Questions: 1. Do different management types (i.e. hay meadow, silage meadow, meadow-pasture, pasture) have different impact on the size and composition of the seed bank of mesic grassland (Arrhenatheretalia)? 2. How strong is the effect of management on the seed bank in relation to above-ground vegetation, edaphic factors and land-use history? 3. Are there differences in C-S-R plant strategy types and seed longevity under different management regimes?

Location: Lahn-Dill Highlands in central-western Germany.

Methods: Above-ground vegetation and the soil seed bank of 63 plots (at 21 sites) in mesic grasslands were studied. Differences between management types in quantitative seed bank traits and functional characteristics were tested by ANOVA. The impact of management, above-ground vegetation, site conditions and land-use history on seed bank composition were analysed by partial CCA.

Results: Management had no significant impact on species richness and density of the seed bank but significantly influenced their floristic composition and functional characteristics. CCA revealed that even after adjustment for soil chemical parameters and above-ground vegetation management still had significant impact on seed bank composition. ANOVA revealed that silage meadows contained higher proportions of R-strategy compared to hay meadows. In contrast, in hay meadows and meadow-pastures proportions of S-strategy were higher than in silage meadows.

Conclusions: The type of grassland management has little impact on quantitative seed bank traits. Management types with a high degree of disturbance lead to an increase of species following a ruderal strategy in the seed bank. Irrespective of management type only a limited proportion of characteristic grassland species is likely to re-establish from the seed bank after disappearance from above-ground vegetation.

Nomenclature: Wisskirchen & Haeupler (1998).

Questions: 1. How does the composition and diversity of established seedlings compare to that of larger size classes in tropical forests? 2. How do species abundances in the seedling layer vary with adult abundance and life history strategies?

Location: Barro Colorado Island (BCI), Panama.

Methods: We inventoried woody seedlings ≥ 20 cm tall and < 1 cm DBH in ca. 20 000 1-m² quadrats within the BCI 50-ha Forest Dynamics Plot, where all trees and shrubs ≥ 1 cm DBH have been identified. We compared diversity and composition of seedlings to that of larger size classes and tested whether adult abundance, growth form, and shade tolerance contributed to variation in seedling abundance among species.

Results: We encountered 60056 seedlings of 332 tree, shrub, and liana species. Diversity of tree seedlings was lower than that of trees ≥ 1 cm DBH. Species abundances in the seedling layer increased non-linearly with reproductive adult abundance, such that per capita seedling abundance declined with adult abundance. Per capita seedling abundance was highest for canopy tree species and lowest for understorey trees. For canopy trees, shade-tolerant species had significantly higher per capita seedling abundance than more light-demanding species.

Conclusions: The woody seedling layer on BCI is composed of a subset of the species present in larger size classes. Most species were present in less than 1% of seedling plots, suggesting strong recruitment limitation. Tree species abundance in the seedling layer is largely a function of reproductive adult abundance, but is also influenced by life history strategies and compensatory processes.

Nomenclature: Correa et al. (2004).

Question: Herbivores can play a fundamental role in regulating the composition and structure of terrestrial plant communities. Relatively inconspicuous but nevertheless ubiquitous gastropods and small mammals are usually considered to influence grassland communities through distinct modes. 1. Do terrestrial gastropods and small mammals, either alone or in combination, influence plant community composition of an intact annual grassland? 2. Do these herbivores influence the plant size structure of the dominant grass Avena?

Location: Jasper Ridge Biological Preserve (37°24′ N, 122°13′ W, elevation 150 m) in northern California.

Methods: Animal exclosures were used to examine the single and combined influences of these herbivores on annual grassland production, community composition, and plant size structure during the growing season of an intact annual grassland.

Results: The removal and exclusion of the herbivores increased the prevalence of grasses relative to legumes and non-legume forbs; increased total production of above-ground plant biomass; and increased mean plant size and exacerbated size hierarchies in populations of Avena. The effect of both gastropods and small mammals, alone and in combination, was characterized by temporal oscillations in the relative dominance of grasses in plots with vs. without herbivores.

Conclusions: Both groups of herbivores are important controllers of California annual grassland that exert similar influences on production and composition. While other factors appear to determine the absolute number of individuals in this plant community, selective consumption of grasses by gastropods and small mammals partially offsets the competitive advantages associated with their early germination.

Question: Is soil hydrology an important niche-based driver of biodiversity in tropical forests? More specifically, we asked whether seasonal dynamics in soil water regime contributed to vegetation partitioning into distinct forest types.

