R.H. Marrs

Biotic homogenization and changes in species diversity across human-modified ecosystems

Changing land use and the spread of ‘winning’ native or exotic plants are expected to lead to biotic homogenization (BH), in which previously distinct plant communities become progressively more similar. In parallel, many ecosystems have recently seen increases in local species (α-) diversity, yet γ-diversity has continued to decline at larger scales. Using national ecological surveillance data for Great Britain, we quantify relationships between change in α-diversity and between-habitat homogenizations at two levels of organization: species composition and plant functional traits. Across Britain both increases and decreases in α-diversity were observed in small random sampling plots (10–200 m2) located within a national random sample of 1 km square regions. As α-diversity declined (spatially in 1978 or temporally between 1978 and 1998), plant communities became functionally more similar, but species-compositional similarity declined. Thus, different communities converged on a narrower range of winning trait syndromes, but species identities remained historically contingent, differentiating a mosaic of residual species-poor habitat patches within each 1 km square. The reverse trends in β-diversity occurred where α-diversity increased. When impacted by the same type and intensity of environmental change, directions of change in α-diversity are likely to depend upon differences in starting productivity and disturbance. This is one reason why local diversity change and BH across habitats are not likely to be consistently coupled.

The conservation value of bracken Pteridium aquilinum (L.) Kuhn-dominated communities in the UK, and an assessment of the ecological impact of bracken expansion or its removal

Bracken Pteridium aquilinum is generally regarded by farmers, foresters and conservationists as a problem species. Bracken stands are usually poor floristically, except where they protect a relict, woodland vernal flora. Similarly, bracken generally has a poor fauna; in some instances stands can provide cover for some birds and animals, but have a high conservation value only in a few specific cases. Further spread of bracken will lead to the loss of many areas of other habitats with a high conservation value, e.g. moorland and lowland heathland, and thus a decline in many uncommon species. There is therefore a choice between allowing the continuing spread of bracken and thus reducing the ecological and landscape diversity of Britain, or controlling it on a large scale. Bracken can be controlled by herbicide use or mechanical means; both give the same on-site selectivity and intensity of treatment, but are expensive. Irrespective of how it is controlled there will be difficulties with revegetation of semi-natural communities. The release of alien biological control agents is an alternative strategy; this could be cheap and offers the possibility of reducing bracken vigour on a countrywide scale. However, there would be no control over which sites were affected or the effects at a particular site.

An assessment of change in Calluna heathlands in Breckland, eastern England, between 1983 and 1991

Recent studies in the Netherlands have suggested that Calluna heathlands are being replaced by grassland because of elevated nitrogen levels in polluted rain. This paper reports the results of a survey of Calluna heathlands in the Brecklands of East Anglia, where high nitrogen levels are also reported and the climate is Continental. The Calluna heaths showed a wide spectrum of change, from those which had almost completely changed to grassland between 1983 and 1991 to others where Calluna remained dominant. It was hypothesised that the major factor initiating these changes was the critical part of the Calluna regeneration cycle between building/mature/degenerate phases and the new pioneer phase. In Breckland this transition is under exogenous control, and can be initiated directly by climate, management or insect herbivory. All of these factors can cause large-scale Calluna death, which provides the gaps for grass invasion, and all of them could interact with effects of high pollutant levels.

Determination of buffer zones to protect seedlings of non-target plants from the effects of glyphosate spray drift

Abstract There is an increasing need to protect semi-natural vegetation from the potential effects of herbicide drift. One way to protect sensitive sites is to surround them with a no-spray buffer zone. Earlier estimates of buffer zone size based on bioassay experiments with established perennials suggested zones needed to be 6–10 m wide. In this paper four bioassay experiments are reported, where seedlings grown in trays were exposed downwind of glyphosate applications and taken to a glasshouse for assessment. Three experiments were done with Lychnis flos-cuculi seedlings including one with different surrounding grass structures, and Experiment 4 tested the response of 15 species typical of semi-natural vegetation. The mortality of Lychnis flos-cuculi varied between experiments and appeared more or less unaffected by grassland structure except immediately downwind of the sprayer. The multi-species experiment indicated a wide sensitivity to spray drift, and one species was affected between 15 and 20 m downwind. Thus, seedlings of some species were affected at greater distances than established plants, indicating either greater capture of drift or a greater sensitivity. On sites where seedling establishment is an important mechanism for community regeneration, buffer zones may need to be 20 m wide.

