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

In Britain, Lobelia urens (L.) (the heath lobelia) occurs in rough grasslan d, is rare and only found in southern England, where it is at the northern limit of its range. Emergence and survival of L. urens was investigated at six locations in two geographically distinct sites experiencing spring, aut umn or no grazing in two consecutive years. Four factors were evaluated qua litatively, 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 recruit ment was also assessed using the nearest neighbour distance (NND). Grazing created depressions, removed plant litter and increased the proportion of s ites with higher plant cover. It also resulted in a more open sward with hi gher NNDs. None of these changes stimulated recruitment. Instead, small inc reases in the frequency of some rare or very rare microhabitat types were v ital in making grazed rough grassland more suitable for emergence. Reduced litter loads and a greater quantity of moss were two key responses. Overall , survival of L. urens was less than 1% and was particularly favoured by mo ss and an increase in NNDs. Shading from higher plants, with or without pla nt litter, decreased emergence, but the precise role of litter was complex and most probably related to its quantity. In an experimental seed bed, onl y empty depressions favoured emergence. The microhabitat relationships of L . urens were unusually consistent among locations and consecutive years. L. urens requires 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 dat a from recruitment experiments are also discussed.