Location: Tropical rain forest in northwestern Mato Grosso, Brazil.

Methods: We investigated the distribution of trees and lianas ≥ 1 cm DBH in ten transects that crossed distinct hydrological transitions. Soil water content and depth to water table were measured regularly over a 13-month period.

Results: A detrended correspondence analysis (DCA) of 20 dominant species and structural attributes in 10 × 10 m subplots segregated three major forest types: (1) high-statured upland forest with intermediate stem density, (2) medium-statured forest dominated by palms, and (3) low-statured campinarana forest with high stem density. During the rainy season and transition into the dry season, distinct characteristics of the soil water regime (i.e. hydro-indicators) were closely associated with each vegetation community. Stand structural attributes and hydro-indicators were statistically different among forest types.

Conclusions: Some upland species appeared intolerant of anaerobic conditions as they were not present in palm and campinarana sites, which experienced prolonged periods of saturation at the soil surface. A shallow impermeable layer restricted rooting depth in the campinarana community, which could heighten drought stress during the dry season. The only vegetation able to persist in campinarana sites were short-statured trees that appear to be well-adapted to the dual extremes of inundation and drought.

Nomenclature: Ribeiro et al. (1999).

Questions: (1) What are the roles of regional climate and plant growth rate for seedling establishment during ENSO rainy pulses along the western coast of South America? (2) What is the water threshold for tree seedling establishment in these arid ecosystems?

Location: Atacama Desert, western South America: Piura (5°10′ S, 80°37′ W), Mejia (17°00′ S, 71°59′ W), Fray Jorge (30°41′ S, 71°37′ W).

Methods: We experimentally simulated a gradient of ENSO rainfall in three locations encompassing the total extent of the Atacama Desert to test the relative importance of regional climate for seedling establishment during rainy pulses. We also carried out a common garden experiment to test the role of potential interspecific differences in growth rate among two Prosopis tree species.

Results: Water threshold for seedling survival increased towards the south with less than 27 mm required in Piura, 100 mm in Mejia and 450 mm in Fray Jorge. We found that seedling survival and growth rate (shoots and roots) were much higher in Piura than in the other two sites for both Prosopis species.

Conclusions: Our results indicate that tree establishment during rainy pulses is more likely to be successful in regions where rain falls during warm months and stimulates fast plant growth, and where loose soil texture facilitates deep root growth and therefore access to more stable water sources.

Question: How do various plant functional groups and types are related to regeneration of limestone grasslands and to temporal changes in cover of trees and shrubs? Which mechanisms are responsible for changes in differently treated grasslands?

Location: Southern Poland, 50° 01′N, 19° 50′E.

Methods: A 12-year restoration experiment after the clearing of a 35-yr-old secondary pine wood developed on limestone grassland, and a parallel observation of succession in neighbouring abandoned grassland were carried out. Changes in cover of species with different functional traits and established strategies in relation to time and cover of shrub layer were analysed using multiple linear regression.

Results: Over 12 years the composition and cover of species changed in the studied grasslands, but significant differences still occurred between the old grassland and the grassland restored in former wood gaps and that developed in former closed wood. Despite the very close proximity of the old grassland, the composition of restored grasslands was still much determined by initial conditions. In these grasslands, the cover of Festuco-Brometea species was significantly lower than in the old grassland. In all sites, the cover of species with large leaves and with leaves distributed regularly along the stem, and the cover of CS strategists increased. With increasing shrub cover in restored grasslands, the cover of annuals and biennials, tall species, and R and CS strategists decreased.

Conclusions: Limited availability of seeds seems the principal reason for the weak regeneration of xerothermic limestone grasslands. Trees and shrubs inhibit the developed of both early successional species and perennials occurring in established communities. They significantly hinder regeneration of grasslands and should be cut every five or six years.

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

Questions: 1. How does the spatial structure of the organic layer affect tree sapling physiology? 2. Are the organic layer and Picea mariana height growth spatially structured at different scales? 3. Does microtopography influence the accumulation of organic matter and does organic layer thickness affect height growth?

Locations: Picea mariana forests, northwestern Quebec, Canada.

Methods: We assessed the spatial pattern of each variable in one wildfire site and one harvest site using semivariograms and correlograms. We measured the cross-correlation between relative elevation and organic layer thickness, and between organic layer thickness and growth using cross-correlograms.

Results: Picea mariana height growth was autocorrelated to a greater extent in the wildfire site (103 m) than in the harvest site (43 m). The spatial structure of organic layer thickness was similar in both sites. Deeper depressions in the harvest site, as illustrated by spatial variance in relative elevation at short distances (ca. 50 m), and by high autocorrelation values, increased the accumulation of organic matter within 20 m.

