Comparative population ecology of Senna occidentalis (L.) Link, a monsoon desert annual, in two different habitats

A demographic study of Senna occidentalis (L.) Link, a circumtropical weed, was conducted at two habitats: an old-field (a moister site) and a waste ground (a drier site). The survivorship curves of both the populations closely corresponded to Deevy type I (greater age-specific mortality at higher age). Moisture stress appeared to be the major cause of mortality. Seed rain occurred essentially during October with the highest seed-fall in the second and third weeks. Seed predation by bruchids was greater for the waste ground population compared to that of the old-field. The extent of seed predation differed significantly among individuals between the populations. Seed densities in soil at both the sites declined remarkably in late August due to rainfall and consequent germination while the seed bank was elevated in November due to production and dispersal. The proportional allocation of biomass to roots was small for both the populations. Reproductive allocation of biomass and phosphorus varied considerably between populations. Reproductive allocation of biomass and particularly phosphorus was much greater for the old-field compared to the waste ground population.