Traits related to disturbance frequency and intensity
The opportunistic strategy should be selected in the more disturbed situations [i.e. proximal extremity of the (c) axis, Fig. 3]. In this part of the scheme, the occurrence of any plant species should be determined mostly by an ability to tolerate disturbances and to regenerate in gaps. Traits allowing survival during disturbances are usually grouped under the ‘ruderal’ or ‘r’ strategy. Several authors have defined the key-traits of this strategy:
’ strategists of Southwood (Southwood, 1988
) have a large number of small seeds with wide dispersal ability, and early maturation (i.e. small size and lack of vegetative spread).
Ruderals of the Grime model are of small size with a limited lateral spread, a short life cycle and a high frequency of flowering. These species have numerous small seeds or spores dispersed by wind, and should be able to persist in the seed bank in a dormant form for long periods (Grime, 2002
Ruderals of the Kautsky model are of small size, with a limited lateral spread, and have a short life-span with a large proportion of the annual biomass production spent on sexual reproduction, with no vegetative propagules, but numerous dormant seeds or zygotes (Kautsky, 1988
Traits related to the ‘explerent’ explorative strategy (Rabotnov, 1975
; in Onipchenko, Semenova & Van Der Maarel, 1998
) include a high production of small seeds, a large seed bank and a high relative growth rate, which leads to rapid growth if nutrients are available.
Connell & Slatyer (1977) suggested that large highly disturbed patches should be recolonized mainly by external colonizers, whereas the less disturbed patches should be partly re-colonized by seeds and propagules from the soil reservoir. Field observations partially confirm this prediction, although similarities between seed bank and established vegetation remain relatively high in areas frequently exposed to disturbances (Tabacchi et al., 2005).
When sexual reproduction is effective, seeds are expected to have a high dispersability. In the Salicaceae, for example, dispersability is efficient through small wind-dispersed seeds that must reach suitable habitats rapidly after release in order to compensate for low viability of the diaspores (Guilloy et al., 2002). Mahoney & Rood (1998) developed a model showing that the efficiency of dispersal of such species depends on a narrow timeframe. As a consequence, their recruitment is highly variable from year to year. Water dispersal should be highly favoured, as it increases the opportunity for propagules to reach gaps immediately after the disturbance, but such dispersal requires high buoyancy of propagules (Andersson, Nilsson & Johansson, 2000; Boedeltje et al., 2004; Riis & Sand-Jensen, 2006). Seeds of the helophytes Alisma plantago-aquatica L., Carex flava L. and Cladium mariscus (L.) Pohl., like seeds of the hydrophyte Hippuris vulgaris L., are able to float for >1.5 years (Praeger, 1913). Seeds of the disturbance-tolerant species Berula erecta (Hudson) Coville and Myriophyllum spicatum L. are able to float for about 7 days (Guppy, 1906; Praeger, 1913), while fragments of several species that colonize disturbed habitats are able to float for several weeks (Barrat-Segretain, Bornette & Hering-Vilas-Bôas, 1998; Boedeltje et al., 2003). Conversely, seeds and fragments of plants species that colonize undisturbed habitats tend to have low buoyancy: seeds of Baldellia ranunculoides (L.) Parl., Oenanthe fistulosa L., Oenanthe aquatica (L.) Poir. and Nuphar lutea (L.) Sm. (I. Combroux & G. Bornette, pers. obs.), or fragments of Potamogeton coloratus Hornem. sink immediately or very soon after release (Barrat-Segretain, Henry & Bornette, 1999).
Seeds of plants that colonize disturbed habitats tend to have no dormancy, or dormancy breakage depending on a signal from the disturbance itself (Thompson & Grime, 1979; Jutila, 2001; Karrenberg, Edwards & Kollman, 2002), which enables them to be immediately available when gaps are created. For example, Charophytes are pioneer species, which usually bloom after disturbance (Bornette & Arens, 2002), suggesting that the disturbance itself induces oospore germination. During floods, the abrasive effects of sediment movement can break cuticular dormancy. Seeds of several species that colonize disturbed habitats [Luronium natans (L.) Raf., Potamogeton pectinatus L., and Potamogeton pusillus L., Bornette et al., 1998) show increasing germination if they are scarified (S. Greulich & G. Bornette, pers. obs. for L. natans, and Teltscherova & Hejny, 1973).
Vegetative regeneration is a key function for the maintenance of species subjected to recurrent disturbance, particularly in infertile situations that can prevail in the most disturbed floodplain habitats (Bellingham & Sparrow, 2000; Klimešová & Klimeš, 2007). Several authors have demonstrated the prevalent role of clonal growth in species maintenance after disturbances through survival of deeply anchored roots or rhizomes, spreading from refuges or sprouting from vegetative propagules (Prach & Pyšek, 1994; Henry et al., 1996; Barsoum, 2002). As an example, along U.S.A. rivers, two shrubs common on the channel shelf (a bank feature), Alnus serrulata (Ait.) Willd. and Cornus amomum Mill., are relatively resistant to destruction by flooding because of small, highly resilient stems and the ability to sprout rapidly from damaged stumps (Hupp & Osterkamp, 1985). This high capability of regrowth is also facilitated by the production of adventitious roots that utilize nutrients in alluvial material deposited by floods, allowing for rapid rooting of flood-detached branches (Hupp & Osterkamp, 1996). Plants that produce rhizomatous systems resist flood disturbance by vegetative production of new shoots from resistant rhizomes (Bartley & Spence, 1987; Willby, Abernethy & Demars, 2000; Kotschy & Rogers, 2008). Plants having a high growth rate should also be selected when disturbance frequency increases. High growth rate would be important not only for seedlings, but also for plants that regenerate from plant fragments, or that colonize empty patches by growing in from the edge (Barrat-Segretain & Amoros, 1996; Henry & Amoros, 1996).
Disturbance affects the size of eroded versus deposited patches, as patches tend to be larger when disturbances increase in intensity. Traits linked to vegetative and to sexual reproduction are involved differently in the recolonization process, depending on patch size (Miller, 1982; Belsky, 1986). The growth rate of plants at the patch edge, as well as patch size, determine the contribution of vegetative propagation to recolonization (Connell & Keough, 1985). In large patches, seed colonization tends to dominate, whereas the edge effect tends to be low (Vandvik, 2004). Further, Miller (1982) also suggested that large patches should be colonized mostly by species having high rates of reproduction and high dispersal ability, whereas small patches should be colonized mostly by more competitive species with a high growth rate located around the patch perimeter (edge effect). Consequently, even if the regenerative strategies involved in the colonization process vary according to patch size, large patches (i.e. patches that are usually generated by a high frequency and/or intensity of disturbances) should be colonized mainly by seeds with high dispersal ability.