Growth rate and effect of sheep browsing on young eucalypts in an anthropogenic Themeda grassland

Abstract

This paper reports the fate of both pre-existing ('lignotuberous') and newly-emerged eucalypt seedlings in an anthropogenic Themeda australis grassland on the Central Tablelands of NSW under varying grazing regimes. Compared to reported growth rates on farms elsewhere, the rate of height increase was low for both unbrowsed pre-existing and new seedlings: 16.7 (± 2.5) and 16.6 (± 0.3) cm/a respectively. Heights remained unchanged or declined during the cooler months. Most of the new seedlings were not above sheep grazing height 4.5 years after their presumed time of emergence.

Portions of the seedling population were exposed to a short period of high intensity sheep grazing in spring 1996 and/or autumn 1997. The effect of crash-grazing was more pronounced in spring, when over 90% of available seedlings were browsed, than in autumn when only about half of the available population was browsed. Mortality of new seedlings was higher following grazing in spring than in autumn. The finding that seedlings were not selectively browsed in autumn offers a means of conserving eucalypt regeneration while at the same time deriving some production from pastures. Reasons for different grazing effects in autumn and spring are unknown but could be related to differences between the pastures and/or between eucalypt seedlings on the two occasions.