A Quantitative Analysis of Shoot Phenology and Dominance in Herbaceous Vegetation

Abstract

SUMMARY (1) Variation in standing crop and in litter has been measured by seasonal sampling and sorting of the herbaceous vegetation at thirteen sites in the Sheffield region. The vegetation types examined comprised tall herb, woodland floor and grassland communities. At each site the living fraction in 4-10 replicated 0 25 m2 quadrats was separated into its components, and graphs were plotted showing the shoot phenology of the more common species. (2) At the sites in which herbaceous vegetation was growing under fertile, relatively undisturbed conditions, there was a large peak in standing crop during the summer ( > 400 g m- 2 dry weight), and the vegetation contained few species. The low speciesdensities at these sites appeared to be related to the ability of certain species to exercise competitive dominance, a phenomenon involving the rapid expansion of a dense leaf canopy during the period June-August, coupled with the production of a high density of persistent litter. (3) In the woodland and grassland sites examined, the sum of the maximum standing crop and the litter did not exceed 800 g m- 2, and a variety of plant phenologies was encountered. At two of the woodland sites vernal species were prominent; these plants exhibited truncated periods of shoot growth, which preceded full expansion of the tree canopy and followed immediately the marked decline in density of tree litter in early spring. (4) At the sites where the standing crop was severely restricted by low soil fertility, the commonest phenological pattern was that of the evergreen; in certain of these species, no seasonal peak of shoot expansion could be detected. In two of the limestone grasslands investigated, forbs with mid-summer peaks of shoot expansion were prominent; the majority of these plants had relatively deep root-systems, and appeared to exploit reserves of moisture during periods when many grasses were subjected to desiccation. (5) A consistent feature of the results was the marked amplitude of seasonal variation in the abundance of bryophytes, expansion of which coincided with the moist, cool conditions of spring and autumn. (6) A general conclusion drawn from this study relates to the control of speciesdensity in herbaceous vegetation. The results suggest that the potential for high species-density corresponds approximately to the range of 350-750 g m-2 in the sum of maximum standing crop and litter.