Soil seed bank composition along a gradient from dry alvar grassland to Juniperus shrubland

Abstract. Dry alvar grasslands on limestone on the Baltic island of Öland, SE Sweden, are very species-rich as long as the traditional agricultural exploitation of grazing and fire wood collection continues. After abandonment, encroachment of Juniperus communis starts and a closed woodland can develop within 100 yr. A chronosequence, representing a successional series, was used for the comparison of sites still grazed, and sites ungrazed for about 20, 55 and 80 yr, respectively. Out of the 58 characteristic dry alvar grassland species 55 % disappeared from the established vegetation after 80 yr of abandoning, and 80 % also vanished from the seed bank. Arenaria serpyllifolia, Trifolium repens, Agrostis vinealis, Linum catharticum, Polygala vulgaris, Cerastium fontanum, Luzula campestris, Achillea millefolium and Potentilla tabernaemontani were the only species left in the seed bank. More than 75 % of the dry alvar grassland species were classified as having a transient or short-term persistent seed bank. It is concluded that restoration management, by cutting junipers, of overgrown dry alvar grassland cannot rely on the longevity of seeds in the soil. Seeds have to be dispersed by wind or grazing animals.     

Soil seed bank and driftline composition along a successional gradient on a temperate salt marsh

Abstract. This study focuses on the relationship between vegetation succession and soil seed bank composition on the Schiermonnikoog (The Netherlands) salt marsh over 100 yr. The importance of driftline material in seed dispersal and the relationship with succession is also investigated. The results indicate that the majority of species have a transient or short‐term seed persistent bank. Seeds of most species are able to float over the salt marsh and become concentrated in the driftline higher up the marsh. After plants have established a seed bank forms, which disappears when vegetation is replaced by later‐successional species. Exceptions are Spergularia mar‐itima, which is still present in the seed bank of late successional stages, and Juncus gerardi and Glaux maritima, which appear in the seed bank of early successional stages, but are absent in the vegetation. Based on the results of this study constraints and possibilities for salt‐marsh restoration by de‐embankment are discussed.     

Vegetation and soil seed bank in a 23-year grazing exclusion chronosequence in a Mediterranean dry grassland

Long-term grazing shaped plant diversity in dry Mediterranean grasslands. Abandonment of grazing affects plant diversity especially in the northern Mediterranean. Considerable efforts are, therefore, under way for grassland conservation and restoration. Yet, we do not know at which temporal scales impacts of grazing abandonment appear and in particular how soil seed banks evolve after longer grazing abandonment. Here, we provide detailed data from one of the very few long-term experiments available. These experiments provide data for up to 23 years (1982–2005) of grazing exclusion built in 1982, 1989, 2000 and 2001. Grazing exclusion decreased species richness, modified vegetation structure and changed soil parameters. Decline in species richness appears in communities that experienced 16 and 23 years of grazing exclusion. Only four to nine plant species of this Mediterranean grassland built persistent soil seed banks appearing after grazing exclusion, compared to 40–50 species in the established vegetation of grazed plots. Hence, similarity between vegetation and soil seed bank decreased with time of grazing exclusion. Even 23 years after abandonment, no woody plants colonised the experiments. We conclude that vegetation will recover fast from grazing abandonment in the short-term. Nevertheless, longer abandonment will impact diversity due to reduced soil seed banks.     

Vegetation and seed bank dynamics in a tidal freshwater marsh

Questions: What are the feedbacks among the seed bank, parent vegetation, and landscape structure that control plant species turnover in space and time in a tidal freshwater marsh? How can these feedbacks be used to better understand marsh community dynamics and to establish restoration practices that seek to restore vegetation diversity of this important and widely distributed ecosystem? Location: Potomac River, Virginia, United States (15 km south of Washington, DC). Methods: We sampled the seed bank and standing vegetation in a tidal freshwater marsh and explored similarities between seed bank and vegetation composition through space and time. We then investigated marsh surface elevation, distance to nearest tidal channels, and life history of component species as potential explanations for the observed vegetation patterns. Results: The composition of individual plots changed considerably from year to year; however, the composition at broader spatial (whole marsh) and temporal (4 years) scales was relatively stable. Species composition of the seed bank was dissimilar to both the previous and current year's standing vegetation, and similarity to standing vegetation was particularly low in plots dominated by annual species. Landscape structure and life history characteristics of individual species best explained the spatiotemporal variability in marsh vegetation. Conclusions: Restoration designs should be landscape‐dependent and explicitly incorporate spatially structured elements such as elevation gradients to maximize community diversity in reconstructed tidal freshwater marshes. Optimal designs include areas of high seed input, areas of high species turnover, as well as other areas of greater stability.     

