Why do more plant species use ants for dispersal on infertile compared with fertile soils?*

Myrmecochores are plant species that bear seeds with food-bodies adapting them for dispersal by ants. Within Australia and South Africa, myrmecochores account for a large percentage of the flora in sclerophyll vegetation on very infertile soils. On fertile soils, there are few myrmecochores and more species with fleshy fruits adapted for dispersal by vertebrates. This effect of very infertile soils may also account for the high incidence of myrmecochores in the floras of Australia and South Africa compared with other continents. The taxonomic distribution of myrmecochory shows it has evolved many times within the Australian flora. Evidence has been collected in relation to eight hypotheses that might account for the prevalence of myrmecochory on very infertile soils: (1) Myrmecochore seeds do not appear to be relocated to nutrient-enriched microsites. Seed-removing ant species relocate their nest entrances frequently; active ant nests are not nutrient-enriched; and seedlings emerging after fire are not located in nutrient-enriched soil. (2) Traffic of seed-removing ants is not greater on infertile than on fertile soils. (3) Burial to avoid predation by small mammals is not a significant factor for seeds in Australian vegetation. (4) Experiments in which diaspore traits are manipulated support the idea that a larger food body increases the likelihood that a diaspore will be taken by a seed-disperser ant species that will not eat the embryo, rather than by a seed-predator ant. However, evidence so far does not support the hypothesis that, on very infertile soils, seed-disperser ants are consistently more available than seed-predator ants. (5) Removal rates of fruits adapted for dispersal by vertebrates were, on average, two and a half times greater in vegetation on fertile soils. Birds were the main removers. Our current opinion is that the fruit-removing bird assemblage is more likely to be a consequence of the high incidence of species with vertebrate-adapted fruits, rather than the reverse. (6) Fleshy fruits are more expensive in potassium than food bodies for ants, but not in nitrogen or phosphorus. This might be a reason why fleshy fruits are uneconomic on very infertile soils. (7) Species with smaller embryo-plus-endosperm weight are relatively more likely to be adapted for dispersal by ants rather than vertebrates. However, differences in the frequency distribution of embryo-plus-endosperm weights between infertile and fertile soils are moderate in the Sydney area. An indirect association between soil fertility and dispersal mode via embryo-plus-endosperm weight cannot account for much of the difference in dispersal spectrum. (8) Species growing to less than 2 m tall are relatively more likely to be adapted for dispersal by ants rather than vertebrates, and also account for more of the flora on infertile than on fertile soils. This indirect correlation is capable of accounting for as much as 80% of the higher incidence of myrmecochory on infertile soils.     

Effect of three mound-building ant species on the formation of soil seed bank in mountain grassland

The effect of three ant species (Lasius flavus, Formica spp., Tetramorium caespitum) on soil seed bank formation was studied in temperate mountain grassland. Seed removal experiments, analysis of soil seed content and seed survival experiments were carried out to evaluate the influence of ground ants on the seed fate. In the seed removal experiment seeds of 16 species, including 5 species with elaiosome-bearing seeds (myrmecochores), were exposed and their removal followed for 39 h. On average, ants removed 63.8% of myrmecochorous seeds and 10.9% of seeds without adaptation to ant dispersal. Analysis of soil seed content revealed that myrmecochores, in spite of expectations that they would accumulate in nests of seed dispersing ants, were most abundant in the soil of control plots. Evidence on seed relocation to the ant nests was obtained from a comparison of mounds of seed dispersing and seed non-dispersing ant species, as more seeds were found in the mounds of Formica spp. and Tetramorium caespitum (seed dispersers) in comparison with the mounds of Lasius favus (non-disperser).The soil seed bank of the compared microhabitats (control plots and mounds of 3 ant species) differed in their species composition, seed abundance and vertical distribution. The most distinct qualitative differences were between seed flora of control plots and mounds of Tetramorium caespitum. Control plots had approximately 30,000 propagules per m², which was double the number of seeds found in the ant mounds. In control plots, abundance and diversity of seeds steeply declined with depth; this trend was not observed in the mounds probably due to bioturbation. In the seed survival experiment, more seeds (2 out of 3 species) survived in control plots, which may also contribute to the higher seed abundance in this microhabitat.This study showed that seed relocation by ants does not contribute significantly to seed bank build-up at this study site. Ants may, however, increase the regeneration success of myrmecochores, mainly by dispersal for distance and placement in a larger spectrum of microsites, in contrast to species not adapted for myrmecochory.     

