Question: We know of no study that has linked volcanic eruptions occurring anywhere worldwide and the local population cycles of any species. The keystone saguaro cactus (Carnegiea gigantea) establishes in cohorts. We test whether there is a statistical relationship between Carnegiea gigantea establishment and volcanic eruptions.

Location: Northern Sonoran Desert, Arizona, USA.

Methods: We use both a region-wide dataset made up of 30 populations, and a dataset from a marginal site. We incorporate data for over 750 individuals over an area of more than 50 000 km². We created a 111-year time series of population peaks and troughs and correlated this over the 111-year record with the annual Weighted Historical Dust Veil Index (WHDVI). A t-test compared establishment patterns with the WHDVI.

Results: We found a significant relationship between volcanism and C. gigantea regeneration at both the marginal site, and in the region-wide dataset.

Conclusions: We suggest that while different populations are influenced by temporary global temperature ameliorations to different extents, our results show that populations do derive significant benefits from volcanic eruptions that promote their regeneration over large portions of their range, as is also exhibited locally at our marginal site, where populations are most susceptible to the inhospitable conditions that are witnessed at the edge of their range. This paper draws a link between the population fluctuations and regeneration of a species locally with geologic events from distant parts of the earth.

Questions: How do gap aspect, openness, age and gap density in the surroundings affect diversity and composition of lianas within treefall gaps? Are the variation patterns in liana communities within treefall gaps associated with their climbing mechanisms?

Location: Subtropical montane forests in northwestern Argentina.

Methods: We used ordination analyses (NMDS) and multiple regressions to describe liana communities (species and climbing groups, stems ≥ 1 cm) in 35 gaps located in a 6-ha plot and to assess relationships with aspect, canopy and subcanopy openness, age and density of recent (three to eight years old) and old (> 8 years old) gaps.

Results: Treefall gaps segregated in the ordination diagrams based on their liana species and climbing mechanisms composition: gaps surrounded by a higher density of recent gaps showed higher densities of lianas species using tendrils, spines / hooks and voluble stems. Liana density and richness (independent of stem density) were also positively associated with the density of recent gaps in the surroundings, and negatively associated to gap age. The number of liana species corrected per area was negatively associated to gap aspect and subcanopy openness, and absolute number of liana species was positively associated to gap canopy openness.

Conclusions: This study integrates the analysis of different factors influencing liana communities within treefall gaps, and shows that spatial context of gap density (a variable largely neglected in previous studies) is particularly important on lianas composition and diversity, probably by increasing propagule input and the availability of small trellises for support.

Nomenclature: Zuloaga & Morrone (1999).

Question: What are the physical and chemical effects of plant litter on annual grassland community composition, above-ground net primary production (ANPP), and density?

Location: California annual grassland.

Methods: We manipulated litter and light levels independently and in concert. Litter removal and litter addition treatments tested both the physical and chemical impacts of litter's presence. We additionally simulated the effect of litter physical shading by using shade cloth, and added powdered litter to test for the chemical impacts of decomposing litter.

Results: Increased whole litter and shading decreased grass germination and establishment, but not that of forbs or legumes. Species shifts occurred within all groups across treatments, including a transition from small-seeded to large-seeded grass and legume species with increased shading. ANPP was highest in control plots (473 ± 59 g/m²), and species richness was highest in litter removal plots. While the physical effects of litter via shading were significant, the chemical effects of adding powdered litter were negligible.

Conclusions: This work suggests that over one growing season, the physical impacts of litter are more important than chemical impacts in shaping community structure and ANPP in annual grasslands. Changes in light availability with altered litter inputs drive shifts in species and functional group composition. Litter feedbacks to ANPP and species composition of local patches may help maintain diversity and stabilize ANPP in this grassland.

Nomenclature: Hickman (1993).

Question: How does nutrient limitation vary along major environmental gradients in fens and wet meadows?

Location: West Carpathians (Czech Republic, Slovakia).

Methods: We recorded total plant species composition in 83 plots of 9 m². Living above-ground biomass of total vegetation was sampled. Concentrations of Ca, Fe, N, P, K were determined. Obtained data were analysed by various univariate and multivariate statistical techniques.