Heat shock increases the reliability of a temperate calcareous grassland seed bank study

Reliable assessment of the density and species richness of the viable seeds in the soil is essential to estimate the probability of successful restoration of a particular plant community. Since temperate calcareous grasslands contain several thermophilous species typical of fire prone habitats, heat shock can be expected to break dormancy of target species for calcareous grassland restoration. We tested the effect of heat shock on the estimated density and species richness of seeds in soil samples. Heat shock treatments between 70 and 110°C resulted in both a higher estimated seed density and estimated species richness, in particular of specialist calcareous grassland species. Moreover, 25 species germinated exclusively from the heat-treated samples. This indicates that several species would have been missed by handling soil seed bank samples following the standard germination protocol, resulting in an underestimation of the restoration potential.     

Native-species seed additions do not shift restored prairie plant communities from exotic to native states

Exotic- and native-dominated communities can exist as alternate states in landscapes, but whether exotic-dominated states are persistent in the face of propagule pressure from native species is not well known. Here, we asked whether adding native seeds to low diversity, exotic-dominated patches would shift them to a more diverse, native state by using a long-term experiment with tallgrass prairie species in Iowa, USA. Previous work established that community assembly history led to alternate exotic or native states of perennial species. We added native seeds to plots in the spring after removing aboveground biomass with fire. We found that an experimental seed addition did not cause a shift from exotic to native states. Plots seeded eight years earlier in spring and without a priority effect continued to have the highest abundance and diversity of native species and lowest proportion of exotics. Our results suggest that exotic-dominated states in restorations can persist in the face of native species propagule pressure. Thus, assembly history can play a strong role in generating and maintaining alternate states over long time frames that are relevant to restoration. New restoration projects in exotic-dominated landscapes should maximize effort toward establishing native species during initial stages of restoration.     

The role of the seed bank,seed rain and the timing of disturbance in gap regeneration

Abstract. Question: Does the degree and timing of disturbance contribute significantly to the pattern and process of regeneration in plant communities as a consequence of the availability and number of species of propagules present? Location: Acid grassland at 230 m a.s.l., eastern Scotland, UK. Methods : Plots were surface disturbed or had their soil profile inverted at monthly intervals at 12 dates during a year. Seed bank and seed rain were assessed at each treatment time. The effect of disturbance intensity and timing on the regenerating vegetation was assessed. Results: Removing the seed bank significantly slowed regeneration, as it contributed 43 % of developing cover after one year where it was present. At an individual seed level, seed in the seed rain had a much higher likelihood of contributing to the regenerating vegetation than a seed in the seed bank. Some species showed a reliance on the seed bank for regeneration, and hence there was a significant difference in the vegetation that developed between plots with the seed bank intact and those where it was removed. Winter disturbed plots (little seed rain) had slower rates of re‐vegetation than summer disturbed plots. Timing had little effect on species composition, though a significantly higher cover of perennial forb species developed on the winter disturbed plots. Conclusion: Removing the contribution of the seed bank had a greater effect on the composition of regenerating vegetation than the effect of seasonal variation on the seed rain.     

Restoration of wooded meadows ‐ a comparative analysis along a chronosequence on Öland (Sweden)

Abstract. Wooded meadows on the Baltic Island of Öland result from traditional agricultural management over centuries which has led to a species‐rich vegetation with high species diversity. Today, nearly all of these meadows have been abandoned and became rapidly overgrown by deciduous shrub and tree species forming a closed canopy which resulted in a rapid and strong decrease in species numbers of the herb layer. Recent efforts aim to restore overgrown wooded meadows by cutting single shrubs and trees to open the canopy. However, the effects of abandonment as well as of any restoration management in wooded meadows have rarely been documented until now. Mechanisms driving succession after restoration such as the dispersal potential of the respective species over time and space have not been analysed yet. Therefore, a chronosequence was studied which included a traditionally managed wooded meadow, an overgrown meadow which has been abandoned for more than 100 yr and a meadow which was restored 36 yr ago by cutting and is now grazed. We analysed the soil seed bank of the 3 meadows in comparison with the established vegetation and endozoochorous seed dispersal by cattle and sheep. After abandonment 87% of the typical grassland species vanished from the established vegetation and were replaced by species characteristic of woodland and disturbed grassland communities. The mean number of species decreased from 52 species per plot (4 m²) to 18 species. Mean species number and number of seeds in the seed bank declined significantly from the traditionally managed to the overgrown meadow. Most of the grassland species were assigned to a transient seed bank type while only 1/3 could be classified as having a short‐term persistent seed bank. Thus, restoration of wooded meadows cannot rely on the soil seed bank. Endozoochorous seed dispersal by cattle and sheep was shown for 15% of the species with seed densities per 100 g air dried dung of 737 and 767, respectively. Movement of animals between ancient and restored wooded meadows is recommended since many of the species only occurred in low densities and therefore, will probably not be found in the dung samples.     

