Impact of sheep grazing on nutrient budgets of dry heathlands

Questions: What effect does sheep grazing have on the nutrient budgets of heathlands? Can grazing compensate for atmospheric nutrient loads in heathland ecosystems? What are the conclusions for heathland management? Location: Lüneburg Heath, NW Germany. Methods: During a one-year grazing experiment (stocking rate 1.1 sheep/ha) nutrient balances for N, Ca, K, Mg and P were calculated by quantifying input rates (atmospheric deposition, sheep excrement) and output rates (biomass removal, leaching). Results: Atmospheric nutrient deposition amounted to 22.8 kg.ha⁻¹.a⁻¹ for N and < 0.2 kg.ha⁻¹.a⁻¹ for P. Sheep excrement increased the inputs for N and P by ca. 3.5 and 0.2 kg.ha⁻¹.a⁻¹, respectively. Grazing reduced N- and P-stores in the above-ground biomass by 25.6 and 1.9 kg.ha⁻¹.a⁻¹, respectively. N-and P-losses via leaching amounted to 2.2 and < 0.2 kg.ha⁻¹.a⁻¹. Output:input ratios for P were high, indicating that grazing severely affected P-budgets of heaths. Conclusions: Our results suggest that sheep grazing has the potential to compensate for atmospheric nutrient loads (particularly for current N deposition rates). However, in the long term the combination of elevated N-deposition and P-loss due to grazing may cause a shift from N-(co-) limited to more P-(co-) limited plant growth. To counteract an aggravation of P-deficiency in the long term, grazing may be combined with management measures that affect P-budgets to a lesser extent (e.g. prescribed burning).     

Impact of prescribed burning on the nutrient balance of heathlands with particular reference to nitrogen and phosphorus

Question: Can prescribed winter burning compensate atmospheric nutrient loads for dry heathlands? What effects does prescribed burning have on nutrient balances, particularly as regards the limiting nutrients N and P? Location: Lueneburg Heath, NW Germany. Methods: In two burning experiments (in 10/15 year old Calluna‐stands) nutrient balances (for N, Ca, K, Mg, P) were calculated by analysing nutrient inputs (atmospheric deposition, ash deposition), nutrient stores (above‐ground biomass, organic horizon) and nutrient outputs (biomass combustion, leaching). Results: Atmospheric nutrient deposition amounted to 22.8 kg.ha^‐1.a^‐1 for N and < 0.5 kg.ha^‐1.a^‐1 for P. Nutrient stores in the above‐ground biomass were 95/197 kg.ha^‐1 for N and 5/13 kg.ha^‐1 for P (first/second experiment, respectively). From these stores 90/53% (for N) and 25/14% (for P) were removed by burning. Effects of leaching on nutrient balances were low. In the first two years after burning, leaching rates of N increased by about 4/6 kg.ha^‐1, whereas leaching rates of P did not change significantly. Input/output‐ratios showed that prescribed burning leads to positive nutrient balances for N, Ca and Mg in the long term. For example, the amounts of N removed by prescribed burning are equivalent to ca. five years of atmospheric inputs. Applied in ten‐year cycles, this measure alone cannot prevent N accumulation in the long term. Conclusion: Regarding 10/15 year old Calluna‐heaths, we assume that prescribed burning cannot compensate for atmospheric N inputs, thus making long‐term changes in the nutritional state inevitable. Therefore, prescribed burning should be applied in combination with high‐intensity management measures.     

Seedling emergence after sod cutting in grass heath

Abstract. Experimental plots in grass heath, previously subjected to various sod cutting treatments, contained a seed store of ericoid dwarf shrubs, but seeds of grasses of the former grass heath were not found. The density of emerging dwarf-shrub seedlings in the plots was determined mainly by soil moisture, but with density of seeds stored in the soil and soil acidity as modifying factors. It is concluded that the restoration of a dwarf-shrub heathby sod cutting is relatively successful, as compared to other measures, because the grass seeds become obsolete.     

