Thirteen-year growth of jarrah (Eucalyptus marginata) on rehabilitated bauxite mines in south-western Australia

Abstract

Summary Alcoa's objective is to restore its bauxite mines in south-western Australia to a productive self-sustaining jarrah forest. This paper describes the growth and form of 13-y-old jarrah (Eucalyptus marginata) trees established from seed and planted seedlings, and subject to various fertiliser, understorey seed and spacing treatments. Seeded jarrah had a mean establishment density of 2662 stems ha−1 and a density of 2300 stems ha−1 after 13 y, a survival rate of 86%. Sixty-two per cent of the seeded trees developed into saplings (> 1.5 m tall). The saplings had a mean height of 9.0 m and dbhob of 14.4 cm after 13 y. The mean density of potential sawlogs (trees with a single stem to at least 2 m) was 1297 stems ha−1 after 13 y. The presence of tall native legumes (acacias) decreased the percentage of trees that became saplings from 72% to 53%. When N fertiliser was not provided the legumes had a suppressive effect on the basal area and bole volume of potential sawlogs, reducing them to 60% and 57%, respectively, of the values without legumes. Planted jarrah trees had a mean survival rate over all treatments of 84% after 13 y. Closer tree spacing (particularly 1 m × 1 m) resulted in significantly lower survival (77%) than the wider spacings. Inclusion of tall native legumes with closer tree spacing reduced survival even more (69% at 1 m × 1 m). The mean sapling height across all planted treatments was 9.1 m. The mean dbhob was 15.2 cm with 67% becoming saplings. Closer tree spacing had the strongest effect in significantly reducing all of these parameters. Tall native legumes reduced the dbhob of the jarrah trees and reduced the overall mean height of all jarrah by limiting the proportion of jarrah that became saplings. Absence of added N fertiliser also significantly reduced these measures. Basal areas were similar in rehabilitated and unmined forest after 13 y. Mean annual growth increments (1.52 m2 ha−1 seeded, 2.72 m2 ha−1 planted) were similar to those reported for similar aged regrowth forest (2.62 m2 ha−1) and higher than older thinned (0.45 m2 ha−1) or unthinned (0.19 m2 ha−1) unmined forest. Basal area increments were higher from 8–13 y than 0–8 y, indicating that growth rates were not slowing down over this period. The optimum stocking rate for jarrah in rehabilitated bauxite mines will be a balance between growth rate, tree form, other environmental factors and cost considerations. The current rehabilitation prescription aims to establish 1300 stems ha−1 of ‘total eucalypts’ by broadcast seeding (jarrah and marri — Corymbia calophylla). This study indicates that an initial stocking of 1300 stems ha−1 is optimum and meets Alcoa's mine rehabilitation completion criterion of providing at least 300 potential sawlogs ha−1 at 15 y of age.