Overstorey growth response to thinning, burning and fertiliser in 10–13-year-old rehabilitated jarrah (Eucalyptus marginata) forest after bauxite mining in south-western Australia

Abstract

Summary Silvicultural treatment of dense stands of regrowth jarrah (Eucalyptus marginata) and marri (Corymbia calophylla) established following bauxite mining in the jarrah forests of Western Australia may be required for both timber production and catchment water yield objectives. In 10–13-y-old rehabilitation stands, four thinning treatments (1500–2500 (unthinned), 1111, 625 and 400 stems ha−1 retained) were applied in summer and either burnt by a prescribed fire in the following autumn or left unburnt. Fertiliser was also applied in a factorial combination with thinning and burning treatments. Tree growth was assessed 4 y after treatment. Mean individual tree diameter growth (under bark) was 0.48 cm y−1 in unthinned stands, rising exponentially to 1.44 cm y−1 when stands were thinned to 400 stems ha−1 (4.2 m2 ha−1 basal area under bark (baub)). Application of fertiliser in unburnt plots further increased growth rates of the two heaviest thinnings by 18% and 25% compared to equivalent unburnt and thinned treatments. Maximum stand basal area growth under bark (basal area under bark increment (baubi)) of 2 m2 ha−1 y−1 was achieved with thinning to 1111 stems ha−1 (7.6 m2 ha−1 baub). However, the heaviest thinning to 4.2 m2 ha−1 achieved 76% of maximum stand growth whilst increasing diameter growth of the best 100 retained trees per hectare from 1.4 cm y−1 to 2 cm y−1 under bark, potentially shortening crop rotation. The heaviest thinning also increased the time for leaf area index (LAI) to return post-treatment to pre-treatment levels to a predicted 7 y compared with 5 y when thinned to 1111 stems ha−1, which may extend the duration of any increased streamflow related to thinning. Burning in autumn reduced the effect of thinning such that mean diameter increment under bark was only 1.0 cm y−1 at the heaviest thinning, and there was no fertiliser effect. Autumn burning after thinning also stimulated dense understorey and tree coppice regeneration and is therefore not recommended.