Effects of litters with different concentrations of phenolics on the competition between Calluna vulgaris and Deschampsia flexuosa

We hypothesized that the outcome of competition between ericaceous plants and grasses is strongly affected by the concentrations of phenolics in the litter that they produce. To test the effect of phenolic-rich litter on soluble soil nitrogen concentrations, plant nitrogen uptake and inter-specific competition, we conducted a greenhouse experiment with the shrub Calluna vulgaris and the grass Deschampsia flexuosa and their leaf litters. Two litters of C. vulgaris were used, with equal nitrogen concentration but different (high and low) concentrations of total phenolics. The D. flexuosa leaf litter contained lower concentrations of phenolics, but higher concentrations of nitrogen than the C. vulgaris litters. The plants were grown in monocultures and in mixed cultures. Inorganic and dissolved organic nitrogen were measured monthly during the experiment. After four months, we measured above- and belowground biomass and the nutrient concentrations in above- and belowground plant parts. In monocultures, C. vulgaris produced more shoot and root biomass on its own litter than with no litter. Growth of Calluna was reduced on grass litter. D. flexuosa plants produced most biomass on their own litter type, whether in monocultures or in mixed cultures. Addition of Calluna litter stimulated the growth of D. flexuosa both in monoculture and in mixtures. The grass plants outcompeted Calluna both on shrub litter and on grass litter but not when grown without litter. The two C. vulgaris litter types that differed in their concentration of phenolics did not differ in their effects on the competition between the two species or on the production of inorganic and dissolved organic nitrogen. We conclude that the nitrogen content of the litter is more important as a plant feature driving competition between shrubs and grasses than the concentrations of phenolics.     

A reappraisal of cyclical processes in Calluna heath

Small-scale, short-term cycles constitute an important aspect of vegetation dynamics. It has been widely accepted that such cycles are characteristic of unmanaged heath communities where the dominant, Calluna vulgaris, undergoes an age-related series of changes in its growth and morphology. This is thought to give rise to a repetitive sequence of changes in the occupancy of gaps which form in the Calluna canopy and are first colonized by other species, with Calluna re-establishing later. Recently, doubts have been cast as to whether the changes are in fact often repetitive. A new study has shown much greater variability than was previously suspected in the transitions occurring in gaps. There is evidence that, in some instances though by no means all, Calluna can re-establish and thereby initiate a genuine cycle. It seems, however, that such re-entry is seldom by means of seedling establishment, but more frequently by development of adventitious shoots and roots on stems which have come to lie across the gap and have been covered by moss, moist litter or humus.     

Vegetation re‐establishment on land previously subject to control of Pteridium aquilinum by herbicide

Abstract. Vegetation development after conventional bracken control with herbicide in the uplands is often slow and rarely results in the development of the target community. Two experiments were established on contrasting sites to investigate cost‐effective methods for the re‐establishment of vegetation dominated by Calluna vulgaris (heather), which is the usual target community seen as the desired replacement for Pteridium aquilinum in the uplands by many land managers. At one site, where grazing intensity was low and shelter was sufficient to prevent rapid Pteridium litter loss, the necessary treatments to establish Calluna were to disturb the litter and add a source of Calluna seeds. A difference between continued Pteridium control or a relaxation of control was not observed over the time‐scale of the experiment. At the second site, where grazing intensity was high and shelter minimal, vegetation dominated by Rumex acetosella had developed on a thin but dense litter layer. To enhance Calluna regeneration it was necessary to prevent sheep grazing, disturb the remaining litter layer and add a source of Calluna propagules. A nurse crop hindered Calluna establishment due to its vigorous growth. At neither site was an initial application of fertilizer an aid to Calluna establishment. It is possible to devise methods for the re‐establishment of Calluna at sites where previous attempts at Pteridium control alone have not had the desired effect on vegetation development. However, the precise choice of methods used is dependent on the grazing management at the site. High levels of livestock grazing prevented Calluna establishment, but some trampling may aid litter breakdown. The ability to adjust stocking rates and access to the site by machinery are critical factors to take into account in producing an effective strategy for long‐term Pteridium control and its replacement by target vegetation.     

The leaf tannin of willow-herb [Chamaenerion angustifolium (L.) Scop.]

1. The leaf tannin of willow-herb [Chamaenerion angustifolium (L.) Scop.] has been isolated and separated into two fractions of differing solubility. 2. The tannin contains a penta-O-galloyl-β-d-glucose core to which further galloyl groups are depsidically bound. 3. The unfractionated tannin contains an average of 10·5 galloyl groups/glucose molecule; the soluble fraction has on average 7·6 galloyl groups/glucose molecule and the less soluble fraction has 12·4. 4. The tannin is a mixture of molecules ranging at least from hepta- to trideca-galloyl-β-d-glucose. 5. The tannin forms complexes with proteins and the fact that it is a hydrolysable gallotannin has a bearing on the release of nitrogen from the protein of the dead leaf.     