Ecosystem Development on Naturally Colonized China Clay Wastes: I. Vegetation Changes and Overall Accumulation of Organic Matter and Nutrients

(1) As a result of changes in mining operations and recessions in the china clay industry there are a number of abandoned waste tips in Cornwall ranging in age since last tipping from 16-116 years. These tips have been naturally colonized by vegetation. (2) The vegetation on a series of waste tips is described and four speciesassemblages-two pioneer groups, an intermediate group and a mature woodland group-are identified. It is clear that succession to a woodland ecosystem can occur on these waste tips. (3) The accumulation of organic matter and nutrients was measured and is discussed in relation to successional development. The central role of nitrogen in these new environments is suggested, and the potential inputs to the nitrogen budget examined.

Impacts of climate, management and nitrogen deposition on the dynamics of lowland heathland

. The design of a cell-based spatial simulation model of heathland vegetation dynamics including the impacts of rainfall, nitrogen deposition and management is described. The model includes aspects of life history, colonization and seed dispersal for Calluna vulgaris and Deschampsia flexuosa, a common heathland grass. The interactive effects of rainfall, nitrogen deposition and management by cutting or rotovating on vegetation dynamics, and particularly relative cover of Calluna and Deschampsia are investigated. Management by cutting or rotovating resulted in more stable vegetation dynamics, with pure Calluna stands being persistent in the longer term and Deschampsia expanding more slowly when management was imposed. Cutting and rotovating management interacted differently with rainfall and nitrogen deposition. A smaller management interval was required to maintain high Calluna cover by cutting when rainfall was low. More frequent cutting was also required under conditions of high nitrogen deposition. This effect was greatest in simulations with moderate to high rainfall. The outcome of management by rotovating was little affected by rainfall or nitrogen deposition. It is concluded that heathlands that experience varying climates may show different responses to nitrogen deposition and management. This variation should be taken into account when defining critical loads or designing management prescriptions.

The effects of shade on the plant communities of rides in plantation woodland and implications for butterfly conservation.

Abstract During 1985–6 the vegetation in the rides (forest tracks) of eight English lowland woods, planted on ancient woodland sites was surveyed. Shade was measured in each transect by hemispherical photography. Dicotyledonous species richness and the majority of dicot plant species declined as shade increased. The results for individual species compare favourably with Ellenbergs (L) value in most cases, and a method is suggested for modifying Ellenbergs data in the light of new information. The woodlands contained species of both open and shaded habitats and communities were influenced by soil acidity. Because shade in woodland rides increases as the trees mature, theory states that there will almost certainly be a reduction in plant species diversity. Butterfly populations would also be adversely affected, partly because of the reduction in direct sunlight, and partly because their larval foodplants decline in both quantity and quality since they are generally associated with low to moderate shade.

Heathland and acid grassland creation on arable soils at Minsmere: identification of potential problems and a test of cropping to impoverish soils

Abstract The RSPB is attempting to establish heathland and acid grassland on 158 ha of arable land at Minsmere, ultimately to benefit two species of rare breeding birds, the stone curlew Burhinus oedicnemus and the woodlark Lullula arborea. An initial study of the arable soils was carried out to assess their suitability for the establishment of Calluna heathland and acid grassland. Selected chemical properties of the arable soils were compared to (1) existing healthland and (2) some old-field successions on former arable land, both adjacent to the arable fields to assess the magnitude of the change required. Three soil factors were identified as potential constraints; the arable soils had a much greater soil pH and higher concentrations of exchangeable calcium and extractable phosphorus than heathland soils, almost certainly from previous lime and fertilizer additions. These factors will almost certainly have to be reduced in order to establish heath or acid grassland communities on the arable soils at Minsmere. In the initial stages the RSPB followed the prescription adopted by MAFF for the re-establishment of heathland on arable soils within the Breckland Environmentally Sensitive Area. This involved an arable cropping regime designed to reduce ‘soil fertility’. At Minsmere, linseed, spring and winter barley, and cereal rye were planted, and inorganic nitrogen and potassium fertilizers applied. The aim was to improve crop yield, increase nutrient uptake and hence remove more nutrients from the system. The crops were harvestable in most years, although yields were lower than those found under normal farm management. The cropping removed more nutrients from the system than measured inputs, but there have been no appreciable reductions in soil pH or available nutrients. There was a slight indication that the exchangeable calcium concentrations may be declining after seven years, but no reduction in soil pH was found. Accordingly, cropping must be viewed as a medium to long-term option (> 7 years) for impoverishing the arable soils at Minsmere.

