Question: Does the spatial pattern of nutrient supply modify community biomass responses to changes in both species composition and richness?

Location: Duke University Phytotron (Durham, North Carolina, USA).

Methods: We conducted a microcosm experiment to evaluate individual plant and whole community responses to species richness, species composition and soil nutrient heterogeneity. The experiment consisted of seven levels of species composition (all possible combinations of Lolium perenne, Poa pratensis and Plantago lanceolata) crossed with three levels of soil nutrient distribution (homogeneous, heterogeneous-up, and heterogeneous-down, where up and down indicates the location of a nutrient patch in either the upper or the lower half of the soil column, respectively).

Results: Communities containing Plantago and Lolium responded to nutrient heterogeneity by increasing above- and below-ground biomass. Nutrient heterogeneity also increased size inequalities among individuals of these species. Significant species composition × nutrient heterogeneity interactions on community biomass and individual size inequality were observed when nutrient patches were located in the upper 10 cm of the soil columns. However, root proliferation in nutrient patches was equivalent regardless of the vertical placement of the patch.

Conclusions: Our results suggest that nutrient heterogeneity may interact with plant species composition to determine community biomass, and that small-scale vertical differences in the location of nutrient patches affect individual and community responses to this heterogeneity.

Nomenclature: Tutin et al. (1964–1980).

Question: What is the nature of the relationships between cover, diversity and abundance of biological soil crusts, cover and diversity of vascular plants, and annual rainfall, soil texture and forestry practices in Callitris glaucophylla woodlands?

Location: Arid and semi-arid Callitris glaucophylla-dominated woodlands of eastern Australia.

Methods: We documented soil crust-forming mosses, lichens and liverworts at 83 woodland sites along a gradient of declining rainfall. Linear and non-linear regression were used to examine relationships between soil crust species and attributes of vascular plant communities, and a similarity matrix (species abundance × sites) was subjected to Non-metric Multi-Dimensional Scaling (MDS), and Analysis of Similarities (ANOSIM) to show the degree of association between groups of taxa, and soil texture, rainfall classes and forestry practices.

Results: We collected 86 taxa. Mosses were dominated by the family Pottiaceae, and lichens were dominated by squamulose forms. Average annual rainfall was highly correlated with soil crust community composition, and loamy soils supported a greater cover and diversity of taxa compared with sandy soils. Increases in tree cover were associated with significant, though weak, increases in abundance, but not diversity, of crusts. Crusts tended to be more diverse in areas that (1) had a sparse cover of ground-storey plants; (2) were relatively stable – as indicated by the proportion of perennial and/or native plants; (3) had more stable soil surfaces; and (4) were unlogged. Litter cover, overstorey thinning, and livestock grazing had no appreciable effect on crust diversity or cover.

Conclusions: Callitris glaucophylla woodlands provide substantial habitat for soil crust organisms, and the dense tree cover and closed canopies of Callitris do not appear to have a major influence on the structure of biological crust communities. Unlike other woodland systems, relatively few patches would be required to reserve a high diversity of crust species.

Nomenclature: McCarthy (1991) for lichens; Streimann & Klazenga (2002) for mosses; Scott (1985) for liverworts.

Questions: Do the population dynamics of trees differ among topographic positions and, if so, how does topographic position affect the population dynamics of species that are distributed in a topography-specific manner? Which is the most important life stage in determining vegetation patterns?

Location: Primary and secondary warm temperate evergreen broad-leaved forest (40 - 280 m a.s.l.) on the western part of Yakushima Island, Japan.

Methods: Mortality, recruitment, DBH growth and distribution of stems (≥ 5 cm DBH) in a 2.62-ha plot were surveyed in 1992 and 2002 to determine the relationships between population parameters and (1) topography and (2) distribution patterns of 17 common tree species.

Results: Common species (n = 17) were classified into three distribution pattern groups: group A, distributed mainly on convex slopes; group B, on concave slopes, and group C, not aggregated with respect to topographic position. Stem mortality, recruitment and DBH growth were greater in group A than in group B within each topographic class. The hierarchy of stem mortality among topographic classes for groups A and B was convex > planar > concave. Stem recruitment density was relatively high on the convex and concave slopes, respectively, for groups A and B.

