Seed-shedding in a Scottish heath community

Abstract. For a 28-week period in late 1987 and early 1988, a study of seed-shedding by several heath species was carried out at the Muir of Dinnet in northeastern Scotland. The dominant species in the heath is Calluna vulgaris. Seed-shedding in Calluna began in early September 1987 and was completed in April 1988, with the period of maximum shedding falling between early November and late December 1987. The total numbers of seeds/m² deposited in stands of Calluna in its four growth-phases were: pioneer, 18 910; building, 169010; mature, 198580; degenerate, 33900. Substantial loss of potential seeds results from the shedding of immature flowers. A control area close to, but outside the area of Calluna dominance had a deposition rate of 770 Calluna seeds/m², indicating sufficient seed to colonise nearby available habitats. Seed rain was also recorded for several other heathland species: Erica cinerea, E. tetralix, Carex spp. and Betula pendula. Seeds of Erica cinerea were deposited in all the Calluna stands, 32670/m² in the pioneer stand, 17600/m² in building, 3720/m² in mature, 210/m² in degenerate (120/m² in the control area). Numbers were greatest at the start of the sampling period, declining thereafter. This applied also to Betula seeds. Erica tetralix occurred in the degenerate Calluna stand and yielded 640 seeds/m² (400/m² in the control area). Seeds of Carex spp. were obtained in the control: 4110/m². The method of sampling has a significant effect on the figures obtained. A method using tube collectors, emptied frequently, is recommended.     

Lignotuber size of Erica australis and its relationship with soil resources

Abstract. It has been proposed that resprouting after a disturbance would be favoured under conditions of low soil resource availability. In lignotuberous plants of Mediterranean‐type areas, successful resprouting after disturbances such as fire depends on the size of the lignotuber, but little is known about the role of soil resources in determining the relative size of this organ. In this work we tested whether the relative size of the lignotuber in the resprouting shrub Erica australis is related to the availability of soil resources. At each of 13 different sites in Spain, 10 plants were chosen and the sizes of their various parts (above‐ and below‐ground) measured. Additionally, at each site we evaluated soil fertility, foliar N and P concentrations and plant water potential in the middle of the summer. The relationships between lignotuber dimensions and the various plant parts were assessed. In all cases, significant differences between sites were found indicating consistent differences in lignotuber size across the range of plant sizes. This was particularly the case when lignotuber dimensions were expressed as a function of foliar biomass. Lignotuber dimensions relative to foliar biomass were positively correlated with soil pH and negatively with plant water potentials in midsummer. There were, however, no clear relationships between lignotuber relative dimensions and soil N and P contents or other measures of site productivity. In summary, lignotubers were not smaller at more productive sites, in fact they were relatively larger at sites where soils were less acidic, but where plant water conditions were less favourable. These findings contradict predictions made based on current theory on the role of soil fertility allocation to resprouting.     

The effects of grazing on local colonisation and extinction during early succession

Estimates of colonisation of plant species were made at three spatial scales in an old-field on limestone subject to five experimental sheep grazing regimes. Local extinctions within grazing treatments were estimated in 1-m² permanent quadrats. These data were used to assess the effects of grazing treatment and spatial scale on the process of species change over a period of six years. Colonisation of the lOha field was virtually a random draw of plant species from adj acent vegetation, irrespective of plant life-history traits including dispersal strategy. The effects of grazing on colonisation increased at smaller spatial scales. Colonisation rates changed little during the study on the 10 ha scale, but declined steeply with time at smaller scales. Colonisation rates of short-lived species declined more than those of perennials, and short-lived species were subject to erratic episodes of extinction which did not affect perennials. Short periods of grazing enhanced colonisation rates of all species, but extinction rates were the same as in ungrazed controls. Grazing for longer periods further enhanced colonisation rates, but also increased extinction rates. This produced diversity patterns consistent with a ‘hump-backed model’, except that no grazing treatment was heavy enough to decrease diversity. Ungrazed controls had low species diversity, but areas grazed for longer periods were no more diverse after six years than those grazed for short periods.     

The role of seedbank and sown seeds in the restoration of an English flood-meadow

Abstract. In 1985 an opportunity arose to make use of an area (referred to as Somerford Mead) which had recently been used for intensive grass or cereal production but which originally had been a permanent hay-meadow. Situated on circum- neutral alluvial soils over limestone gravel, it is in close proximity to the River Thames and to species-rich flood meadows, including Oxey Mead (Fig. 1). This paper describes the early stages of an experiment to recreate a ‘typical’ flood-meadow community using seed harvested from Oxey Mead. Before sowing the seed, soil samples were taken and seedling emergence suggested that little of the original flood-meadow seed bank survived. Although Oxey Mead was known to contain at least 57 species, germination of samples of the sown seed in the glasshouse was confined to 12 species. All except Bromus commutatus and Trisetum flavescens and an additional nine Oxey Mead species were recorded in the field, together with 24 arable weed/ruderal species. Soil-nutrient contents (N, P and K) were high; so the restoration of this flood-meadow using only management techniques (hay cutting at the end of June and aftermath grazing) and the local seed bank would take a long time. The use of Alopecuros pratensis-Sanguisorba officinalis seed mixture was justified by the accelerated succession on Somerford Mead towards the parent community.     

A two-phase mosaic in a savanna grassland

A two-phase grassland mosaic was described using species association measures and pattern analysis techniques on local frequency data from six transects of contiguous quadrats. One phase consisted of almost mono-specific patches of Setaria incrassata, which was negatively or not associated with ten common species. The other phase was dominated by Themeda triandra which was positively associated with several species. PQV analyses revealed similar, significant, anisotropic scales of contagion of Setaria and Themeda, with phases alternating over 9 m up-slope and 4 m along the contour. Setaria was associated with a tree canopy and deeper soils of higher organic carbon content than Themeda, which was negatively associated with tree canopies. Excavation of Setaria rhizomes indicated mean densities of 20.5 putative genets/m² and 3.6 ramets per genet. Dense clonal growth, seedling recruitment and a competitive exclusion mechanism may contribute to patch maintenance.     

农业区域市化对植物多样性的影响：遵化的研究

选择河北省遵化市(39°55'～40°22'N,117°34'～118°14'E),调查了城郊和乡村在东南西北4个方向上以及市区的植物多样性,同时考察了不同取样点土壤养分的差异结果表明,西部和北部接近,而东部和南部的土壤养分特殊,城郊与乡村明显差异;西部物种丰富度低于其它方向,而南部最高,这种差异由草本造成;城市与东南西北4个方向的生物多样性差异明显;城市植物多样性与城郊和乡村呈现梯度变化;本研究统计的物种总数只有126种,而与之同纬度的北京东灵山区却有高等植物997种.城市化的影响是引进了一些观赏物种,区域地形和土壤养分等自然特征的影响微弱.