Question: Are there changes in dispersal patterns in floodplain pioneer vegetation with effects on seedling number, species richness and species composition along a gradient of declining river-floodplain connectivity?

Location: Middle Elbe river floodplain, Germany.

Methods: An experiment with five treatments was set up along a gradient of declining river-floodplain connectivity, partitioning seedlings into three groups: (1) emerging solely from water dispersed seeds, (2) from wind/animal dispersed seeds and (3) from the soil diaspore bank. Two controls were established: without any manipulation and exclusion of all seeds. The results were compared with those of vegetation and soil sampling to evaluate the representativeness of the experimental sites in terms of species composition, diversity, seedling number and soil parameters.

Results: Water dispersal and the soil diaspore bank were the major dispersal strategies shaping floodplain pioneer vegetation at the Middle Elbe river. The number of seedlings, species rich-ness and the variation in species composition in these habitats depend on the degree of connectivity. The seedling number and species richness is highest in sites of permanent or almost permanent exchange with the main channel, where water dispersal additionally contributes to the number of seedlings grown from the soil seed bank.

Conclusion: The results underline the importance of river-floodplain ecotones as sink habitats for water-dispersed seeds. Considering the strongly reduced river-floodplain interactions due to dykes and other engineering structures, management strategies are necessary to improve connectivity and the renewal of fluvial land forms.

Nomenclature: Wisskirchen & Haeupler (1998).

Question: Which fraction of the decrease in species richness under fertilization can be explained by assemblage-level thinning?

Location: An alpine meadow on the eastern Tibet plateau.

Methods: 60-m² plots were randomly assigned to a control or one of four levels of ammonium phosphate fertilizer. Treatments were repeated for three years. The effect of assemblage-level thinning was decided based on similarity in quadrats within and between fertilizing levels, bootstrap simulation based on random thinning of the high density (low production, low fertility) quadrats and correlation of species' biomass in low fertility and high fertility.

Results: Fertilization increased production, reduced species richness and reduced density of individuals. Heavily fertilized quadrats are more similar in species composition in 2000 but less similar in 2001 and 2002. Rarefaction showed that a decrease in density can account for 32.3–42.9% decrease of species richness, but the simulated species richness is always significantly higher than the observed one. When production and species richness are similar at two levels of fertilization, species biomass in the higher fertility treatment is positively correlated with biomass at lower fertility. When the two fertilizer levels differed in production and species richness, there was no evidence of correlation in species biomass, suggesting that assemblage level thinning cannot explain all the loss of species.

Conclusion: Although a decrease in density could explain much of the decrease (up to 42.9%) in species richness when this alpine meadow was fertilized, other important mechanisms such as interspecific competition cannot be ignored. Future studies should investigate the effect of assemblage level thinning on species diversity, and search for mechanisms responsible for a decrease in diversity.

Abbreviations: ALT = Assemblage level thinning; GLM = General linear model; ISC = Inter-specific competition.

Nomenclature: Anon. (1998).

Question: How does groundwater input affect plant distribution in Alnus glutinosa (black alder) carrs?

Location: Alder carrs along the river Meuse, SE Netherlands.

Methods: Three types of site, characterized by groundwater flow, were sampled in 17 A. glutinosa carrs. Vegetation and abiotic data (soil and water chemistry) were collected and analysed using a Canonical Correspondence Analysis. Based on the results, a laboratory experiment tested the effect of groundwater input (Ca² ) on pore water chemistry (NH₄ availability).

Results: Environmental factors indicating groundwater input (Ca² and Fe² ), correlating with the NH₄ concentration in the pore water, best explained the variation in plant distribution. NH₄ availability was determined by Ca² input via the groundwater and subsequent competition for exchange sites in the sediment. As a result, nutrient-poor seepage locations fully fed by groundwater were dominated by small iron resistant plants such as Caltha palustris and Equisetum fluviatile. More nutrient-rich locations, fed by a combination of groundwater and surface water, allowed the growth of taller iron resistant plant species such as Carex paniculata. Nutrient-rich locations with stagnating surface water were hardly fed by groundwater, allowing the occurrence of fast growing and less iron tolerant wetland grasses such as Glyceria fluitans and G. maxima.

Conclusion: Groundwater input affects plant composition in A. glutinosa carrs along the river Meuse by determining nutrient availability (ammonium) and concentrations of toxic iron.

Abbreviation: GLM = Generalized linear model.

Question: What is the value of using Rhinanthus minor in grassland restoration and can restrictions on its establishment be overcome?

Location: England (United Kingdom).