Effects of increased temperature and nitrogen availability on the displacement of Calluna vulgaris by Pteridium aquilinum

Abstract. Atmospheric greenhouse gas concentrations will continue to rise for the foreseeable future, and, due to radiative forcing, surface temperatures will also increase. If soils warm sufficiently, this will result in increased litter decomposition rates, while nitrogen availability will also rise due to increased deposition. A field-based experiment manipulating air temperatures using polythene tents and nitrogen availability by adding 50 kg N ha^-1 yr^-1 has shown that this will result in further encroachment of Pteridium into Calluna moorland. Although this experiment, treating plots on and around the boundary between Pteridium and Calluna, did not show any statistically significant effect on the movement of the boundary, there were clear indications of Calluna dieback at the boundary. In addition, the frequency of Pteridium fronds increased in the Calluna-dominated areas, while Calluna became less abundant in the pure Pteridium plots. Calluna dieback was not due to litter accumulation as is frequently assumed, but the dieback is likely to be caused by shading of Calluna by the increased growth of Pteridium fronds. Botanical diversity was higher in the Pteridium plots, but considering the importance of grouse shooting for the maintenance of Calluna moorland, displacement of Calluna by Pteridium may result in a lower conservation and economic value of moorlands.     

The influence of ammonium nitrate on the root growth and ericoid mycorrhizal colonization of Calluna vulgaris (L.) Hull from a Danish heathland

Conversion of European heathlands to grassland has been reported as a response to increased nutrient availability, especially of nitrogen; a direct effect upon mycorrhizal colonization has been proposed as an likely explanation.This hypothesis was tested in a random block experiment with four blocks and four replicates on a Danish inland heath, Hjelm Hede. Ammonium nitrate was applied (0, 35, 50 and 70 kg N ha^–1 year^–1) to a stand of Calluna vulgaris (L.) Hull four times annually for 2 years. Calluna roots were sampled on four occasions in the 2nd year of the nitrogen treatment. The extent of ericoid mycorrhizal colonization was determined by direct observation of the roots using a line-intersection method. The nitrogen content of the current-year shoots of Calluna increased when they were treated with nitrogen. Nitrogen fertilization had no significant effects on ericoid mycorrhizal colonization of Calluna nor on root biomass. The seasonal variation in mycorrhizal colonization of the Calluna roots was highly significant. The spatial variability of mycorrhizal colonization, both in replicated plots and in the two contrasted soil horizons – the mor layer and the bleached sand – within the plots, were considerable. I conclude that heather decline under enhanced nitrogen input is unlikely to be caused by a direct impact on the ericoid mycorrhizae of Calluna. Received: 1 December 1998 / Accepted: 7 December 1999     

Early stages of Calluna vulgaris regeneration after burning of coastal heath in central Norway

Questions: How does Calluna vulgaris regenerate after burning wet and dry heaths of different age? Location: Central Norway. Methods: Patches of Calluna dominated old and young wet heath and old dry heath were burned. The mean temperatures reached in the categories of heath were measured. Cover, frequency, height and number of seedlings of Calluna were studied in plots. Results: The mean burning temperatures varied from 680 to 740°C, and were highest in wet heath that had not been burned for at least 50 years. Regeneration of Calluna was evident in the year of burning, but only from seeds. Even heath with a short burning interval lacked vegetative regeneration. Calluna cover increased yearly and exceeded 50 % three years after burning. Regeneration was evident in the year of burning, and was highest in heath with a short burning interval. The height increased regularly at all sites, but was most rapid in dry heath, probably because of better microclimatic conditions. There were more seedlings in the heath with a short burning interval. Conclusions: Calluna only regenerates from seeds afterburning in central Norway. The density of Calluna increased rapidly, which is positive for the conservation and management of coastal heaths in central Norway.     

PATTERNS OF LEAF AND FLOWER REMOVAL: THEIR EFFECT ON FRUIT GROWTH IN CHAMAENERION ANGUSTIFOLIUM (FIREWEED)

The response of Chamaenerion angustifolium (fireweed) plants to different patterns of artificial leaf and flower herbivory was examined to determine if the effects of localized herbivory were confined within vertical sectors of the plant. The effect of leaf and flower removal on fruit development was compared for removals within sectors and distributed among sectors. Fruit development did not differ in plants subjected to different patterns of leaf and flower removal. These results suggest that in disturbed plants carbohydrate movement is not confined within vertical sectors.     