Results: Major environmental gradients correspond to the poor-rich vegetation gradient, from poor Sphagnum-fens to calcareous fens, and the fen-meadow gradient from sedge-moss fens to forb-rich wet meadows. Ca-concentration in above-ground biomass was strongly positively correlated with the poor-rich gradient, while Fe- and N-concentrations had an opposite trend. Poor-fen vegetation contained little calcium and much iron. The lowest P-concentrations were found in calcareous tufa-forming fens. Variation in N:K ratio indicated a slight decrease of K-availability towards poor fens. Along the fen-meadow gradient, vegetation uptake of P, K and Ca increases and the uptake of Fe decreases.

Conclusions: Our results suggest that variation in the type of nutrient limitation is not a dominant cause of the poor-rich gradient in fens due to the important role of calcium and iron. Nevertheless, species distribution along the poor-rich gradient is influenced by phosphorus shortage in tufa-forming fens and relatively high N-uptake and low K-availability in poor fens. Additional P- and K-supply to Ca-rich fens can alter species composition towards forb-rich meadows.

Nomenclature: Frey et al. (1995) for bryophytes; Kubát et al. (2002) for vascular plants.

Question: What are the effects of different aspects of data collection and analysis on the spatial partitioning of ordination results by multiscale ordination?

Locations: Heterogeneous pasture at Marchairuz in the Jura mountains, Switzerland and Mixed hardwood-pine forest in Oostings Natural Area, NC, USA.

Methods: We evaluated the efficiency of different sampling designs for identifying the spatial structure of plant communities and analysed two datasets with multiscale ordination. We compared the effects of quadrat size, the number of species included in the analysis, data type, detrending and ordination method on the shape and precision of the community variogram summarizing spatial community structure.

Results: A three-block sampling design provided a more even distribution of the number of pairs of observation per distance class than random, transect or grid designs. The precision of variogram estimates depended more strongly on the number of species than on the number of quadrats. In contrast, the choice of data type (abundance transformation) had little influence on the shape of the variogram. Detrending reduced the range of spatial autocorrelation. An increase in quadrat size resulted in a smoother variogram and stronger spatial autocorrelation. Principal components analysis (PCA) and Redundancy analysis (RDA) resulted in a larger range of spatial autocorrelation values than did Correspondence analysis (CA) and Canonical correspondence analysis (CCA), but the shape of the variograms was similar.

Conclusion: Random samples as well as transects and regular grids may not be efficient sampling designs for spatial analysis of community structure. While the number of species considered strongly affects the precision of a community variogram, its shape depends on the size of the sampling units. Earlier studies may have overestimated the spatial scale of internal organization in plant communities.

Question: Detecting species presence in vegetation and making visual assessment of abundances involve a certain amount of skill, and therefore subjectivity. We evaluated the magnitude of the error in data, and its consequences for evaluating temporal trends.

Location: Swedish forest vegetation.

Methods: Vegetation data were collected independently by two observers in 342 permanent 100-m² plots in mature boreal forests. Each plot was visited by one observer from a group of 36 and one of two quality assessment observers. The cover class of 29 taxa was recorded, and presence/absence for an additional 50.

Results: Overall, one third of each occurrence was missed by one of the two observers, but with large differences among species. There were more missed occurrences at low abundances. Species occurring at low abundance when present tended to be frequently overlooked. Variance component analyses indicated that cover data on 5 of 17 species had a significant observer bias. Observer-explained variance was < 10% in 15 of 17 species.

Conclusion: The substantial number of missed occurrences suggests poor power in detecting changes based on presence/absence data. The magnitude of observer bias in cover estimates was relatively small, compared with random error, and therefore potentially analytically tractable. Data in this monitoring system could be improved by a more structured working model during field work.

Nomenclature source: Karlsson (1998).

Question: Does the distribution of plant species found in forests correlate with variation in the Humus Index (based on a ranking of humus forms) and, if so, do the species exhibit different responses according to phyletic lineages?

