Bryophytes live in microhabitats determined by the physical environment, usually modified by the vascular plant vegetation, and seemingly in ‘ecological isolation’ from other plants. However, bryophytes are involved in a variety of competitive, parasitic, symbiotic, mutualistic and as yet unspecifiable interactions with vascular plants, algae, fungi, lichens, cyanobactcria and autotrophic and heterotrophic bacteria. In only very few cases have these interactions been analysed functionally. Yet, such information may be essential for a better understanding of (1) such aspects of bryophyte ecology as mineral nutrition, carbon economy, herbivory, and growth and development of the gametophyte, and (2) the ecological role of bryophytes in terrestrial ecosystems.

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The effects of vegetation structure on population processes in five shortlived species occurring in two Dutch chalk grasslands are analyzed on the basis of some Iabaratory and field experiments. The main factors regulating population numbers appear tobe light quantity available for seedlings in dense microsites and water availability, which affects seedling survival, growth and fecundity of established plants in open micro-habitats.Two broad categories are recognized, "spenders" and "savers". These are based on seed weight, mortality of seedlings, presence of a persistent seed bank and variation in life span. We suggest that coexistence of the short-lived species is due to niche-sharing rather than niche-differentiation.

SUMMARYNanocyperion communities (s.l.) are considered here as "warp-and-woof" communities;the Nanocyperion components are described separately as synusiae. On