Robin J. Pakeman

Leaf traits capture the effects of land use changes and climate on litter decomposability of grasslands across Europe

Land use and climate changes induce shifts in plant functional diversity and community structure, thereby modifying ecosystem processes. This is particularly true for litter decomposition, an essential process in the biogeochemical cycles of carbon and nutrients. In this study, we asked whether changes in functional traits of living leaves in response to changes in land use and climate were related to rates of litter potential decomposition, hereafter denoted litter decomposability, across a range of 10 contrasting sites. To disentangle the different control factors on litter decomposition, we conducted a microcosm experiment to determine the decomposability under standard conditions of litters collected in herbaceous communities from Europe and Israel. We tested how environmental factors (disturbance and climate) affected functional traits of living leaves and how these traits then modified litter quality and subsequent litter decomposability. Litter decomposability appeared proximately linked to initial litter quality, with particularly clear negative correlations with lignin-dependent indices (litter lignin concentr tion, lignin:nitrogen ratio, and fiber component). Litter quality was directly related to community-weighted mean traits. Lignin-dependent indices of litter quality were positively correlated with community-weighted mean leaf dry matter content (LDMC), and negatively correlated with community-weighted mean leaf nitrogen concentration (LNC). Consequently, litter decomposability was correlated negatively with community-weighted mean LDMC, and positively with community-weighted mean LNC. Environmental factors (disturbance and climate) influenced community-weighted mean traits. Plant communities experiencing less frequent or less intense disturbance exhibited higher community-weighted mean LDMC, and therefore higher litter lignin content and slower litter decomposability. LDMC therefore appears as a powerful marker of both changes in land use and of the pace of nutrient cycling across 10 contrasting sites.

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. Nomenclature: Stace (1997).

Rehabilitation of degraded dry heather [Calluna vulgaris (L.) Hull] moorland by controlled sheep grazing

Many areas of heathland in Europe have seen a decline in the area and condition of Calluna vulgaris (heather)-dominated vegetation, with subsequent declines in the associated faunal interest. Grazing, alongside burning, is still the predominant means of managing heathland vegetation, and, therefore, it is by manipulating this management that cost-effective improvements in vegetation condition can be made. This paper investigates the suitability of different grazing treatments for rehabilitating degraded ‘dry heath’. Treatments varied in the intensity (0–1.9 sheep ha−1 year−1) and timing (summer vs. winter) of sheep grazing. These treatments were compared with the behaviour of vegetation outside the fenced area kept under the previous management (open access all year round). As rabbits were common on the site, fences were erected to prevent access to the sheep grazed plots. Vegetation composition remained stable outside the fenced area, whilst all the fenced treatments showed a decrease in heather utilisation and an increase in the relative frequency of heather over the 5 years of the experiment. The increase was in proportion to the reduction in stocking rate, with only slow increases in relative frequency observed in the high grazing treatments (winter and summer). Other species that benefited from reducing grazing included Empetrum nigrum and Vaccinium myrtillus, whilst declines were observed for Agrostis capillaris and total monocotyledonous species. Only small overall differences were observed between the winter low, summer low and no sheep grazing treatments. However, a difference in response was present between the sheep exclosures and the sheep+rabbit exclosures, indicating that rabbits were having a noticeable effect on heather recovery at this site. On this degraded ‘dry heath’ system, imposing a reduction in stocking density improved the condition of the dwarf shrubs present and reduced the grass component of the vegetation. There was little effect of the timing of grazing, such that a reduction in sheep numbers to 0.8/0.9 sheep ha−1 year−1, to give utilisation levels of below 20%, can achieve the desired result of improving vegetation condition whilst still achieving some economic return from grazing. However, the wide range of ‘sustainable’ stocking densities for different heathland systems highlights the need to base effective management on measured utilisation rather than on stock numbers.

