Grassland In Southern England Research Articles

Species indicators for naturally-regenerating and old calcareous grassland in southern England

Habitat restoration requires realistic goals. To naturally regenerate European lowland calcareous grassland, whose extent has severely declined, over a century may be required for vegetation to become indistinguishable from that of old calcareous grassland. Progress of natural regeneration can be characterized using member species of the reference vegetation as indicators of favourable site condition. Chronosequence studies have suggested that calcareous-grassland species differ predictably in their ability to colonize ex-arable land, with some usually colonizing early on, and others in later stages. If such patterns are affected by gradually-attenuating establishment limitation, this would have important implications for restoration practice and indication of progress. Particularly, late-colonizing species might be better indicators of favourable site conditions than early colonizers.To explore these aspects, we have reanalysed chronosequence data previously used to investigate causal mechanisms affecting calcareous-grassland restoration progress. We carried out an indicator species analysis to determine which species are indicative of particular stages of natural regeneration. Using correlation analyses, we tested whether species colonization patterns matched those found by previous chronosequence studies that were geographically more limited or relied on more informal approaches to determine species order of colonization. Correlation analyses were also used to test whether order of colonization could be explained by establishment limitation or by dispersal limitation, or by established plant strategies that underlie such limitations.We identified 30 species as indicative of particular stages of natural regeneration, including nine that specifically indicate old calcareous grassland. Correlation results confirmed high congruence with species order of colonization in previous chronosequence studies, and indicated that establishment limitation plays a role in shaping species order of colonization, potentially mediated through differential stress tolerance. We failed to demonstrate a role of dispersal limitation in shaping order of colonization. Based on our results, we derived three categories of indicator species for passively-restored calcareous grassland, mirroring the regeneration stage during which these species usually colonize. This includes a category labelled by us as ‘old-grassland indicators’ that achieve notable abundance only in old grassland. We conclude by discussing how such a categorization can benefit the measurement of restoration progress, the tentative identification of old grassland and its conservation, e.g. through linking agri-environment payments to the occurrence of old-grassland indicators, thus fostering positive change in farmer attitudes towards old grassland.

Changes in plant species richness and productivity in response to decreased nitrogen inputs in grassland in southern England

Biomass production and plant species diversity in grassland in southern England was monitored before and after a change from conventional to organic farming. Our 18-year study, part of the UK's Environmental Change Network long-term monitoring programme, showed that the cessation of artificial fertiliser use on grassland after conversion to organic farming resulted in a decrease in biomass production and an increase in plant species richness. Grassland productivity decreased immediately after fertiliser application ceased, and after two years the annual total biomass production had fallen by over 50%. In the subsequent decade, total annual grassland productivity did not change significantly, and yields reached 31–66% of the levels recorded pre-management change. Plant species richness that had remained stable during the first 5 years of our study under conventional farming, increased by 300% over the following 13 years under organic farm management. We suggest that the change in productivity is due to the altered composition of species within the plots. In the first few years after the change in farming practice, high yielding, nitrogen-loving plants were outcompeted by lower yielding grasses and forbs, and these species remained in the plots in the following years. This study shows that grassland can be converted from an environment lacking in plant species diversity to a relatively species-rich pasture within 10–15 years, simply by stopping or suspending nitrogen additions. We demonstrate that the trade-off for increasing species richness is a decrease in productivity. Grassland in the UK is often not only managed from a conservation perspective, but to also produce a profitable yield. By considering the species composition and encouraging specific beneficial species such as legumes, it may be possible to improve biomass productivity and reduce the trade-off.

Plant functional group composition modifies the effects of precipitation change on grassland ecosystem function.

