The population biology of wrinklegrass (Ischaemum rugosum Salisb.) - Seed and seedling dynamics

B.B. Baki and L.A.N. Nabi

Abstract

Population regulation, an important process determining the fate of individuals, prevails in an environment. Quite often the environmental sieves regulating the population fluxes of individuals in a population are either density- or time-mediated through the phases of seed production, seedling emergence and establishment. Studies were conducted to assess the density- and time-mediated dynamics of synthetic seed and seedling populations of wrinklegrass (Ischaemum rugosum Salisb.). Seed production capacity of the first, and second generation synthetic populations of wrinklegrass did not vary significantly with the initial parental population, although the mean seed rain per plant was inversely related to the corresponding increase in sowing or seed rain density. A single genet of wrinklegrass grown devoid of neighbours, produced up to 6 000 seeds within a growth cycle of 4 months or 18 000 seeds per year. The parallel figures for the respective sowing densities of 1 000–6 000 seeds/m² in the first generation synthetic population ranged from 48–62 seeds/plant. Invariably, the seed production capacity of wrinklegrass was reduced by 50% with twice the corresponding increase in seed rain density. The respective success rate of seeds from the seed rain emerging as seedlings were 85% and 90% in the first and second generation populations. Population regulation via density-dependent mortality or self-thinning, and time-mediated decrease prevailed among emerged seedlings in the first and second generation synthetic populations. The success rates of emerged seedlings becoming established plants in the first and second generation populations were 80.8% and 22.8%. The mean total seed weights of two successive generations of wrinklegrass were in line with the Kira’s Law of Constant Final Yields.

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