Restoration of high species density in calcareous grassland: the role of seed rain and soil seed bank

Abstract. A small-scale restoration experiment concerning a species-rich calcareous grassland was conducted over a 7-yr period. 20 yr of abandonment (1971-1990) resulted in both a strong reduction of the number of species and an absolute dominance of the shrub Crataegus monogyna and the grass Brachypodium pinnatum - which contributed 98% of the above-ground herb phytomass. The plot (3 m x 6 m) was cleared in September 1990 and thereafter yearly mown in autumn. Small soil seed bank samples were taken immediately after clearing and again in the autumn of 1996. The inner part of the plot showed a reduced soil seed bank compared to the plot underneath the vegetation outside the plot in 1990. Six years later an increased number of seeds in the soil in this inside area could be recorded mainly in the upper soil layer, although the amount was only 40% compared to the outside or control area. Ca. 20% of the species present in the restored vegetation were recorded in seed traps during the main seed dispersal period in 1997. Restoration of high species richness in the plot was a fast process during the first few years: from seven species in 1990 towards 39 three years later. This was significantly faster than the restoration process of the calcareous grassland at the same slope 20 yr earlier. This was probably due to the increased size of the community species pool at the site as a result of appropriate management during the last decades. Relative species number increase was highest at the largest scale during this experiment, viz. at the total plot area. The success of restoration of species-rich, dry grassland vegetation largely depends on the interaction between several conditions, e.g. seed availability, either originating from the seed rain or soil seed bank, favourable germination conditions and a successful development to a mature, seed producing individual. Germination and establishment strongly depend on suitable vegetation structure as a result of sustainable management.     

Relationships among the seed rain,seed bank and vegetation of a Hawaiian forest

Abstract. Hawaiian ecosystems are prone to invasion by alien plant species. I compared the seed rain, seed bank, and vegetation of a native Hawaiian forest to examine the potential role that seed ecology plays in allowing alien species to invade native forest. Absolute cover of seed plants in the forest was 126 %, annual seed rain was 5 713 seeds m^-2 yr^-1, and the mean density of seedlings emerging from the seed bank averaged across four seasons was 1 020/m². The endemic tree Metrosideros polymorpha was the most abundant species in the vegetation, seed rain and winter seed bank. Overall, native seed plants comprised 95 % of the relative cover in the vegetation and 99 % of the seeds in the seed rain, but alien species comprised 67 % of the seeds in the seed bank. Alien species tended to form persistent seed banks while native species formed transient or pseudo-persistent seed banks. Dominance of the seed bank by alien species with persistent seed banks suggests that aliens are favorably placed to increase in abundance in the vegetation if the forest is disturbed.     

Germinable soil seed banks in abandoned grasslands in central and western Norway and their significance for restoration

Questions: Could the seed bank increase biodiversity during restoration of abandoned, species‐poor, formerly cultivated vegetation? Is it possible to identify how climate, soil and former and present management and vegetation affected the seed bank? Location: The study sites were eight abandoned grasslands, four in Orkdal, central Norway and four in Gaular, western Norway. Methods: 144 seed bank samples were collected from three depths. Each sample was sown and placed in a greenhouse. After three months, the trays were dried and stored at 4°C in a dry place for two months. This was repeated twice. Results: There was a separation of the two regions along the first DCA axis in both the seed bank and in the vegetation analysis and also a clear separation of the seed bank from the vegetation along the second axis. These results are caused by differences in former management as well as temperature, precipitation and soil type between Gaular and Orkdal. We found more annuals, short‐lived species and species demanding light open conditions in the seed bank than in the vegetation probably because these species have the capacity for producing persistent seeds. Most of the species found only in the seed bank were found in very few samples and with few individuals. Conclusion: These results suggest that it may be difficult to increase vegetation biodiversity through restoration of grasslands such as those investigated if the natural soil seed bank is the main seed source.     