Conclusions: The topographical positions of adult trees were not always most suited for adult survival and growth. For group A, the distribution pattern of adults was determined in the juvenile stage, while this was not the case for group B. Studies of juvenile stages are important for understanding the demographic basis of vegetation distribution patterns.

Nomenclature: Mitsuta & Nagamasu (1984).

Abbreviations: AIC = Akaike's information criterion; BA = Basal area; IC = Index of Convexity; G = DBH growth; M = Stem mortality; R = Stem recruitment.

Question: Are there hot spots of algal mat deposition in space and time at the marsh scale and, if so, how does this affect the coexistence of a dominant (Spartina anglica) and gap dependent (Salicornia europaea) species?

Location: The Rattekaai salt marsh in the Scheldt estuary in the southwestern Netherlands (NW Europe).

Methods: Mat cover and the abundance of the gap dependent species Salicornia europaea were monitored at the scale of a marsh. The effects of mat cover on the vegetation structure were studied by applying three mat removal treatments over three growing seasons.

Results: The low marsh border was found to be a hot spot of algal mat deposition during the growing season, which had a correlated spatial pattern between two successive years at a 20 m × 20 m scale. The combination of duration, timing and repetition of mat cover determined growth inhibition of the competitive dominant Spartina anglica, and thereby the abundance of subordinates such as Salicornia europaea. Mat cover reduced the storage of carbon reserves in Spartina and our results imply that repetition of non-lethal mat cover can lead to ‘gap creation’. Gaps gave only temporary habitat to less dominant species since Spartina quickly re-invaded them. The gap dependent annual Salicornia was most abundant at intermediate levels of disturbance measured as a function of both space and time.

Conclusions: In addition to disturbance level, the spatial and temporal distribution of disturbance are important in creating and maintaining habitat for gap dependent species. Relatively small disturbances will have a large effect on diversity if the spatial and temporal distribution of the disturbances leads to ‘disturbance hot spots’.

Question: How may sampling time affect exhaustiveness of vegetation censuses in interaction with observer effect and quadrat species richness?

Location: French lowland forests.

Methods: Two data sets comprised of 75 timed, one-hour censuses of vascular plants carried out by five observers on 24 400-m² forest quadrats were analysed using mixed-effect models.

Results: The level of exhaustiveness increased in a semi-logarithmic way with sampling time and decreased with quadrat species richness. After one hour, 20 to 30% of the species remained undetected by single observers. This proportion varied among observers and the discrepancy increased with increasing sampling time. Fixing the sampling time may make richness estimates vary less between observers but the time limit should be at least 30 min to reduce the bias in exhaustiveness between rich and poor quadrats.

Conclusions: We advocate the use of sampling methods based on spatially or temporally-replicated censuses and statistical analyses that correct for the lack of census exhaustiveness in vegetation studies.

Nomenclature: Kerguélen (2002).

Abbreviation: AICc = consistent Akaike information criterion.

Question: What are the qualitative and quantitative long-term changes in the vascular epiphyte assemblage on a particular host tree species?

Location: Lowland rain forest of the San Lorenzo Crane Plot, Republic of Panama.

Methods: We followed the fate of the vascular epiphyte assemblage on 99 individuals of the palm Socratea exorrhiza by three censuses over the course of five years.

Results: The composition of the epiphyte assemblage changed little during the course of the study. While the similarity of epiphyte vegetation decreased on individual palms through time, the similarity analysed over all palms increased. Even well established epiphyte individuals experienced high mortality with only 46% of the originally mapped individuals surviving the following five years. We found a positive correlation between host tree size and epiphyte richness and detected higher colonization rates of epiphytes per surface area on larger trees.