Comparison of seed removal by ants in vegetation on fertile and infertile soils

Plants which have seeds adapted for dispersal by ants (myrmecochores) are more common in some vegetation types than others. In Australia, myrmecochores are common in sclerophyll shrublands which are associated with infertile soils but not in forests on fertile soils. One hypothesis to explain this difference is that ants are less abundant or less active in seed removal on fertile soils. A test of this hypothesis was carried out on four sites in the Royal National Park, south of Sydney, by measuring the number of myrmecochorous seeds removed by ants in a 24 h period. It was found that the removal rates of seeds were slightly higher on fertile sites although this difference was not significant. The hypothesis that a lower removal rate on fertile soils could account for the lower percentage myrmecochory was therefore rejected. Apart from the gross removal rate of seeds by ants, other differences in ant behaviour might be important. It is possible that a greater proportion of ant species unfertile soils are seed predators or ineffective seed dispersers, and evidence obtained indicates that this is worthy of further research.     

Are gastropods, rather than ants, important dispersers of seeds of myrmecochorous forest herbs?

Seed dispersal by ants (myrmecochory) is widespread, and seed adaptations to myrmecochory are common, especially in the form of fatty appendices (elaiosomes). In a recent study, slugs were identified as seed dispersers of myrmecochores in a central European beech forest. Here we used 105 beech forest sites to test whether myrmecochore presence and abundance is related to ant or gastropod abundance and whether experimentally exposed seeds are removed by gastropods. Myrmecochorous plant cover was positively related to gastropod abundance but was negatively related to ant abundance. Gastropods were responsible for most seed removal and elaiosome damage, whereas insects (and rodents) played minor roles. These gastropod effects on seeds were independent of region or forest management. We suggest that terrestrial gastropods can generally act as seed dispersers of myrmecochorous plants and even substitute myrmecochory, especially where ants are absent or uncommon.     

Effects of Nutrient Manipulations and Grass Removal on Cover,Species Composition,and Invasibility of a Novel Grassland in Colorado

Cover and richness of a 5‐year revegetation effort were studied with ,respect to small‐scale disturbance and nutrient manipulations. The site, originally a relict tallgrass prairie mined for gravel, was replanted to native grasses using a seed mixture of tall‐, mixed‐, and short‐grass species. Following one wet and three relatively dry years, a community emerged, dominated by species common in saline soils not found along the Colorado Front Range. A single species, Alkali sacaton (Sporobolus airoides), composed nearly 50% of relative vegetation cover in control plots exhibiting a negative relationship between cover and richness. Seeded species composed approximately 92% of vegetation cover. The remaining 8% was composed of weeds from nearby areas, seed bank survivors, or mix contaminants. Three years of soil nutrient amendments, which lowered plant‐available nitrogen and phosphorus, significantly increased relative cover of seeded species to 97.5%. Fertilizer additions of phosphate enhanced abundance of introduced annual grasses (Bromus spp.) but did not significantly alter cover in control plots. Unmanipulated 4‐m² plots contained an average of 4.7 planted species and 3.9 nonplanted species during the 5‐year period, whereas plots that received grass herbicide averaged 5.4 nonplanted species. Species richness ranged from an average 6.9 species in low‐nutrient, undisturbed plots to 10.9 species in the relatively high‐nutrient, disturbed plots. The use of stockpiled soils, applied sparingly, in conjunction with a native seed mix containing species uncommon to the preexisting community generated a species‐depauperate, novel plant community that appears resistant to invasion by ruderal species.     