Methods: Two experiments were established to determine the efficacy of inoculating R. minor on a suite of four agriculturally improved grasslands and the efficacy of using R. minor in grassland restoration. In Experiment 1, the effect of herbicide gap creation on the establishment and persistence of R. minor in grasslands ranging in productivity was investigated with respect to sward management. In Exp. 2, R. minor was sown at 1000 seeds/m² in conjunction with a standard meadow mix over a randomized plot design into Lolium perenne grassland of moderate productivity. The treatment of scarification was investigated as a treatment to promote R. minor.

Results: Gap size had a significant role in the establishment and performance of R. minor, especially the 30 cm diameter gaps (Exp. 1). However, R. minor failed to establish long-term persistent populations in all of the agriculturally improved grasslands. In Exp. 2, establishment of R. minor was increased by scarification and its presence was associated with a significant increase in Shannon diversity and the number of sown and unsown species. Values of grass above-ground biomass were significantly lower in plots sown with R. minor, but values of total above-ground biomass (including R. minor) and forb biomass (not including R. minor) were not affected.

Conclusions: The value of introducing R. minor into species-poor grassland to increase diversity has been demonstrated, but successful establishment was dependent on grassland type. The scope for using R. minor in grassland restoration schemes is therefore conditional, although establishment can be enhanced through disturbance such as sward scarification.

Nomenclature: Stace (1997).

Questions: What are the effects of fire on the relationships between community composition and environment? Does fire affect β-diversity and rates of species turnover along spatial and environmental gradients? Do these effects differ among functional groups?

Location: Linville Gorge, in the southern Appalachians of western North Carolina, USA.

Methods: We sampled all vascular plants in 20 plots (200 to 400 m²) before and after a wildfire. We used Non-metric Multidimensional Scaling to compare pre- and post-fire relationships between community composition and elevation, fire history, and topographic moisture. We compared Whittaker's β_w and quantified rates of species turnover along geographic and environmental gradients for all plants, trees, shrubs, and herbs. Randomization tests were used to determine whether rates of species turnover changed after the fire.

Results: Correlations between community composition and environmental variables generally increased after fire. The largest increase was for elevation, reflecting its potential to influence fire severity. β_w decreased only slightly and the only statistically significant change in species turnover rates occurred for herbs over geographic distance.

Conclusions: Large changes in community composition occurred after fire due to immigration of herbaceous species. Community-environment correlations increased but β-diversity did not change because fire effects and immigration were influenced by the same environmental gradients that structured vegetation pre-fire. Results differed for herbaceous species that responded to environmental but not spatial gradients post-fire. Dispersal limitation and community resilience through resprouting or regeneration from serotinous cones in dominant woody vegetation maintain β-diversity and species turnover after fire.

Nomenclature: Kartesz (1994).

Questions: 1. Can the importance and the intensity of competition vary independently along a nutrient gradient? 2. Are these variations species dependent?

Location: Sub-alpine pastures of the northern French Alps.

Methods: Competition intensity measures how much competition decreases the performances of an organism. Competition importance measures how much competition contributes to affect performance, among other processes (such as environmental stress or disturbance). Competition intensity and importance were measured on three co-occurring species: Festuca rubra, a perennial grass, and two forbs of contrasting basal area, Chaerophyllum hirsutum and Alchemilla xanthochlora. A neighbour removal experiment was performed on Festuca rubra in three sub-alpine grassland communities differing in fertility and on Chaerophyllum hirsutum and Alchemilla xanthochlora in the two more fertile of these communities. The importance of competition was quantified using an index proposed by Brooker et al. (2005).

Results: Competition intensity and importance showed different patterns of variation along the fertility gradient for Festuca rubra: competition importance decreased with decreasing fertility whereas competition intensity did not change. The largest forb was the least affected by competition. Our results suggest that the importance of competition for all three species depended on their individual tolerance to low nutrient availability.

Conclusions: 1. The distinction between the importance and the intensity of competition is helpful to explain conflicting results obtained on the variations of competition indices along productivity gradients. 2. The choice of a phytometer can affect the conclusions drawn from empirical studies.

Abbreviations: C_imp = Competition importance; C_int = Competition intensity; DM = Dry matter production; NI = Nitrogen nutrition index; RNE = Relative neighbour effect; SLA = Specific leaf area.

Questions: Do ordination patterns differ when based on vegetation samples recorded in plots of different size? If so, how large is the effect of plot size relative to the effects of data set heterogeneity and of using presence/absence or cover-abundance data? Can we combine plots of different size in a single ordination?