Multivariate identification of plant functional response and effect traits in an agricultural landscape

Plant functional traits have been proposed as a linkage between the environmental control of vegetation and ecosystem function. Identification of traits that mediate the response of plant species to the environment is well established, but the identification of effect traits and the linkage between the two sets is less developed. This was attempted for a study of eight contrasting land uses in a marginal agricultural landscape where data on vegetation, management controls of the disturbance regime, and soil characteristics, including nitrogen release, were measured simultaneously with measures of ecosystem function such as litter decomposition rates and primary productivity on 30 sites. Trait data were assembled from databases, and an iterative multivariate approach using the three table (species, trait, environment) method RLQ was employed to identify a parsimonious set of traits that predict plant species responses to the environment and a parsimonious set of traits that link vegetation to ecosystem function. The lists of response and effect traits were similar, and where differences were observed, traits were usually highly correlated with at least one trait in the other list. This approach identified a small number of traits (canopy height, leaf dry matter content, leaf size, and specific leaf area) that provide a means of linking vegetation responses to environmental change with changes in ecosystem function. Other response traits included vegetative spread strategy, start of flowering, and seed terminal velocity, but within the system studied these traits were all significantly correlated to the traits shared between the response and effect lists.

Low intensity, mixed livestock grazing improves the breeding abundance of a common insectivorous passerine

Abstract Livestock grazing is a major driver of ecosystem change and has been associated with significant declines in various bird species in Britain and worldwide. However, there is little experimental evidence to show how grazing affects bird populations. We manipulated livestock densities in a replicated field experiment and found that mixed sheep and cattle grazing, at low intensity, improved the breeding abundance of a common upland passerine, the meadow pipit Anthus pratensis, after two years. Plots stocked with sheep alone (at high or low density) or not stocked at all held fewer pipit territories. Despite a year-on-year decline in pairs of meadow pipits in intensively grazed plots, we found no effect of sheep number on breeding abundance. Our results support the hypothesis that mixed species of herbivores generate greater heterogeneity in vegetation structure, which modifies prey availability, resulting in a greater abundance of birds. The results of our study should inform the management of grassland areas and enhance the abundance of some bird species, particularly in areas that have seen significant shifts from mixed livestock grazing to grazing dominated by single species of animals.

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.

Rehabilitation of degraded Calluna vulgaris (L.) hull-dominated wet heath by controlled sheep grazing

Abstract Many upland areas of the British Isles have seen declines in the area and condition of heather ( Calluna vulgaris )-dominated heathland vegetation. To reverse this decline, management regimes must be designed to rehabilitate areas that have seen this decline. As most of this heathland vegetation is primarily managed by grazing, such management has to determine what stocking levels can maintain the vegetation in a desired state. This paper describes how to reverse this decline through suitable grazing management. A degraded ‘wet-heath’ system, previously grazed at 2.1 sheep ha −1 , was subject to a range of grazing treatments over a 5-year period. Treatments varied in intensity (0–1.4 sheep ha −1 ) and timing (summer only, winter only, or year round) of grazing. Grazing levels were maintained at 2.1 sheep ha −1 outside the fenced areas. Vegetation composition remained stable outside the fenced treatments. All the fenced treatments showed an increase in the relative frequency of the evergreen Calluna vulgaris , with the greatest increase being in the ungrazed treatment, and the least in the year round 1.4 sheep ha −1 treatment. This increase was in line with a reduction in heather utilisation to relatively low and sustainable levels. Other species that benefited from reduced grazing included Carex nigra , Deschampsia flexuosa and to a lesser extent Galium saxatile and Erica tetralix , whereas a range of moss species including Hypnum jutlandicum and Rhytidiadelphus loreus were more frequent at higher grazing levels. Though the recovery of heather was similar in the two seasonally grazed treatments, the vegetation showed different overall trajectories. Winter only grazed allowed a substantial increase in the cover of the deciduous Molinia caerulea , whereas this species was kept in check by summer only grazing. A stocking level of between 0.7 and 1.4 sheep ha −1 appears to be appropriate to maintain and even enhance the cover of heather on degraded wet heath. Complete removal is not necessary. Grazing restricted to the winter period is inappropriate in areas where M. caerulea occurs. Setting appropriate stocking levels to maintain the condition of the vegetation must take into account site conditions, especially the presence of species that can affect the utilisation of heather.