Temperate grassland ecosystems face a future of precipitation change, which can alter community composition and ecosystem functions through reduced soil moisture and waterlogging. There is evidence that functionally diverse plant communities contain a wider range of water use and resource capture strategies, resulting in greater resistance of ecosystem function to precipitation change. To investigate this interaction between composition and precipitation change we performed a field experiment for three years in successional grassland in southern England. This consisted of two treatments. The first, precipitation change, simulated end of century predictions, and consisted of a summer drought phase alongside winter rainfall addition. The second, functional group identity, divided the plant community into three groups based on their functional traits- broadly described as perennials, caespitose grasses and annuals- and removed these groups in a factorial design. Ecosystem functions related to C, N and water cycling were measured regularly. Effects of functional groupidentity were apparent, with the dominant trend being that process rates were higher under control conditions where a range of perennial species were present. E.g. litter decomposition rates were significantly higher in plots containing several perennial species, the group with the highest average leaf N content. Process rates were also very strongly affected by the precipitation change treatmentwhen perennial plant species were dominant, but not where the community contained a high abundance of annual species and caespitose grasses. This contrasting response could be attributable to differing rooting patterns (shallower structures under annual plants, and deeper roots under perennials) and faster nutrient uptake in annuals compared to perennials. Our results indicate that precipitation change will have a smaller effect on key process rates in grasslandscontaining a range of perennial and annual species, and that maintaining the presence of key functional groups should be a crucial consideration in future grassland management.

Spatial heterogeneity and plant species richness at different spatial scales under rabbit grazing

Herbivores influence spatial heterogeneity in soil resources and vegetation in ecosystems. Despite increasing recognition that spatial heterogeneity can drive species richness at different spatial scales, few studies have quantified the effect of grazing on spatial heterogeneity and species richness simultaneously. Here we document both these variables in a rabbit-grazed grassland. We measured mean values and spatial patterns of grazing intensity, rabbit droppings, plant height, plant biomass, soil water content, ammonia and nitrate in sites grazed by rabbits and in matched, ungrazed exclosures in a grassland in southern England. Plant species richness was recorded at spatial scales ranging between 0.0001 and 150 m(2). Grazing reduced plant height and plant biomass but increased levels of ammonia and nitrate in the soil. Spatial statistics revealed that rabbit-grazed sites consisted of a mixture of heavily grazed patches with low vegetation and nutrient-rich soils (lawns) surrounded by patches of high vegetation with nutrient-poor soils (tussocks). The mean patch size (range) in the grazed controls was 2.1 +/- 0.3 m for vegetation height, 3.8 +/- 1.8 m for soil water content and 2.8 +/- 0.9 m for ammonia. This is in line with the patch sizes of grazing (2.4 +/- 0.5 m) and dropping deposition (3.7 +/- 0.6 m) by rabbits. In contrast, patchiness in the ungrazed exclosures had a larger patch size and was not present for all variables. Rabbit grazing increased plant species richness at all spatial scales. Species richness was negatively correlated with plant height, but positively correlated to the coefficient of variation of plant height at all plot sizes. Species richness in large plots (<25 m(2)) was also correlated to patch size. This study indicates that the abundance of strong competitors and the nutrient availability in the soil, as well as the heterogeneity and spatial pattern of these factors may influence species richness, but the importance of these factors can differ across spatial scales.

Evaluating the reliability of point estimates of wetland reference evaporation

Abstract. The Penman-Monteith formulation of evaporation has been criticised for its reliance upon point estimates so that areal estimates of wetland evaporation based upon single weather stations may be misleading. Typically, wetlands comprise a complex mosaic of land cover types from each of which evaporative rates may differ. The need to account for wetland patches when monitoring hydrological fluxes has been noted. This paper presents work carried out over a wet grassland in Southern England. The significance of fetch on actual evaporation was examined using the approach adopted by Gash (1986) based upon surface roughness to estimate the fraction of evaporation sensed from a specified distance upwind of the monitoring station. This theoretical analysis (assuming near-neutral conditions) reveals that the fraction of evaporation contributed by the surrounding area increases steadily to a value of 77% at a distance of 224 m and thereafter declines rapidly. Thus, point climate observations may not reflect surface conditions at greater distances. This result was tested through the deployment of four weather stations on the wetland. The resultant data suggested that homogeneous conditions prevailed so that the central weather station provided reliable areal estimates of reference evaporation during the observation period March–April 1999. This may be a result of not accounting for high wind speeds and roughness found in wetlands that lead to widespread atmospheric mixing. It should be noted this analysis was based upon data collected during the period March-April when wind direction was constant (westerly) and the land surface was moist. There could be more variation at other times of the year that would lead to greater heterogeneity in actual evaporation. Keywords: evaporation, Penman-Monteith, automatic weather station, fetch, wetland

