The growth strategies and distribution of isoetids in Scottish freshwater lochs

The isoetids are one of the commonest ecological groupings of aquatic macrophytes in Scotland. Their growth strategies involve a range of features associated with Grime's classification of plant strategies. Subularia aquatica L. is a characteristic ruderal, although all of the isoetids occur in habitats that are disturbed. Littorella uniflora (L.) Aschers. and Isoetes lacustris L. are relatively stress tolerant with respect to low light. Littorella uniflora is also better able to withstand competition than the other isoetids. All are stress tolerant when considering growth form and the nutrient limitation of the habitats in which they commonly occur. Littorella uniflora has a much wider range of distribution than the other isoetids with regard to alkalinity or nutrient status of any loch. An experimental growth study confirmed that Littorella uniflora is faster growing and has a greater response to increasing nutrients than Lobelia dortmanna L., and this is used to explain its greater tolerance of mesotrophic environments. Decline of the isoetids in Scotland is found to be due to drainage, reservoir construction, acidification and eutrophication. The latter factor is thought to be the most important, although evidence on these factors is limited. This is discussed in relation to the growth strategies of these plants.