TILLER DYNAMICS OF SPARTINA-MARITIMA IN SUCCESSIONAL AND NONSUCCESSIONAL MEDITERRANEAN SALT-MARSH

Tiller demography was compared in two populations of Spartina maritima present at similar elevations in the coastal saltmarshes of Odiel (Hu elva, S.W. Spain). The successional population consisted of colonizing tussocks in a littoral lagoon, and the non-successional population co mprised a stable award that had fringed a major channel for 40 years. At both sites S, maritima was replaced by Arthrocnemum perenne at high er elevation, where sediments were less reducing. Rapid, consistent se diment accretion confirmed the successional nature of the lagoon site but there was little net accretion in the stable sward. Census of perm anent quadrats at the successional site chronicled moving concentric ' waves' of high tiller density as tussocks expanded. Initially high den sities declined after one year to low values at the end of the second year but they had almost recovered after 3 years. The decline represen ted a combination of reduced numbers of births and increased numbers o f deaths. Tiller densities were substantially higher in the stable swa rd and showed relatively small fluctuations with time. The underlying risk of tiller mortality was similar in the two populations for much o f the time but after two years there was increased mortality, mainly a ssociated with flowering, at the successional site; very few tillers f lowered in the sward. This mortality contributed to a shift to a young er age structure in the successional population. Data aggregated over consecutive 3-monthly periods were examined for density dependence. No ne was found in the successional population. In the sward population t here was evidence of density-dependent adult and juvenile mortality of tillers, particularly over the first Is months of the study, when the re were compensatory responses to subtle variations in density. The la ck of density dependence and relatively low peak density of about 2000 m(-2) near to the leading edges of the expanding tussocks at the succ essional site suggest that tiller placement there was regulated mainly by physiological mechanisms affecting rhizome growth and bud developm ent in well integrated clones.