Ant diversity in an Amazonian savanna: Relationship with vegetation structure,disturbance by fire,and dominant ants

Abstract The savannas of South America support a relatively diverse ant fauna, but little is known about the factors that influence the structure and dynamics of these assemblages. In 1998 and 2002, we surveyed the ground‐dwelling ant fauna and the fauna associated with the woody vegetation (using baits and direct sampling) from an Amazonian savanna. The aim was to evaluate the influence of vegetation structure, disturbance by fire and dominant ants on patterns of ant species richness and composition. Variations in the incidence of fires among our 39 survey plots had no or only limited influence on these patterns. In contrast, spatial variations in tree cover and cover by tall grasses (mostly Trachypogon plumosus), significantly affected ant species composition. Part of the variation in species richness among the study plots correlated with variations in the incidence of a dominant species (Solenopsis substituta) at baits. Ant species richness and composition also varied through time, possibly as an indirect effect of changes in vegetation cover. In many plots, and independently of disturbance by fire, there was a major increase in cover by tall grasses, which occupied areas formerly devoid of vegetation. Temporal changes in vegetation did not directly explain the observed increase in the number of ant species per plot. However, the incidence of S. substituta at baits declined sharply in 2002, especially in plots where changes in vegetation cover were more dramatic, and that decline was correlated with an increase in the number of ground‐dwelling species, a greater turnover of bait‐recruiting species and the appearance of the little fire ant Wasmannia auropunctata. The extent to which these changes in fact resulted from the relaxation of dominance by S. substituta is not clear. However, our results strongly suggest that the ant fauna of Amazonian savannas is affected directly and indirectly by the structure of the vegetation.     

Does Seeding a Locally Adapted Native Mixture Inhibit Ingress by Exotic Plants?

Non‐native plant species often colonize retired agricultural lands, creating monocultures with low species diversity that provide poor wildlife habitat. We assessed whether sowing a mix of 29 locally adapted native species reduced invasion of non‐native plant species compared to allowing vegetation to colonize naturally following tillage. There was a sampling date × treatment interaction for canopy cover of perennial exotic plant species. Plots that were not sown to natives had two to six times greater canopy cover of exotic species than did plots with both preparation (woody vegetation removed, plowed, and disked) and control (no preparation or sowing) plots. Canopy cover of exotic plants was similar in prepared‐only and control treatments from October 2008 to June 2010, ranging from 8 to 40%. Percent absolute canopy cover of native vegetation was 10–20 times greater on prepared and planted plots than on prepared‐only plots during March 2009 to June 2010. Sowing a mix of locally adapted native species may inhibit encroachment by non‐native species for up to two years after sowing on retired agricultural land in the Lower Rio Grande Valley of Texas.     

Relations between the floristic composition of isolated young woods and their proximity to ancient woodland

Abstract. Recent studies indicate that, in the present-day agricultural landscape, the floristic composition of young woodland communities can be fully developed if the woods are situated adjacent to ancient woodlands. Four 70-yr-old deciduous woods in the Carpathian foothills were examined in relation to three adjacent ancient oak-hornbeam and oak-pine woodlands, which are the nearest source of woodland species diaspores. On the basis of data from 208 plots, the frequencies of various species groups in the field layer of the woods were analysed. The dependence of vegetation differentiation within the recent woods on (a) distance to the border with the ancient woodlands and (b) light intensity was examined by Partial Detrended Canonical Correspondence Analysis (DCCA). A significant relation between distance to ancient woodland and species composition was found for recent woods on rich brown soils. The vegetatively propagating species, myrmecochores and small autochores attained higher cover values near ancient woodland; endozoochores and anemochores were most abundant further away. Within recent, more open woods on poor podzolic and leached brown soils, colonisation is strongly inhibited by dense growth of Carex brizoides; here, vegetation regeneration is much slower than in woods on rich soils much further away from the source of diaspores.     

