Shoot regeneration after fire or freezing temperatures and its relation to plant life-form for some heathland species

Shoot regeneration after prescribed burning or following the freezing temperatures of winter was monitored for nineteen heathland species present in an Arctostaphyleto-Callunetum community in northeast Scotland. Species whose renewal buds were near the surface of the ground started to grow earlier in the spring than species with renewal buds above the surface, but grouping species according to the position of their renewal bud (i.e. their life-form) did not account for all of the interspecific variation apparent. In the case of shoot regeneration after fire, species whose renewal buds were destroyed by fire because they were above-ground started to regenerate about the same time as species with belowground buds, protected from fire, but reached their maximum frequency of occurrence later. Grouping species by life-form was of limited value as a means of interpreting this interspecific variation in the timing of shoot regeneration after fire. It would be unwise to use plant life-form as the sole basis for interpreting or predicting a species' response to temperature stress when extreme temperatures occur regularly, as they do in heathland. The possible use of other plant traits to interpret and predict interspecific variation in the regeneration rate of heathland plants is discussed.Nomenclature follows Tutin et al. (1964–1980) for vascular plants. Acknowledgements. The Nature Conservancy Council and Mr J. J. Humphries kindly allowed Dinnet Moor to be used for the work presented here. One of us (RJR) received financial support for field work from the Natural Sciences and Engineering Research Council of Canada.     

Calluna and its associated species: Some aspects of co-existence in communities

Summary Using heath communities (dominated byCalluna vulgaris) as an example, an attempt is made to identify those characteristics of other species wiich enable them to co-exist with the dominant. In the course of its life-span,Calluna passes through a series of growth phases (pioneer, building, mature, degenerate) during which there are marked changes in the condition affecting associated species. These display a variety of ‘complementary strategies’ which enable them to co-exist withCalluna during a part, or the whole, of the sequence. Changes with time in the flora of even-aged stands ofCalluna, produced as a result of management by burning, are accounted for in similar terms. It is also shown that invasion of heath communities by tree species (e.g.Betula spp.,Pinus sylvestris), which may eventually eliminateCalluna, is related to the sequence of growth-phases. Contribution to the Symposium on plant species and plant communities, held at Nijmegen, 11–12 November 1976, on the occasion of the 60th birthday of Professor Victor Westhoff-to whom the author conveys the good wishes of Scottish botanists and conservationists, and particularly those of the University of Aberdeen. Nomenclature follows Clapham, Tutin & Warburg (1962) for vascular plants, Watson (1968) for bryophytes, and Duncan (1970) for lichens. Following common practiceCalluna vulgaris is referred to simply asCalluna. Grateful acknowledgement is made to several present and former research students who have kindly permitted the use of their data: particularly P. Barclay-Estrup, Edith M. French, Gong Wooi Khoon, C.J. Legg, Evelyn W. Paterson, S.D. Ward. I am especially grateful to C.J. Legg for valuable discussions on the subject of this paper.     

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.     

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.     

A model of Calluna population dynamics; the effects of varying seed and vegetative regeneration

Regeneration of Calluna vulgaris in heathlands occurs from both seed and layering, although the relative importance of these two strategies appears to vary. A population model based on transitions between growth phases has been devised and parameter values obtained from published and original work. The effects of differing amounts of seed and vegetative regeneration on the population changes were then tested.The model predicts that a population with a relatively high occurrence of layering will tend towards a steady state, with little temporal variation in population density and cover. Conversely, low layering capacity increases temporal variation with little effect on mean cover. Increased seed regeneration on the other hand, increases both the amplitude of temporal variation in population density, and the frequency of cycles, whereas low seed regeneration results in a stable age distribution at less than 100% cover.