Methods: Two homogeneous and two heterogeneous data sets were sampled in Czech forests and grasslands. Cover-abundances of plant species were recorded in series of five or six nested quadrats of increasing size (forest 49-961 m²; grassland 1-49 m²). Separate ordinations were computed for plots of each size for each data set, using either species presences/absences or cover-abundances recorded on an ordinal scale. Ordination patterns were compared with Procrustean analysis. Also, ordinations of data sets jointly containing plots of different size were calculated; effects of plot size were evaluated using a Monte Carlo test in constrained ordination.

Results: The results were consistent between forest and grassland data sets. In homogeneous data sets, the effect of presence/absence vs. cover-abundance was similar to, or larger than, the effect of plot size; for presence/absence data the differences between ordinations of differently sized plots were smaller than for cover-abundance data. In heterogeneous data sets, the effect of plot size was larger than the effect of presence-absence vs. cover-abundance. The plots of smaller size (≤ 100 m² in forests, ≤ 4 m² in grasslands) yielded the most deviating ordination patterns. Joint ordinations of differently sized plots mostly did not yield patterns that would be artifacts of different plot size, except for plots from the homogeneous data sets that differed in size by a factor of four or higher.

Conclusions: Variation in plot size does influence ordination patterns. Smaller plots tend to produce less stable ordination patterns, especially in data sets with low β-diversity and species cover-abundances. Data sets containing samples from plots of different sizes can be used for ordination if they represent vegetation with large β-diversity. However, if data sets are homogeneous, i.e. with low β-diversity, the differences in plot sizes should not be very large, in order to avoid the danger of plot size differences distorting the real vegetation differentiation in ordination patterns.

Question: Is there a relationship between size and death in the long-lived, deep-rooted tree, Acacia erioloba, in a semi-arid savanna? What is the size-class distribution of A. erioloba mortality? Does the mortality distribution differ from total tree size distribution? Does A. erioloba mortality distribution match the mortality distributions recorded thus far in other environments?

Location: Dronfield Ranch, near Kimberley, Kalahari, South Africa.

Methods: A combination of aerial photographs and a satellite image covering 61 year was used to provide long-term spatial data on mortality. We used aerial photographs of the study area from 1940, 1964, 1984, 1993 and a satellite image from 2001 to follow three plots covering 510 ha. We were able to identify and individually follow ca. 3000 individual trees from 1940 till 2001.

Results: The total number of trees increased over time. No relationship between total number of trees and mean tree size was detected. There were no trends over time in total number of deaths per plot or in size distributions of dead trees. Kolmogorov-Smirnov tests showed no differences in size class distributions for living trees through time. The size distribution of dead trees was significantly different from the size distribution of all trees present on the plots. Overall, the number of dead trees was low in small size classes, reached a peak value when canopy area was 20 - 30 m², and declined in larger size-classes. Mortality as a ratio of dead vs. total trees peaked at intermediate canopy sizes too.

Conclusion: A. erioloba mortality was size-dependent, peaking at intermediate sizes. The mortality distribution differs from all other tree mortality distributions recorded thus far. We suggest that a possible mechanism for this unusual mortality distribution is intraspecific competition for water in this semi-arid environment.

Nomenclature: Barnes et al. (1997).

Question: What is the effect of gap size on the seedling emergence, growth and survival of four common tree species in wooded pastures?

Location: A pasture in the Jura mountains, Switzerland.

Methods: Seeds were sown in a complete three-way factorial design with eight blocks in May 2003. Each block consisted of a competition treatment (four gap sizes including zero) and a mowing treatment (mown and unmown). Emergence, survival and total biomass of tree seedlings of three species (Picea abies, Acer pseudoplatanus and Fagus sylvatica) were measured. A fourth species (Abies alba) failed to germinate.

Results: Gaps had a positive influence on the early stages of tree development for all species. Larger gaps favoured growth and survival more than small gaps. Seedling growth was higher when vegetation around the openings was mown. Mowing the vegetation at gap size zero enhanced both growth and survival compared to unmown vegetation. Mown gaps larger than zero had increased seedling desiccation but decreased seedling predation. Species showed similar trends in their emergence and growth responses to gap size and mowing treatments but for Picea emergence rate was higher and survivorship was lower than for Acer and Fagus.

Conclusions: Gap size does matter for tree seedling success but even in more favourable large gaps only a small percentage of seedlings emerged and survived. The effects of gaps on tree seedling establishment are complex as a result of interactions between biotic and abiotic changes caused by gaps.

Nomenclature: Lauber & Wagner (2000).

Abbreviations: LG = Large gap; LME = Linear mixed-effects model; MG = Middle gap; NG = No gap; REML = Restricted maximum likelihood estimation; SG = Small gap.

