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Figure 3.3 (a-e) Plant life form analyses for the 5 relevées whose positions in the primeval forest of Fiby urskog are shown in Fig. 2.7. Relevas 2 and 3 each had an area of 0.4 ha, those of relevas 1, 3 and 5 were 0.3 ha. (f) A comparison of the 5 relevas by means of the S0rensen coefficient of community (see text for details).
glaucum, turfs (like the pile of a carpet) e.g. Dicranum majus and Polytricum formosum, canopy formers (producing a raised leafy canopy or dendroid form such as Climacium dendroides and Thamnium alopecurum), mats and wefts (dense and loose interwoven shoots, respectively). Rough mats, smooth mats and dendroid forms are the commonest types on rocks, but a wider variety grow on soil. The influence of light levels and soil conditions on bryophyte distributions is discussed in Section 6.5.1.
Biological (or life form) spectra for particular areas are constructed by expressing the numbers of species in each life form class as percentages of all the species present. These can be compared with the normal spectrum that Raunkiaer derived from 1000 plants taken as a representative sample of the world flora (Fig. 3.3h). There is a strong correlation between the climate of an area and the life forms of the plants present; a phytoclimate is characterized by the life form which most greatly exceeds the percentage for its class in the normal spectrum. Like that of most of the cool temperate zone including Britain, the phytoclimate of Raunkiaer's native Denmark is hemicryptophytic. This is because although the vegetation may be visually dominated by trees, in the phytoclimate classification each species carries equal weight in this system, regardless of abundance or importance. Thus, although the relatively few species oftrees native in Britain since the last glaciation were dominant almost everywhere until the advent of large-scale agriculture, these phanerophytes appear quite insignificant when compared with those of the normal spectrum.
The regions of the world in which the four major world climates occur, with their subdivisions, can be delimited by lines along which biological spectra are similar. Where rainfall is not deficient the phytoclimate of the tropics is phaner-ophytic, and within this zone a higher proportion of the larger forms are present in the wetter areas. Subtropical desert areas are therophytic and burst into life after very occasional periods of heavy rain. Geophytes are best represented in regions with a Mediterranean climate where the unfavourable season is the hot
Caption for Figure 3.3 (cont.)
(g) Biological spectrum of the entire reserve compared with (h) the normal spectrum taken by Raunkiaer (1934) from the world list. Figures in parentheses are numbers of species. S, stem succulent phanerophytes; E, epiphytic phanerophytes; MM, meso- and megaphanerophytes (perennating buds 8-30 m and > 30 m above ground, respectively); M, microphanerophytes (short woody plants with buds 2-8 m above ground); N, nanophanerophytes (smaller shrubs with buds 0.25-2m above ground); Ch, chamaephytes; H, hemicryptophytes; G, geophytes; Hel, helophytes; Hyd, hydrophytes; HH, helophytes and hydrophytes; Th, therophytes. (Data of Packham, Hytteborn, Claessen and Leemans. From Packham et al., 1992. Functional Ecology of Woodlands and Forests. Chapman and Hall, Fig. 2.3. With kind permission of Springer Science and Business Media.)
dry summer, but do not give their name to a phytoclimate. Plants with this life form are at a disadvantage where the soil warms slowly in spring and the growing season is short. Resting buds of hemicryptophytes in the cool temperate zone are often protected by snow in hard winters, but warmed by the sun as soon as it melts. The chamaephytes give their name to the cold zones near the poles where cushion forms in particular derive protection from snow, growing again as soon as it melts in spring. A survey of temperate broadleaved woodlands in Britain and North America showed that hemicryptophytes were abundant in all, but especially in northern areas, while cryptophytes with their buried buds were more important in southern and drier sites.
3.1.2 Local life form variation within a primitive forest
Figure 3.3 illustrates life form variation within the primeval forest of Fiby urskog, Sweden. This shows the number of species for each life form in five areas of the reserve whose positions are shown in Fig. 2.7 (note that a releve is a description of a visually uniform area). Releve; 4 is in species-rich spruce forest of the edge buffer zone. This has been subject to considerable interference in the past, has many more herb species than the other conifer-dominated relevees and the greatest number of additional species are hemicryptophytes. Other parts of the forest are equally species-rich, however. The next most important group is that of the geophytes. Many are rhizome geophytes such as oak fern Gymnocarpium dryopteris, baneberry Actaea spicata, wood anemone Anemone nemorosa, the liverleaf Hepatica nobilis, May lily Maianthemum bifolium and chickweed-wintergreen Trientalis europaea. Abundant in northern spruce forests, wood-sorrel Oxalis acetosella is found as an herbaceous chamaephyte, a rosette-hemicryptophyte or a rhizome geophyte according to the particular environment of the individuals concerned. Helophytes (marsh plants permanently rooted in mud) such as tufted sedge Carex elata, water horsetail Equisetum fluviatile, common marsh-bedstraw Galium palustre, yellow loosestrife Lysimachia vulgaris and marsh cinquefoil Potentilla palustris are common in releve 5, a damp downy birch Betula pubescens woodland now being invaded by Norway spruce Picea abies. Therophytes are absent here.
The index of similarity, otherwise known as the Sorensen coefficient of community, has been used to illustrate the differences between these communities. It is calculated from the number of species found in one community (a), the number in the other (b), and the species common to both (c):
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