Deposition, germination and spatio-temporal patterns of charophyte propagule banks: a review

Charophyte oospore banks have been studied in different habitat types, whereas only one detailed study of bulbil banks has been recorded from the literature. The density of oospore banks generally appears to be very high, dominating the propagule bank in many wetlands. Oospore banks are characterised by high spatial variability within and between the sites. Bulbils and oospores are not uniformly distributed with sediment depth, oospores being generally more numerous in the top few centimetres. Animal activities can mix the sediment, bury propagules and/or homogenise their distribution. Burial deeper than 2 cm limits oospore germination, but burial to a depth of 5 mm can improve their germination capacities. Oospore banks are highly variable between years. They are generally perennial, cumulating several generations of oospores, and buffering the risk of reproductive failure with time. The limited germination rates recorded for cospores from the bank could be related to this perennial strategy, whereas up to 100% of freshly harvested oospores may germinate. Germination rates and patterns and the conditions required for germination vary greatly from one study to another as a result of the large range of species and habitats investigated. Certain water birds consume charophytes and could be efficient dispersal agents for oospores. Various abiotic factors can affect the charophyte biomass and therefore the oospore production which then in turn could affect the propagule bank. The oospore bank density appears to influence charophyte abundance in the field, but above a threshold, its abundance is not related to the number of oospores in the soil. Below a threshold of density, cbarophytes may be unable to establish dense stands of vegetation. (C) 2002 Elsevier Science B.V. All rights reserved.