Possible positive-feedback mechanisms: plants change abiotic soil parameters in wet calcareous dune slacks

In this paper the results are presented from a mesocosm study of the effects of typical dune slack plants on the soil solution nutrient contents. In dune slack succession, early successional species often show radial oxygen loss (ROL) whereas their successor species do not show ROL. ROL has impact on abiotic soil parameters and therefore, affect the competitiveness of both species. Mesocosms with Littorella uniflora and Carex nigra, used as respectively a ROL and a non-ROL species, showed remarkable differences in soil solution parameters. Special attention was given to nitrogen, as it is the limiting resource in dune slack succession. Mesocosms with L. uniflora showed a higher nitrate content in the soil than mesocosms with C. nigra and the control. Moreover, estimating the nitrogen balance, a significantly higher fraction of nitrogen was missing in L. uniflora (57%) than in C. nigra (5%). The enhanced nitrogen loss in mesocosms with L. uniflora could act as a positive-feedback mechanism for early successional stages that slows down the vegetation development in early stages of dune slack succession towards the more-productive later stages. The mechanism could even lead to alternative stable states in dune slack succession.     

Alkalinity generation and sediment CO2 uptake influence establishment of Sparganium angustifolium in softwater lakes

1. Softwater lakes are generally dominated by slow growing, small, isoetid plant species that are adapted to the carbon‐ and nutrient‐limited conditions in these lakes. We investigated the strategy of a fast growing species, Sparganium angustifolium, for occupying softwater lakes. A field survey was carried out in Norwegian carbon‐limited Isoëteto‐Lobelietum softwater lakes to compare abiotic conditions at locations with and without S. angustifolium. In addition, long term abiotic changes (1995–2008) related to the sudden establishment of the species on experimentally limed plots were studied. Based on the results, the carbon acquisition mechanism of S. angustifolium was tested in eco‐physiological laboratory experiments. 2. The redox potential was significantly lower at locations with S. angustifolium (220 ± 2.3) compared to locations without S. angustifolium (338.1 ± 13.9). The lower redox potential was accompanied by significantly higher concentrations of HCO₃^?, CO₂ and Fe²⁺ in the sediment pore water, indicating in‐lake alkalinity generation due to higher iron reduction rates in the generally iron‐rich sediments. In addition, the lower redox potential was accompanied by a higher nutrient availability (NH₄⁺ and PO₄^3?) in the sediment pore water. Since there were no differences in water quality between the lakes, the ability of S. angustifolium to grow in softwater lakes very likely depends upon the higher dissolved inorganic carbon (DIC) and nutrient concentrations present in the sediment pore water. 3. Results from the liming experiment revealed that appearance of S. angustifolium on limed plots was related to the dissolution of Ca and Mg carbonates and development of a lower redox potential in the sediment. These processes were accompanied by a sustained increase in the availability of DIC in the sediment pore water. 4. The eco‐physiological experiments indicated that S. angustifolium can increase in biomass and produce floating leaves at a relatively high DIC availability in the root medium. In addition, it appeared that S. angustifolium can take up CO₂ by the roots. As far as we know, the ability to use sediment CO₂ has only been described as an adaptation typical for isoetid plant species. Use of the relatively large sediment CO₂ pools present in these sediment types (>1000 μmol L^?1) to enable development of long floating leaves for additional uptake of atmospheric CO₂ is a very different strategy to colonise softwater lakes as compared to isoetid plant species.     

Nitrification and nitrogen mineralization in a lowland rainforest succession in Costa Rica,Central America

Summary Nitrogen availability is a critical component of productivity in successional lowland rainforests, and nitrogen losses from a given system may largely depend on rates of nitrification in soils of the system. Two hypotheses were tested in a study of a 6-point secondary rainforest sere in the coastal lowlands of Costa Rica: that nitrification and N mineralization change in a directed fashion in lowland rainforest successions, and that nitrification is regulated by ammonium availability at all points along the sere. Nitrate and mineral N production were measured in short-term laboratory incubations of soils from different stages of secondary succession corresponding to 0, 3, 8, 16, 31 and 60 + years following disturbance. Results indicate that nitrification increases through the first 4 successional stages and then declines somewhat before leveling off. In soil from all sites, most of the N mineralized was nitrified, and added NH₄Cl strikingly stimulated net nitrate production. Added NaH₂PO₄, CaCO₃, and CaSO₄ did not stimulate net nitrate production or did not result in a greater proportion of nitrate than in controls. These results suggest that nitrification and N mineralization may tend to increase through secondary rainforest succession and that ammonium availability along the sere regulates rates of nitrification.     

