THE EFFECT OF TREE DENSITY AND VIGOROUS LEGUMES ON UNDERSTOREY SPECIES RICHNESS, DENSITY AND COVER IN REHABILITATED BAUXITE MINES

The target tree establishment density for rehabilitated mine pits in the jarrah forest is currently 1300 stems ha-1 and is designed to ensure that 300 stems ha-1 of sawlogs are available for future timber production. However, tree density may affect other ecosystem parameters such as water use and understorey species richness. Rehabilitated sites were historically seeded with an understorey mix that included vigorous legumes to establish rapid vegetation cover and fix atmospheric nitrogen. Similar to high tree density, these vigorous legumes may have the undesirable effect of out-competing the small understorey species, which are the main contributors to the species richness of rehabilitated mined sites. The major objectives of this study were to determine 1) if high tree densities reduce the richness, density and cover of understorey species and 2) if the competitive effect of a vigorous native legume seed mix on smaller understorey species is evident 15 years post-rehabilitation. A complete factorial experiment of four tree densities (625, 1250, 2500 or 10 000 stems ha-1) and two seed treatments (a high rate of vigorous native legume seed [+L] and no vigorous legume seed [-L]) were established in five replicate sites in 1988 i