The potential for lowland heath regeneration following plantation removal

Historical records show that large areas of lowland heath have been planted with coniferous woodland. Such sites will increasingly be targeted for heathland restoration in order to achieve the objectives set by current conservation policy. Therefore information on management to achieve effective restoration is a priority for research. In this study we examined the size, composition and distribution of the seed bank beneath conifer plantations of different ages at two important heathland sites in Britain. Changes in the physical and chemical properties of heathland soil under the plantations were also described. There were significant, exponential declines in the mean density of viable heather seeds with plantation age at both sites. However, relatively large seed banks survived for 40 years and under some circumstances for more than 70 years, suggesting that restoration on first rotation sites is unlikely to be seed limited. There were small, but significant differences in the soil nutrient status under the plantations compared to the adjacent heaths. It was concluded that the main constraint on heathland regeneration is likely to be the burial of the seed bank by a thick layer of conifer leaf litter. The mechanical removal of this layer has been shown to be practically feasible.

Rabbit endozoochroy and seedbank build-up in an acidic grassland

The sources of seed for seedbank build-up in an acidic grassland were identified from analysis of differences in seedbank build-up over one year between plots where the input of rabbit pellets to ‘seed-free’ soil had either been left or removed. In parallel, the flux of seed arriving in rabbit pellets was monitored. Pellet seed content and total seed input were highest in late summer/early autumn and again in the spring. The seed content of the pellets was dominated by a small number of species: Sagina apetala, Senecio jacobaea, Urtica dioica and Veronica arvensis. Smaller seeded species were more likely to be present as germinable seed in the pellets. Seedbank build-up as a result of wind, splash or adhesive dispersal totalled 547xa0seedsm2. The additional effect of allowing seed input in pellets was 267xa0seedsm2, though this increase was not significant. The total increase in seedbank over one year was equivalent to 15–20% of the seedbank present in undisturbed soil. Eight species showed a significant build-up of seed in the seedbank over one year as a result of all means of dispersal, but only Myosotis discolor showed a significantly higher soil germinable seed content in the plots where pellets had been allowed to remain in situ. The build-up of seed in the seedbank is contrasted with the build-up of vegetation on disturbed areas within the same system.

Species spread and persistence: implications for experimental design and habitat re-creation

Abstract The patterns of species establishment, persistence and spread in a long‐term experiment investigating the re‐creation of chalk grassland using different seed mixtures on ex‐arable land were analysed with the intention of providing enhanced re‐creation/restoration prescriptions and a critique of experimental designs. Species had a wide range of behaviours; (1) species that did not establish from seed e.g. Blackstonia perfoliata, (2) establishment but poor persistence in closed sward e.g. Hieracium pilosella, (3) establishment and persistence but little spread e.g. Onobrychis viciifolia and (4) good establishment and spread over all treatments e.g. Centaurea nigra and Trisetum flavescens. Persistence appeared greater in species with lower germination rates in both light and dark. Spread was greater in species with higher seed mass and lower dark germination rates. However, results from small‐plot experiments should be critically analysed. Though initial results show the potential for re‐establishment of vegetation, subsequent vegetation dynamics may be more closely related to the invasion of species from other treatments than the original species sown. Experimental designs should employ either large plots and/or wide guard rows to extend their useful life span. Long‐term data show which species are useful for cost‐effective restoration. Some species need to be sown over the whole area to provide cover (a ‘grass matrix’) and competition with arable weeds. Others need to be sown over wide areas as they spread poorly. Some species can be sown at low densities or in small patches as they have the ability to spread in closed vegetation. Nomenclature:Clapham et al. (1987).