Conclusions: Epiphyte assemblages on individual S. exorrhiza trees were highly dynamic while the overall composition of the epiphyte vegetation on the host tree species in the study plot was stable. We suggest that higher recruitment rates, due to localized seed dispersal by already established epiphytes, on larger palms promote the colonization of epiphytes on larger palms. Given the known growth rates and mortality rates of the host tree species, the maximum time available for colonization and reproduction of epiphytes on a given tree is estimated to be ca. 60 years. This time frame will probably be too short to allow assemblages to be ever saturated.

Nomenclature: Angiosperms: D'Arcy (1987); Ferns: Lellinger (1989).

Questions: Do species planted outdoors in native soil differ in the timing of their period of vegetative spread during the growing season?

Location: Cleish Hills, Fife, Scotland, UK.

Methods: Patches (20 cm × 20 cm) of Agrostis capillaris, Festuca rubra, Holcus lanatus, Lolium perenne and Poa trivialis were arranged in plots so that every species patch had all the remaining species as neighbours. The plots were cut every 14 days at 3 cm and photographed monthly from above. The photographs of species boundaries were traced and digi-tised. Measurements were made on the digitised images of the distances each species spread and of the area occupied by each species.

Results: Fitting some simple models to the data indicated firstly that species varied in their vegetative competitive ability, with A. capillaris and P. trivialis being the most and least competitive respectively, and secondly that the ability of a species to colonize can differ from its ability to resist colonization, the most discrepant species being H. lanatus. Our analysis also indicated statistically significant variation in the data over and above the simple models, providing evidence of additional, genuine, complexity in the seasonal patterns of spread.

Conclusion: This study provides evidence to support the hypothesis that clonal grass species do not spread vegetatively at the same times within a growing season. Since sward height is known to affect species spread, there is some opportunity for the manipulation of species composition simply through temporal control of sward height.

Nomenclature: Stace (1991).

Abbreviations: s.e.d. =standard error of difference.

Question: What are the changes in vegetation structure, soil attributes and mesofauna associated with grazing in mesic grasslands?

Location: Southern Campos of the Río de la Plata grasslands, in south-central Uruguay.

Methods: We surveyed seven continuously grazed and ungrazed paired plots. Plant and litter cover were recorded on three 5-m interception lines placed parallel to the fence in each plot. We extracted soil fauna from a 10 cm deep composite sample and analysed the oribatids. Soil attributes included bulk density, water content, organic carbon (in particulate and mineral associated organic matter) and nitrogen content and root biomass at different depths. Changes in floristic, Plant Functional Types and mesofauna composition were analysed by Nonmetric Multidimensional Scaling.

Results: Species number was lower in ungrazed than in grazed plots. Of 105 species in grazed plots only three were exotics. Shrub and litter cover were significantly higher inside the exclosures, while the cover of Cyperaceae-Juncaceae was lower. Grazing treatments differed significantly in plant and oribatid species composition. Grazing exclusion significantly reduced soil bulk density and increased soil water content. Carbon content in particulate organic matter was lower in the upper soil of ungrazed sites, but deeper in the profile, grazing exclosures had 8% more carbon in the mineral associated organic matter.

Conclusions: Our results generally agree with previous studies but deviate from the results of previous analyses in (1) the increase of shrub cover in ungrazed sites; (2) the redistribution of the soil organic carbon in the profile and (3) the low invasibility of the prairies regardless of grazing regime.

Abbreviations: BMRPP = Blocked multi-response permutation procedures; INDVAL = Indicative values; MAOM = Mineral associated organic matter; NMDS = Non-metric multi dimensional scaling; POM = Particulate organic matter.

Nomenclature: Cabrera & Zardini (1978).

Questions: Boreal forests along small streams are bryophyte diversity hotspots because they are moist, productive and relatively high pH. Do these factors also explain the large differences in species richness and species composition found among streamside sites? Do the species of species-poor sites represent nested subsets of the species of more species-rich sites? How do the results apply to conservation?

Location: Forests along small streams in mid-boreal Sweden.

Methods: Survey of the flora of liverworts and mosses and habitat properties, including calculation of a pH-index based on species indicator values, in 37 sites (1000-m² plots).

