Effects of naturally regenerated Acacia dealbata on the productivity of a Eucalyptus nitens plantation in Tasmania, Australia

Twenty-two plots were established in a Eucalyptus nitens plantation and measured over three years to investigate the effects of naturally regenerated Acacia dealbata weeds on productivity. Under favorable moisture conditions, A. dealbata grew at a rate close to (and sometimes exceeding) that of E. nitens, bringing about canopy closure by two years of age where A. dealbata stem densities were high. Competition peaked at age 2-6 years before declining as the A. dealbata canopy was suppressed, in part by intra-specific Acacia competition. E. nitens basal area was a highly significant inverse correlate of A. dealbata stem density from ages 2-8 years. However, a simple correlation between E. nitens and A. dealbata basal areas was significant only in young stands (age 2-4 years). The effects of early-age competition by A. dealbata on E. nitens productivity were substantial, reducing standing volume compared to Acacia-free plots by almost 25% by age eight years. Soil nitrogen status was positively correlated with A. dealbata presence at both ages sampled (four and eight years) and negatively correlated with E. nitens basal area. A. dealbata was, therefore, considered to be a net contributor of nitrogen during this period, but competition for other resources negated any positive effect of this contribution.