Across the chronosequence, feathermosses were replaced by shade and desiccation tolerant slower growing hummock Sphagna and then by faster growing hollow Sphagna. These changes were linked with both increasing light availability and the movement of the water table into the forest floor.

Conclusions: As water table rise is dependent on forest floor thickness, which is in turn influenced by the presence of Sphagna, this successional sequence represents an example of facilitation. Furthermore, it emphasizes the importance of water table rise in determining stand level, and landscape level variables such as carbon balance.

Nomenclature: Anderson (1990) for Sphagnum, and Crum & Anderson (1981) for other mosses.

Abbreviations: CWD = Coarse woody debris; DCA = Detrended Correspondence Analysis; LCR = Live crown ratio; PAR = Photosynthetically active radiation; pCCA = Partial Canonical Correspondence Analysis; TSF = Time since fire; VPD = Vapour pressure deficit.

Question: Temporal and spatial shifts in competitive and facilitative interactions among plants have important implications for species coexistence and community diversity. Many studies have focused on inter-seasonal variation in these interactions, but very few have examined short-term intra-seasonal shifts between competition and facilitation. In the central Caucasus Mountains the subalpine climate changes considerably over the season, with a relatively benign (humid and cooler) first part followed by a much more stressful (drier and warmer) second part. We ask: do plant interactions shift from competitive to facilitative during the growing season as environmental conditions change from mesic to dry?

Location: The central Caucasus Mountains, Georgia.

Methods: We experimentally investigated shifts in the balance of positive and negative interactions in plant communities over the course of a single growing season by conducting sequential removal experiments on two co-dominant species.

Results: We found that during the wet and cool first half of the growing season, target plants without neighbours accumulated significantly more biomass than individuals with neighbours, indicating competition. However, in the drier second half of the growing season competitive interactions were shifted to facilitation as individuals without neighbours accumulated significantly less biomass.

Conclusions: In general, these results support the view that competitive and facilitative effects exist in dynamic tension in plant communities with facilitation intensifying as abiotic stress increases, also within a growing season.

Abbreviation: EP = Effective precipitation.

Nomenclature: Nakhutsrishvili (1999).

Questions: The objectives of this study were to clarify the extent to which environmental factors and geographical distance account for tropical floristic composition, and propose a methodology for delimiting the boundaries of floristic types based on species similarity.

Location: The Panama Canal watershed.

Methods: To assess which factors (climate, topography, geology and geographical distance) account for floristic composition, we performed Mantel tests on distance matrices and partitioned variation in species composition using canonical analysis. We used a permutation-based regression model computed on distance matrices and a hierarchical clustering of the tree composition to construct a predictive map of forest types of the Panama Canal Watershed.

Results: We found that spatial variation alone explained 22–27% of species variation, while the fraction of species variation explained by environmental variables was smaller (10–12%); 13–19% of the variation was accounted for by the joint effect of environmental variation and geographic distance. The similarity-based map emphasizes the principal division in tree flora between the drier Pacific side and the wetter Caribbean slopes.

Conclusions: The distribution of Panamanian tree species appears to be primarily determined by dispersal limitation, then by environmental heterogeneity. ‘Environmental segregation’ processes do play an important role. Maps of broad-scale vegetation patterns based on thorough tree inventories can be used in conservation planning in the tropics.

Abbreviation: PCW = Panama Canal Watershed.

Question: Did disturbance, no density-dependence of the dominant species, and negative neighbourhood interactions on rare species affect tree demography during a decline in species diversity associated with the increase of Acer saccharum from 1939–2001?

Hypotheses: 1. The rise in dominance of A. saccharum was because of its advantage in disturbances and lack of density-dependence of its demography. 2. Rare species were not favoured by disturbances, including those from Dutch elm disease, and demonstrated negative neighbourhood interactions with A. saccharum.

Location: Brownfield Woods in Illinois, USA.