Effects of microbial mats on germination and seedling survival of typical dune slack species in the Netherlands

The role of microbial mats in wet dune slack succession is often discussed. We tested if presence of microbial mats may retard dune slack succession by lowering the germination and seeding survival of successor species. This hypothesis was tested on a set of typical dune slack species of the Frisian Islands in two climate chamber experiments. The species were separated into early-, intermediate- and late successional species. There were large differences in germination rates between species (2% – >200% compared to the reference), but within a species the high germination rates were mostly found on sand without a microbial mat. Only the germination of Agrostis stolonifera appeared to be stimulated by the presence of a well-developed microbial mat, they were even higher than in the reference. Seedling survival also did not show different responses between successional stages. Seedlings placed on top of a microbial mat showed for most species lower survival rates compared to seedlings that were planted or placed on top of the sand. Growth was the only measured variable that differed between successional groups. Species of the early- and intermediate successional stages grew significantly better if a microbial mat was present whereas late successional species were not stimulated. Early and intermediate successional species seem to be favored by the presence of a microbial mat. An explanation for this may be that they can profit from the enhanced nitrogen availability caused by N₂-fixation by cyanobacteria in the microbial mat. This revised version was published online in June 2006 with corrections to the Cover Date.     

The central role of Clark's nutcracker in the dispersal and establishment of whitebark pine

Summary Plant and soil water relationships in a typical nebraska Sandhills prairie were examined to 1) explain the observed distribution patterns of several dominant grasses along a topographic gradient, and 2) show how spatial and temporal variations in soil moisture are critical to community organization on a sandy substrate. An experimental transect encompassing the major community and soil types along a steep, west-facing vegetated dune was established. Maximum available water was shown to be significantly higher in the fine textured surface soils of the lowland sites than the coarse textured sands of the dune sites. Seasonal (1979) patterns of available soil moisture of the sampling sites on the transect showed that in the upper elevation dune sands, moisture was available in the entire profile with surface depletions not occurring until mid to late summer. In contrast, moisture in the surface 60–80 cm in the fine textured lowland soils was exhausted by early to mid-summer with the entire profile nearly dry by late summer. Deep-rooted, C₄ species, Andropogon hallii and Calamovilfa longifolia which are common on upper, coarser sandy soils showed significantly greater water stress on fine textured soils than on dune sands. C₃, shallowrooted species, Agropyron smithii, Stipa comata, and Koeleria cristata always experienced lower mid-day and predawn leaf water potentials than the C₄ species. The C₃ species, with the exception of Koeleria are most abundant on finer textured soils that provide substantial moisture during their peak activity in the spring. It appears that the C₄ species show more conservative water use patterns than the C₃ species as significantly lower leaf conductances in the C₄'s were measured when soil water was abundant. The C₃ species appear to be opportunistic with available water and rapidly deplete surface soil moisture as a result of high transpiration rates. These data suggest that the temporal and spatial distribution of available water along this gradient controls species distribution according to rooting morphology, photosynthetic physiology, and water deficits, incurred by transpirational losses. Competitive interactions between species that utilize soil moisture differently may be an important factor in community organization.     