Results: The number of bryophyte species per plot ranged from 34 to 125. Neither soil moisture nor basal area of trees (a proxy for productivity) correlated significantly with species richness and composition, whereas pH-index and cover of boulders did. Species richness and composition were more strongly correlated with pH-index for mosses than for liverworts. The richness and composition of bryophyte species most frequently found on moist ground, stream channel margins and, most unexpected, woody debris were all more strongly associated with the pH-index than with other habitat properties. Although species composition was significantly nested, there was still some turnover of species along the first ordination axis.

Conclusions: To attain high numbers of species, streamside forests need to have boulders and at least pockets with higher soil and stream-water pH. The number of Red list species was weakly correlated with total species richness and the most species-rich sites contained many species found more in non-forest habitats. Hence, bryophyte conservation in streamside forests should not focus on species-rich sites but on the quality and quantity of substrate available for assemblages of forest species that are strongly disfavoured by forestry.

Abbreviation: LOWESS = Locally Weighted Regression Scatterplot Smoothing.

Nomenclature: Söderström & Hedenäs (1998); Söderström et al. (2000).

Questions: The Cross Timbers are a mosaic of savannas, grasslands and upland forests, occupying a significant portion of south-central North America. Our questions here were (1) how does a severe tornado affect the two most dominant tree species of the area Quercus marilandica and Q. stellata with respect to damage and mortality; (2) how do such patterns vary as a function of tree size? What are the implications of disturbance for codominance in species-poor systems?

Location: The Cross Timbers in Oklahoma, USA.

Methods: We established a 14.48-ha permanent plot following a severe tornado in 2003. We identified, numbered and tagged each tree and recorded its diameter at breast height (DBH), spatial coordinates, status (dead or alive), and damage type. We examined (1) relative abundance before and after the tornado; (2) differences in damage and mortality, and (3) the influence of tree diameter on the probability of damage and mortality for each species.

Results: Differences in species identity and tree characteristics were significantly related to tree mortality following the tornado, after accounting for spatial locations. The odds of mortality were 12.0 times greater for Q. marilandica than for Q. stellata. Such greater vulnerability of Q. marilandica versus Q. stellata was also reflected in changes in density and basal area. Tree diameter clearly influenced the damage and mortality pattern in Q. stellata; larger trees sustained more damage and mortality. However, Q. marilandica did not exhibit size-dependent mortality.

Conclusion: The tornado affected the two dominant species differently. The intra- and inter-specific differences in wind-storm susceptibility may allow coexistence of the two species and are potentially important in the dynamics of the Cross Timbers. Species more damaged might finally benefit from the wind disturbance due to their resprouting ability.

Nomenclature: Kartesz (1999).

Abbreviations: TGPP = Tallgrass Prairie Preserve.

Questions: 1. Do harvester ants (Messor barbarus) promote seed mortality in Mediterranean grassland?; 2. Is this effect greater in large-seeded species?

Location: Central Spain.

Methods: We established an ant-exclusion experiment of five circular (1.5 m diameter) plots from where ants were excluded during one year, along with ten control plots. We recorded the seed bank of all species in the plots both before and after the treatment. The effect of seed length and weight was analysed after transforming data into phylogenetically independent contrasts, and alternatively by dividing the species data set into morphological groups.

Results: Longer and heavier seeded species significantly increased in the seed banks under the exclusion treatment, although ants did not significantly modify overall seed densities.

Conclusions: Although the ants do not collect large numbers of seeds, they differentially affect the composition of the seed banks by selecting the longest or heaviest seeds, or both. The persistence of this short-term effect in the seed bank may result, over a number of years, in the system evolving towards a predominance of small-seeded annuals, congruent with the species composition actually observed in Mediterranean grasslands.

Abbreviations: PIC = Phylogenetically independent contrast.

Nomenclature: Castroviejo (1986–2005), except in the taxa yet to be covered which are named following Tutin et al. (1964–1980).

Question: What are the importance of elevation and fire in maintaining ecotones of Florida scrub assemblages along a gradual topographic gradient?