Methods: Historical maps of trees (≥ 7.6 cm DBH) from 1939, 1951, 1988, and 2001 in 16 quadrats (48 m × 68 m) were used to compare demography of eight tree species. Effects of disturbances, density-dependence, and neighbourhood interactions on mortality and recruitment of tree species within a 6-m radius of individual target trees were studied.

Results: A. saccharum demonstrated a demographic advantage over rare species. It had lower mortality and higher recruitment rates. Disturbances facilitated recruitment of A. saccharum, but did not enhance rare species. Density-dependence of both mortality and recruitment of A. saccharum occurred, but population projection models indicated that ecological conditions became more favourable for A. saccharum in the past 62 years. Furthermore, negative neighbourhood interactions were detected between rare species and A. saccharum. The increase in neighbouring A. saccharum significantly increased mortality and reduced recruitment of the rare species.

Conclusions: The general disturbance regime, enhanced by Dutch elm disease, in Brownfield Woods facilitated the rise of dominance of A. saccharum. Meanwhile, rare species declined as a result of their disadvantage in disturbance and negative neighbourhood interactions with A. saccharum.

Nomenclature: Mohlenbrock (1986).

Questions: As a consequence of socio-economic changes, many Castanea sativa coppices have been abandoned and are now developing past their usual rotation length. Do we have to expect changes in stand structure and composition of abandoned Castanea sativa coppice invaded by other species? Is a tree ring-based approach adequate to early recognise changes in inter-specific competitive interaction?

Location: Lowest alpine forest belt of the southern Swiss Alps.

Methods: We selected a 60-year old abandoned Castanea sativa coppice stand with sporadic Fagus sylvatica and Quercus cerris mixed in. Using tree-ring based indices we analysed differences in the species-specific response to competition. Analyses were performed by comparing how subject dominant trees (10 Castanea, 5 Fagus, 5 Quercus) have differently faced competition from their immediate Castanea coppice neighbourhood, taking into account the changes over time and space.

Results: Although no species appears yet to have made a difference in the surrounding coppice mortality, there are species-specific differences in growth dominance, which indicate potential successional processes. Castanea sativa growth dominated in the early stages of stand development. However, after approximately 30–35 yr Fagus sylvatica and Quercus cerris became much more dominant, indicating a change in competitive potential that does not favour Castanea sativa.

Conclusions: Without interventions this coppice will develop into a mixed stand. A tree-ring based approach allows an early recognition of forthcoming changes in stand composition and structure and is likely to be an important tool for forest landscape management.

Nomenclature: Aeschimann et al. (2004).

Abbreviations: BAI = Basal area increment; TD = Tree density.

This article investigates whether the Braun-Blanquet abundance/dominance (AD) scores that commonly appear in phytosociological tables can properly be analysed by conventional multivariate analysis methods such as Principal Components Analysis and Correspondence Analysis. The answer is a definite NO. The source of problems is that the AD values express species performance on a scale, namely the ordinal scale, on which differences are not interpretable. There are several arguments suggesting that no matter which methods have been preferred in contemporary numerical syntaxonomy and why, ordinal data should be treated in an ordinal way. In addition to the inadmissibility of arithmetic operations with the AD scores, these arguments include interpretability of dissimilarities derived from ordinal data, consistency of all steps throughout the analysis and universality of the method which enables simultaneous treatment of various measurement scales. All the ordination methods that are commonly used, for example, Principal Components Analysis and all variants of Correspondence Analysis as well as standard cluster analyses such as Ward's method and group average clustering, are inappropriate when using AD data. Therefore, the application of ordinal clustering and scaling methods to traditional phytosociological data is advocated. Dissimilarities between relevés should be calculated using ordinal measures of resemblance, and ordination and clustering algorithms should also be ordinal in nature. A good ordination example is Non-metric Multidimensional Scaling (NMDS) as long as it is calculated from an ordinal dissimilarity measure such as the Goodman & Kruskal γ coefficient, and for clustering the new OrdClAn-H and OrdClAn-N methods.

Abbreviations: AD = Abundance/dominance; NMDS = Non-metric Multidimensional Scaling.