Effects of grazing and depth on two wetland plant species

Abstract

Wetland plants in Llangothlin Lagoon, northern New South Wales, are subject to grazing and trampling by cattle, sheep and waterbirds and to fluctuating water levels. Myriophyllum variifolium J. Hooker, an aquatic dicotyledon with dispersed meristems, exhibited different morphological changes to the emergent monocotyledon Eleocharis acuta R. Br, under simulated and natural grazing at different water depths. Responses were principally determined by position and number of meristems.

Growth point production (numbers of shoots and branches) increased under light, frequent clipping (25% every 14 or 7 days) in non-submerged plants only. Node production, total plant or shoot length, and above- and below-ground biomass decreased under similar clipping treatments. E. acuta did not increase shoot production or above-ground biomass under any clipping treatment, and only for the lightest clipping treatment (clipped once to 7 cm when non-submerged) was no decrease in total shoot length observed. More intense and frequent clipping treatments and submersion to 15 cm prevented both species from replacing lost tissues. Interaction between clipping and submersion occurred in both species, indicating that growth responses are complex.

The distribution and abundance of the two species reflect the greater tolerance of M. variifolium than E. acuta to grazing and inundation. Low intensities of cattle and sheep grazing may be beneficial by increasing species diversity.