Herbage reponse to tree and shrub thinning in Eucalyptus populnea shrub woodlands

Abstract

Effects of tree and shrub densities on production of native grasses and herbs in woodlands dominated by Eucalyptus populnea were investigated in two experiments. Densities were controlled experimentally by selectively killing individual trees and shrubs with chemical compounds. In an experiment in which girdling or ring barking was compared with chemical treatment, the herbage response was more rapid and of greater magnitude on plots treated chemically. Throughout the experiment the herbage biomass was always highest on chemically treated and lowest on untreated plots. The highest dry weight of herbage from areas in which trees were chemically treated was 2600 kg/ha; comparable yields from ring barked and untreated areas were 1540 and 820 kg/ha respectively.

In a second experiment, living trees and shrubs were reduced to several density levels by chemical thinning. Herbage biomass fluctuated with season and with rainfall, but the relationship between herbage weight and woody plant density consistently had the same form and closely fitted a transition sigmoid curve. Trees had a greater effect than shrubs on herbage production: limits for maximum herbage biomass were 2–6 and 360 per hectare respectively.

Practical implications of the findings for the management of E. populnea woodlands are discussed.