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.     

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.     

Flood‐deposited wood creates regeneration niches for riparian vegetation on a semi‐arid South African river

Question : The formation of large woody debris (LWD) piles during floods has significant impacts on riparian succession through pioneering plants often establishing in association with wood. We assess the importance of LWD for seed regeneration of riparian plants after a century‐scale flood disturbance in a semi‐arid environment. Location : The Sabie River within Kruger National Park in the semi‐arid northeast of South Africa. Methods : Our approach was to quantify the riparian soil seed bank, to record the frequency of establishment of riparian plants in woody debris piles, and to conduct experimental out‐plantings of common riparian trees in plots with and without LWD. Results : We found the abundance and diversity of seedlings were higher in soils taken from wood piles than from open reference areas, and most seedlings were herbaceous species. Surveys indicated that numbers of seedlings recorded within woody debris were significantly greater than in open reference areas or within established vegetation. Seedling establishment in various cover‐types also varied for different riparian tree species. Experimental out‐planting of seedlings of two riparian tree species (Philenoptera violacea and Combretum erythrophyllum) revealed that, after 433 days, planted seedlings survived only in woody debris piles. Conclusion : LWD formed after a large flood creates heterogeneous patches that may influence post‐disturbance regeneration of riparian vegetation by providing a variety of environmental niches for seedlings establishment. We suspect that higher seedling survival in LWD is due to increased moisture (particularly in the dry season) and nutrients, and protection from seasonal flooding and herbivory.     

Effects of artificial plant cover on plant colonization of a bare peat surface

Abstract. This paper describes the effect of artificial plant cover on plant colonization of a bare peat surface, resulting from peat harvesting. Plant species colonization was compared on plots supplied with plastic models simulating Vaccinium vitis-idaea plants and plots without this artificial cover. After two growing seasons, species composition and total biomass of the established plant cover were similar in the two plot types. However, the number of established seedlings in the plots with artificial cover was significantly higher than that in the plots without cover. Out of 13 species observed four differed significantly in their performance on the two plot types. Betula spec. had both higher seedling numbers and higher biomass on the test plots; Deschampsia cespitosa had a higher biomass, whereas the seedlings were too numerous to be counted; Salix phylicifolia had higher seedling numbers. On the contrary, the number of seedlings of Epilobium angustifoliwn was lower on plots with artificial cover. It is suggested that colonization by Betula, D. cespitosa and S. phylicifolia was facilitated mainly by the improved microclimatic and soil moisture conditions under the artificial plant cover. On the other hand, germination of E. angustifolium may be negatively influenced by the increased shade on the test plots.     

Effects of disturbance caused by traditional Spanish rural land use on the regeneration of Cytisus multiflorus

Abstract. Cytisus multiflorus is a leguminous matorral shrub native to the NW Iberian Peninsula, where it is one of the most important species in the extension of matorral at the expense of set-aside agricultural land. Dehesas have traditionally been used for extensive livestock raising and matorral was periodically burnt, cut or pulled out. The two latter practices are now out of use. However, burning is more frequent than in the past. The effects of fire, cutting, and pulling out of C. multiflorus on its regeneration was studied in order to ascertain whether the presently increasing distribution of the species relates to fire-stimulated regeneration or to the reduction of other traditional practices. Three years after treatment two sets of parameters were determined: 1. Plant origin: seedlings and different ramet types (ramets=resprout clumps), density, weight, and biomass as well as the percentage of resprouting. 2. Seed persistence at various soil depths. The possible mechanisms of breaking dormancy and plant emergence in different years after fire were studied in other experiments. The results suggest that the regeneration mechanism in C. multiflorus is stimulated by fire, but it is not an exclusive relationship. Stimulation occurs also after other disturbances leading to the total elimination of aerial biomass. The present expansion of the species does not appear to result from the abandonment of some traditional practices, such as cutting or pulling out, but from frequent fires (resulting in aerial-biomass elimination).     

Dispersal,diversity and distribution patterns in pioneer vegetation: The role of river‐floodplain connectivity

Question: Are there changes in dispersal patterns in floodplain pioneer vegetation with effects on seedling number, species richness and species composition along a gradient of declining river‐floodplain connectivity? Location: Middle Elbe river floodplain, Germany. Methods: An experiment with five treatments was set up along a gradient of declining river‐floodplain connectivity, partitioning seedlings into three groups: (1) emerging solely from water dispersed seeds, (2) from wind/animal dispersed seeds and (3) from the soil diaspore bank. Two controls were established: without any manipulation and exclusion of all seeds. The results were compared with those of vegetation and soil sampling to evaluate the representativeness of the experimental sites in terms of species composition, diversity, seedling number and soil parameters. Results: Water dispersal and the soil diaspore bank were the major dispersal strategies shaping floodplain pioneer vegetation at the Middle Elbe river. The number of seedlings, species richness and the variation in species composition in these habitats depend on the degree of connectivity. The seedling number and species richness is highest in sites of permanent or almost permanent exchange with the main channel, where water dispersal additionally contributes to the number of seedlings grown from the soil seed bank. Conclusion: The results underline the importance of river‐floodplain ecotones as sink habitats for water‐dispersed seeds. Considering the strongly reduced river‐floodplain interactions due to dykes and other engineering structures, management strategies are necessary to improve connectivity and the renewal of fluvial land forms.