Growth patterns of Juncus gerardi clonal populations in a coastal habitat

Juncus gerardi populations demonstrated a logistic growth curve during the colonization stage. Shoot production by vegetative multiplication was virtually continuous from December to Tune. Experiments suggested that the stabilisation stage of the demographic curve reflected water deficit. Taller, fertile, winter and early spring cohorts could be distinguished from shorter, infertile end of spring and beginning of summer cohorts. Shoot emergence began in March and terminated at the end of June, when water becomes a limiting factor due to a period of water shortage, typical of the thermo-atlantic climate. Spatial extension of populations was due to rhizome growth, which ceased during flowering. Flowering in May temporarily checked growth in shoot height of all emerged cohorts. No cost of reproduction was demonstrated concerning the rate of appearance of new shoots. Although fertile shoots were taller than vegetative shoots, their growth rates were significantly lower from April onwards. The tallest fertile shoots produced the most capsules. Energy allocation to seed production is the only possible means for long-term establishment of new genotypes, and vegetative multiplication appears as the principal source of recruitment of new modules in Juncus gerardi. Resource allocation patterns in this clonal species are discussed in relation to the ecological background in the concerned marshlands and with theoretical proposals derived from models of spatial colonization strategies in clonal plants.