A Strategy for Restoration of Montane Forest in Anthropogenic Fern Thickets in the Dominican Republic

Deforested tropical areas are often colonized by competitive ferns that inhibit forest succession. In thickets of such a fern (Dicranopteris pectinata), we investigated methods for initiating restoration of tropical montane forest in the Ébano Verde Scientific Reserve (Dominican Republic). In clearings in the thickets, growth and survivorship of 18 common early‐ and late‐successional woody species were tested, with and without fertilizer (poultry litter). Three years after sowing, life history did not affect survivorship, but early‐successional species grew faster than late‐successional species (height increase 153 ± 103 cm vs. 81 ± 67 cm [mean ± 1 SD]). Inga fagifolia, a late‐successional species, and Alchornea latifolia, an early‐successional species, had 160 ± 62 cm mean height increase, and low mortality rates (<4%). In contrast, four late‐successional species (Cyrilla racemiflora, Myrcia deflexa, Prestoea acuminata var. montana, and Mora abbottii), and one early‐successional species, Ocotea leucoxylon, had approximately 39% mortality and height increase of 43 ± 48 cm. Brunellia comocladifolia had high mortality (55%), but averaged 340 ± 201 cm height increase, and was the only species whose growth was improved by fertilization. Fertilization improved survivorship of only one species, Piper aduncum. After three years, soils in the clearings had low pH and available P and did not differ significantly from soils in thickets. However, based on the growth rates and survivorship of sown woody plants, these soils did not appear to be a barrier for restoration. Although a complementary study demonstrated substantial natural regeneration, active planting should be used to increase plant density and diversity, especially where natural regeneration is poor.     

Soil aeration in relation to soil physical properties, nitrogen availability, and root characteristics within an arctic watershed

The seasonal change in soil oxygen availability was determined in several habitats along a topographic moisture gradient in an arctic watershed. The effect of changes in soil aeration on soil chemical and plant properties was examined by comparison of the driest (tussocks) and wettest (wet sedge tundra) sites along this gradient. Spatial variability and seasonal change in soil oxygen availability was closely linked to the hydrologic regime and the thickness of the organic soil horizon. The greatest extension of the aerobic soil layer was found beneath well-drained tussocks, while less than 10% of the unfrozen soil layer is aerated in flooded wet sedge tundra. Intertussock areas and watertracks (channels of water drainage) have intermediate levels of aeration. In tussock tundra, soil oxygen diffusion is restricted in the mineral soil layer below the organic horizon due to reduced pore space. Organic matter constituents and their change with depth were similar beneath tussocks and in wet sedge tundra, indicating that factors other than soil aeration (e.g. low soil temperatures, short growing season) are the primary controls on decomposition in these two arctic tundra systems. NH₄ ⁺, the dominant form of inorganic N, was more available in wet sedge tundra than in tussock tundra. At both sites, extractable and soil solution NO₃ ^- concentrations increased 4 to 8 fold in the second part of the growing season, indicating increased nitrifier activity with improved soil oxygen availability. Although soils thawed as deep as 60 cm, approx. 90% of the root biomass was concentrated within 20 cm of the surface. Despite the anaerobic soil environment in wet sedge tundra, the dominant species there, Eriophorum angustifolium, reached slightly greater rooting depths than E. vaginatum, whose roots grow in the elevated, aerobic portion of tussocks. E. angustifolium had a root porosity of 31%, within the range found for wetland species, while roots of E. vaginatum had a porosity close to 12%. Rhizome porosity were low in both species (11%).     

Growth and allocation of the arctic sedges Eriohorum angustifolium and E. vaginatum: effects of variable soil oxygen and nutrient availability

In arctic tundra soil, oxygen depletion associated with soil flooding may control plant growth either directly through anoxia or indirectly through effects on nutrient availability. This study was designed to evaluate whether plant growth and physiology of two arctic sedge species are more strongly controlled by the direct or indirect effects of decreased soil aeration. Eriophorum angustifolium and E. vaginatum, which originate from flooded and well-drained habitats, respectively, were grown in an in situ transplant garden at two levels of soil oxygen, nitrogen, and phosphorus availability over two growing seasons. In both species, N addition had a stronger effect on growth and biomass allocation than P addition or soil oxygen depletion. Net photosynthesis and carbohydrate concentrations were relatively insensitive to changes in these factors. Biomass reallocated from shoots to below-ground parts in response to limited N supply was equally divided between roots (nutrient acquisition) and perennating rhizomes (storage tissue formation) in E. angustifolium. E. Vaginatum only increased its allocation to rhizomes. In the flood-tolerant E. angustifolium, growth was improved by soil anoxia and biomass allocation among plant parts was not significantly affected. Contrary to our initial hypothesis, whole-plant growth in E. vaginatum improved in flooded soils; however, it only did so when N availability was high. Under low N availability growth in flooded soils was reduced by 20% compared to growth in the aerobic environment. Reduced biomass allocation to rhizomes and thus to storage potential under anaerobic conditions may reduce long-term survival of E. vaginatum in flooded habitats.     