Location: Archbold Biological Station (ABS), 12 km south of Lake Placid, Florida, USA.

Methods: Vegetation cover of upland Florida shrublands was quantified using the line-intercept method along 20 transects traversing similar elevation gradients, stratified by time since fire (TSF). We objectively identified shrubland ecotones using a split moving windows boundary analysis (SMW) with three different window widths. Non-metric multidimensional scaling ordination was used to determine relationships among plant assemblages defined by SMW.

Results: We located up to four ecotones per transect, the majority of which were wide, highly heterogeneous zones. Relative elevation controlled the distribution of plant assemblages in upland Florida shrublands. Ecotones in shrublands > 30 years TSF had relatively low dissimilarity values in SMW, indicating that previously discrete plant assemblages with longer TSF were becoming more similar with time.

Conclusions: Split Moving Windows (SMW) analysis identified ecotones relatively well although patches generated by oak clonal growth were sometimes identified as ecotones. Fire suppression caused ecotones to become more diffuse, suggesting that without fire at least every 30 years, discrete plant assemblages within upland Florida shrublands will be more continuous.

Abbreviations: ABS = Archbold Biological Station; SMW = Split moving window; TSF =Time since fire; NMDS = Non-metric multi-dimensional scaling; OSF = Open Scrubby Flatwoods; RS = Rosemary Scrub; SF = Scrubby Flatwoods; F = Flatwoods; SCB = Squared city block dissimilarities.

Nomenclature: Wunderlin (1998); Abrahamson et al. (1984).

Questions: How does tree growth in a tropical woodland savanna vary as a function of size, and how is it affected by competition from neighbours, site attributes, and damage caused by disturbance?

Location: western Zimbabwe.

Methods: Trees of common species were tagged, mapped, and measured annually between 2001 and 2003 in a Kalahari sand woodland savanna. Diameter increments were analysed with mixed model regressions for the largest ramet in each genet. Stem diameter and damage, soil texture, and indices of competition at multiple spatial scales were used as covariates.

Results: Stem diameter increased initially and then declined as a function of size in undamaged trees, which grew faster than damaged trees. Growth in damaged trees declined with size. No site differences were detected, and there was evidence for between-tree competition on growth only in the fastest-growing species, Brachystegia spiciformis. In several species the growth rate of the largest ramet increased as a function of the basal area of secondary ramets, contrary to expectations. For many species, the growth models showed poor explanatory power.

Conclusions: Growth in Kalahari sand savanna trees varies as a function of size and changes in tree architecture caused by disturbance agents such as fire, frost, and elephant browsing. Disturbance may thus play an important role on vegetation dynamics through its effects on growth in the post-disturbance phase. Growth is highly stochastic for some species in this system, and more deterministic in others. It is hypothesized that this dichotomy may be driven by differences in rooting depth among species.

Nomenclature: Coates Palgrave (1977).

Abbreviations: LRT = Likelihood ratio test; PC1 = Axis 1 of principal components analysis.

Questions: In the absence of herbivores, what is the importance of shrub facilitation for the colonization of grasslands by Fagus sylvatica and Quercus pubescens? Is there an indirect facilitative effect of shrubs on tree seedlings by limiting herb competition?

Location: Causse du Larzac, southern Massif Central, France.

Methods: We conducted field experiments on the facilitative role of shrubs in seedling emergence and survival, in relation to potential negative effects of shading and the presence of herbs. The effects of shade and herbs on allocation and root morphology were analysed in a pot experiment.

Results: According to these experiments, the establishment of both tree species was facilitated by shrubs, but the two species differed in the processes underlying this facilitation. Shade directly facilitates the emergence rate of both species. Shade also indirectly facilitates Fagus survival by limiting herb competition. No indirect facilitation of Quercus survival was detected. These differences reflect variation in the tolerance of herb competition by seedlings of the two species. The tolerance of herb competition by Quercus seedlings allows regeneration over a wide area under each shrub and some regeneration events in grasslands at low grazing intensity. In contrast, for Fagus, only a narrow area under each shrub is suitable; regeneration is zero in grassland. The high tolerance of herb competition by Quercus seedlings may result from the avoidance of root competition.

