Phosphorus supply influences heathland responses to atmospheric nitrogen deposition

On an upland moor dominated by pioneer Calluna vulgaris and with an understorey of mosses and lichens, experimental plots were treated with factorial combinations of nitrogen (N) at +0 and +20kg Nha(-1)yr(-1), and phosphorus (P) at +0 and +5kg Pha(-1)yr(-1). Over the 4-year duration of the experiment, the cover of the Calluna canopy increased in density over time as part of normal phenological development. Moss cover increased initially in response to N addition but then remained static; increases in cover in response to P addition became stronger over time, eventually causing reductions in the cover of the dominant Calluna canopy. Lichen cover virtually disappeared within 4 years in plots receiving +20kg Nha(-1)yr(-1) and also in separate plots receiving +10kg Nha(-1)yr(-1), but this effect was reversed by the addition of P.     

Linking field experiments to long-term simulation of impacts of nitrogen deposition on heathlands and moorlands

The results from three long-term field manipulation studies of the impacts of increased nitrogen deposition (0–120 kg N ha^?1 yr^?1) on lowland and upland heathlands in the UK were compared, to test if common responses are observed. Consistent increases in Calluna foliar N content and decreases in litter C:N ratios were found across all sites, while increases in N leaching were not observed at any site over the range 0–80 kg ha^?1 yr^?1. However, the response of Calluna biomass did vary between sites, possibly reflecting site differences in nutrient status and management histories. Five versions of a simulation model of heathland responses to N were developed, each reflecting different assumptions about the fate and turnover of soil N. Model outputs supported the deduction from mass balance calculations at two of the field sites that N additions have resulted in an increase in immobilisation; the latter was needed to prevent the model overestimating measured N leaching. However, this version of the model significantly underestimated Calluna biomass. Model versions, which included uptake of organic N by Callunaand re-mobilisation of N from the soil organic store provided some improvement in the fit between modelled and field biomass data, but re-mobilisation also led to an overestimation of N leaching. Quantification of these processes and their response to increased N deposition are therefore critical to interpreting experimental data and predicting the long-term impacts of atmospheric deposition on heathlands and moorlands.     

Impacts of atmospheric pollution on the plant communities of British acid grasslands

Air pollutants are recognised as important agents of ecosystem change but few studies consider the effects of multiple pollutants and their interactions. Here we use ordination, constrained cluster analysis and indicator value analyses to identify potential environmental controls on species composition, ecological groupings and indicator species in a gradient study of UK acid grasslands. The community composition of these grasslands is related to climate, grazing, ozone exposure and nitrogen deposition, with evidence for an interaction between the ecological impacts of base cation and nitrogen deposition. Ozone is a key agent in species compositional change but is not associated with a reduction in species richness or diversity indices, showing the subtly different drivers on these two aspects of ecosystem degradation. Our results demonstrate the effects of multiple interacting pollutants, which may collectively have a greater impact than any individual agent.     

Effects of increased deposition of atmospheric nitrogen on an upland moor: leaching of N species and soil solution chemistry

This study was designed to investigate the leaching response of an upland moorland to long-term (10 yr) ammonium nitrate additions of 40, 80 and 120 kg N ha(-1) yr(-1) and to relate this response to other indications of potential system damage, such as acidification and cation displacement. Results showed increases in nitrate leaching only in response to high rates of N input, in excess of 96 and 136 kg total N input ha(-1) yr(-1) for the organic Oh horizon and mineral Eag horizon, respectively. Individual N additions did not alter ammonium leaching from either horizon and ammonium was completely retained by the mineral horizon. Leaching of dissolved organic nitrogen (DON) from the Oh horizon was increased by the addition of 40 kg N ha(-1) yr(-1), but in spite of increases, retention of total dissolved nitrogen reached a maximum of 92% and 95% of 80 kg added N ha(-1) yr(-1) in the Oh and Eag horizons, respectively. Calcium concentrations and calcium/aluminium ratios were decreased in the Eag horizon solution with significant acidification mainly in the Oh horizon leachate. Nitrate leaching is currently regarded as an early indication of N saturation in forest systems. Litter C:N ratios were significantly lowered but values remained above a threshold predicted to increase leaching of N in forests.     

Physiological proteins in resource-limited herbivores experiencing a population die-off

Nutrient availability is predicted to interact with herbivore population densities. Competition for low quality food at high density may reduce summer food intake, and in turn winter survival. Conversely, low population density may favor physiological recovery through better access to better quality spring forage. Here, we take advantage of the long-term study of the Soay sheep population of St. Kilda (Scotland) to measure plasma protein markers and immunity in two consecutive summers with contrasting population densities. We show that, following a winter die-off resulting in a shift to low population density, albumin and total proteins increased, but only in adult sheep. The effect was not solely attributable to selective disappearance of malnourished sheep. Similarly, the concentration of antibodies was higher following the die-off, potentially indicating recovery of immune function. Overall, our results are consistent with the physiological recovery of surviving individuals after a harsh winter.     

