Rhizome elongation and seagrass clonal growth

A compilation of published and original data on rhizome morphometry, horizo ntal and vertical elongation rates and branching patterns for 27 seagrass s pecies developing in 192 seagrass stands allowed an examination of the vari ability of seagrass rhizome and clonal growth programmes across and within species. Seagrass horizontal rhizomes extend at rates ranging between 1.2 a nd 574 cm yr(-1), develop a branch, with an angle from 19 to 72 degrees, fo r every 6 to 1800 horizontal internodes, and add a new shoot for every 1.1 to 7.5 cm of rhizome produced. Vertical rhizomes elongate at rates between 0.1 and 34 cm yr(-1) and the probability that they will branch varies over 3 orders of magnitude. Much (between 40 and 173%) of the variability of sea grass horizontal rhizome and clonal growth programmes is species-specific, largely (21 to 63% of the variance) associated with differences in size amo ng species, although seagrasses also show important intraspecific variabili ty. The broad repertoire of seagrass rhizome and clonal growth programmes e xplains the different rates and efficiency at which the species occupy spac e. The implications of specific growth programmes for space occupation were examined by simulating the development of seagrass rhizome networks of 3 s eagrass species encompassing the range of horizontal rhizome growth (Haloph ila ovalis, Thalassodendron ciliatum, Posidonia oceanica). This exercice sh owed that small, fast-growing species achieve a much lower spread efficienc y (m(2) of ground covered m(-1) of rhizome produced) than the large, slow-g rowing species. Differences in rhizome branching angles greatly constrained the form of rhizome networks. The results show that clonal growth patterns are a primary component of seagrass productivity and, therefore, the key t o the development and maintenance of seagrass meadows.