Location: Paris Basin, France, with a temperate Atlantic climate

Methods: Mosses and vascular plants (herbs, ferns) were inventoried in two broad-leaved forests with contrasting soil conditions, where 15 and 16 sites were investigated, respectively. Variety of stand age and prevailing soil conditions were analysed in 5 plots and 20 subplots in a grid at each site. Mantel tests were used to estimate correlations between the Humus Index and plant species richness, taking into account spatial autocorrelation.

Results: The local (plot, subplot) species richness of moss communities increased with the Humus Index, i.e. when humus forms shifted from mull to moder. The reverse phenomenon was observed in vascular communities. The opposite response of these two plant groups could be explained by opposite strategies for nutrient capture which developed in the course of their evolutionary history.

Conclusions: Although not necessarily causative, the Humus Index predict fairly well changes in species richness which occur in forest vegetation, provided that phyletic lineages and geographical position are taken into consideration.

Nomenclature: Rameau et al. (1989); Hill et al. (2006); Lambinon et al. (1999).

Questions: How do tree age, microhabitat characteristics and epiphytic competitors affect the occurrence of crustose lichens associated with old oaks? How do microhabitat characteristics and microclimate affect the cover of competitors (bryophytes and macrolichens)? How do microhabitat characteristics correlate with microclimatic variables?

Location: Southeast Sweden.

Methods: Eight crustose lichen species were surveyed on 165 Quercus robur trees, 17–478 years old, at three study sites. The occurrence patterns of these species were examined at two spatial scales: among trees and within trees. Occurrence patterns within trees were examined in 10 cm × 10 cm plots at all four cardinal aspects at different heights from 0.5 to 4.5 m above the ground.

Results: At the tree level, age-related factors were the most important predictors of species occurrence. All species were more frequent on trees > 100 years than on younger trees. At the plot level, the frequency of occurrence increased with increasing bark crevice depth. The frequencies of all study species decreased with increasing cover of bryophytes. Bryophytes were in turn more frequent in plots that were exposed to rainwater and showed a low evaporation rate. Patches most exposed to rainwater were directed upwards, and the lowest evaporation rates occurred on the northern side of the trunks.

Conclusions: For many crustose lichens the association with old oak trees seems at least partly to depend on their preference for the deep bark crevices that only occur on old trees. Trees represent epiphyte habitat patches that differ in size due to within-tree variability in habitat quality, such as bark crevice depth and microclimate. This study shows that variability at a finer scale, within habitat patches, contribute to explain species occurrence patterns at habitat patch level.

Nomenclature: Santesson et al. (2004).

Questions: Do large trees improve the nutrient content and the structure of the grass layer in savannas? Does the magnitude of this improvement differ with locality (soil nutrients) and season (water availability)? Are grass structure and species composition beneath tree canopies influenced by soil fertility and season?

Location: South Africa.

Methods: We compared grass leaf nutrient contents and grass sward structure beneath and outside tree canopy areas in three savannas of different soil fertility during the dry and the wet seasons.

Results: Grass nitrogen contents were twice as high during the wet season as compared to the dry season, being more strongly elevated underneath tree canopies during the wet season. Grasses had significantly less stem material and provided less dead material underneath trees on the high soil fertility site. Grass species composition differed significantly beneath and outside tree canopies, with more nutritious grass species found sub-canopy. Grass species richness was significantly lower beneath than outside of trees at the site of high soil fertility.

Conclusions: Trees improve overall quality of savanna grasses by enhancing grass growth and nutrient uptake during the wet season, and by delaying grass wilting in the dry season. The positive effect of trees on the grass layer might attract grazing herbivores in otherwise nutrient-poor savannas. Hence, single standing large trees should be maintained to sustain high grass quality and, consequently, grazer populations in savanna habitats.

Nomenclature: Trees: Palgrave (1983); Beentje et al. (1994) Grasses: Van Oudtshoorn (1999).

Question: How did the vascular plant species composition of a nunatak in the alpine vegetation belt change over a time span of 100 years?

Location: A 5.6-ha nunatak, Isla Persa in the Swiss Alps, that remained ice-free during the last maximum glacier advance in the 1850s and is today partly covered with climactic alpine grassland and dwarf heath shrubs.

Methods: Floristic inventories in 1906, 1927, 1972, 1995, 2003 and 2004 and a comparative analysis of the species composition over the period 1906–2004.