Effects of grazing by re-introduced Equus hemionus on the vegetation in a Negev desert erosion cirque

Abstract. Asiatic wild asses, Equus hemionus, were driven to extinction in Israel in the early 20th century. In 1983, a herd of these animals was re-introduced to the wild in Makhtesh Ramon, a large erosion cirque in the central Negev desert, Israel. The population has grown steadily ever since and now numbers some 100 animals. In order to determine whether the wild asses are having a significant impact on the vegetation, we have monitored the plant communities in Makhtesh Ramon since 1992, using McAuliffe's log-series survey method. Our study involves 11 pairs of plots along the length of the altitudinal gradient in Makhtesh Ramon. The altitudinal gradient results in a rainfall gradient from an average of 95 mm rain per year to an average of about 40 mm per year. Each pair of plots consists of: (1) an unfenced plot, and (2) a fenced plot that excludes wild asses but not the other large mammalian herbivore, the dorcas gazelle Gazella dorcas. The wild asses have not had a significant impact on vegetation cover, species richness, diversity or dominance. Three plant species showed significant increases in percentage cover in fenced plots, while one species showed a significant increase in percentage cover in unfenced plots. Furthermore, eight plant species invaded fenced plots, three species invaded unfenced plots and one species disappeared from unfenced plots during the study. Using Detrended Correspondence Analysis, we found that the major differences among plots are due to position along the altitudinal gradient. The Detrended Correspondence Analyses indicated that the wild asses have had no significant effect on vegetation community structure.     

Changes in the composition of understorey vegetation after harvesting eucalypts for sawlogs and pulpwood in East Gippsland

Cover and abundance of all understorey vascular plant species were studied on permanent plots on two areas (Maramingo and Reedy Creek) in the foothill forests of East Gippsland, eastern Victoria. The areas were harvested for sawlogs and pulpwood in 1976, and parts were burned to assist regeneration. Plots were assessed before harvesting and at intervals until 1980. Changes in total species composition were minimal, although there were many positive and negative changes in crown cover and abundance of individual species. Total cover of understorey vegetation was reduced initially but recovered well on both areas. By 1980, cover at Maramingo was slightly greater than the low levels before harvesting. Some species there were favoured by burning while others were not. At Reedy Creek the original dense understorey had not regenerated fully by 1980. Snig tracks had revegetated to 60% of original average cover but log landings were slower to revegetate. Introduced species were initially a very minor component of the vegetation on both areas: they remained so at Reedy Creek (confined mainly to tracks) but at Maramingo three species proliferated along with other small herbaceous plants.     

Germination in Baltic coastal wetland meadows: similarities and differences between vegetation and seed bank

The abundance and variety of seedlings in Baltic coastal grasslands was studied in cattle grazed and ungrazed areas in seashore and delta on the western coast of Finland. The vegetation, seed bank and environmental conditions of the same sites were also studied. Altogether 4609 seedlings were observed in 79 field plots (20 cm× 20 cm) making an average of 1458.54 seedlings/m². The majority of the seedlings were dicots (67.5%) and perennials (96.8%). Altogether 44 species and an average of 4.14 species per plot were identified as seedlings. The most abundant taxon, Carex formed 23.2% of all the seedlings observed in the field. In the delta the seedling density was two times that found at the seashore. Also, the numbers of species of seedlings per plot were significantly higher in the delta than in the seashore, but in total, more species germinated from the seashore than from the delta even though the number of plots was higher in the delta (n = 51) than in the seashore (n = 28). The numbers of species and seedlings per plot were smaller in grazed sites than in ungrazed ones similar to the pattern earlier reported concerning the mature vegetation and seed bank. The mean numbers of seedlings were the highest at lower geolittoral plots (about 20 cm elevation), where the seed bank was also densest. About 60.0% of the mature species in the plots were also detected as seedlings and 91.4% of the seedling species were found in the mature vegetation of the plots. The number of seedlings was positively correlated with the cover of mature vegetation giving support to the nurse effect theory. Seedling numbers were negatively correlated with the abundance of litter indicating that litter restricts germination because it is a mechanical barrier but also decreases the amount of light. The number of seedlings and species, both in total and per unit area, were greater in the seed bank than in the field seedling population. The seedling population was more similar to the mature vegetation than to the seed bank. This revised version was published online in August 2006 with corrections to the Cover Date.     

