Long-term growth responses of three Flindersia species to different thinning intensities after selective logging of a tropical rainforest

Abstract

Studies in low-diversity temperate forests clearly establish that thinning leads to increased growth of remaining trees. However, few studies have investigated the long-term impacts of thinning regimes on the growth responses of tropical timber species. In this study, we analysed data collected over a period of 46 years to assess the long-term growth responses and growth patterns of remnant trees of three key commercial species (Flindersia brayleyana F. Muell., Flindersia bourjotiana F. Muell. and Flindersia pimenteliana F. Muell.). We also examined how initial tree size, crown size and crown position within canopy, and neighbourhood competition affected growth rates of the three Flindersia species. Within these species, we assessed growth responses of trees above 10 cm diameter at breast height (DBH) in an Australian tropical rainforest, to different intensities of thinning i.e. logging only as a control and low-, medium- and high-intensity thinning (L, LT, MT and HT, respectively). Medium and high intensity treatments facilitated recruitment of all three Flindersia species into the >10 cm DBH class while medium intensity thinning resulted in the greatest average annual basal area increments (BAIs). The lower individual BAI of Flindersia species in HT was due to fewer large Flindersia trees remaining after thinning, combined with large numbers of recruits and slow growth of small remnant trees. BAI in HT increased initially, with a peak during 1981–1989 and then decreased. BAI was constant and varied little over time within the other treatments. High intensity thinning had prolonged impacts on the growth patterns of Flindersia trees over 46 years. Tree initial DBH, crown size and crown position within canopy explained most variation in individual BAIs of Flindersia trees. Both thinning intensity and measurement periods explained variation in individual BAIs of Flindersia trees. Larger remnant Flindersia trees with broad and dominant crowns tended to grow faster. BAIs of individual trees were negatively correlated with neighbourhood competition. If the goal of tropical native forest management is to improve productivity of target species, the results indicate that thinning should be sufficiently intense to reduce neighbourhood competition while maintaining large diameter commercially desirable trees with large and dominant crowns, which may consequently facilitate the timber volume and above-ground biomass recovery after selective logging.