Effects of nutrient supply,light availability and herbivory on the growth of heather and three competing grass species

Grasses are becoming more abundant in areas in NE Scotland which until recently were dominated by heather (Calluna vulgaris). However, it is not clear if grasses are aggressive competitors which are now able to outcompete the dwarf shrub due to changes in environmental factors (such as grazing pressure and increasing nutrient inputs), or just opportunistic invaders, occupying gaps in the canopy which occur when heather reaches the degenerate stage. Experiments in turves and in field plots were carried out in order to investigate the performance of three grass species, Nardus stricta, Deschampsia cespitosa and Deschampsia flexuosa growing in competition with heather. These three species were selected because they differ in their nutrient requirements, palatability to herbivores and tolerance of shading. The grasses were planted in heather canopies of different structure, either turves of heather of different height and age, or moorland plots with or without heavy grazing by sheep and deer. Fertiliser (NPK) was applied to half the experimental plants. The growth of the grass species and the heather in response to the fertiliser and grazing treatments was measured, together with the light levels penetrating the canopy and received by the grass plants.Results indicated that heather was likely to be outcompeted by grasses only when there are gaps in the canopy, resulting either from heavy grazing or from the heather being in the mature or degenerate phase. Fertiliser enhanced plant growth whereas fencing out herbivores led to strong competition for light as the heather canopy closed. It is concluded that grasses require gaps in the canopy to successfully invade heather moorland, or they tend to be shaded out. Thus better management of heather moorlands to maintain a dense canopy structure may help to preserve heather cover even under increasing nutrient inputs.     

A laboratory assessment of the relative susceptibility of Molinia caerulea (L.) Moench and Calluna vulgaris (L.) Hull to a range of herbicides

There is a concern that Molinia caerulea (L.) Moench may be increasing in upland moorland communities at the expense of Calluna vulgaris (L.) Hull and other ericaceous species. In order to develop a strategy for the control of Molinia, laboratory studies were carried out to determine the dose-response relationships of seven graminicides and glyphosate on both Molinia caerulea and Calluna vulgaris. Plants were grown under laboratory conditions and sprayed with increasing doses of herbicide using a precision sprayer. Results were highly variable, indicating the great morphological plasticity of these two species, making determination of the ED₅₀ difficult. Glyphosate, quizalofop-ethyl and sethoxydim successfully reduced various measures of Molinia growth to 50% of control levels (ED₅₀ estimates ranged from 0.41–0.67 kg a.i. ha^-1, 0.18–1.20 kg a.i. ha^-1and 0.37–0.49 kg a.i. ha^-1respectively). In contrast, only glyphosate reduced Calluna growth significantly (ED₅₀s ranged from 0.025–0.45 kg a.i. ha^-1), whilst selective herbicides left it undamaged. A comparison of the results for Molinia and Calluna suggests that there is no application rate of glyphosate which will reduce Molinia effectively whilst leaving Calluna undamaged. Recommendations for field testing of selective herbicides are discussed.     

Competition between heather and grasses on Scottish moorlands: Interacting effects of nutrient enrichment and grazing regime

Abstract. Considerable losses and degradation of heathlands (in moorlands and lowlands) have been reported across Europe, with Calluna vulgaris (heather) being replaced by other species, often grasses. Increasing atmospheric nitrogen deposition and overgrazing have been suggested as the driving factors behind this change. This possibility was investigated in a study of the interacting effects of nutrient inputs and grazing on heather and three grass species (Nardus stricta, Deschampsia cespitosa and D. flexuosa) in the field, on a moorland in northeastern Scotland. In addition, the interacting effects of increasing nutrients and Calluna canopy height on N. stricta and D. cespitosa were studied using turves in an outdoor experimental area. In the field, fencing had a larger effect than fertilizer on the growth of all species, except for N. stricta, the species most unpalatable to herbivores. Fencing led to an increase in the height of the Calluna canopy, which may reduce light availability for the grasses. In the turf experiment, the height of the Calluna canopy affected the diameter of the grass tussocks and percentage of green matter (i.e. live leaf material), with plants under the more closed Calluna canopies being smaller. This study suggests that the slow‐growing, evergreen Calluna is a more effective competitor than the faster growing grasses when it has a tall, intact canopy, even at increased levels of nutrient supply. However, overgrazing promotes gap formation in the Calluna canopy, providing an opportunity for grasses to take advantage of increased nutrients. Thus the conservation of heather moorlands requires an understanding of the grazing level which allows Calluna to maintain sufficient canopy structure to outcompete grasses for light.     

