Their remarkable tolerance to climatically extreme conditions, their capacity to colonize both the exterior and the interior of rocks, and their relative success as epiphytes allow many poikilohydrous nonvascular autotrophs to survive and grow in the absence of strong competition by homoiohydrous plants. This is evident with the numerous lichen and bryophyte communities formed in polar regions, high mountains, and on rock outcrops, everywhere where vascular plants fail to grow. In real desert lands (Shields et al. 1957, Scott 1982), and also the Mediterranean and steppe regions, deserted land in southern Australia (Rogers and Lange 1971) and on dunes, bryophytes, lichens, and soil algae cover the soil as crusts with characteristic community structure such as for instance of many others like the famous Fulgensietum fulgentis in Europe or the Crossidio crassinervis-Tortuletum obtusatae in the Irano-Turanian territory (Galun and Garty 2001). These types of vegetation, especially the crusts, are ecologically crucial because they reduce erosion and contribute to the preservation of hydric, chemical, and physical properties of the soil (Danin and Garty 1983, Belnap and Lange 2001).
A particular phenomenon in semiarid regions is the occurrence of erratic (vagrant) lichens (Kappen 1988, Rosentreter 1993, Perez 1997) and bryophytes (Scott 1982). With their curled lobes or shoots, they are blown by the wind to shallow depressions and await a flush of water or just a bit of rain to unfold their thalli and to become productive. Without roots and not dependent on any soil formation, lichens and bryophytes typically form pioneer communities on young volcanic material or areas around receding glaciers as long as these sites are prepared for the establishment of homoiohydrous plants (Kappen 1988). In many habitats, mosaics are formed between vascular plant and cryptogam communities like in the boreal region, or lichens and bryophytes together with poikilohydrous vascular plants (see Figure 2.1) form communities in temporarily moist habitats with shallow soil cover (Gaff 1977, Volk 1984, Muller 1985, Lazarides 1992, Belnap and Lange 2001).
Communities with dominant poikilohydrous angiosperms were recognized since Lebrun (1947) who described a Craterostigmetum nanolanceolati, comprising Craterostigma lanceo-latum, C. plantagineum, C. hirsutum, Lindernia philcoxii together with Riccia and Cyperaceae on lateritic crusts in Zaire. Typical are communities between resurrection plants and bryophytes and elsewhere lichens that have exploited temporal ponds or run-offs on Inselbergs in subtropical and tropical regions, such as the Xerophytetum humilis in Namibia (Volk 1984) where Xerophyta (= Barbacenia humilis) is associated with Riccia, Bryum, and cyanobacteria and locally with C. plantagineum, some succulents, and grasses. Similar communities with dominating poikilohydrous Scrophulariaceae were described for the African savannah region (Fischer 1992). Volk (1984) observed that communities of the Nanocyperion teneriffae are typical of pioneering vegetation that combines the strategies of therophytes and resurrection plants. In these places, the resurrection plants can outcompete therophytes, as they are able to defend their place against invasion of seeds and seedlings. Forming stable herds, turfs, or mats, their dominant position on inselbergs was demonstrated by Porembski and Barthlott (1997).
A transition from dominance to exclusion of poikilohydrous vascular plants was found in southern Africa as a function of soil depth (Gaff 1977). Initial stages are characteristic with poikilohydrous water plants (Chamaegigas intrepidus in Namibia or Craterostigma monroi in Zimbabwe) and lead to grass-herb communities over several steps to, finally, a pluvio-therophytic grass vegetation or associations between Xerophyta species and perennial succulent life forms. Although they grow slowly, they are also very successful in cracks and rock ledges with changing water supply once they are established. Species-rich communities of nonvascular autotrophs are frequently formed on tree trunks, branches, and even leaves in wet forests, particularly on mountain slopes.
Algae, lichens, and bryophytes grow potentially everywhere because of their small and easily transportable vegetative and generative propagules (Kappen 1995). The evolution of a great variety of asexual means of reproduction has made nonvascular plants extremely successful in the colonization of remote and difficult sites. We have only limited knowledge about the reproductive strategies and success of poikilohydrous vascular plants. Myrotham-nus flabellifolius forms perianthless inconspicuous monoecic flowers, which are most likely anemogamous (Puff 1978, Child 1960). According to Puff (1978), pollen tetrades may increase the fertility on a receptaculum (success of mating) in this species. The seeds are extremely small and can be dispersed over long distances by the wind, but we have no records about periods and conditions of flowering. The flowers of poikilohydrous Scrophulariaceae do not differ much from those of the homoiohydrous members of the family (Heil 1925, Hickel 1967, Smook 1969, Gaff 1977). However, the rapidity of producing flowers is remarkable in species such as Chamaegigas intrepidus, as flower buds appeared simultaneously with the floating levels. As expected, the reproductive phase occurred predominantly during the wet season, and a general requirement was a period of photosynthetic activity before reproduction, as was shown for Vellozia schnitzleinia (Nigeria: Owoseye at Sandford 1972) and poikilohydrous grass species in Australia (Lazarides 1972). Nevertheless, a few grass species can reproduce during the dry period. Chamaegigas intrepidus has a high genetic variability within and between populations on one site. Although gene flow over long distances is low, there is no evidence of genetic isolation of populations by distance (Heilmeier et al. 2005). Describing the reproductive biology and ecology of the Velloziaceae (Kubitzki 1998, Ibisch et al. 2001) helps to interpret the ecological strategy of poikilohydrous plants and to compare their relative success with that of homoiohydrous co-occurring species.
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