Impact of reindeer grazing on ground-dwelling Carabidae and Curculionidae assemblages in Lapland

Reindeer Rangifer tarandus L. grazing shapes forest vegetation, microclimate, and soil respiration in Lapland, especially due to grazing on lichens (Cladina). We studied how these changes and their magnitude affect ground‐dwelling species of beetle families Carabidae (predators) and Curculionidae (herbivores), by using pitfall traps to collect invertebrates from pairs of grazed and ungrazed study plots over a wide range of site types. Changes in abundance, composition, richness and diversity of beetle assemblage were tested in relation to magnitude of the impacts on vegetation. The species compositions of Carabidae and Curculionidae differed between grazed and ungrazed plots in all sites. The relative difference between grazed and ungrazed plots in the number of individuals increased linearly with the impact of reindeer on vegetation cover. Carabid beetles, as a family, were more common in grazed plots in all sites. Curculionid beetles were more common in ungrazed plots in the birch dominated sites. This difference was mainly due to the species that feeds on deciduous leaves. In the pine dominated sites with high Cladina cover and more changes in ground vegetation, the number of curculionids feeding on conifers was higher in grazed plots. Species richness and diversity (H’) of both families were higher in grazed plots. Of the total 27 species, 11 were found only in grazed plots, while not a single species was found only in ungrazed plots. The relative difference between plots in diversity and evennes (H’/H'max) had humped response to the difference in Cladina cover. The diversity values were greater in grazed plots at the intermediate levels of grazing impact, and only in sites with very low or extremely high Cladina cover difference was the diversity higher in ungrazed plots. The response of beetle diversity resembled the hypotheses suggested for the relationship between grazing and vegetation diversity: greatest positive effect at intermediate grazing intensity and negative effects at unproductive sites.     

Shrubland Restoration Following Woody Alien Invasion and Mining: Effects of Topsoil Depth, Seed Source, and Fertilizer Addition

Invasion by woody alien plants, construction, and mining operations are among the major disturbances degrading vegetation in the Cape Floristic Kingdom, South Africa. The aim of this study was to assess whether native fynbos shrubland vegetation could be restored following dense alien invasion and disturbance by mining. An area supporting dense alien trees was cleared and topsoil was stripped and stockpiled to simulate mining disturbance. A field trial investigated the effects of topsoil depth, seed mix application, and fertilizer on native species recruitment and vegetation development over a three‐year period. Soil‐stored seed banks contributed 60% of the species recruited, indicating that areas invaded for three decades have good restoration potential. The addition of a fynbos seed mix, which included serotinous overstory species, improved both the richness and structural composition of the vegetation. Most species sown in untopsoiled plots established, but survival and growth was low compared to topsoil plots. Poor growth in combination with a lack of soil seed bank species, indicate that restoring a diverse and functional cover of indigenous vegetation on subsoil is not possible in the short‐term. Soil amelioration is required to improve rooting conditions and initiate ecosystem processes. Shallow and deep topsoil treatments yielded high plant density, richness, and projected canopy cover, but canopy cover was higher in deep topsoil plots throughout the trial. Fertilizer addition increased canopy cover in untopsoiled and shallow topsoil plots via an increase in alien annual species. Fertilizer addition ultimately may lead to increased native vegetation cover in untopsoiled areas, but as it increased proteoid mortality on deep topsoil plots, it is not recommended for sites where topsoil is available. A species‐rich and structurally representative fynbos community may be restored on topsoiled areas provided that the native disturbance regime is simulated and seeds of major structural guilds not present in the soil seed bank are included in the seed mix.     

Relationships between seed banks and spatial heterogeneity of North American alvar vegetation

Abstract. This is the first quantitative study of seed bank characteristics in North American alvar habitats. We assessed seed bank density, species richness, and species composition in 75 plots distributed among five alvar sites in Bruce Peninsula National Park, Ontario, Canada, each of which displayed areas of high and low vegetation cover within the alvar and a fully forested perimeter area. Forested habitats immediately adjacent to alvar patches contained minimal seed banks for species restricted to the alvar patches. Open alvars contained less than 1% seeds from woody forest species. This suggests that forest is not invading adjacent alvar habitat via seeds and that adjacent forest does not contain a reservoir of alvar seeds. When compared to areas on the alvar with high vascular plant cover, areas with low cover contained a slightly smaller viable seed bank, but seed banks from high and low vegetation cover plots had similar species composition and species richness. High vegetation cover plots had slightly higher mean and maximum soil depths compared with low cover plots, but no differences in other physical and chemical parameters. Thus, spatial heterogeneity in plant cover is associated only weakly with heterogeneity in below‐ground factors. Despite the availability of seed and soil resources, vegetation dynamics are constrained in areas with low plant cover, and thus alvar community development seems to respond non‐linearly to resource availability.     