Results: 31 species that were not recorded in the first inventory were found in the following surveys. However, among them only six were common by 2004. Generally, the new species prefer warmer conditions than those previously present and most newcomers are associated with montane or sub-alpine grasslands and woodlands. In particular, the observed increase of Vaccinium myrtillus and the arrival of shrub and tree species further substantiate a trend towards vegetation composition of the lower altitudinal belt. Ferns represented 26% of the newcomers, probably due to the high dispersal ability of their lightweight spores. The observed species enrichment was globally small compared to previously inventoried summits.

Conclusion: Floristic change strongly suggests warmer climatic conditions as the main factor contributing to species compositional change. The relative stability of species richness may be explained by several factors: the isolation of the nunatak and the difficulties for plants to reach the site, the colder local climate, a limited available species pool and interactions of established alpine plants with newly immigrating taxa. Supplementary data collected at a similar altitude would be necessary to better understand the influence of climate change on alpine grasslands.

Nomenclature: Aeschimann et al. (2004).

Questions: 1. How many traits associated with persistence and regeneration are necessary to predict the response of plants to soil fertility and disturbance? 2. Are correlated changes in trait expressions linked to the response of functional groups to fertility and disturbance?

Location: Lower Frankonia, Germany.

Methods: On 120 plots located in managed and abandoned grasslands, fields, and vineyards, we recorded species composition, disturbance intensity, soil water and nutrients, and ten candidate traits for 75 species. We used a novel method which is based on three steps: (1) logistic regression to separate responsive from non-responsive species; (2) iterative clustering of all possible combinations of the candidate traits including regression of each cluster in response to the environmental variables; (3) selection of the trait combination that performed best with respect to goodness of fit of all clusters from this combination. Bivariate trait relationships across functional groups were analysed with reduced major axis regression (RMA).

Results: The parsimonious trait combination consisted of life span, specific leaf area (SLA), canopy height, and seed number. The ‘acquisitive’ functional groups in terms of SLA and height were linked to higher fertility and earlier disturbance, while the ‘retentive’ groups related to lower fertility and later disturbance. Investment in reproduction, however, displayed a reverse relation. SLA and canopy height showed correlated shifts in two pairs of co-occurring functional groups.

Conclusions: A small number of traits is sufficient to predict the response of species. Plants need a higher increment in SLA to reach the same height, if start of disturbance is earlier. Linkages between traits shift from generative to vegetative with increasing fertility and earlier start of disturbance. Functional groups enable shifts in scaling relationships between traits to be analysed, in contrast to the analysis of single traits.

Nomenclature: Rothmaler (1994).

Question: How might contrasting plant groups interact to confer ecosystem stability during the development of a wet heath: i.e. in response to two major environmental drivers, past hydrologic change and burning?

Location: Rannoch Moor, Scotland.

Methods: Three peat cores were used to reconstruct vegetation change, surface moisture and burning phases, during wet heath development for a radiocarbon-dated period from ca. 9500–1000 yr BP.

Results: Statistical analysis of peat cores revealed significant relationships between burning phases and past periods of inferred surface dryness. Vegetation elements were related in the palaeoecological record to surface dryness (Ericaceae), wetness (Sphagnum, monocotyledons, and Racomitrium) and periods of burning (i.e. negative correlation of Racomitrium and monocotyledon remains with charcoal). However, correlations between plant groups could not be adequately explained by their equivalent direct response to hydrology and burning, and the effects of species interactions are invoked.

Conclusions: The results indicate millennial-scale stability of the wet heath ecosystem in response to past environmental change (i.e. hydrology and burning). This stability is conferred by interaction between a diversity of plant groups, enabling local shifts in vegetation composition in response to environmental drivers. This long-term stability of the mire ecosystem forms a template against which present-day threats (e.g. pollution, climate change) should be critically assessed.

Question: The drivers of spatial patterning among plant species and the implications of those patterns for the structure and function of plant communities are of ongoing interest and debate. Here we explore the spatial patterning shown by individual species in species-rich plant communities. We (1) compare the levels of aggregation in these communities to those observed in other species-rich communities, in particular tropical rain forests, and (2) consider how abiotic conditions might influence the levels of aggregation observed.