Growth and mineral nutrition of plant species from clearings on different horizons of an iron-humus podzol profile

Summary Four plant species of woodland clearings — Avenella flexuosa, Chamaenerion angustifolium, Rumex acetosella, and Senecio sylvaticus — were grown on five different horizons of an iron-humus podzol profile. Reasonable growth was achieved only on the A₀ horizon, which was rich in organic matter. The growth reduction on the other horizons was correlated with low concentrations of manganese and phosphorus in soils and plant organs. The restriction of so-called acidophilous species to acid soils is interpreted as meaning that they have a need for a high supply of manganese.     

An experimental study of old-field succession in relation to different environmental factors

From 1984 to 1986, old-field succession on sterilized sand and loam was studied under different water- and nutrient regimes. Within one month, moss and phanerogam species appeared on all experimental plots but further succession was rather varied. Salix species established quickly on loam and formed within 3 years a shrub layer up to 3 m in height. On sand, woody plant species were observed only at a high ground-water level. On loam, the well-known old-field succession from short-living therophytes to long-living phanerophytes of clearings and woodlands proceeded very quickly. In contrast, on sand, therophytes, hemicryptophytes and herbaceous chamaephytes of ruderal- and grassland communities were still dominant after three years. A high ground-water level as well as mineral fertilization had sometimes positive, sometimes negative effects on this succession. Periodic estimates of cover, made during the succession were supplemented at the end of the experiment by the measurements of phytomass and bioelement storage. The highest amount of biomass was measured on the three loamy soils where shrub layers were well developed. In comparison with data published elsewhere, the above-ground biomass of 2.2–2.8 kg dry matter m^-2 and the below-ground biomass up to 7.2 kg dry matter m^-2 were both extraordinarily high. Over the three years, the vegetation on sandy soils accumulated between 1.2 and 5.1 g N m^-2 yr^-1 and on loamy soils between 17.1 and 24.7 g N m^-2 yr^-1.     

ECOLOGICAL CONSTRAINTS TO SEEDLING ESTABLISHMENT ON THE PUMICE PLAINS,MOUNT ST. HELENS,WASHINGTON

For both Epilobium angustifolium and Anaphalis margaritacea, the two dominant species in primary succession on the Pumice Plains on Mount St. Helens, density of the seed rain far exceeds the density of colonists. To test the hypothesis that colonization rate is limited by processes occurring during seedling establishment, we investigated the effects of subsurface moisture (using a wet and a dry site), microtopography (fine and coarse pumice particles), and seed size (five size classes), and monitored the fates of seedlings originating from experimentally sown seed for 2 years. Subsurface moisture had the strongest effect on seedling emergence and survival. By the second year there were over 20 times as many seedlings in the wet site as in the dry site; survival in the latter site was nearly zero. Coarse pumice plots had greater establishment than did fine pumice plots. Emergence rate increased significantly with increasing seed size, but this initial difference disappeared by the second year. A. margaritacea established under a wider range of conditions than did E. angustifolium. We conclude that colonization by these species, and hence rate of primary succession, is limited by availability of safe sites for germination and establishment and not by seed dispersal. Safe sites are defined primarily by the level of subsurface moisture and secondarily by microtopography and seed size, and have a greater effect on seedling emergence than on seedling survival.     