Threatened vascular plant species in spontaneously revegetated post‐mining sites

The occurrence of threatened vascular plant species has been occasionally reported from some disturbed sites. Nevertheless, almost nothing is known about the conservation potential of post‐mining habitats for vascular plant species at a landscape scale. We asked how much threatened species occur in the artificial habitats, and if the participation of particular threatened species categories differ over the course of succession. We studied 10 successional series running in 321 post‐mining sites across the Czech Republic, Central Europe. In total, 2,597 vegetation samples comprising 935 vascular plant species were analyzed. We found 235 threatened species (14% of the country's threatened flora). The majority of the threatened plants, namely 223, avoided plots overgrown by woody species, in which only 48 species occurred. The importance of spontaneously revegetated habitats for nature conservation differed among successional stages formed by discontinuous vegetation cover (not fully developed and still open, up to approximately 25 years since site abandonment), which harbored especially critically endangered species of open habitats, and continuous vegetation cover (fully developed and closed, over approximately 25 years), which was suitable for specialized threatened species typical of fully established vegetation. A mosaic of various successional stages comprising both categories of vegetation cover should be applied as an effective restoration solution to maintain a variety of habitats for threatened species.     

Ecology of the Pteridophytes on the Southern Slopes of Mt. Kilimanjaro. Part II: Habitat Selection

Abstract: Based on the evalutation of 957 vegetation plots on the southern slope of Mt. Kilimanjaro, habitat preferences for 140 species of pteridophytes were evaluated. Using the average percentage cover value, and taking into account the pteridophyte flora's composition, life form spectra and its spectra of seasonal growth pattern, eight vegetation formations were recognized. Ferns contributed less than 1 % of the vegetation cover of salt marshes, ruderal vegetation, grasslands and (sub-)alpine heathlands. In contrast, pteridophytes constituted the most important vascular plant group on rocks, where 64 species were found, forming about two-thirds of the vegetation cover. With respect to alpha and beta diversity and fern biomass, luxuriant montane forest was the main habitat for pteridophytes on Mt. Kilimanjaro. Here 130 pteridophyte species (93 % of the whole pteridophyte flora of the study area), on average, contributed 16 % of the total vegetation cover. Epiphytic ferns, tree ferns and filmy ferns had their main distribution between 1900 and 2400 m, in a zone coinciding with the maximum rainfall on Mt. Kilimanjaro's southern slope.
Poikilohydrous species were typical of dry habitats, such as on rocks, in meadows or along roadsides, but they also occurred in the often sun-exposed epiphyte layer in moist montane forests. Deciduous species, which were in many cases fire resistant, had a similar distribution; however, inside the forest belt they were restricted to the lower and upper parts, where fires are a common phenomenon. Evergreen species were the dominant group in swamps, forests and forest clearings.
Compared to other volcanoes in East Africa, Mt. Kilimanjaro is distinctly richer in fern species in general and in filmy ferns, tree ferns and epiphytic ferns in particular, suggesting that the forest belt of the southern slope of Mt. Kilimanjaro is wetter than those of other high mountains in East Africa.     

Dispersal distance as a benefit of myrmecochory

Summary Nutrient-enrichment and predator avoidance are generally considered the major benefits of myrmecochory, but this is apparently not so in Australia where some of the greatest known concentrations of myrmecochorus plants occur. Here I demonstrate that distance dispersal is a potential benefit of myrmecochory in the Australian environment. Although mean dispersal distance at a site in southeastern Australia was only 2.1 m, the dispersal curve was characterised by a narrow peak and long tail. A dispersal curve of this shape has been shown by Green (1983) to be optimal when safe sites for seedling establishment are rare, as is typically the case for Australian myrmecochores in the absence of fire. Both mean disperal distance and shape of the dispersal curve are influenced strongly by nest density and dispersion, population size, and territoriality of seed-dispersing ants. I argue that distance dispersal is likely to be a benefit of myrmecochory throughout Australia, independent of any targeting of seeds to ant nests.     