The effect of increased nutrient availability on leaf turnover and aboveground productivity of two evergreen ericaceous shrubs

Summary Leaf turnover and aboveground productivity in relation to nutrient availability were studied in the evergreen shrubs Erica tetralix and Calluna vulgaris. In monospecific stands of these species four levels of nutrient (NPK) availability were created during three growing seasons. Percentage survival and life expectancy of Erica leaves decreased with increasing nutrient availability. For Calluna there was no effect. Winter mortality of Erica leaves was smaller than growing season mortality. These was no difference for Calluna. The timing of leaf mortality of both species was not affected by nutrient treatment. At the end of the experimental period current year leaf biomass, total biomass and current year second year and third year biomass of both species showed a significant increase with increasing nutrient availability. The relative increase was greater for Calluna, except for second and third year biomass. Stem production and stem mortality of both species increased with increasing nutrient availability. The increased stem mortality resulted also for Calluna in an increased leaf turnover (per unit ground area) with increasing nutrient availability. Nutrient cycling in ecosystems dominated by these species will increase with increasing nutrient availability, because of increased leaf and stem turnover and productivity. This phenotypic effect is similar to the effect of the shift in dominance between different species which occurs along natural gradients of nutrient availability.     

Germination and establishment of seedlings in different phases of theCalluna life cycle in a Scottish heathland

Previous authors have suggested that cyclical succession takes place inCalluna-dominated heathland, and is determined by the different growth phases of the dominant. Characteristic species are said to invade at different stages of the life cycle ofCalluna, particularly in the initial and pioneer phases and in the degenerate phase. In the last phase, gaps are formed due to separation and dying of the oldest frame branches.This paper aims to analyze in which microhabitat and in which phase of theCalluna life cycle seedling emergence and establishment is possible.More seedlings appear in pioneerCalluna than in matureCalluna stands in experimentally manipulated micro-sites. In pioneerCalluna establishment is affected only by application of litter, in matureCalluna by increased illumination. The differences between treatments disappeared after three years, but not the difference in establishment between stands.Seedling emergence from species sown experimentally was different in the successive phases ofCalluna. Highest emergence was in the pioneer stand.In a seedling survey also most seedlings appeared and established in the pioneer phase. The number of recently germinated seeds was high in all phases. However, seedling survival was very low, except in the pioneer stands. This study does not of itself show evidence for cyclical succession inCalluna-dominated heath vegetation. Colonization of gaps by seedlings ofCalluna in mature or degenerate phases is possible, but must be an infrequent occurrence.Nomenclauture follows Clapham, Tutin & Warburg (1962).We wish to thank Dr G. R. Miller for stimulating discussions during the start of the experiments. E. de Hullu wishes to thank the Botany Department in Aberdeen for their hospitality during the research period.     

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.     

Isolation of pomolic acid from Chamaenerion angustifolium and the evaluation of its potential genotoxicity in bacterial test systems

The lipophilic components of the medicinal plant, the fireweed (Chamaenerion angustifolium), have been examined using the raw materials of a three-year harvest. Twenty eight aliphatic and six triterpene acids have been detected for the first time by chromatography—mass spectrometry analysis. It has been shown using the Ames test and the SOS chromotest that pomolic acid possesses no genotoxic and mutagenic properties.     

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 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.     

Charcoal as a habitat for microbes and its effect on the microbial community of the underlying humus

Wildfires produce a charcoal layer, which has an adsorbing capacity resembling activated carbon. After the fire a new litter layer starts to accumulate on top of the charcoal layer, which liberates water‐soluble compounds that percolate through the charcoal and the unburned humus layer. We first hypothesized that since charcoal has the capacity to adsorb organic compounds it may form a new habitat for microbes, which decompose the adsorbed compounds. Secondly, we hypothesized that the charcoal may cause depletion of decomposable organic carbon in the underlying humus and thus reduce the microbial biomass. To test our hypotheses we prepared microcosms, where we placed non‐heated humus and on top one of the adsorbents: non‐adsorptive pumice (Pum), charcoal from Empetrum nigrum (EmpCh), charcoal from humus (HuCh) or activated carbon (ActC). We watered them with birch leaf litter extract. The adsorbing capacity increased in the order Pumorg in the litter extract, respectively. After one month, all adsorbents harboured microbes, but their amount and basal respiration was largest in EmpCh and HuCh, and smallest in Pum. In addition, different kinds of microbial communities with respect to their phospholipid fatty acid and substrate utilization patterns were formed in the adsorbents. The amount of microbial biomass and number of bacteria did not differ between humus under different adsorbents, although different microbial communities developed in humus under EmpCh compared with Pum, which is obviously related to the increased pH of the humus under EmpCh, and also ActC. We suggest that charcoal from burning can support microbial communities, which are small in size but have a higher specific growth rate than those of the humus. Although the charcoal layer induces changes in the microbial community of the humus, it does not reduce the amount of humus microbes.     

