Effect of Drought During Different Stages in the Life-Cycle on the Growth and Biomass Allocation of Two Aira Species

(1) The effects of drought treatments applied in different stages of the life-cycle on the growth and allocation of biomass in the vegetative and reproductive phases of Aira caryophyllea and A. praecox have been studied. (2) Drought treatment in the vegetative phase had no effect on growth in the reproductive phase in either species. In the reproductive phase A. praecox grew exponentially, while A. caryophyllea did not grow. The biomass allocation showed greater differences between the species than between the treatments, especially in the reproductive phase. The proportion of biomass allocated to reproductive organs was affected by the drought treatments. The production of caryopses was reduced by drought if it occurred in the vegetative as well as in the reproductive phase. (3) In contrast to A. caryophyllea, A. praecox showed both somatic and reproductive growth after inflorescence emergence. (4) A. caryophyllea seems better adapted to reproduction in a habitat with a restricted growing season, while the continued vegetative growth in the reproductive phase of A. praecox increases its survival during drought. Its continued vegetative growth may lead to a higher reproductive yield. (5) The two species have different mechanisms for maintaining their existence in the same habitat. While A. caryophyllea is mainly adapted to a growing season of which the duration is restricted by drought, A. praecox is better adapted to drought itself. For that reason A. praecox can be expected to occur in drier sites in a dune habitat than A. caryophyllea.