The effect of overwood competition on the long-term survival, growth and stocking of underplanted tree species in logged tropical rainforest in north Queensland, Australia

Enrichment of selectively logged rainforest by planting seedlings of commercially valuable species is often recommended to improve productivity, ecological functioning, and biodiversity. However, the survival and subsequent growth of enrichment plantings depend on how they respond to competition from neighbours. For successful forest restoration or commercial forest management, the challenge is to determine appropriate frequencies and intensities of silvicultural interventions, principally competition reduction. In this study, we use long-term data (49–60 years) from four enrichment planting experiments established after selective logging and girdling of non-commercial trees in a tropical rainforest in north Queensland, Australia, to assess the effects of competition on the survival and growth of enrichment plantings. We found that residual overwood development severely reduced the survival, diameter and volume growth of two widely planted rainforest species - Flindersia brayleyana (Rutaceae) and Agathis robusta (Araucariaceae). Both species responded well to overstory removal, either as an initial treatment or thereafter. Use of Hegyi’s competition index confirmed the strong suppressive effect of neighbours. The most abundant competitor was the fast-growing, early-secondary species Acacia aulacocarpa, which only became apparent after cessation of silvicultural treatments, 7 years after planting. Our results reaffirm the importance of an initial or early-age removal of the competing overstory for the success of enrichment planting. Further silvicultural treatments may be required for extended periods to control excessively increased overwood basal area and to remove late emerging pioneer or early secondary species.