Location: We describe the spatial structure of four species-rich Mediterranean-type shrubland communities near Eneabba, Western Australia. The four sites each contain > 10000 plants and up to 113 species, and differ in substrate-type, species richness and composition.

Methods: We analysed the spatial patterning of all species with more than 20 individuals (233 species patterns), and used point process models for aggregated patterns to separate first-order gradient effects from second-order clustering.

Results: Aggregated distributions were most common at all sites, but especially at the site with the highest resource availability and heterogeneity and lowest species richness. A Poisson cluster process best described the majority of aggregated species, suggesting that local interactions drive fine-scale patterns in these communities.

Conclusions: As with many previous studies, we found that most species showed strong local aggregation. The proportion of species showing aggregation was less than has been described in species-rich tropical rainforests but was higher than observed in many temperate plant communities. The highest proportion of aggregated species was seen at the most resource-abundant site; this is in direct contrast to conceptual models that suggest that competition should be weakest, and aggregation most prevalent, in the most resource-limited sites.

Questions: Do lianas alter the relative success of tree species during regeneration? Are the effects of lianas on tree seedlings moderated by canopy openness? How are patterns of biomass allocation in tree seedlings affected by liana competition?

Location: Tropical moist semi-deciduous forest in Ghana.

Methods: Seedlings of the trees Nauclea diderrichii (pioneer), Khaya anthotheca (non-pioneer light demander) and Garcinia kola (non-pioneer shade bearer) were planted with the lianas Acacia kamerunensis (fast growing) and Loeseneriella rowlandii (slow growing) in large and small gaps (ca. 15% and 8% PAR respectively) and in the forest understorey (ca. 4% PAR). Seedling survival, growth and biomass allocation were measured.

Results: Canopy openness moderated the interaction between liana and tree seedlings. The nature of the interaction was both liana and tree species specific and displayed temporal variation. Acacia competition effects were stronger in sites with greater canopy openness. In big gaps, Acacia reduced significantly the biomass of Nauclea by 32% and Khaya by about 50%. Khaya growth in leaf area was five times greater without Acacia, while Nauclea and Garcinia were not affected. Acacia was more plastic than Loeseneriella in response to the environment and the tree species. Our results show that while Loeseneriella, with lower rates of growth, did not affect seedling growth of the three species evaluated, Acacia could alter the relative success of tree species during regeneration.

Conclusions: There is evidence that competitive effects by Acacia on tree regeneration through competition could modify tree species capacity to establish. Effects by lianas at the regeneration phase may have important implications for forest management.

Nomenclature: Hall & Swaine (1981) and Hawthorne (1995).

Questions: Is the area of influence of individual trees determined by tree size? Does competition, inferred from spatial pattern between neighbouring trees, affect adult tree spacing patterns in an tropical forest? At what size-class or stage is competition between neighbours most likely to affect adult tree spacing patterns?

Location: Kibale National Park, western Uganda.

Methods: Relationships between focal tree size and nearest neighbour distance, size, density, and species in a 4-ha permanent plot, using point pattern analyses.

Results: We found non-random patterns of distribution of nearest tree neighbours (stems > 10 cm DBH). Independent of identity, tree density was highest and neighbours were regularly spaced within 3–5m of an individual. Tree densities were lower and relatively constant at distances >5m and neighbours were typically randomly spaced. In general, conspecific patterns conformed to the latter trends. Thus, individual area of influence was small (within a radius of 3–5 m). Rarer species were more clumped than common species. Weak competitive thinning occurred among more densely packed small trees (<20 cm DBH), and rapidly disappeared with increasing tree size and distance from an individual. The clumping and density of individuals was not significantly affected by tree size.

Conclusions: Negative effects of competition among trees are weak, occur within the crown radius of most individuals, and are independent of adult tree size and identity. The density of neighbouring trees (aggregation) did not decline with increasing focal tree size at either the conspecific or the community level and tree diameter (tree size) was not a good estimator of the implied competitive influence of a tree. Mechanisms operating at the recruitment stage may be important determinants of adult tree community diversity and spacing patterns.

Nomenclature: Hamilton (1991).