Modelling long-term transpiration measurments from grassland in southern England

Observations of latent heat, sensible heat and momentum flux from eddy correlation equipment are presented, along with approximately weekly measurements of soil moisture content. The measurements were made over a pasture field in the south of England for 3 years, covering two dry years, 1995 and 1996 and a more normal year, 1997. The data show that, overall, approximately 80% of the measured net radiation was used for evaporation. During July and August of the two dry years, soil moisture deficits of over 150 mm developed within the top 1400 mm, and at these deficits the evaporation was reduced. During the summer months, overall between 71 and 76% of the net radiation was used for evaporation. During the winter months, the latent heat flux exceeded the net incoming radiation. An energy closure better than 10% suggests that the measurements were well founded. The surrounding trees may, however, have influenced the momentum exchange; the estimated roughness length of 38 mm is considerably more than the commonly accepted 10% of vegetation height. This extensive dataset provides an excellent test for the current UK Meteorological Office Surface Energy Scheme (MOSES) when applied to this site, the results of which are presented here. In its unoptimised form, the model can explain 72% of the variance of the hourly evaporative flux data, and with excellent agreement with the soil moisture measurements. Optimisation can improve the model fit, but only up to a maximum of 76%. The largest remaining errors occur during mid-summer when high temperatures, vapour pressure deficits and soil moisture deficits occur.

Seasonal trends of gross N mineralization in a natural calcareous grassland

AbstractThe effect of climate change on biologically mediated soil processes is likely to be complex and difficult to predict. The direct effect of changes in soil moisture and temperature may be obscured or even reversed by changes in the return of organic substrates to the soil. This paper is the first in a series reporting the results from a medium‐term investigation into the effect of simulated climate change on soil N mineralization in a seminatural calcareous grassland in southern England. Gross rates of N mineralization were determined by isotope dilution in plots subjected to winter warming (3 °C above ambient), enhanced or decreased summer rainfall and combinations of these treatments. The results from the control treatment, reported here, show a strong seasonality of gross N mineralization with rates highest in the Spring and Autumn and lowest in Summer. They indicate that water availability is the main restraint on microbial processes and plant growth and that, in the short term, enhanced summer rainfall is likely to be the main factor influencing nutrient turnover. Further papers will report the results from the imposed treatments.

Factors determining winter densities of birds on Environmentally Sensitive Area arable reversion grassland in southern England, with special reference to skylarks ( Alauda arvensis)

In the South Downs and South Wessex Downs Environmentally Sensitive Areas (ESAs) in the UK, conservation emphasis was on maintenance and restoration (arable reversion) of grassland for landscape, archaeological and botanical purposes. This work investigated the effect of large-scale arable reversion on nationally declining grassland birds, especially skylarks, using repeated surveys (over 225 grassland and arable fields, totalling 40 km 2) and an experiment, over the winters 1994/95–1996/97. Densities of wintering skylarks were higher on arable reversion fields where the sward was above 10 cm than on ones with shorter sward, and decreased with sward closure. Higher densities of skylarks foraged in cereal stubbles than on arable reversion grass, reflecting the availability of broad-leaved weed seeds. Any beneficial effect of experimentally opening the sward on established arable reversion fields was at best short-term. The results contributed to revised management prescriptions for the two ESAs in 1997 (South Downs) and 1998 (South Wessex Downs), which should improve conservation benefits to grassland birds.

Soil N dynamics in a natural calcareous grassland under a changing climate

This paper reports the results from a medium-term field scale investigation into the effects of simulated climate change on soil N mineralisation in a semi-natural calcareous grassland in southern England. The experiment utilised soil warming cables, automatic rainshelters and a watering system to examine two climate change scenarios: warmer winters with summer drought and warmer winters with enhanced summer rainfall. Gross N mineralisation rates in treated plots were determined, using 15N pool dilution techniques, at 6-weekly intervals over a 3-year period. Results from control plots showed a strong seasonality of mineralisation with highest rates in autumn and winter and lowest rates in summer. They suggest that water availability is the main constraint on microbial processes and plant growth. Unexpectedly, additional summer rainfall had no direct effect on N mineralisation at the time of application (summer). The treatment did, however, significantly (<0.05%) reduce rates in subsequent autumn and winter months. In contrast, summer drought significantly increased N mineralisation rates in autumn and winter. Winter warming similarly had no direct effect on N mineralisation in winter but decreased rates in spring. We hypothesise that the observed treatment effects result from changes in organic C and N input, in plant litter, resulting from the direct impact of climatic manipulation on perennial plant growth, death and senescence. This paper compares and contrasts the response to climate manipulation in the grassland system with results from other ecosystem types such as northern forests.

