Germination Effects on Pasture Composition in a Dry Monsoonal Climate

Abstract

Germination and establishment, in annual self-seeded swards of the legume Stylosanthes humilis and the associated grass weed Digitaria ciliaris, were studied in 4 seasons in a dry monsoonal climate at Katherine, Northern Territory, Australia. Digitaria germinated in a series of false starts followed by a single major wave of germination. Stylosanthes germinated in a single major wave. Changes in the species ratio during germination-establishment are defined in terms of a series of filters. At least 3 filters appear to operate. The 1st filter (ad,s) describes the change from hard to soft seed and the change in the soft seed population caused by false start germination waves; the 2nd filter (as,g) describes the germination of seeds in the main germination event; the third filter(ag,e) quantifies establishment of seedlings. Germination in this dry monsoonal climate usually occurs in 1 major wave, and therefore the change in species ratio from seed production to establishment can be described by a serial and consecutive arrangement of the 3 component filters. The 1st and 3rd component filters favored Stylosanthes. As a result, the combined filter (ad,e) also favored Stylosanthes. Values of the combined filter (ad,e) varied from year to year, but were consistently higher on run-off than on run-on areas.