Effects of fertilization and cutting on the chemical composition of vegetation and soils of mountain heathlands in Spain

Abstract. In the mountains of northern Spain, patches dominated by Calluna vulgaris are scarce and they may disappear or change as a result of continued lack of management and possibly increasing nutrient availability through atmospheric deposition. The effects in the soil properties and in the composition of Calluna vulgaris and Erica tetralix shoots on heathlands dominated by Calluna and Erica subjected to fertilization and experimental cutting were studied in three mountain passes in northern Spain. A total of 90 1‐m² plots received different combinations of cutting and twice the estimated atmospheric deposition of nitrogen (5.6 g.m^?2.yr^?1) as ammonium nitrate. One of the dominant ericaceous species (Calluna and Erica) was selectively cut by hand at ground level and their nitrogen shoot content were compared in the presence or absence of the other. Treatments were carried out in April 1998. In each plot one soil sample was taken in the original situation and 12, 24 and 36 months after the treatments. Soil properties such as organic matter, total nitrogen, available phosphorus and pH were determined. In every plot five shoots of Calluna and Erica were also taken to analyse total nitrogen content in the original situation and 12, 24 and 36 mo after the treatments. Nitrogen addition does not necessarily lead to increased levels in the soil, and a clear pattern was not found in the three areas. A gradual decrease in available phosphorus content was detected in the three areas until two years after treatment, although values tend to recover in two of the areas in the third study year. An increase in organic matter content was observed in all areas. It is concluded that increased nutrients alone, at twice the rate of the estimated current atmospheric deposition for the area, which is relatively low, will not alter significantly the soil characteristics of the mountain heathland stands. A clear increase in plant N‐content is observed in the fertilized plots in comparison with the non‐fertilized ones and Calluna always has higher nitrogen content than Erica. This increase is most pronounced one year after the treatments started in one of the areas and after two years in the other two areas. In some cases the elimination of one species is seen to favour nitrogen increase in the other.     

Population dynamics of Gentiana pneumonanthe and Rhynchospora fusca during wet heathland restoration

Abstract. The population dynamics and reproductive strategies of two rare wet heathland species, Gentiana pneumonanthe and Rhynchospora, were studied in experimental permanent plots in a wet heathland near Bremen, NW Germany, to assess how effective sod cutting and mowing are in promoting these species. In one experiment, small plots (0.25 m² - 4 m²) were sod-cut or mown; in the second, one large plot 30 mx 50m) was sod-cut. The development of the vegetation and the number of shoots of the two target species were recorded annually. Sod cutting lead to the highest shoot numbers of Gentiana in the long run, whilst mowing was more effective at the beginning of the experiment. Seedlings and adult shoots slowly became more and more abundant after sod cutting. In contrast, Rhynchospora soon formed closed stands after sod-cutting, first through seed germination and then through fast clonal growth of the established individuals. Moist soil or short inundations promoted the germination and seedling establishment of Gentiana, whereas drought negatively affected seedling recruitment. Long periods of inundation severely reduced the population at first, but high numbers of seedlings were found in the following growing season. For the disturbance-dependent Gentiana and Rhynchospora, the availability of gaps within the vegetation is of crucial importance. To promote existing populations, we suggest small-scale sod cuttings which create gaps without disturbing existing flowering individuals too much. For degenerated stands of wet heathland we recommend large-scale sod cutting to activate the seed bank. Additionally, seed introduction may be helpful to encourage the development of a wet heathland with characteristic floristic composition.     

Experimental evidence from Cantabrian mountain heathlands suggests new recommendations for management of Vaccinium myrtillus L

Background: Management actions in mountain heathlands oriented to increasing the cover of Vaccinium myrtillus promote mowing of Calluna vulgaris to avoid competition. However, such action ignores the fact that plant–plant interactions range from competition to facilitation under different stress conditions.

Aims: To test whether the interactions between these two species are related to competition or facilitation, which would change the perspective for their management in mountain heathlands.

Methods: A total of 40 experimental plots located in a montane heathland (northern Spain) were selected. Calluna or Vaccinium plants were either cut at ground level or not cut, and plots were fertilized with nitrogen or not in a three-factorial design. Vegetative and reproductive shoot mass and length of the current year’s shoots were estimated in two consecutive years for both species. Herbivory on Vaccinium by browsing ungulates was compared in control plots and plots where Calluna had been cut 2 yrs ago.

Results: The two study species co-occurred in 72.4% of the sampled plots, although Calluna was more abundant. Vaccinium growth was greater in plots with Calluna than in plots where Calluna had been cut. Browsing on Vaccinium was also higher in plots were Calluna had been cut, and the effect of fertilization was only significant on the reproductive biomass of Calluna.

Conclusions: Contrary to initial proposals, our results suggest that Vaccinium does not always benefit from the cutting of Calluna in heathlands. Thus, the management of Cantabrian mountain heathlands should consider maintaining Calluna in order to facilitate the growth of Vaccinium.     