Conclusions: The interaction between microtopography and organic matter accumulation led to paludification and poor growth of Picea mariana at the harvest site. Paludification at the wildfire site was independent of microtopography and was probably a result of stand development.

Objective: This empirical study was designed to explore the role of ecological features of species in the spatial patterning of a grassland community.

Location: Banks of the river Rhône in France.

Material and Methods: First, we explored the spatial pattern of 29 species recorded in the community using spatial autocorrelation analysis of species cover values. Second, we then explored the relationship between the patterns found and a set of life attributes that characterized the ecological features of species for resource foraging or dispersion. Finally, we explored the spatial relationship of groups of species that shared the same ecological features using cross-correlation analysis.

Results: We found a significant relationship between the spatial pattern and life attributes of the species highlighting three groups of species: (1) species characterized as competitors, reproducing by runner clonal organs and forming large, dense patches; (2) species characterized as competitive-ruderals, dispersing exclusively by seed production and forming small periodic patches; and (3) species classified as CSR, characterized by rosette morphology and short rhizomes as clonal organs without any significant spatial autocorrelation. Spatial segregation was found between group 1 and group 2 up to 14 m; no significant cross-correlation between groups 1 and 3 between 0 and 3.5 m, and association between groups 2 and 3 up to 14 m.

Conclusions: These results helped to understand how species attributes (relative to stature or dispersion abilities); external factors (such as disturbance) and biotic processes (competition) interplay in structuring the plant community under study in space.

Question: How are the effects of mineral soil properties on understory plant species richness propagated through a network of processes involving the forest overstory, soil organic matter, soil nitrogen, and understory plant abundance?

Location: North-central Arizona, USA.

Methods: We sampled 75 0.05-ha plots across a broad soil gradient in a Pinus ponderosa (ponderosa pine) forest ecosystem. We evaluated multivariate models of plant species richness using structural equation modeling.

Results: Richness was highest at intermediate levels of understory plant cover, suggesting that both colonization success and competitive exclusion can limit richness in this system. We did not detect a reciprocal positive effect of richness on plant cover. Richness was strongly related to soil nitrogen in the model, with evidence for both a direct negative effect and an indirect non-linear relationship mediated through understory plant cover. Soil organic matter appeared to have a positive influence on understory richness that was independent of soil nitrogen. Richness was lowest where the forest overstory was densest, which can be explained through indirect effects on soil organic matter, soil nitrogen and understory cover. Finally, model results suggest a variety of direct and indirect processes whereby mineral soil properties can influence richness.

Conclusions: Understory plant species richness and plant cover in P. ponderosa forests appear to be significantly influenced by soil organic matter and nitrogen, which are, in turn, related to overstory density and composition and mineral soil properties. Thus, soil properties can impose direct and indirect constraints on local species diversity in ponderosa pine forests.

Question: What are the main forces driving natural regeneration in burned mature Mediterranean forests in the mediumlong term and what are the likely successional trajectories of unmanaged vegetation?

Location: Valencia Region, eastern Spain.

Methods: A wildfire burned 33 000 ha of Pinus halepensis and P. pinaster forest in 1979, and subsequent smaller wildfires took place between 1984 and 1996. The study was designed to sample the range of environmental and disturbance (fire recurrence and land use) conditions. The territory was classified into 17 different geomorphological and fire-recurrence units. Vegetation cover and floristic composition were measured on a total of 113 plots (1000 m² each) randomly selected within these units.

Results: The results show that 23 years after the fire the regenerated vegetation consists of successional shrublands, and that forest ecosystem resilience can be very low. The vegetation presents a strong correlation with most of the environmental variables, but fire (one or two fires), soil type and land use (in that order) are the main drivers of vegetation composition. Quercus coccifera shrublands persist on limestone soils while diverse types of other shrublands (dominated by seeder species) are found on marl soils.

Conclusions: The results of this study indicate that disturbance factors strongly coupled to human activities, such as land use and fire, play a critical role in the current state of vegetation. Fire creates vegetation patches in different successional states while land use and soil type define the different types of shrubland in terms of their specific composition.

Question: Community structure may be influenced by patterns of dispersed seeds (seed rain) because they contribute to the template of plant community development. We explored factors influencing seed rain in a system dominated by tidal water, where direction and magnitude of water flow are difficult to predict, unlike many other hydrochorous systems where water flow is directional. We posed three main questions: (1) are patterns in seed rain linked to effects of hydrodynamic variability; (2) do these patterns in seed rain reflect distribution of seed sources and seed production; and (3) what are the implications for the assembly of tidal communities?

