The role of seedbank and sown seeds in the restoration of an English flood-meadow

Abstract. In 1985 an opportunity arose to make use of an area (referred to as Somerford Mead) which had recently been used for intensive grass or cereal production but which originally had been a permanent hay-meadow. Situated on circum- neutral alluvial soils over limestone gravel, it is in close proximity to the River Thames and to species-rich flood meadows, including Oxey Mead (Fig. 1). This paper describes the early stages of an experiment to recreate a ‘typical’ flood-meadow community using seed harvested from Oxey Mead. Before sowing the seed, soil samples were taken and seedling emergence suggested that little of the original flood-meadow seed bank survived. Although Oxey Mead was known to contain at least 57 species, germination of samples of the sown seed in the glasshouse was confined to 12 species. All except Bromus commutatus and Trisetum flavescens and an additional nine Oxey Mead species were recorded in the field, together with 24 arable weed/ruderal species. Soil-nutrient contents (N, P and K) were high; so the restoration of this flood-meadow using only management techniques (hay cutting at the end of June and aftermath grazing) and the local seed bank would take a long time. The use of Alopecuros pratensis-Sanguisorba officinalis seed mixture was justified by the accelerated succession on Somerford Mead towards the parent community.     

The effects of grazing on local colonisation and extinction during early succession

Estimates of colonisation of plant species were made at three spatial scales in an old-field on limestone subject to five experimental sheep grazing regimes. Local extinctions within grazing treatments were estimated in 1-m² permanent quadrats. These data were used to assess the effects of grazing treatment and spatial scale on the process of species change over a period of six years. Colonisation of the lOha field was virtually a random draw of plant species from adj acent vegetation, irrespective of plant life-history traits including dispersal strategy. The effects of grazing on colonisation increased at smaller spatial scales. Colonisation rates changed little during the study on the 10 ha scale, but declined steeply with time at smaller scales. Colonisation rates of short-lived species declined more than those of perennials, and short-lived species were subject to erratic episodes of extinction which did not affect perennials. Short periods of grazing enhanced colonisation rates of all species, but extinction rates were the same as in ungrazed controls. Grazing for longer periods further enhanced colonisation rates, but also increased extinction rates. This produced diversity patterns consistent with a ‘hump-backed model’, except that no grazing treatment was heavy enough to decrease diversity. Ungrazed controls had low species diversity, but areas grazed for longer periods were no more diverse after six years than those grazed for short periods.     

Competitive effects of herbaceous vegetation on tree seedling emergence,growth and survival: Does gap size matter?

Question: What is the effect of gap size on the seedling emergence, growth and survival of four common tree species in wooded pastures? Location: A pasture in the Jura mountains, Switzerland. Methods: Seeds were sown in a complete three‐way factorial design with eight blocks in May 2003. Each block consisted of a competition treatment (four gap sizes including zero) and a mowing treatment (mown and unmown). Emergence, survival and total biomass of tree seedlings of three species (Picea abies, Acer pseudoplatanus and Fagus sylvatica) were measured. A fourth species (Abies alba) failed to germinate. Results: Gaps had a positive influence on the early stages of tree development for all species. Larger gaps favoured growth and survival more than small gaps. Seedling growth was higher when vegetation around the openings was mown. Mowing the vegetation at gap size zero enhanced both growth and survival compared to unmown vegetation. Mown gaps larger than zero had increased seedling desiccation but decreased seedling predation. Species showed similar trends in their emergence and growth responses to gap size and mowing treatments but for Picea emergence rate was higher and survivorship was lower than for Acer and Fagus. Conclusions: Gap size does matter for tree seedling success but even in more favourable large gaps only a small percentage of seedlings emerged and survived. The effects of gaps on tree seedling establishment are complex as a result of interactions between biotic and abiotic changes caused by gaps.     

Convergent succession of phytoplankton in microcosms with different inoculum species composition

Summary Different initial mixtures of phyto-and zooplankton from different lakes were grown under identical chemical and physical conditions in medium size (8-and 12–1) laboratory microcosm cultures until convergence of phytoplankton species composition was attained. Five such experiments with four (four experiments) or three (one experiment) microcosm cultures were run. Three experiments were performed with weak stirring which permitted sedimentary elimination of the diatoms. Two experiments were conducted with stronger stirring to prevent sedimentation. In the three sedimentation intensive experiments, the final phytoplankton community was composed of the filamentous chlorophyte Mougeotia thylespora together with a smaller biomass of nanoplanktic algae. In the two sedimentation free experiments the final phytoplankton community consisted of pennate diatoms. Both dissolved nutrient concentrations and the chemical composition of biomass suggested strong nutrient limitation of algal growth rates in the final phase of the experiments. The zooplankton communities at the end of the experiments were composed of species that were apparently unable to ingest the large, dominant algae and that presumably fed on the nanoplanktic undergrowth and the bacteria. There was a distinct sequence of events in all experiments: first, the large zooplankton species (Daphnia and Copepoda) were replaced by smaller ones (Chydorus, Bosmina, rotifers); second, all cultures within one experiment developed the same nutritional status (limitation by the same nutrient); and third, the taxonomic composition of phytoplankton of the different cultures within one experiment converged. The last took 7–9 weeks, with is about 2–3 times as long as the time needed in a phytoplankton competition experiment to reach the final outcome.     