Dynamics of species richness and composition in a limestone grassland restored after tree cutting

Abstract. After abandonment many limestone grasslands have been overgrown by trees and shrubs; as a result, species-rich communities with many regionally rare and endangered species are vanishing. Some studies suggest that, in cases where grassland is being restored, the species composition and rate of change is strongly dependent on the initial conditions, i.e. the earlier presence of grassland species and the opportunity for colonization of new sites by grassland species. These hypotheses were tested in a five-year restoration experiment after the clearing of a 35-yr-old secondary pine wood developed on abandoned grassland. Tree cutting induced rapid changes in the floristic composition and species cover. The number of grassland species from the class Festuco-Brometea increased significantly in the restored grassland, but their cover was much lower than in the old grassland. Canonical Correspondence Analysis showed significant differences in species composition between the grassland restored in former wood gaps and that developed in former closed wood. In wood gap sites the cover of species from the class Molinio-Arrhenatheretea and tufted perennials was much higher, whereas the cover of Festuco-Brometea species was lower. Significantly more shrubs, woodland species, ruderal and nitrophilous species as well as annual and biennial species occurred in the former closed wood site. It was found that richness and composition of the restored grassland depended strongly on the community composition before tree cutting, as well as on the presence of grassland species in the neighbourhood. Periodical tree cutting enables the maintenance of a temporal-spatial mosaic of scrub-grassland communities in isolated habitats and the preservation of local species diversity.     

The dynamics of species richness in an experimentally restored calcareous grassland

Abstract. This paper concerns the hypothesis that shoot (light) competition is the main interaction determining the community change during succession from a relatively species-poor deciduous forest (an overgrown former grassland) to a species-rich grassland, while root (nutrient) competition is of little importance. In a 4-yr restoration experiment, clearcutting, mowing and root trenching were used as treatments. The results did not reject the hypothesis. However, the significance of separating two kinds of shoot competition - ‘coarse-scale (between different growth forms) and ‘fine-scale’ (between similar growth forms) became evident. Release from the ‘coarse-scale’ shoot competition (between different growth forms) increased species richness at the beginning of the experiment. This change was interpreted as the replacement of one species pool (shade-tolerant herbaceous perennials) by another (light-demanding herbaceous perennials), the second pool containing considerably more species. The importance of ‘fine-scale’ shoot competition increased gradually - the levelling of competition by mowing resulted in a more pronounced increase in species richness during successive years. The elimination of ‘coarse-scale’ root competition seemed to be important to some extent only in combination with another treatment - mowing. Initial colonization of the cleared area by individual species was a stochastic process which had little relevance to life-history traits. True grassland species were able to colonize quickly. On the community scale, the developing community still remained relatively poor in species. In all plots which were cleared but not mown, succession already started to reverse towards woody vegetation in the third year.     

The persistence of calcareous grassland species in the soil seed bank under developing and established scrub

The relationships between the composition of the soil seed bank, the field layer vegetation, and the scrub canopy were investigated along a 69 m transect, grading from incipient woodland, through scrub, into intensively rabbit-grazed calcareous grassland. The results are used to assess the persistence of species associated with open calcareous grassland in the seed bank under developing scrub. Scrub age, composition and density, changed along the transect from the woodland to open grassland. A total of 35 forb and grass species were found in the field layer. The pattern evident in the scrub layer was also reflected in the herbaceous vegetation. The field layer in the most closed portion of the transect, where the scrub was oldest, was dominated by shade-tolerant species normally associated with woodland habitats. The abundance of these species decreased along the transect as the scrub age declined, and the field layer became increasingly dominated by species typical of open grassland. A total of 47 species germinated from the seed bank. Few species were recorded in the seed bank along the entire length of the transect. Overall, the seed bank was dominated by Hypericum perforatum and Centaurium erythraea, which accounted for 38.2% and 28.6% of emerging seedlings respectively. As with a number of similar studies, the composition of the seed bank had a low correspondence with the composition of the field layer vegetation. The results also emphasise that the composition of the seed bank can be viewed as an ecological palimpsest, with germinable seed of species from each stage of the old-field succession occurring in the soil. The seed bank is an important component in the re-vegetation of an area after disturbance such as scrub removal. This study supports the findings of previous research in showing that relatively few characteristic calcareous grassland species form persistent seed banks. The soil seed bank would therefore appear to be of limited value in the restoration of such grassland following scrub removal.     

