Conclusions

Marginal areas present a challenge to successful reproduction. It is particularly noticeable that at their limits of geographical distribution flowering plants can be particularly diverse in the strategies that are employed to aid reproduction and perennation, and a wide range of different strategies have evolved for overcoming the limitations of short growing seasons, disturbance through natural causes such as flooding and erosion, and other aspects of environmental uncertainty. The overall result of these evolutionary responses has been to maintain biodiversity despite the frequent physical constraints on reproduction. Hybridization, pheno-logical and ecotypic specialization, sexual and asexual reproduction all combine in flowering plants to overcome the environmental and biotic limitations to reproduction. Depending on the nature of the environmental stress, flowering and fruiting can be accelerated sufficiently to accomplish seed production. The dispersal of propagules is vital in marginal areas and many diverse solutions are found from plants at river margins that employ fish, to high mountain forests where seed-hoarding birds serve to distribute seeds both up and below the treeline. Vegetative layering is also common in the ultra-short environmental window that is available for reproduction in semi-deserts as well as in the short growing seasons of the High Arctic. In polar regions, reliance on slow maturing perennials as opposed to quick growing annuals allows the development of floral structures to take place over several seasons. Asexual reproduction is particularly noticeable both in woody and non-woody species at high latitudes, and like sexual reproduction can take many forms ranging from apo-mictic seed production to pseudo-vivipary, bulbil formation and vegetative spread through stolons and tillers. Despite the reliance by many species in marginal areas on vegetative reproduction, genetic diversity is generally well maintained (see Chapter 2) and clonal longevity coupled with sporadic sexual production appears to be sufficient in many cases for the avoidance of loss of species variation.

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