The effects of herbivory and vegetation on laboratory-raised Dactylorhiza practermissa (Orchidaceae) planted into grassland in southern England : S. L. McKendrick, Biological Conservation, 73(3), 1995, pp 215–220

The effects of herbivory and vegetation on laboratory-raised Dactylorhiza practermissa (Orchidaceae) planted into grassland in southern England : S. L. McKendrick, Biological Conservation, 73(3), 1995, pp 215–220

The effects of herbivory and vegetation on laboratory-raised Dactylorhiza praetermissa (Orchidaceae) planted into grassland in Southern England

Symbiotically-propagated seedlings of the southern marsh orchid Dactylorhiza praetermissa were planted into a meadow at Wakehurst Place, Southern England and followed for 30 months. Most mortality in the first growing season could be attributed to either mammals or pheasants. Fencing was effective in eliminating herbivory by mammals. Application of molluscicide did not always prevent mollusc grazing. No seedlings survived the first winter in areas dominated by dense Juncus articulatus. The majority of the remaining plants flowered or produced a second rosette of leaves within three years of planting. The practical implications of these results in the introduction of D. praetermissa and other orchids to natural vegetation are discussed.

Control of Relative Abundance of Perennials in Chalk Grassland in Southern England. III. Shoot Phenology

(1) The shoot phenology of five graminoid and five dicotyledonous species is described from the time-course of leaf birth and death on marked individuals (tillers and rosettes) in chalk grassland in Sussex at intervals between February 1982 and March 1983. (2) Pronounced seasonal patterns in leaf number per unit studied were found for the dicotyledons but were much less obvious for the graminoids; shoot phenological patterns are more likely to be expressed at the level of tiller populations. (3) Considerable variation in the cycles of leaf birth and death were found for all species; differences in phase of these cycles results in the phenological patterns observed for leaf number. (4) It is concluded that interspecific differences in shoot phenology, particularly in relation to early-season and asynchronous growth, may be of significance in the determination of relative abundance of perennials in chalk grassland.

Control of Relative Abundance of Perennials in Chalk Grassland in Southern England. II. Vertical Canopy Structure

(1) The vertical distribution of leaves was recorded using a point quadrat, at intervals throughout 1981 and 1982 from grazed chalk grasslands in Sussex and Wiltshire; the data were analysed to describe the overtopping relationships between leaves of different species. (2) In most cases there was a positive correlation between the relative height of rosette leaves of each species ('relative leaf position') and their relative abundance. (3) Despite the short turf of these chalk grasslands (< 10 cm excluding inflorescence stalks) there is pronounced vertical structure; the tallest species (often graminoids) tend to be the most abundant, and the shortest species (often dicotyledons) the least abundant. (4) It is suggested that relative abundance is partly determined by competition for light and that there is a steady-state relationship between species with taller and with shorter rosette leaves. The balance between species is determined by grazing with taller species favoured between grazing events and the shorter species at the time of grazing. (5) In grazed chalk grasslands in southern England, the balance falls in favour of species with taller and widely spreading growth-forms because grazing intensity is low and there is often a long time between successive grazing periods. (6) The species with shorter growth-forms persist, and the species with taller growthforms do not come to ecological dominance, because of chronic, differential defoliation.

Control of Relative Abundance of Perennials in Chalk Grassland in Southern England: I. Constancy of Rank Order and Results of Pot- and Field- Experiments on the Role of Interference

SUMMARY (1) A study of the relative abundance of perennial species in chalk grassland in southern England revealed great constancy (P < 0.001) in rank order: (a) across any one site (twenty stands of 0-48 m2 in an area of 3480 m2), (b) from year to year (1980-82) and (c) between two sites 150 km apart. Comparison of these records with transformed data from phytosociological accounts in the literature provides evidence of high constancy of rank order over much larger areas and longer periods. Ancient chalk grassland is thus an ideal community for investigation of the mechanisms determining the relative abundance of species. (2) Two experiments were carried out to determine the extent to which the order of relative abundance may be explained by short-term (1-2 years) ability to interfere with growth of plants of other species in the vegetative phase. (3) In an outdoor pot experiment, fourteen dicotyledons were grown in monocultures and in mixtures with one or two standard species (a grass and a dicotyledon). After 18 weeks there was a significant positive correlation between interference ability in the experiment and mean relative abundance in the field. Essentially, the same results were found after 70 weeks. (4) In a field experiment, six dicotyledons were grown in monocultures, and in all possible mixtures of two species, in plots on an area of chalk grassland cleared of turf. After 62 weeks, the order in interference effects paralleled the order of relative abundance in the grassland. There was also a positive correlation between interference ability and the mean height of rosette leaves of each species. This suggests the importance of aboveground interference. (5) After 115 weeks, and for 3 years afterwards, additional elements of the perennial life-history became important in determining performance in the plots, especially longevity of adult individuals, degree of lateral spread and the frequency of establishment of seedlings. (6) We suggest that the control of relative abundance in chalk grassland involves steady-state relationships: (i) between species with taller (and more widely-spreading) and shorter individuals and (ii) between species with longer- and shorter-lived individuals.

