DEMOGRAPHIC VARIABILITY IN TILLER POPULATIONS OF 2 PERENNIAL PAMPA GRASSES

The general objectives of this study were: (1) to investigate the impo rtance of internal influences in regulating the tiller dynamics in nat ural populations of the warm-season perennial grasses Paspalum dilatat um and Sporobolus indicus, coexisting in Argentine flooded pampa, in a s much as they act independently of the underlying external environmen t, and (2) to evaluate the extent to which interactions between intern al and external factors affect the variation in tiller dynamics within such populations. Within-population variation in seasonal development of plants and tillers with different neighbour composition was studie d for an annual growth cycle. Tiller survival and tillering were signi ficantly influenced by tiller size. Tiller age influenced tiller fate, as suggested by the additive effects of age and size of tillers. Thes e relationships varied with season and with species. Size and age of t illers showed additive effects with their neighbouring species on the tiller fate of P, dilatatum, but the effects of age and size of S. ind icus changed according their neighbourhood. Tiller survival of S. indi cus during the early growth season was more size-dependent when the co ld-season species Poa lanigera, was the principal neighbour. Flowering and tillering probabilities were positively related through their com mon positive responses to tiller size. Tiller survival and recruitment between different seasons were strongly related. Independently of nei ghbour composition, tiller survival was generally inversely related to tiller recruitment in previous seasons. Therefore, significant densit y-dependent mortality of tillers was found for both species during the early summer when tiller density was expressed by basal area units.