Question: Is a mosaic structure apparent in the spatial distribution of trees in old-growth Abies amabilis forests?

Location: Montane forests of the western Cascade Range, Washington, USA.

Methods: Maps of tree locations were created for study areas located in two, 300-year old stands and a single 600-year old stand. Stand structure parameters were calculated using several subsample quadrats sizes (56.25 - 306.25 m²), which were drawn randomly with replacement at a density of 250 quadrats per ha from the stem maps in the computing environment. Spatial cross-covariance functions between different canopy strata were estimated using the spline cross-correlogram.

Results: Negative spatial correlation (segregation) between subcanopy tree density and areas of high overstorey occupancy was detected. Understorey and midstorey tree densities were positively spatially correlated. These general trends were apparent across the range of observational scales investigated. Significant spatial correlation between canopy strata was observed at spatial scales of 12 - 44 m and extended to the largest scales in the 600-year old stand.

Conclusion: The observed spatial segregation between canopy strata supports the hypothesis that old A. amabilis forests form fine-scale structural mosaics. Structural segregation at small scales may be due to competitive interactions as well as exogenous forcing of tree locations (e.g. by mortality due to pathogens or disturbance), however segregation at large scales in the 600-year old stand is likely due to exogenous factors alone. This study reinforces the idea that horizontal heterogeneity is an emergent property of old-growth forests.

Nomenclature: Hitchcock & Cronquist (1973).

Questions: 1. What are the distribution and habitat associations of non-native (neophyte) species in riparian zones? 2. Are there significant differences, in terms of plant species diversity, composition, habitat condition and species attributes, between plant communities where non-natives are present or abundant and those where non-natives are absent or infrequent? 3. Are the observed differences generic to non-natives or do individual non-native species differ in their vegetation associations?

Location: West Midlands Conurbation (WMC), UK.

Methods: 56 sites were located randomly on four rivers across the WMC. Ten 2 m × 2 m quadrats were placed within 15 m of the river to sample vegetation within the floodplain at each site. All vascular plants were recorded along with site information such as surrounding land use and habitat types.

Results: Non-native species were found in many vegetation types and on all rivers in the WMC. There were higher numbers of non-natives on more degraded, human-modified rivers. More non-native species were found in woodland, scrub and tall herb habitats than in grasslands. We distinguish two types of communities with non-natives. In communities colonized following disturbance, in comparison to quadrats containing no non-native species, those with non-natives had higher species diversity and more forbs, annuals and short-lived monocarpic perennials. Native species in quadrats containing non-natives were characteristic of conditions of higher fertility and pH, had a larger specific leaf area and were less stress tolerant or competitive. In later successional communities dominated by particular non-natives, native diversity declined with increasing cover of non-natives. Associated native species were characteristic of low light conditions.

Conclusions: Communities containing non-natives can be associated with particular types of native species. Extrinsic factors (disturbance, eutrophication) affected both native and non-native species. In disturbed riparian habitats the key determinant of diversity is dominance by competitive invasive species regardless of their native or non-native origin.

Nomenclature: Stace (1991).

Questions: Which factors best predict the probability of elephant and frost damage in Kalahari sand woodland savanna? What is the association between tree mortality and the disturbance regime?

Location: Western Zimbabwe.

Methods: Elephant and frost damage, topkill, and whole-plant mortality were quantified in ten common tree species in a Kalahari sand savanna in Zimbabwe. Individual trees were tagged in 20 plots and monitored over a two-year period. A model selection approach was used to test the association between the probability of damage and size, prior damage, and neighbourhood effects, and to investigate the effect of damage on mortality.

Results: Elephant damage differed strongly among species, and was not influenced by neighbourhood effects or prior disturbance. Frost damage also varied across species, and declined as a function of stem size and neighbourhood tree cover, and – against expectations – prior disturbance. Topkill increased as a function of elephant and frost damage, but was lower in previously damaged than in undamaged trees.

Conclusions: Frost and elephant damage are influenced by community composition, and frost damage is also correlated with community structure and prior disturbance. Frost is an important and generally overlooked disturbance agent in southern African woodlands, where it may play a key role in association with other disturbance factors – such as elephant herbivory – that reduce woodland canopy cover.

Nomenclature: Coates Palgrave (1977).

Question: How is tundra vegetation related to climatic, soil chemical, geological variables and grazing across a very large section of the Eurasian arctic area? We were particularly interested in broad-scale vegetation-environment relationships and how well do the patterns conform to climate-vegetation schemes.