Destruction at the ground and the diurnal cycle of concentration of ozone and other gases

AbstractSeveral gaseous pollutants show marked diurnal variations, with daily maxima and nightly minima of concentration. The nocturnal decrease is held to be due to deposition at the ground, with isolation of the lowest layers of the atmosphere from the air above by the nocturnal inversion. In the morning, deep mixing is re‐established and brings material down to the surface from higher layers. Ozone exhibits this behaviour clearly and is convenient to study as an example, but it has been suggested by some authors that changes in the rates of gas‐phase reactions that produce and destroy ozone are responsible for the daily cycle of concentration. We have studied the destruction of ozone at the surface and conclude that this process can probably explain the nocturnal decline of concentration near the ground.Deposition rates for ozone were deduced from profiles of concentration, windspeed and temperature measured over grassland in southern England. The mean deposition velocity was 0.58cms−1 by day and 0.29cms−1 by night. The aerodynamic and surface components of resistance both increased at night, but the behaviour of the surface resistance was not consistent with stomatal control.Observations of the nocturnal decrease of ozone concentration at the same site are reported. A computer simulation of the behaviour of ozone, with a description of the nocturnal boundary layer due to Wyngaard, duplicated the major features observed, and a procedure for predicting the height of the mixed layer and the rate of decline of ozone concentration at the ground gave good agreement with observations on many occasions.The nocturnal behaviour has important consequences for the measurement of mean concentrations in the atmosphere, and for long‐range transport. Observation of the daily cycle of concentration may indicate the likely importance of the surface sink for many gases.

Population Changes in Neophilaenus lineatus (L.) (Homoptera: Cercopidae) in Different Parts of its Range

The majority of published population studies of animals consider the circumstances pertaining to a single population in a given type of habitat. Yet authors such as Huffaker (1958), Clark et al. (1967) and Klomp (1962) have emphasized the importance of distinguishing between 'salubrious and hazardous environments' (Huffaker 1958), in interpreting a study and in particular when attempting to reconcile the seemingly polarized views of the theorists. Of course, it will not necessarily be possible in a particular case to define a habitat as salubrious or hazardous since there will be every gradation between these extremes. It should, however, be possible to study a population living in a habitat which is representative of the broad conditions under which this species is observed to live and thrive and to contrast this with a population which has been specifically chosen because it is living at the extreme edge of the species' range. In 1965 I published an account of a short-term study of a cercopid (Neophilaenus lineatus (L.)) living at high altitude in the Pennine hills of northern England (Whittaker 1965a, b). This did appear to be on the verge of extinction as judged by the fact that it became temporarily extinct at only slightly higher altitudes. This same species occurs throughout Britain in areas of uncultivated grassland. The study has since been extended with the aim of comparing these populations with one in a low-altitude grassland in southern England at Wytham Woods, Berkshire. As far as can be judged from knowledge of the distribution and biology of N. lineatus, this latter habitat is favoured by the species whilst the populations on the Pennines are living in a peripheral, hazardous environment. The grassland known as Upper Seeds at Wytham Woods also supports two other species of cercopid, and one of these, Philaenus spumarius (L.), shares a pipunculid parasite Verrallia aucta Fallen with Neophilaenus lineatus. They differ, however, in the important respect that N. lineatus feeds exclusively on monocotyledons and Philaenus spumarius on dicotyledons. This account will compare and contrast the changes occurring in two populations of Neophilaenus lineatus at the edge of the species' range on the Pennine hills (over a period of 9 years) with a population from lowland southern England during 6 years.