Location: Salt marshes on the Wadden island of Schiermonnikoog, The Netherlands.

Methods: Species compositions of vegetation, seed rain, seed production and driftlines along a chronosequence of communities were compared. We also studied seed movement by sowing Astroturf® mats with seeds and checking for seeds remaining after a single tidal inundation.

Results: Storm surges had a significant effect on seed-rain patterns as the highest density and diversity of captured seeds were found during a stormy period. Seed rain of the youngest communities was more influenced by storms than that of older communities. Patterns in seed rain generally followed similar patterns in the distribution of established plants, and seed production.

Conclusions: Results suggested mostly local dispersal of seeds. However, there was some evidence of long-distance dispersal occurring during storm surges in younger communities that are regularly inundated with tidal water. The possible role of seed retention in constraining community development, rather than dispersal per se, is further discussed.

Nomenclature: van der Meijden (1996).

Question: Factors influencing seedling establishment are known to vary between open sites and those protected by plant cover. In many desert regions, protected microhabitats below shrubs are essential for establishment of many cactus species. Very little is known about these factors for Andean cacti and how the importance of vegetation cover varies with cactus species. Are Andean cacti associated more frequently to vegetation cover than to open ground? Are they associated to certain shrub species? Is the distributional pattern in relation to cover similar for different cactus species? In what microhabitat (below or away from shrubs) are cactus seeds more abundant? These questions are addressed for the case of an Andean semi-desert.

Location: Semi-arid tropical Andes, La Paz department, Bolivia.

Methods: We examined 132 isolated shrubs ≥ 50 cm along a line across two microhabitats: areas below and away from shrubs/trees. Shrub crown size was measured. The among-shrub samples were taken from open spaces contiguous to each of the sampled shrubs. In both microhabitats, all cactus species were recorded. The cardinal direction of the cacti was also registered. Correlation between canopy diameter and number of beneficiaries was evaluated for Prosopis flexuosa. The cactus seed bank in each microhabitat was also studied.

Results and Conclusions: The four cactus species found behaved differently in relation to shrub canopies. These distributional differences could be due to differences in growth form. Columnar cacti apparently need the shade of shrubs. Only the columnar species is able to grow near the base of the tallest nurse species. The opuntioid cacti studied seem more facultative: although apparently preferring shrub under-canopies, they are able to establish in open ground. The globose cactus is the most indifferent to the presence of plant cover. These patterns parallel others found in North America. The capacity of different cacti to appear in open spaces could be related to vegetative propagation, and not necessarily to seedling tolerance of heat.

Nomenclature: Anderson (2001) for cacti; Beck & Valenzuela (1991) for the other families.

Questions: Can gender of nurse plants affect regeneration patterns and spatial population structure? Is there a seedseedling conflict in the regeneration process? What factors are responsible for the clumped spatial population structure observed for adult trees?

Location: Mediterranean cold semi-arid high mountains in Spain.

Methods: The spatial pattern of adult Juniperus thurifera trees was studied by means of Ripley's K-analysis. χ² analyses were used to test for natural seedling frequency in each of three main microhabitats: (1) under female and (2) male tree canopies and (3) in open interspaces. The observed pattern was explained experimentally by studying seed and seedling survival for two years. Survival probabilities were calculated across life stages for each of three main microhabitats.

Results: Adult J. thurifera trees were aggregated in space. Most seedlings were found underneath female J. thurifera trees. Experimental studies demonstrated that from seed dispersal to seedling survival all life stages showed the same positive or negative trend within a given microhabitat, indicating stage coupling and no seed-seedling conflicts. Attraction of frugivorous birds by reproductive female junipers and improvement of environmental conditions beneath tree canopies were the main factors responsible for the variation in seedling density among microhabitats; highest underneath female trees and lowest in open interspaces.

Conclusions: In dioecious species, the gender of nurse plants can significantly determine the spatial population structure. In J. thurifera forests, facilitation beneath female trees occurs among all life stages without any sign of seed-seedling conflict. The most critical factors shaping the spatial population structure were directed seed dispersal and environmental amelioration beneath female conspecific trees.

Nomenclature: Castroviejo et al. (1986).

Question: Plant invasions result from complex interactions between species traits, community characteristics and environmental variations. We examined the effect of these interactions on the invasion potential of two invasive Senecio species, S. inaequidens and S. pterophorus, across three Mediterranean plant communities in a natural park.