Growth response of altitudinal ecotypes of mountain birch to temperature and fertilisation

High- and low-altitude ecotypes of mountain birch (Betula pubescens ssp. czerepanovii) showed clear differences in their responses to various experimental conditions, including two temperature regimes and four fertilisation rates. There was, however, no simple way to characterise the elevational ecotypes in terms of relative growth rate, nitrogen (N) productivity, or root N uptake rate. The leaf N concentration was generally higher in the high-altitude seedlings than in the low-altitude seedlings. At low temperature, high-altitude mountain birch maintained a relatively high growth rate by combining high root N uptake rate and high leaf N concentration with high N productivity. An increase in temperature and/or fertiliser rate resulted in a marginal increase in N productivity in the high-altitude seedlings but resulted in a strong increase in N productivity in the low-altitude seedlings. In parallel, increased temperature resulted in a pronounced decrease in leaf N concentration only in the low-altitude seedlings. Our results suggest that the weak growth response to increased temperature in high-altitude mountain birch is functionally related to high leaf N concentration. The high leaf N concentration of high-altitude mountain birch is genetically determined and has an adaptive value in a cold environment. This suggests that there is a trade-off between high N productivity at low temperature and a strong response of N productivity to temperature. Received: 21 March 1998 / Accepted: 1 December 1998     

Root turnover as determinant of the cycling of C,N, and P in a dry heathland ecosystem

Root production and turnover were studied using sequential core sampling and observations in permanent minirhizotrons in the field in three dry heathland stands dominated by the evergreen dwarfshrub Calluna vulgaris and the grasses Deschampsia flexuosa and Molinia caerulea, respectively. Root biomass production, estimated by core sampling, amounted to 160 (Calluna), 180 (Deschampsia) and 1380 (Molinia) g m^-2 yr^-1, respectively. Root biomass turnover rate in Calluna (0.64 yr^-1) was lower compared with the grasses (Deschampsia: 0.96 yr^-1; Molinia 1.68yr^-1)). Root length turnover rate was 0.75–0.77 yr^-1 (Deschampsia) and 1.17–1.49 yr^-1 (Molinia), respectively. No resorption of N and P from senescing roots was observed in either species. Input of organic N into the soil due to root turnover, estimated using the core sampling data, amounted to 1.8 g N m^-2 yr^-1(^Calluna), 1.7 g N m^-2 yr^-1 (Deschampsia) and 19.7 g N m^-2 yr^-1 (Molinia), respectively. The organic P input was 0.05, 0.07 and 0.55 g P M^-2 yr^-1, respectively. Using the minirhizotron turnover estimates these values were20–22% (Deschampsia) and 11–30% (Molinia) lower.When the biomass turnover data were used, it appeared that in the Molinia stand root turnover contributed 67% to total litter production, 87% to total litter nitrogen loss and 84% to total litter phosphorus loss. For Calluna and Deschampsia these percentages were about three and two times lower, respectively.This study shows that (1) Root turnover is a key factor in ecosystem C, N, and P cycling; and that (2) The relative importance of root turnover differs between species.     

Heathlands functioning in a perspective of climate warming – Estimation of parameters,elements for discrete event simulation

Three Calluna vulgaris (L.) Hull heathlands (1100, 1500 and 1780 m altitude) are studied in order to establish and estimate parameters which will be integrated in a discrete event simulation of heathland functioning in the case of a 2 °C climate warming. The sites, situated in the Chaîne des Puys and the Massif du Sancy (France), present similar conditions for Calluna growth: bedrock (trachyt), exposure and slope, so that they can be compared on a climatic basis. Main parameters sampled are: age distributions, life expectancies, layering probabilities, annual growths, biomasses (standing crop), C/N rates and potential respiratory of soils. In addition, experiments were carried out on germination, hypocotyle elongation and growth of mycorrhizal fungi in order to estimate the inhibitory-to-growth action against the substitution process. On one stand (exhibiting gaps) the patch structure of Calluna plants was recorded and mapped. The rising of biomass expected is calculated according to the Aerts's model, as a function of nitrogen availability which would increase under a scenario of climate warming. From a similar point of view, layering probabilities, life expectancies and inhibition of seedlings survival modelling – under a linear model assumption – are proposed.     

Competition for nutrients between Calluna vulgaris (L.) Hull and Molinia caerulea (L.) Moench

Summary A combination of a removal and fertilization experiment in the field and a replacement experiment in containers in a cold frame was carried out to investigate the role of nutrient supply in the interactions between Calluna vulgaris and Molinia caerulea. It is concluded that the growth of Calluna as well as that of Molinia increased with increasing nutrient availability. However, the increase in biomass of Molinia was much larger than that of Calluna. It is also concluded that increased nutrient availability favours the competitive vigour of Molinia in interaction with Calluna.