Long‐term effects of exclusion of grazing stock on degraded herbaceous plant communities in a riparian Eucalyptus camaldulensis forest in south‐eastern Australia

Abstract Stock grazing has degraded many riparian ecosystems around the world. However, the potential for ecosystem recovery following the removal of grazing stock is poorly known. We developed a conceptual model to predict the responses of native and exotic herbaceous plants to grazing exclusion, based on site productivity and the degree of initial vegetation degradation. The effects of excluding grazing stock on richness, cover and composition of herbaceous plants were examined over 12 years in the degraded understorey of a riparian forest in Gulpa Island State Forest in south‐eastern Australia. We predicted that grazing exclusion would lead to limited changes in vegetation cover, richness and composition, owing to presumed low site productivity and the high degree of understorey degradation. Results showed that the cover, richness and composition of native and exotic species varied significantly among years. Over all plots, regions and years, total cover was slightly but significantly lower in grazed than in ungrazed plots (43.4% vs. 50.8%). While the cover of native plants increased over time in both treatments, the rate of increase was slightly greater in ungrazed plots. Grazing exclusion had no effect on the richness of native or exotic species, but had a significant but minor impact on plant composition, with different common species (mostly exotics) being promoted or diminished in ungrazed plots. The composition of grazed and ungrazed areas did not become more different over time. Overall, the effects that could be attributed to grazing exclusion were relatively minor and transient. Results are consistent with predictions based on site productivity and initial degradation, and should not be extrapolated to other more productive, or less degraded, riparian systems.     

How reliable are our vegetation analyses?

Abstract. Two sets of 40 relevés, made independently by two observers on the same 5m x 5m sample plots, were compared to estimate the sampling error and to assess the effect of this sampling error on (1) estimates of species richness and diversity (2) results of multivariate analyses, and (3) estimation of species turnover in repeated sampling. The relevés were made according to the standard Braun-Blanquet method. The sampling error was estimated for (1) recording of species in sample plots and (2) visual estimation of the degree of cover (or of the general population size). Despite the fact that the sample plots were searched thoroughly for 30 - 40 min, the number of overlooked species was high with a discrepancy of 13% between corresponding relevés. Regarding multivariate analysis, the error caused by missing species was at least as important as the error in visual estimation of species cover. The estimates of degree of cover using the Braun-Blanquet scale are sufficiently reliable for use in multivariate analysis when they are subjected to ordinal transformation. When average cover values are used, the patterns detected are based solely on dominants. Species richness and species diversity could be reliably estimated from the relevés, but the estimates of equitability are very unreliable. The classical relevé method remains one of the most efficient survey methods for recognition of vegetation types on the macro-community and landscape scales.     

How reliable is the monitoring of permanent vegetation plots? A test with multiple observers

Questions: A multiple plot design was developed for permanent vegetation plots. How reliable are the different methods used in this design and which changes can we measure? Location: Alpine meadows (2430 m a.s.l.) in the Swiss Alps. Methods: Four inventories were obtained from 40 m² plots: four subplots (0.4 m²) with a list of species, two 10m transects with the point method (50 points on each), one subplot (4m²) with a list of species and visual cover estimates as a percentage and the complete plot (40 m²) with a list of species and visual estimates in classes. This design was tested by five to seven experienced botanists in three plots. Results: Whatever the sampling size, only 45‐63% of the species were seen by all the observers. However, the majority of the overlooked species had cover < 0.1%. Pairs of observers overlooked 10‐20% less species than single observers. The point method was the best method for cover estimate, but it took much longer than visual cover estimates, and 100 points allowed for the monitoring of only a very limited number of species. The visual estimate as a percentage was more precise than classes. Working in pairs did not improve the estimates, but one botanist repeating the survey is more reliable than a succession of different observers. Conclusion: Lists of species are insufficient for monitoring. It is necessary to add cover estimates to allow for subsequent interpretations in spite of the overlooked species. The choice of the method depends on the available resources: the point method is time consuming but gives precise data for a limited number of species, while visual estimates are quick but allow for recording only large changes in cover. Constant pairs of observers improve the reliability of the records.