Chapter 1: The Characeae Plant

21
do they occur?’ are central to such studies and have been vital in the formulation of
hypotheses concerning characeae distribution and abundance. In other studies ecological
experiments have been undertaken, in order to understand the mechanisms (i.e. ‘how?’) and
reasons (i.e. ‘why?’) for the diversity and distribution of characeae that we see (Blindow et al.
2002).
1.5.2 Processes controlling characeae distribution
Many characeae species develop a long-lived bank of oospores in the soil of permanent and
temporary wetlands and riparian zones (Casanova and Brock 1990). The density of oospores
in this ‘seed’ bank is dependent on 1) the longevity of oospores; 2) the rate of oospore
addition to the bank; 3) the loss from the seed bank via germination of new plants, death or
dispersal (via invertebrate ingestion, or dispersal of the substrate (mud) attached to animals)
(Fig. 1.16). Oospore density can vary from just a few oospores/m 2 (Casanova and Brock
1990) to hundreds of thousands of oospores/m 2 or per litre of surface soil (van den Berg
1999; Porter 2007). The presence and density of oospores in the seed bank depends on
oospore production by adult plants, and in most systems there is likely to be a mass
deposition of oospores of a single age in the late summer and autumn, or as temporary water
bodies dry up, as this is the time when the oospores of most species mature (Casanova and
Brock 1990; Casanova 1994; Casanova and Brock 1999a). In permanent water bodies there
can be immediate germination of fresh oospores, although many oospores are dormant upon
release (Casanova and Brock 1997). Oospores can remain viable in seed banks for years
(Casanova and Brock 1997) and even decades (Simons et al. 1994). There is one record of
oospores germinating from sediment estimated to be at least 45 years old (Rodrigo et al.
2010). Oospore viability and subsequent germination generally decreases over time
(Casanova and Brock 1994), as even very low respiration rates can exhaust the oospore starch
reserves over time, and there is likely to be loss to bacterial infection of oospores as well as
direct herbivory. Germination followed by failure to establish is also a mechanism by which
oospore banks can be depleted (de Winton et al. 2004). As a result of deposition of oospores
over many seasons and the longevity of oospores, oospore banks can contain oospores of
various ages, accumulated over time. Germination from the seed bank, and the differential
germination requirements of characeae species can partly explain characeae distribution in
the landscape (Casanova and Brock 1996).