The effects of castration, sex ratio and population density on social segregation and habitat use in Soay sheep

We analysed 16 years of census data gathered on the island of Hirta (archipelago of St. Kilda) to investigate the effects of castration, population density, sex ratio, season and group type on habitat use and social segregation of Soay sheep. From 1978 to 1980, 72 male lambs were castrated. We used this experiment to study how a change in reproductive status could affect sociality and habitat choice of these males. Males, females and castrates were all segregated outside the rutting season in autumn. Castrates were the least segregated from females in spring and summer but were most segregated from them during the pre-rut. The more equal the sex ratios, the higher was the degree of social segregation. The three sex classes used similar habitat types, namely, Holcus agrostis, Agrostis festuca and Calluna habitats. Holcus agrostis and Agrostis festuca were top- and second-ranked in female and castrate habitat use, while Holcus agrostis and Calluna were the two top habitat types used by rams. It is unclear why males included Calluna heath habitats, but it cannot be excluded that they might have shifted their use depending on forage availability. A lack in differences in habitat use between castrates and females suggests that body size differences alone cannot be the driving factor for habitat segregation in male and female Soay sheep and that there are reasons other than body size that could motivate reproductive males to use additional habitat types, such as Calluna heath. Although habitat use shifted from one habitat type to the next between low- and high-population-density years and between seasons, there was no clear link between population density and how different groups (male, female or castrate) used these areas. We discuss effects of reproductive status, population density and sex ratio on social segregation and habitat use and suggest that these factors need to be taken into account when investigating causes of sexual segregation in ungulates.     

Effects of increased deposition of atmospheric nitrogen on an upland moor: nitrogen budgets and nutrient accumulation

This study was designed to investigate the effect of long-term (11 years) ammonium nitrate additions on standing mass, nutrient content (% and kg ha(-1)), and the proportion of the added N retained within the different compartments of the system. The results showed that more than 90% of all N in the system was found in the soil, particularly in the organic (Oh) horizon. Added N increased the standing mass of vegetation and litter and the N content (kg N ha(-1)) of almost all measured plant, litter and soil compartments. Green tissue P and K content (kg ha(-1)) were increased, and N:P ratios were increased to levels indicative of P limitation. At the lowest treatment, most of the additional N was found in plant/litter compartments, but at higher treatments, there were steep increases in the amount of additional N in the underlying organic and mineral (Eag) horizons. The budget revealed that the proportion of added N found in the system as a whole increased from 60%, 80% and up to 90% in response to the 40, 80 and 120 kg N ha(-1) year(-1) treatments, respectively.     

Estimating the functional form for the density dependence from life history data

Two contrasting approaches to the analysis of population dynamics are currently popular: demographic approaches where the associations between demographic rates and statistics summarizing the population dynamics are identified; and time series approaches where the associations between population dynamics, population density, and environmental covariates are investigated. In this paper, we develop an approach to combine these methods and apply it to detailed data from Soay sheep (Ovis aries). We examine how density dependence and climate contribute to fluctuations in population size via age- and sex-specific demographic rates, and how fluctuations in demographic structure influence population dynamics. Density dependence contributes most, followed by climatic variation, age structure fluctuations and interactions between density and climate. We then simplify the density-dependent, stochastic, age-structured demographic model and derive a new phenomenological time series which captures the dynamics better than previously selected functions. The simple method we develop has potential to provide substantial insight into the relative contributions of population and individual-level processes to the dynamics of populations in stochastic environments.     

Modelling nitrogen saturation and carbon accumulation in heathland soils under elevated nitrogen deposition

A simple model of nitrogen (N) saturation, based on an extension of the biogeochemical model MAGIC, has been tested at two long-running heathland N manipulation experiments. The model simulates N immobilisation as a function of organic soil C/N ratio, but permits a proportion of immobilised N to be accompanied by accumulation of soil carbon (C), slowing the rate of C/N ratio change and subsequent N saturation. The model successfully reproduced observed treatment effects on soil C and N, and inorganic N leaching, for both sites. At the C-rich upland site, N addition led to relatively small reductions in soil C/N, low inorganic N leaching, and a substantial increase in organic soil C. At the C-poor lowland site, soil C/N ratio decreases and N leaching increases were much more dramatic, and soil C accumulation predicted to be smaller. The study suggests that (i) a simple model can effectively simulate observed changes in soil and leachate N; (ii) previous model predictions based on a constant soil C pool may overpredict future N leaching; (iii) N saturation may develop most rapidly in dry, organic-poor, high-decomposition systems; and (iv) N deposition may lead to significantly enhanced soil C sequestration, particularly in wet, nutrient-poor, organic-rich systems.