Ecological study of pine forest clearings along the French Atlantic sand dunes: Perspectives of restoration

The 18th century afforestation campaign carried out in western France to fix sand dunes has left only a small surface of non-forested dunes. As dune plantations have only a small conservation value, it would be of great interest to restore grey dunes. But, for the moment, there is no proof that pine felling would lead to grey dunes. So, a study has been carried out on pine forest clearings along the French Atlantic shoreline. Floral and ecological data were carried out and analysed with two statistical tools, CA (canonical analysis) and CCA (canonical correspondence analysis). The clearings appear very different from one bank of the Gironde river to the other. South of the river, the clearing vegetation develops on acid soils and possess some thermophilous species. North of the river, the clearings are composed mostly of species growing best on calcareous soils. Apart from geographical variations, clearings showed two patterns of response: development of closed thickets and tall heathlands; or an open vegetation similar to growing heathlands and grasslands. The cover, the height, the floral richness per strata as well as pedological characteristics are the parameters which explain the best floristic composition of pine clearings. The vigour of pine forests and the degree of exposure to coastal influences determine whether close or open vegetation develops. Where there is an open vegetation, restoration of grey dunes may be possible. In the other case, alternative solutions, such as the restoration of dune woodlands or dune heath, may provide best conservation values.     

Shrubs as ecosystem engineers in a coastal dune: influences on plant populations,communities and ecosystems

Question: How do two shrubs with contrasting life‐history characteristics influence abundance of dominant plant taxa, species richness and aboveground biomass of grasses and forbs, litter accumulation, nitrogen pools and mineralization rates? How are these shrubs – and thus their effects on populations, communities and ecosystems – distributed spatially across the landscape? Location: Coastal hind‐dune system, Bodega Head, northern California. Methods: In each of 4 years, we compared vegetation, leaf litter and soil nitrogen under canopies of two native shrubs –Ericameria ericoides and the nitrogen‐fixing Lupinus chamissonis– with those in adjacent open dunes. Results: At the population level, density and cover of the native forb Claytonia perfoliata and the exotic grass Bromus diandrus were higher under shrubs than in shrub‐free areas, whereas they were lower under shrubs for the exotic grass Vulpia bromoides. In contrast, cover of three native moss species was highest under Ericameria and equally low under Lupinus and shrub‐free areas. At community level, species richness and aboveground biomass of herbaceous dicots was lower beneath shrubs, whereas no pattern emerged for grasses. At ecosystem level, areas beneath shrubs accumulated more leaf litter and had larger pools of soil ammonium and nitrate. Rates of nitrate mineralization were higher under Lupinus, followed by Ericameria and then open dune. At landscape level, the two shrubs – and their distinctive vegetation and soils – frequently had uniform spatial distributions, and the distance separating neighbouring shrubs increased as their combined sizes increased. Conclusions: Collectively, these data suggest that both shrubs serve as ecosystem engineers in this coastal dune, having influences at multiple levels of biological organization. Our data also suggest that intraspecific competition influenced the spatial distributions of these shrubs and thus altered the distribution of their effects throughout the landscape.     

Effects of Ca- and Fe-rich Seepage on P Availability and Plant Performance in Calcareous Dune Soils

Ca- and Fe-rich seepage in wet dune slacks often sustains oligotrophic, species rich vegetation as a result of reduced P availability. While this effect has been attributed to pH buffering, we tested whether Ca- and Fe-rich seepage also immobilised P in calcareous soils with a strong pH buffer. Two oligotrophic species (Carex flacca and Schoenus nigricans) and two eutrophic species (Calamagrostis epigejos and Molinia caerulea) were planted in experimental sods. After 4 months supply with water of seepage or infiltration quality, with or without the addition of P, soil P fractions and a number of plant physiological responses were measured. A field validation was performed in a flow-through lake in calcareous dunes where the seepage flow had been restored recently. The readily available water soluble P fraction (Pw) was reduced by more than 80% by seepage, both in the greenhouse experiment and in the field, but the P Olsen-inorganic fraction was unaffected. All four test species had elevated N: P ratios in aboveground tissues when treated with seepage, indicating that seepage water had indeed reduced P availability to plants. Formation of dauciform roots by Carex flacca was diminished by P addition to less than 25% of treatments without P addition, indicating sensitivity to P availability, while seepage on average halved production of these root structures. Dauciform root formation by Schoenus nigricans was unaffected by the P addition and the hydrological treatment. Biomass of the test species in the experiment as well as vegetation biomass and relative abundance of oligotrophic species in the field were unrelated to seepage patterns, suggesting that compensatory mechanisms enabled the plants to sustain biomass production within the time frame of this experiment. In conclusion, Ca- and Fe-rich seepage can lower P availability in soils with a strong pH buffer. In the long term, this may create favourable conditions for species that have low P requirements or efficient P uptake. In the short term, however, existing vegetation seems to be resilient to changes in P supply.     