Conclusions: In grasslands with severe drought stress, and almost obligatory shrub facilitation for tree seedling regeneration, the tolerance of herb competition may alter the precise role of facilitation in the colonization process.

Abbreviations: LMF = Leaf mass fraction; PAR = Photo-synthetically active radiation; R = Germination rate; RCI = Relative competition intensity; RLC = Relative length of coarse roots; RMF = Root mass fraction; SLA = Specific leaf area; SMF = Stem mass fraction; SRL = Total specific root length; SWC = Soil water content.

Nomenclature: Tutin et al. (1964–1993).

Question: How do patterns in colonization and patch expansion of an invasive woody plant (Larrea tridentata, Zygophyllaceae) differ between two grassland ecosystems at a biome transition zone?

Location: Semi-arid/arid transition zone in central New Mexico.

Methods: Frequency of occurrence, height, and surface area of saplings (n = 134) and patches of adult plants (n = 247) of the invasive shrub, L. tridentata, were measured within a mosaic of ecosystems dominated either by the Chihuahuan Desert species, Bouteloua eriopoda (Poaceae), or the shortgrass steppe species, B. gracilis, located within 1 km of the L. tridentata-dominated ecosystem. Distances between L. tridentata patches and patch area were used to estimate connectivity as a measure of propagule pressure. Sapling age (estimated from height using previously established relationships) and distance to the L. tridentata-dominated ecosystem was used to evaluate patterns in dispersal. Cover by species or functional group inside each L. tridentata patch was compared with surrounding vegetation to estimate changes in species composition with patch expansion.

Results: L. tridentata saplings (< 1%) and adult patches (15%) occurred less frequently in B. gracilis-dominated ecosystems than expected based on areal extent of this ecosystem type. Propagule pressure did not differ with distance from the core ecosystem dominated by L. tridentata. Evidence for both local and long-distance dispersal events was found. Similar relationships between number of plants and patch area in both grassland types indicate similar patterns in patch expansion. Cover of perennial forbs was higher and cover of dominant grasses was lower in L. tridentata patches compared with the surrounding vegetation for both ecosystem types.

Conclusions: Spatial variation in L. tridentata saplings and patches at this biome transition zone is related to the different susceptibilities to invasion by two grassland ecosystems. The persistence of grasslands at this site despite region-wide expansion by L. tridentata may be related to the spatial distribution of B. gracilis-dominated ecosystems that resist or deter invasion by this woody plant.

Questions: Do plant characteristics predict microsite colonization in severe habitats dominated by abiotic factors? Specifically, does colonization of microsites differ among shrubs, forbs and grasses or between wind- and water-dispersed plants, non-native and native plants, or N-fixing and non-N-fixing plants?

Location: Kowhai River floodplain, Kaikoura, South Island, New Zealand.

Methods: Five microsite characteristics were measured for > 1000 individuals representing 27 colonizing plant species on a two-year old surface of a primary succession on a New Zea-land floodplain. The microsite characteristics included surface contour (convex, concave, or flat), the position of the plant (e.g., upstream, downstream) relative to the closest rock with > 20 cm maximum dimension, the distance to that same rock, the depth of the base of the stem below the surface of a plane resting on the adjacent microrelief, and soil particle size (gravel, pebbles or sand).

Results: All plants preferred concave microsites near large rocks relative to systematically placed null points. We found no clear preferences for microhabitats by dispersal mode, native vs. non-native status, or plants with or without nitrogen-fixing symbionts, but grasses preferentially colonized fine soil particles. Highly variable responses among species contributed to these results. Better predictability of microsite preference was obtained for individual species than for plants grouped by characteristics.

Conclusions: Our results suggest that in severe habitats with strong abiotic filters and low microsite availability, such as found in early primary succession, coarse categories of species characteristics are poor predictors of colonization success.

Nomenclature: Allan (1961) with amendments suggested by Connor & Edgar (1987); Webb et al. (1988); Edgar & Connor (2000).