Influence of management on the seed production and seed bank of calcareous fen species

Abstract. The dynamics of the seed production and seed bank of two dominant perennial species of calcareous fens were investigated in response to different mowing frequency, fallow or pasture. The production of fertile shoots responded strongly to management practices. On sites where mowing alternated with fallow, autumn mowing led to an increase of fertile shoot density in Schoenus ferrugineus and a decrease in Molinia caerulea in the subsequent season. Repeated annual mowing after long-term fallow raised the density in Schoenus and inhibited initiation of flowering shoots in Molinia. Seed set and viability were influenced mainly by climatic or other factors. Variation of seed production between sites and in consecutive years at the same site could thus be large. The relationship between total seed production and seed bank was weak, but it improved when germination rates were included in the calculation. As the seed bank was generally smaller than the viable seed production, low survival and a high turnover of seeds in the upper soil layers is assumed for both species. Implications for recruitment strategies are discussed.     

Effects of cutting and nitrogen deposition on biodiversity in Cantabrian heathlands

Questions: Does the diversity of heathland vegetation change when subjected to experimental disturbances such as cutting and nitrogen fertilization? Do changes in the vegetation structure negatively affect the regeneration of the dominant species Calluna vulgaris? Is cutting an alternative method of conserving the diversity and maintaining the structure of heathlands in the Cantabrian Mountains? Location: Calluna vulgaris heathlands on the southern slopes of the Cantabrian Mountain range, NW Spain. Methods: A total of 60 plots were treated with different combinations of cutting and twice the estimated atmospheric deposition of nitrogen (56 kg‐N.ha^?1.yr^?1). The changes in the cover values of the species present were monitored over a five year study period. The cover values were used to calculate abundance and species richness. Results: Fertilizing with nitrogen allows biodiversity to increase over time. However, the greatest biodiversity is associated with the cutting plus fertilization treatment, since cutting allows gaps to be opened that are easily colonized by pioneer annual species, while fertilization mainly favours an increase in the mean number of perennial herbs (graminoids and forbs). Increased perennial herb richness also corresponds to a rise in their cover values. The recovery of the dominant woody species in these communities, Calluna vulgaris, is not impeded by the increase in perennial herbs species' cover values. Conclusions: In the Calluna vulgaris heathlands studied, cutting plus fertilization allowed an increase in biodiversity over time. No displacement of the dominant woody species, Calluna vulgaris, is observed due to the presence of the perennial herbs. Cutting patches of heathland is recommended as a mechanism for maintaining high vegetation diversity, when grazing is not possible.     

Random Forest characterization of upland vegetation and management burning from aerial imagery

Aim The upland moorlands of Great Britain form distinctive landscapes of international conservation importance, comprising mosaics of heathland, acid grassland, blanket bog and bracken. Much of this landscape is managed by rotational burning to create gamebird habitat and there is concern over whether this is driving long‐term changes in upland vegetation communities. However, the inaccessibility and scale of uplands means that monitoring changes in vegetation and burning practices is difficult. We aim to overcome this problem by developing methods to classify aerial imagery into high‐resolution maps of dominant vegetation cover, including the distribution of burns on managed grouse moors. Location  Peak District National Park, England, UK. Methods Colour and infrared aerial photographs were classified into seven dominant land‐cover classes using the Random Forest ensemble machine learning algorithm. In addition, heather (Calluna vulgaris) was further differentiated into growth phases, including sites that were newly burnt. We then analysed the distributions of the vegetation classes and managed burning using detrended correspondence analysis. Results Classification accuracy was c. 95% and produced a 5‐m resolution map for 514 km² of moorland. Cover classes were highly aggregated and strong nonlinear effects of elevation and slope and weaker effects of aspect and bedrock type were evident in structuring moorland vegetation communities. The classification revealed the spatial distribution of managed burning and suggested that relatively steep areas may be disproportionately burnt. Main conclusions Random Forest classification of aerial imagery is an efficient method for producing high‐resolution maps of upland vegetation. These may be used to monitor long‐term changes in vegetation and management burning and infer species–environment relationships and can therefore provide an important tool for effective conservation at the landscape scale.     

Photosynthesis of ground vegetation in different aged pine forests: Effect of environmental factors predicted with a process‐based model

Question: How do stand age and environmental factors affect the species‐specific photosynthesis of ground vegetation? Location: Five different aged pine forests in Southern Finland. Methods: We measured photosynthesis of common species of ground vegetation during the growing season of 2006. Results: The measured vascular species, especially those with annual leaves, had a clear seasonal cycle in their measured photosynthetic activity (P_maxⁱ). A simple model that uses site‐specific temperature history, soil moisture and recent frost as input data was able to predict the changes in photosynthetic activity in dwarf shrubs with perennial leaves. The P_maxⁱ values of mosses did not have a clear seasonal cycle, but low values occurred after rain‐free periods and high values after precipitation. We modified the model for mosses and included temporary rain events. The model was able to predict most of the large changes in P_maxⁱ of mosses resulting from varying weather events but there was still some uncertainty, which was probably due to difficulties in measuring fluxes over a moss population. Conclusions: Temperature history, recent frosts and soil moisture determine the changes in P_maxⁱ of dwarf shrubs with perennial leaves. The P_maxⁱ of mosses depends mostly on recent precipitation.     