Urea hydrolysis and transformations in coastal dune sands and soil

Summary Urea hydrolysis was studied in samples taken from a coastal sand dune succession, from uncolonized sand; the rhizosphere ofAmmophila arenaria and soil from the mature dune. Comparisons were made with urea hydrolysis in a fertile loam soil. Urea was hydrolyzed in all sand and soil samples, with complete hydrolysis occurring after 6 and 3 weeks in the rhizosphere sand and dune soil compared with only 4 days in the fertile loam. A third of the added urea, however, was still present in the uncolonized sand samples 6 weeks after the beginning of the incubation period. Urea hydrolysis broadly correlated with urease activity.The liberated NH ₄ ⁺ was oxidized to NO ₃ ^– –N in all samples. Urea stimulated the release of N from native organic matter in the two soils, but not sands, due presumably to the low organic matter content of the latter. Nitrite accumulated in the dune sands and soil, but not in the fertile loam.Although N-Serve (Nitrapyrin) had no effect on urea hydrolysis in any of the treated samples, it inhibited the nitrification of released NH ₄ ⁺ –N. The relevance of these findings to the use of urea as a fertilizer to improve plant growth and dune stabilization is commented upon.     

Soil nutrients are not responsible for arrested succession in disturbed coastal dune forest

Coastal dune forest succession frequently proceeds via the Acacia karroo pathway, but may become arrested. We examine whether soil fertility arrests forest succession in A. karroo stands in coastal dune forest in KwaZulu-Natal province, South Africa. We examined soil fertility of A. karroo stands, the adjacent forest, and forested dune slacks at Cape Vidal, and four rehabilitating A. karroo stands (13- to 28-yr-old) at Richards Bay. The effect of nitrogen supplementation on growth of three tree species (a forest pioneer, a late successional forest species, and A. karroo) was compared between A. karroo stands and adjacent dune forest at Cape Vidal. Soil fertility in A. karroo stands and the adjacent forest at Cape Vidal was similar and neither total nor readily mineralisable nitrogen were limiting in either habitat. At Richards Bay, where the dunes were previously strip-mined, total nitrogen accumulated rapidly (2.1–8.0 g N m⁻² yr⁻¹) and the oldest rehabilitating A. karroo stands (26–28 yr) had similar total nitrogen and other soil nutrient levels as stands twice their age at Cape Vidal. Seedling growth was unaffected by nitrogen supplementation. All species grew fastest in A. karroo stands demonstrating that soil nutrient levels in disturbed forest colonised by A. karroo are suitable for the growth of forest tree species. Soil fertility, including available nitrogen, is not limiting secondary succession at Cape Vidal, yet forest species are not replacing A. karroo stands at this site. Post-emergence factors, such as herbivory, are likely responsible for the arrested succession of forest in A. karroo stands.     

An ericoid shrub plays a dual role in recruiting both pines and their fungal symbionts along primary succession gradients

Relative importance of positive and negative interactions between plant species may change along disturbance and resource gradients. Positive interactions are suggested to prevail in low resource, low productivity (high stress) conditions and negative interactions in high resource availability. A dwarf shrub, mountain crowberry Empetrum nigrum ssp. hermaphroditum, is known to have allelopathic impacts on both Scots pine Pinus sylvestris and its ectomycorrhizal symbionts. We aimed to study if the outcome of Empetrum impacts on Scots pine changes along primary succession gradients on the dune shores of Bothnian Bay, in Finland, where abiotic stress gradually changes to biotic stress along the succession. We found that Empetrum may act as a facilitator despite its allelopathic effects, since the proportion of Scots pine seedlings established in Empetrum patches was higher than in patches without the shrub in early and mid succession stages, whereas patches without Empetrum were preferred in late succession. The amount of mycelial fungal biomass (ergosterol) in the soil in the vicinity of the seedling roots was higher in Empetrum patches than in patches without Empetrum and it increased along the succession gradient. Proportion of pine root tips colonised by suilloid morphotypes with abundant external mycelia and the diversity of ectomycorrhizal morphotypes were higher in mid successional stage in Empetrum patches compared to patches without Empetrum. Our results suggest that in the harsh physical conditions of the dune shore Empetrum facilitates pine seedling establishment in the early and mid stages of succession by providing mechanical and physical shelter whereas in late succession negative interactions (competition and allelopathy) between the shrub and the pine are dominating. To our knowledge we present the first finding that an ericoid mycorrhizal shrub could enhance both the performance of the ectomycorrhizal host tree and the tree's fungal symbionts.     