Location: Catalonia, NE Spain.

Methods: We carried out two series of experimental seedling transplantations, in the spring and fall of 2003, in grassland, shrubland and Quercus ilex forest. Competition with neighbouring plants and water availability were manipulated. We evaluated the survival, growth and reproduction with respect to each treatment combination.

Results: Any habitat can be colonised if disturbance occurs. In the absence of disturbance, shrubland enhanced the survival of seedlings. Competition with resident vegetation dramatically reduced survival in grassland and forest when establishment occurred in the spring. However, establishment in the fall promoted invasion in grassland and shrubland, even in the undisturbed treatment. Grassland allowed the highest growth and reproductive performance of both species while forest was the most resistant habitat to invasion. S. inaequidens had a higher growth rate and a shorter pre-reproductive period than S. pterophorus. S. pterophorus produced more biomass and was more dependent on water availability than S. inaequidens.

Conclusions: In the light of our results, we recommend surveying open shrublands and grasslands after periods of rainfall. Special attention should be paid to S. pterophorus, which is currently spreading. A preliminary assessment of the invasiveness of this plant is given in this study.

Question: Land-use change has a major impact on terrestrial plant communities by affecting fertility and disturbance. We test how particular combinations of plant functional traits can predict species responses to these factors and their abundance in the field by examining whether trade-offs at the trait level (fundamental trade-offs) are linked to trade-offs at the response level (secondary trade-offs).

Location: Central French Alps.

Methods: We conducted a pot experiment in which we characterized plant trait syndromes by measuring whole plant and leaf traits for six dominant species, originating from contrasting subalpine grassland types. We characterized their response to nutrient availability, shading and clipping. We quantified factors linked with different land usage in the field to test the relevance of our experimental treatments.

Results: We showed that land management affected nutrient concentration in soil, light availability and disturbance intensity. We identified particular suites of traits linked to plant stature and leaf structure which were associated with species responses to these environmental factors. Leaf dry matter content separates fast and slow growing species. Height and lateral spread separated tolerant and intolerant species to shade and clipping.

Discussion and Conclusion: Two fundamental trade-offs based on stature traits and leaf traits were linked to two secondary trade-offs based on response to fertilization shade and mowing. Based on these trade-offs, we discuss four different species strategies which could explain and predict species distributions and traits syndrome at community scale under different land-uses in subalpine grasslands.

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

Over the last 20 years, several studies comparing recent survey data with historical data from the early 20th century documented an increase in species numbers on high mountain summits of the European Alps. This increase has more or less explicitly been attributed to an upward migration of plant species due to anthropogenic climate warming. However, a reconsideration of the historical and recent data has revealed that more than 90% of the recent species occurrences on mountain summits concern species that were already present at the same or even at higher altitudes within the study region at the time of the historical surveys. This finding suggests that suitable habitats already occurred on these summits under the mesoclimatic conditions prevailing at the beginning of the 20th century and that these habitats were, at least in part, occupied by these plant species. Consequently, the observed increase in species number during the last century does not require the additional temperature increase due to anthropogenic climate change. We therefore consider the phenomenon of increasing species number on high mountain summits to be primarily the result of a natural dispersal process that was triggered by the temperature increase at the end of the Little Ice Age and that is still in progress mostly due to the dispersal limitation of the species involved. Since both the natural dispersal process and a potential upward migration due to anthropogenic climate warming would take place at the same time, we suggest seeding and transplanting experiments in order to assess their respective roles in the increase in species number on mountain summits.

Nomenclature: Aeschimann & Heitz (1996).

Questions: Is tree diversity higher in the southern hemisphere? Are latitudinal asymmetries in diversity sensitive to sampling effects?

Location: 198 forested locales worldwide.

Methods: I re-analysed the Gentry database, which I augmented with an additional survey from New Zealand. Data were used to test whether latitudinal declines in tree diversity differ between the northern and southern hemispheres. Data were also used to test whether hemispheric asymmetries in diversity are sensitive to sampling effects, or geographic variation in tree densities.

Results: Area-based measurements of species diversity are higher in the southern hemisphere. However, southern forests house denser plant populations. After controlling for geographic variation in tree densities, diversity patterns reverse, indicating tree diversity is higher in the northern hemisphere.

Conclusions: Latitudinal changes in tree diversity differ between hemispheres. However, the nature of hemispherical asymmetries in species diversity hinges on how diversity is defined, illustrating how different definitions of diversity can yield strikingly different solutions to common ecological problems.

Nomenclature: Allan (1961) and Moore & Edgar (1970).