Clonal reproduction is particularly suitable for plants in marginal areas and there are many examples of relict species where their continued survival appears to be due largely to being able to reproduce through clonal spread and fragmentation. It is therefore not surprising that marginal areas, whether they be on mountains, or at high latitudes, or in more widespread temperate habitats such as riverbanks and lakesides, are frequently rich in clonal species. Clones enable plants to resist environmental change and survive prolonged periods during which sexual reproduction does not take place. Asexual reproduction, as it is less hazardous and costs less in terms of resources, can aid survival in resource-limited situations. Clonal growth forms also permit the redistribution of resources from senescent to actively growing parts of the plant.
Arctic and alpine habitats are among the most hostile environments for sexual reproduction for flowering plants. Growing seasons that are short and unpredictable can interfere with flowering and also hinder insect pollination. The long arctic winter and the constant risk of soil disturbance from erosion and cryoperturbation also makes seedling establishment hazardous. Thus, despite the fact that the Arctic in summer can be a botanist's delight in the variety and profusion of plants in flower, the amount of viable seed and established seedlings is often low. The fact that a large number of arctic species possess a capacity for vegetative reproduction has led some ecologists to assume that asexual reproduction is more important than sexual reproduction in arctic vegetation (Billings & Mooney, 1968; Grime, 2001). However, the use of molecular methods in population studies now reveals ample evidence of gene flow in the Arctic and therefore sexual reproduction can no longer be dismissed as of only minor importance. Marginal areas, from the deserts to arctic and alpine regions, have many examples of long-lived plants and therefore the contribution of sexual reproduction to the ecological success of such species cannot be assessed over short time spans.
As with sexual reproduction, asexual plants exhibit specialized mechanisms with regard to the manner and timing of asexual reproduction which usually reflect adaptations to particular types of low temperature habitats. The degree of reliance on asexual as compared with sexual reproduction also varies with the nature of the habitat and usually neither form of reproduction is completely excluded. The longevity of many clones makes it difficult to deduce from direct observation as to whether or not sexual reproduction can take place at spaced out and irregular intervals. A study of the alpine sedge Carex curvula in which clonal structure was analyzed from randomly amplified DNA (RAPDs) showed that one particularly large clone with more than 7000 tillers was estimated from annual growth rates to be around 2000 years old (Steinger et al., 1996). Recruitment in this species from sexually produced seeds is extremely rare and the age of this plant demonstrates an ability for survival on one particular spot through a period of diverse warm and cold climates from mild periods in the early Middle Ages throughout a series of climatic changes including the Little Ice Age.
There are nevertheless many examples of arctic species that are very dependent on asexual reproduction. One of the most remarkable is creeping salt marsh grass (Puccinellia phryganodes) which dominates salt marshes over wide areas and has a circumpolar distribution, yet throughout its entire range the species is almost always completely sterile (Lid & Lid, 1994). The alpine bistort (Polygonum vivípara) has often been considered as a species in which asexual reproduction by bulbils is so predominant that ecologically sexual reproduction by seed can be ignored. However, recent research on high arctic, subarctic, and alpine populations of Polygonum vivipara has detected intermediate to high levels of genetic diversity within these various populations as well as showing physiological differentiation between locations. Plants from Scandinavia require considerably longer photoperiods for floral induction than plants from lower latitudes (Alps). Although Polygonum vivipara appears to reproduce almost exclusively asexually by bulbils, seed development can occasionally be observed even in arctic and alpine populations and may be sufficient to account for the differentiation of ecotypes and medium to high levels of genetic diversity found with the arctic and alpine populations (Bauert, 1996).
A similar situation is found in two circumpolar arctic species, Saxifraga cernua (Fig. 4.37), and S.foliosa, where it is sometimes assumed that both species have abandoned sexual reproduction. However, there are records of cases where seed is set irregularly although at low rates in both species. Field studies in northern Sweden have found that both species have an andro-dioecious mating system (a species which has both male and hermaphrodite individuals), a phenomenon which is extremely rare in natural plant populations (Molau, 1992). The two gender classes within each species are maintained by self-incompatibility in hermaphrodites, and a higher rate of vegetative propagation in female-sterile plants. Hermaphrodites are rare, especially in S. cernua, and most populations consist exclusively of female-sterile plants. The sparsity of fruiting material in herbaria may be a result of low frequency of collecting, due to the inconspicuous nature of the plants at this stage.
The tendency for asexual plants to have a more northern range than closely related sexual plants has been shown to be true for 76% of examined species (Bierzychudek, 1985) and has been described as
geographic parthenogenesis. A computer model designed to examine the causes of asexual plants being favoured in the north and at higher altitudes and in marginal resource-poor environments has suggested that as individuals move from areas from where they are well adapted to where they are poorly adapted then sexual reproduction may reduce fitness (Peck et al., 1998). The mal-adaptation of sexually reproducing migrants due to lower population densities when they move to marginal areas can lead to a loss of fitness. Asexual populations will be protected from this tendency and this may account for their success in marginal habitats in the northern hemisphere and elsewhere and may be the ultimate cause for geographic parthenogenesis.
A very successful circumpolar species is Poa alpina, commonly known as alpine meadow grass or alpine blue grass (North America). The species occurs in two forms: P. alpina var. alpina which reproduces sexually, and P. alpina var. vivipara which produces pseudo-viviparous vegetative spikelets (Fig. 4.38) that are then shed at the end of the growing period and root themselves in the ground. The spikelets are very hardy and can survive over winter frozen into snow and ice before rooting in the soil in the spring. Their ability to endure prolonged ice encasement, which deprives their tissues of access to oxygen, has already been described in Chapter 3 (Fig. 3.26). This tolerance of oxygen deprivation helps to explain their tolerance of prolonged freezing. The species is overall very adaptable in its choice of habitat, having low nutrient requirements as well as being tolerant of drought.
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