Molinia

Abstract: SUMMARY (1) A comparative study was made of the seasonal variation in floristic composition of the diets selected by sheep and cattle grazing together on Agrostis-Festuca, Nardus, and Molinia grassland communities in southern Scotland. Fenced plots at each site were grazed in sequence with the experimental animals moved between sites as appropriate. (2) For each period at each site, sward biomass, species and morphological composition and canopy structure were characterized. Diet samples were collected from three to five oesophageally-fistulated animals of each species from four to seven, separate, two-week periods of observation at each site over three years. (3) Sheep diets were more variable than cattle diets, both between and within animals, for all sites. Sheep and cattle differed significantly for almost all major dietary components; exceptions were Molinia leaf, grass sheath and dead material at the Molinia site. (4) At all sites in summer, sheep diets contained more forbs and less grass flowerstem than did cattle diets. (5) At the Agrostis-Festuca and Nardus sites, sheep diets contained more live components than did cattle diets on all sampling dates. At the Molinia site, sheep and cattle diets contained similar proportions of live components in early summer, but in autumn after the Molinia and Holcus leaves died, sheep grazed other grass species and their diets contained less live components than those of the cattle which grazed Junlcus spp. (6) At the Nardus site, both sheep and cattle preferentially grazed the Festuca and Deschamnpsia (from between the Nardus tussocks) and the small patches of broad-leaved grasses. The progressive reduction in herbage biomass and height on the preferred areas in successive grazing periods was associated with a marked increase in the Nardus content of cattle diets but not of sheep diets. (7) Differences between sheep and cattle diets were explicable by (i) a difference in the height at which the animals grazed in relation to differences in the distribution of plant species within the sward canopy, (ii) the greater ability of sheep to select from fine-scale mixtures; and (iii) the greater readiness of cattle to graze tall, more fibrous components. (8) The implications of the differences in diet selection between sheep and cattle for sward responses and grazing management are discussed.

Abstract: We studied the effects of N deposition on shrub-moss competition and the establishment and growth of invasive Betula pubescens and Molinia caerulea in intact bog vegetation removed from a site subject to 40 kg N ha(-1) yr(-1). Mesocosms with and without introduced Betula seedlings and Molinia sprouts were kept under a roof and received an equivalent of 0, 40 and 80 kg N ha(-1) yr(-1) for two growing seasons. N concentration in both interstitial water and Sphagnum decreased when N input ceased and increased when N input was doubled. Molinia biomass was positively related to the inorganic N concentration in the interstitial water. Adding N increased production of Molinia and prolonged survival of Betula seedlings in the first year. Sphagnum height increment showed a hump-shaped relationship with light interception by vascular plants. N deposition encouraged vascular plants to grow by enhancing N availability in the rhizosphere. Water table level and the availability of P were found to be important in explaining species-specific responses to N deposition. The underlying mechanisms and the reversibility of N effects are discussed.

Abstract: The term relative nutrient requirement is introduced in order to measure and to compare the nutrient losses from different perennial plant populations and the amount of nutrient that they need to absorb to compensate these losses. The relative nutrient requirement (L) is defined as the amount of the growth-limiting nutrient that must be taken up to maintain or replace each unit of biomass during a given time interval (e.g., mgN g-1 biomass year-1). It is derived that in a plant community with two competing perennial plant populations, species1 will become dominant if the relative competition coefficient k 12 (sensu De Wit 1960) exceeds the ratio between the relative nutrient requirements of the two species (L 1/L 2), whereas species 2 will become dominant, if k 12 is below this critical ratio. The above-ground litter production was measured inwet heathland communities dominated by Erica tetralix or by Molinia caeruleain order to estimate N and P losses from theaboveground biomass and to calculate the relative N and P requirements of these species. Molinia lost during one year 63% and 34%, respectively, of the amount of N and P present in the above-ground biomass at the end of the growing season. These losses were in Erica 27% and 31%, respectively. The relative N requirements of the two species show the same difference: 7.5 and 2.6 mg N g-1 yr-1, respectively, in Molinia and in Erica. Also the relative P requirement of Molinia is higher as well as that of Erica (0.18 versus 0.08 mg P g-1 yr-1). The relative competition coefficient of Molinia with respect to Erica (k me ) is equal to unity under unfertilized conditions but increases with increasing nutrient supply. Under nutrient-poor conditions k me is below the critical ratio of the relative nutrient requirements of the two species (L m /L e =2.9 or 2.3), so that Erica will be the dominant species. After an increase in nutrient availability k me increases and exceeds this critical limit which results in Molinia replacing Erica. During the last 20 years this replacement of Erica-dominant communities by monocultures of Molinia has been observed in almost all wet heathlands in The Netherlands along with a strong increase in nitrogen availability.

Abstract: Summary 1 In order to test whether the observed invasion of ombrotrophic bogs in the Netherlands by Molinia caerulea and Betula pubescens is the result of long-term high nitrogen (N) loads, we conducted a 3-year fertilization experiment with Sphagnum fallax turfs. Six different N treatments were applied ranging from 0 (control) to 4 g N m−2 year−1. 2 During the experimental period, ammonium concentrations in the peat moisture remained very low due to high uptake rates by Sphagnum. Tissue N concentrations in S. fallax showed a linear response to the experimental N addition. Excess N was accumulated as N-rich free amino acids such as arginine, asparagine and glutamine, especially at N addition rates of 0.25 g m−2 year−1 or higher, indicating N-saturation. 3 Despite the high tissue N : P ratio (above 35), above-ground biomass production by Molinia was still stimulated at N addition rates of 4 g m−2 year−1, and foliar nutrient concentrations were unaffected compared to the control. In contrast to Molinia, Betula was unable to increase its above-ground biomass. Foliar N concentrations in Betula were significantly higher at N addition rates of 4 g m−2 year−1 and excess N was stored in foliar arginine, making up 27% of the total N concentration. Evapotranspiration was increased at higher N addition rates due to stimulated total above-ground biomass production of the vegetation. 4 N addition at the actual Dutch deposition rate of 4 g m−2 year−1 stimulated the growth of Molinia in this experiment, providing evidence that the observed dominance of Molinia on ombrotrophic bogs in the Netherlands is caused by high N deposition levels. Based on the observed changes in biomass production and tissue nutrient concentrations, we assume that a long-term deposition of 0.5 g N m−2 year−1, or higher, leads to undesirable changes in species composition and increased risk of desiccation.