Plant colonization of a bare peat surface: population changes and spatial patterns

Abstract. Changes in size and spatial arrangement of plant populations established on an initially bare peat surface were described over a period of 5 yr by following plant individuals on a 1-cm grid in an area of 10 m x 25 m. The spatial pattern of populations and association between species was analyzed statistically. The study site was very slowly colonized by 14 perennial plant species. The early successional stage was dominated by Carex rostrata, with a clumped spatial distribution, and the homogeneously distributed Eriophorum vaginatum and Pinus sylvestris. Both the growth in size of populations and changes in their spatial distribution were interpreted as a result of species dispersal ability, tolerance to severity of the substrate and pattern of reproduction.     

Effects of nitrogen limitation on species replacement dynamics during early secondary succession on a semiarid sagebrush site

Summary A soil nitrogen (N) availability gradient was induced on a disturbed sagebrush site in northwestern Colorado by fertilizing with nitrogen (high available N), applying sucrose (low available N), and applying neither nitrogen nor sucrose (control). Species composition was studied for 3 years. At the end of the study, N concentration of aboveground tissue of 3 major species was determined. The rate of species replacement was most rapid on plots receiving the sucrose treatment and was slowest on plots receiving the N treatment. Early-seral dominats had greater tissue N concentrations when availability of the resource was high but lower tissue N concentrations when available soil N became limited. Midseral dominants displayed the opposite pattern. These results suggest that the supply of available soil N, and therefore the dynamics of N incorporation in perennial plant tissue, is a primary mechanism in controlling the rate of secondary succession within this semiarid ecosystem.     

Mammals,reptiles and crabs on the Krakatau Islands: Their roles in the ecosystem

In 1982 I studied the changes which had taken place in the fauna of the Krakatau Islands over the last fifty years, and elucidated the roles of terrestrial consumers and reducers in the developing ecosystems of these islands. Only two species of rat were found (Rattus rattus andR. tiomanicus) as observed by Dammerman (1948) in 1933, butR. tiomanicus had newly established its population on Sertung. No rats were found on Anak Krakatau. Although many skinks (Mabuya multifasciata) were observed on R. Besar, this species was not found on the other islands. The rat and skink are considered strong competitors to the sand crab (Ocypode kuhli), because while the crab was restricted to the sandy beach on the three islands where rat and skink live it had expanded inland on Anak Krakatau where there are no rats and skinks. The two banded monitor (Varanus sarvator) was found on every island and it mainly fed on crabs (Gecarcoidea natalis andMetasesarma aubryi). Thus it is suggested that the crab plays an important role in the process of faunal succession on the Krakatau Islands.     

Competition among the pioneers in a seasonal soft-bottom benthic succession: field experiments and analysis of the Gilpin-Ayala competition model

Summary Controlled experiments, designed to assess the effects of pioneers on succession on an intertidal sandflat, provided evidence for interspecific competition between juvenile Hobsonia florida (Polychaeta, Ampharetidae) and oligochaetes. The field data were fitted to both the linear Volterra and non-linear Gilpin-Ayala competition equations. With its greater number of parameters, the Gilpin-Ayala model must provide a better fit to observed population abundances. The Gilpin-Ayala model is flawed as an explanation of the population trajectories of the H. florida and oligochaetes, because its non-linearity parameter affects only intraspecific competion. With either model our field data demonstrate a solution to Hutchinson's paradox. With competition coefficients near unity and similar carrying capacities, the predicted population trajectories are heavily dependent on initial conditions. The predicted times to competitive exclusion are long and can easily exceed the typical period of environmental constancy. Our study offers evidence for Neill's competitive bottleneck: competition acts primarily on the developmental stages of one of a pair of competing species. The permanent meiofauna may act as a competitive bottleneck for the population growth of benthic macrofauna. The mechanism of this competitive interaction probably involves exploitative interspecific competition for benthic diatoms.