Nitrate production in the rhizosphere of Plantago species

Summary The production of nitrate in an old established dune grassland soil and its uptake by plants was studied by comparing amounts of mineral nitrogen and numbers of nitrifying bacteria in the rhizosphere on the one hand, and on the other accumulated nitrate and levels of nitrate reductase (NaR) of individual plants of three Plantago species,i. e., P. major, P. lanceolata andP. coronopus. For these three Plantago species andP. media basal levels of NaR in the absence of nitrate were determined in plants grown in culture solutions. The basal NaR levels ofP. major andP. media (species occurring on nutrient-rich soils) were significantly higher than those ofP. lanceolata andP. coronopus (species found on nutrient-poor soils). NaR activity increased in the presence of nitrate and was suppressed by ammonium.From the numbers of nitrifying bacteria in the rhizosphere and NaR activity in the leaves it was concluded that nitrate was produced in the root environments of the three Plantago species and that the compound was taken up by the plants. NaR activities and numbers of nitrifying bacteria were higher for individuals ofP. major than for those ofP. lanceolata andP. coronopus. No correlation was found between the ammonium levels and the numbers of nitrifying bacteria in the soil, and no indications of inhibition of nitrifying bacteria in the rhizosphere were obtained. For individuals ofP. lanceolata a correlation was found between the numbers of nitrifying bacteria in the soil and NaR activity in the leaves. The results are discussed in relation to the ecological habitats of the three species.Grassland Species Research Group Publication No.38.     

The role of cattle in maintaining plant species diversity in wet dune valleys

The succession of species-rich wetland vegetation in dune valleys into species-poor dwarf shrub vegetation was followed by means of permanent vegetation plots, in which the cover of vascular plant, moss and lichen species were recorded over a period of up to 33 years. Low density cattle grazing is an effective substitute for rabbits in stalling this succession, thus preserving the local plant species diversity. The influence of direct rabbit grazing pressure on the vegetation was studied in exclosures. The differences were significant, but not long-lasting after the exclosures had been opened again to the rabbits. Cladina lichen-rich Empetrum-heathland in the dunes is a stage in a probably cyclic succession that might be triggered by grazing. The total vascular plant, moss and lichen diversity of the dune ecosystem can be maintained by a combination of extensive cattle grazing and a regular but limited re-creation of pioneer situations.     

Nutrient pools and fluxes of the ground vegetation in coniferous forests due to fertilizing,liming and amelioration

The effects of fertilization and amelioration treatments on some nutrient pools and fluxes of ground vegetation in mature pine and spruce stands on acid soils in South Germany are described. In N-limited pine forests with moderate canopy density and with Deschampsia flexuosa an additional N-accumulation in biomass of 20–40 kg ha^-1 occurred 3 years after pure N-fertilization. The N, P, K-cycling through ground vegetation was stimulated more than 10 years by a combined N + CaCO₃ + P treatment leading toa shift in dominance from cryptogams and Ericaceae towards Deschampsia flexuosa and ruderal species like Epilobium angustifolium. The effect of a lupine treatment (combined with initial soil preparation, liming and P supply) was far stronger than the effect of the other experimental procedures. But the fertilizer and amelioration effects on the herb layer of pine forests tended to decline after two decades for different reasons.The shade-tolerant ground vegetation in a nitrogen-saturated spruce forest was not able to prevent heavy additional nitrate losses from upper mineral soil after dolomitic liming. But the Ca, Mg and K fluxes through ground vegetation were strongly elevated in the third year after treatment.