Long‐term seed bank dynamics in a temperate forest under conversion from coppice‐with‐standards to high forest management

Questions: How do changes in forest management, i.e. in disturbance type and frequency, influence species diversity, abundance and composition of the seed bank? How does the relationship between seed bank and vegetation change? What are the implications for seed bank dynamics? Location: An ancient Quercus petraea — Carpinus betulus forest in conversion from coppice‐with‐standards to regular Quercus high forest near Montargis, France. Methods: Seed bank and vegetation were sampled in six replicated stand types, forming a chronosequence along the conversion pathway. The stand types represented mid‐successional stages of stands in transition from coppice‐with‐standards (to high forest (16 plots) and early‐ and mid‐successional high forest stands (32 plots). Results: Seed bank density and species richness decreased with time since last disturbance. Adjusting for seed density effects obscured species richness differences between stand types, but species of later seres were nested subsets of earlier seres, implying concomitant shifts in species richness and composition with time since disturbance. Later seres were characterized by species with low seed weight and high seed longevity. Seed banks of early seres were more similar to vegetation than to later seres. Conclusions: Abandonment of the coppice‐with‐standards regime altered the seed bank characteristics, as well as its relationship with vegetation. Longer management cycles under high forest yield impoverished seed banks. For their persistence, seed bank species will increasingly rely on management of permanently open areas in the forest landscape. Thus, revegetation at the beginning of new high‐forest cycles may increasingly depend on inflow from seed sources.     

Soil seed bank and species turnover in a limestone grassland

Abstract. The vegetation and soil seed bank in a limestone grassland in southern Sweden were studied in permanent 1-m² subplots which had been either grazed or not grazed for 17 yr. Of the 92 species recorded, 18 were present only in the seed bank, 28 were more frequent and 24 were less frequent in the seed bank than in the vegetation and 22 were not detected in the seed bank. Among the species present in the seed bank, therophytes were over-represented. Species turnover in the vegetation was estimated from presence/absence data collected in 1980, 1986 and 1990. Turnover was high, but there were no differences between grazed and ungrazed subplots. The turnover for individual species was also high in many cases. There was no clear relationship between the turnover of a species and its presence in the persistent seed bank.     

Soil seed bank and floristic diversity in a forest‐grassland mosaic in southern Spain

Abstract. Soil seed bank and floristic diversity were studied in a forest of Quercus suber, a forest of Quercus canariensis and a grassland, forming a vegetation mosaic in Los Alcornocales Natural Park, southern Spain. The soil seed bank was estimated by the germination technique. In each community patch, diversity, woody species cover and herbaceous species frequency was measured. Three biodiversity components – species richness, endemism and taxonomic singularity – were considered in the vegetation and the seed bank. Forest patches had a soil seed bank of ca. 11 200–14 100 seed.m^?2 and their composition had low resemblance to (epigeal) vegetation. The grassland patch had a more dense seed bank (ca. 31 800 seed.m^?2) and a higher index of similarity with vegetation, compared with the forests nearby. The complete forest diversity was 71–78 species on 0.1 ha, including 12–15 species found only in the seed bank; the grassland species richness was higher (113 species on 0.1 ha). We discuss the role of soil seed banks in the vegetation dynamics and in the complete plant biodiversity of the mosaic landscape studied.     

Ecological correlations between the persistence of the soil seed bank and several plant traits,including seed dormancy

It was empirically showed that seed size and life history correlate with the formation of a soil seed bank. Although no empirical data are available that indicate a close relationship between seed dormancy and the soil seed bank, dormancy has been considered essential to the formation of a soil seed bank. I have considered the formation of the soil seed bank and survival of seeds for more than a year in the soil, and the persistence and survival of the seed bank for more than 5 years. These periods were derived from the definition of a persistent seed bank and the criterion for seed banks of long-term persistence. Plant traits that are closely related to the formation or persistence of a seed bank and their relationships to dormancy were analysed using two pre-existing databases of seed longevity in soil and comparative ecology. The integrated database comprised 18 plant traits and seed bank formation or persistence data. This approach was used to identify more reliable general empirical rules. The results of a regression tree analysis and common statistical tests of plant traits indicated that only life history and seed size were closely related to seed bank formation, and dormancy was not essential for the formation and persistence of a seed bank. However, the contribution of dormancy differed slightly between dormancy types. Scarification or dry storage requirements to break dormancy slightly enhanced the formation and persistence of a seed bank, whereas a chilling requirement decreased the formation and persistence of a seed bank. In contrast, fluctuating temperature requirements clearly contributed to the formation and persistence of a seed bank.