Lignin signatures show the effects of changes in heather and bracken cover on the composition of organic matter in a moorland soil profile

Bracken (Pteridium aquilinum) is aggressively displacing heather (Calluna vulgaris) on many moorlands in Britain. We investigated the use of lignin derivatives to identify the distribution of soil organic matter (SOM) derived from bracken in moorland soil profiles formed under heather. Phenylpropanoids extracted from recently senesced litters, roots and SOM, using alkaline CuO oxidation, showed distinct signatures for bracken and heather, with vanillyl moieties dominating bracken litter extracts and vanillyl and syringyl dominating heather litter extracts. Ratios of vanillyl and syringyl concentrations characterised the SOM derived from heather and bracken better than the concentrations of the individual moieties. The analysis showed up to a depth of 5 cm under pure bracken cover, and at the interface between heather and bracken, the SOM was largely derived from bracken litter but below that depth SOM was apparently derived from heather. The use of these methods to identify the plant origin of SOM not only enables understanding the effects of changing vegetation cover on organic matter dynamics in moorland soils but could also facilitate management techniques in moorland/heathland restoration which involve the removal of comparatively nutrient-rich SOM derived from bracken. Received: 30 November 1997 / Accepted: 11 April 1998     

Effects of browsing and grazing on cyclic succession in nutrient‐limited ecosystems

Abstract. This paper deals with browsing and grazing as forces driving cyclic succession. Between 1989 and 1994 reciprocal transitions between the dwarf shrub Calluna vulgaris and the grass Deschampsia flexuosa were monitored in permanent plots in a cattle grazed grass‐rich Dutch heathland on podsolic soils in which tree encroachment was prevented. Heather beetles killed Calluna in four of the nine plots during 1991/1992. The monitoring revealed reciprocal transitions and cycles between Calluna and Deschampsia on a subplot scale. Beetles and cattle had additional and complementary effects on the two competing species. Defoliation by beetles and trampling by cattle‐killed Calluna and favoured grass invasion. Grazing and gap creation by cattle in Deschampsia favoured the establishment and recovery of Calluna. Analysis of the causal mechanisms suggests that indirect, resource‐mediated herbivory effects may be as important for the replacement processes as direct effects of defoliation and trampling. Herbivory created differential light and nutrient levels in Calluna and Deschampsia gaps. Grazing and browsing improved the resource‐capturing abilities of Calluna and its resistance to herbivory and abiotic disturbances. The emerged Calluna‐Deschampsia cycle and its driving forces are summarized in a conceptual triangular resource‐mediated successional grazing cycle (RSGC) model, a limit cycle involving herbivore‐plant‐plant resource interactions. It offers a deterministic equilibrium model as alternative for stochastic transitions between the meta‐stable states with dominance of Calluna and Deschampsia respectively. The validity range of the RSGC model and its management implications are briefly discussed.     

Effects of N-deposition on ion trapping by B-horizons of Danish heathlands

Coastal and inland heaths constitute a dominating fraction of the relatively few semi-natural ecosystems in Denmark. At the inland heath, the soil forming factors have been active with the same vegetation type during the last 2000 years, whereas at the coastal heath, these soil forming factors have only been active over a couple of hundred years, due to the activity of windblown sand. Effects on ion-balance and soil processes of nitrogen deposition on a young dune heath and an old inland heath have been investigated. Different levels of NH₄NO₃have been applied to the two heathlands, and rain, throughfall water and soil water chemistry below the experimental plots have been analysed. Results show that the ability to retain added, as well as deposited, nitrogen requires the presence and the integrity of a humifyed H-subhorizon. When the mor-layer for some reason breaks down, the nutrients will be retained by the B-horizons as a kind of back-up, and most probably be reused by the new vegetation. One year after destructive heather beetles attack on the inland heath, the mor-layer started to decompose. A large translocation of organic compounds, complexed associated cations and nitrogen-species, started from the mor-layer to the top of the B-horizon where they precipitate. The podzolic B-horizons can be seen as nutrient trap to the mor-layer. This revised version was published online in June 2006 with corrections to the Cover Date.