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diaspore bank forest floor

Box 7.3 Fig. 1. Differences in frequency of occurrence of the most common bryophyte species in the diaspore bank and forest floor in a Swedish boreal forest (1 = <10%, 2 = 10-50%, 3 = >50%) (data from Jonsson 1993).

Similarly, During and ter Horst (1983) found that competitive pleurocarps, which dominated the above-ground bryophyte layer in a Dutch chalk grassland were seldom represented within the diaspore bank. By contrast, many acrocarpous species rarely found above ground composed the bulk of the bank. This discrepancy may reflect changes in the above-ground vegetation, but also differences in the life strategies: colonists, annuals and short-lived shuttle species are well represented, whereas perennials are rare.

Shuttle species typically produce a few, large spores that may be adapted to cyclic habitat conditions. In particular, seed dormancy, a well-known strategy of vascular plants to bridge unfavourable periods, has been recently demonstrated in bryophytes (Hock et al. 2004). In the annual shuttle Phascum cuspidatum, for example, the number of germinated spores increases strikingly after one year of storage, suggesting that some spores had been in a dormant phase (Box 7.3 Fig. 2). Bryophyte spores may thus be able to remain viable in the soil over decades (During 1997) and development of dormancy might facilitate this process.

% cover

100i-

ou_u_u_u control 6 months 12 months Duration of storage

Box 7.3 Fig. 2. Differences in percentage cover of Phascum cuspidatum emerging from soil samples spread-out and kept in a greenhouse under appropriate growth conditions over 3.5 months immediately after soil sampling (control) and after 6 and 12 months of laboratory storage under air-dry conditions (reproduced from Hock et al. 2004 with permission of Journal of Bryology).

Diaspore banks reflect, to some extent, the composition of past rather than present vegetation. They accumulate genetic variability over generations and display more diversity than that present in the actual above-ground populations (Hock et al. 2008). Diaspore banks, therefore, play a key role in maintaining the high diversity of habitats that are naturally disturbed on a regular basis and in the restoration of degraded habitats.

time (years)

Fig. 7.18. The fugitive life strategy (During 1992). The vertical bar symbolizes the end of the period during which the habitat is suitable for the species. Reproduced from During (1979) with permission of Lindbergia.

time (years)

Fig. 7.18. The fugitive life strategy (During 1992). The vertical bar symbolizes the end of the period during which the habitat is suitable for the species. Reproduced from During (1979) with permission of Lindbergia.

Fig. 7.19. The colonist life strategy (During 1992). The vertical bar symbolizes the end of the period during which the habitat is suitable for the species. Reproduced from During (1979) with permission of Lindbergia.

Often, annual species produce cleistocarpous capsules, wherein no specialized release mechanism exists and spores are directly included within the diaspore bank after the decay of the capsule. Cleistocarpy has evolved in parallel in several lineages of mosses (e.g. in the genera Archidium (Archidiaceae) and Tortula (Pottiaceae) and liverworts (all Sphaerocarpales and many Marchantiales). Annual shuttles are characteristic for cyclic habitats that disappear at varying rates but reappear at the same place (e.g. mud flats at the bottom of dried-out ponds and arable fields). Medium- and long-lived shuttles occur in somewhat longer-lasting micro-habitats. Typical examples of medium-lived shuttles include Splachnum species, which are restricted to dung and produce spores dispersed by flies over rather short distances (Box 4.2).

Fig. 7.20. Frequency (common to occasional versus rare to unknown) of sporophyte production within each of the six classes of life history strategies defined by During (1992) in the British moss flora (data from Longton 1997).
Fig. 7.21. The annual shuttle life strategy (During 1992). The vertical bar symbolizes the end of the period during which the habitat is suitable for the species. Reproduced from During (1979) with permission of Lindbergia.

time (years)

Fig. 7.22. The perennial life strategy (During 1992). The vertical bar symbolizes the end of the period during which the habitat is suitable for the species. Reproduced from During (1979) with permission of Lindbergia.

time (years)

Fig. 7.22. The perennial life strategy (During 1992). The vertical bar symbolizes the end of the period during which the habitat is suitable for the species. Reproduced from During (1979) with permission of Lindbergia.

Conversely, stress-tolerant species invest much in gametophytic development, which enables them to survive periods of stress, and less in reproduction. Stress-tolerant species typically occur in somewhat permanent habitats (e.g. grasslands, peat bogs and forest floor), where they may survive for centuries (Bates 1989, Rydin 2009). Stress-tolerant species can be further divided into dominants and perennials. Dominants, which are only represented by a few Sphagnum species among bryophytes, produce few, large spores. Perennials, by contrast to dominants, produce small spores that serve for the occasional establishment of new populations (Fig. 7.22). Perennials can be slow-growing in dense vegetation such as peat bogs (e.g. Polytrichum strictum) or more aggressive, more competitive and capable of rapid, opportunistic growth (e.g. large pleurocarps such as Brachythecium rutabulum).

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