Methane (CH4) emission from a tidal marsh in the Min River estuary, southeast China

The total methane emission to the atmosphere and hydrosphere, and its seasonal variation, were estimated using an enclosed static chamber technique from a tidal marshes dominated by Phragmites australis (common reed) in the Min River estuary, southeast China. Measurements were taken at three tidal stages (before flood, during the flooding and ebbing process, and after ebb). Potential rates of methane production from the marsh sediment layers were also measured using an incubation technique. This P. australis tidal marsh was a net methane source, emitting 32.59 and 6.87 g CH4· m−2· yr−1 to the atmosphere and hydrosphere, respectively. There was considerable monthly variation with emissions greater before flood in some months, whereas at other months emission was greater after ebb. The average methane fluxes were 5.13, 5.06 and 4.74 mg CH4 m−2 h−1 before flood, during flooding and ebbing, and after ebb, respectively. Emissions to the tidewater and the atmosphere during the flooding and ebbing process were 2.98 and 2.08 mg CH4 m−2 h−1,respectively. Sediment methane production potential (0–40 cm depth) ranged from 0.028–0.123 μ g CH4· g−1· d−1, with the greatest production was in the surface soil. Methane fluxes had a significant correlation with atmospheric, sediment temperature and above ground biomass. The implications of these data for global warming are discussed briefly.

The influence of soil-type, drought and nitrogen addition on interactions between Calluna vulgaris and Deschampsia flexuosa: implications for heathland regeneration

Increasing dominance of grass species such as Deschampsia flexuosa on lowland dry heathlands is a major problem for land managers. This problem is especially acute in north-west Europe where replacement of dwarf-shrubs by grasses has been linked to high levels of nitrogen deposition. Interactions between Calluna vulgaris and D. flexuosa were examined using pot experiments under a variety of watering (drought vs. no drought), nitrogen addition (10 vs. 50 kg N ha−1 yr−1) and soil type (peaty vs. sandy) regimes. Two experiments using identical replacement series designs were used to investigate interactions between these two species at the seedling and young plant stage. Response of the two species to the environmental variables was different for seedlings and young plants. In the seedling experiment D. flexuosa responded significantly to soil type only, with greater cover on peaty soil than on sandy soil after 6 months. C. vulgaris germination showed a significant response to watering regime, with fewer seedlings in plots that had a drought treatment imposed. When the experiment was repeated using young plants of C. vulgaris and D. flexuosa both species responded significantly to soil type only. Height, % cover and shoot biomass of both species were greater when grown in peaty soil than when grown in sandy soil. Watering and nitrogen addition had few significant effects. Species interactions were mainly one-way. C. vulgaris biomass yield was reduced when grown in mixture with D. flexuosa, whilst D. flexuosa yield was unaffected by the presence of C. vulgaris. The yield density curve for D. flexuosa indicated that intra-specific competition was occurring at planting densities above 29 plants m−2 whilst this was not seen for C. vulgaris. The effect of soil type on species interactions was also investigated. C. vulgaris was best able to compete with D. flexuosa on sandy soils where the shoot biomass ratio was greatest (C. vulgaris:D. flexuosa= 1:4.4). Differences in the responses of these species to the environmental variables investigated may help to explain heathland vegetation changes seen under field conditions.