Microhabitat availability and seedling recruitment ofLobelia urens: a rare plant species at its geographical limit

Abstract

AbstractIn Britain,Lobelia urens(L.) (the heath lobelia) occurs in rough grassland, is rare and only found in southern England, where it is at the northern limit of its range. Emergence and survival ofL. urenswas investigated at six locations in two geographically distinct sites experiencing spring, autumn or no grazing in two consecutive years. Four factors were evaluated qualitatively, as a means of characterizing microhabitats for germination and survival: all permutations of higher plant cover, bryophytes, plant litter and surface depressions. The potential effect of adjacent plants on recruitment was also assessed using the nearest neighbour distance (NND). Grazing created depressions, removed plant litter and increased the proportion of sites with higher plant cover. It also resulted in a more open sward with higher NNDs. None of these changes stimulated recruitment. Instead, small increases in the frequency of some rare or very rare microhabitat types were vital in making grazed rough grassland more suitable for emergence. Reduced litter loads and a greater quantity of moss were two key responses. Overall, survival ofL. urenswas less than 1% and was particularly favoured by moss and an increase in NNDs. Shading from higher plants, with or without plant litter, decreased emergence, but the precise role of litter was complex and most probably related to its quantity. In an experimental seed bed, only empty depressions favoured emergence. The microhabitat relationships ofL. urenswere unusually consistent among locations and consecutive years.L. urensrequires high soil surface temperatures but also adequate water for large scale recruitment, and such conditions are encouraged by grazing. The particular problems of experimental design and statistical analysis of data from recruitment experiments are also discussed.