Competition between plantation Eucalyptus nitens and Acacia dealbata weeds in northeastern Tasmania

In this study, we investigated canopy structure, foliar gas exchange and water use in mixed stands of planted Eucalyptus nitens and naturally occurring Acacia dealbata in order to identify the presence or absence of competition. Early competition in A. dealbata was primarily for light but results from leaf level gas exchange and water use experiments indicated that individual eucalypts out-competed A. dealbata after several years. During early stand development (2–4 years), acacias and eucalypts occupied equivalent strata in the mixed canopy and exhibited similar leaf area indices (LAI). However, by 8 years of age, acacias were clearly subdominant and contributed only 20% of stand LAI. Both species exhibited similar maximum photosynthetic rates (about 25 μmol m −2 s −1) and neither species indicated adaptation to low-light sub-canopy environments. The onset of water stress responses occurred earlier in the day and earlier in the summer dry season for acacias than for eucalypts. Although the acacias occupied a limited temporal niche in the forest during young stand development, this interval provided sufficient time and opportunity for seed production. We concluded that periodic disturbance via plantation establishment maintains limited diversity in these eucalypt/acacia systems, eucalypts ultimately overtopping the acacias and out-competing them for light, until the next disturbance cycle again permits a brief period of intense competition.