Material and Methods: We sampled vegetation in 1132 plots from 16 sites from different parts of the Eurasian tundra. Clustering and ordination techniques were used for analysing compositional patterns. Vegetation-environment relationships were analysed by fitting of environmental vectors and smooth surfaces onto non-metric multidimensional scaling scattergrams.

Results: Dominant vegetation differentiation was associated with a complex set of environmental variables. A general trend differentiated cold and continental areas from relatively warm and weakly continental areas, and several soil chemical and physical variables were associated with this broad-scaled differentiation. Especially soil chemical variables related to soil acidity (pH, Ca) showed linear relationships with the dominant vegetation gradient. This was closely related to increasing cryoperturbation, decreasing precipitation and cooler conditions. Remarkable differences among relatively adjacent sites suggest that local factors such as geological properties and lemming grazing may strongly drive vegetation differentiation.

Conclusions: Vegetation differentiation in tundra areas conforms to a major ecocline underlain by a complex set of environmental gradients, where precipitation, thermal conditions and soil chemical and physical processes are coupled. However, local factors such as bedrock conditions and lemming grazing may cause marked deviations from the general climate-vegetation models. Overall, soil chemical factors (pH, Ca) turned out to have linear relationship with the broad-scale differentiation of arctic vegetation.

Nomenclature: Hultén & Fries (1986); Ignatov & Afonina (1992); Konstantinova et al. (1992); Vitikainen et al. (1997).

Abbreviation: NMDS = Non-metric Multidimensional Scaling.

Question: How does geothermal activity influence terrestrial plant colonization, species composition and community development in the Antarctic?

Location: South Sandwich Islands, maritime Antarctic.

Methods: Bryophytes were documented during a biological survey of the archipelago in January and February 1997. Particular attention was given to sites under current or recent influence of geothermal activity. Temperature profiles obtained across defined areas of activity on several islands were linked with the presence of specific bryophytes.

Results: Greatest bryophyte richness was associated with geothermally influenced ground. Of 35 moss and nine liverwort species recorded, only four mosses were never associated with heated ground, while eight of the liverworts and 50% of the mosses were found only on actively or recently heated ground. Some species occur in unheated sites elsewhere in the maritime Antarctic, but were absent from such habitats on the South Sandwich Islands. Several species occurred in distinct zones around fumaroles. Maximum temperatures recorded within the upper 0.5 cm of the vegetation surface were 40 - 47 °C, with only Campylopus introflexus tolerating such temperatures. Maximum temperatures 2.5 or 5 cm below the vegetation surface of this moss reached 75 °C. Other bryophytes regularly present in zoned vegetation included the mosses Dicranella hookeri, Sanionia georgicouncinata, Pohlia nutans and Notoligotrichum trichodon, and the liverworts Cryptochila grandiflora and Marchantia berteroana. Surface temperatures of 25 - 35 °C and subsurface temperatures of 50 - 60 °C were recorded in these species.

Conclusions: These exceptional plant communities illustrate the transport of viable propagules into the Antarctic. Individually ephemeral in nature, the longer term existence of geothermal habitats on islands along the Scotia Arc may have provided refugia during periods of glacial expansion, facilitating subsequent recolonization of Antarctic terrestrial habitats.

Nomenclature: Ochyra et al. (in press) for mosses, Bednarek-Ochyra et al. (2000) for liverworts, Øvstedal & Lewis Smith (2001) for lichens.

The experimental study of the relationship between bio-diversity and ecosystem function has mainly addressed the effect of species and number of functional groups. In theory, this approach has mainly focused on how extinction affects function, whereas dispersal limitation of ecosystem function has been rarely discussed. A handful of seed introduction experiments, as well as numerous observations of the effects of long-distance dispersal of alien species, indicate that ecosystem function may be strongly determined by dispersal limitation at the local, regional and/or global scales.

We suggest that it is time to replace biodiversity manipulation experiments, based on random draw of species, with those addressing realistic scenarios of either extinction or dispersal. Experiments disentangling the dispersal limitation of ecosystem function should have to take into account the probability of arrival. The latter is defined as the probability that a propagule of a particular species will arrive at a particular community. Arrival probability depends on the dispersal ability and the number of propagules of a species, the distance a species needs to travel, and the permeability of the matrix landscape. Current databases, in particular those in northwestern and central Europe now enable robust estimation of arrival probability in plant communities.

We suggest a general hypothesis claiming that dispersal limitation according to arrival probability will have ecosystem-level effects different from those arising due to random arrival. This hypothesis may be rendered more region-, landscape- or ecosystem-specific by estimating arrival probabilities for different background conditions.