Recovery of a logged Australian tropical forest followed by different intensity silvicultural treatments over 48 years: trade-offs between conservation values and commercial timber volume ​​​​​​​

Abstract

Currently, more than 400 million hectares of tropical forests in the world are managed for timber production. Silvicultural treatments following logging that remove stems of low commercial value to reduce competition were considered as an effective tool to enhance the growth and abundance of commercial trees. However, these silvicultural treatments will reduce biomass, likely initially reduce species diversity and may also cause changes in species composition. With a stronger focus on the conservation values of tropical forests today, how to maintain a balance between timber extraction and the conservation values of tropical forests is a central problem in sustainable tropical forest management.Permanent plots were established in a tropical rainforest in north Queensland in 1967. Following selective logging in 1968/69, four treatments were applied in this experiment: selective logging only with no subsequent silvicultural treatment as a control (L), logging followed by low-intensity silvicultural treatment (LT), logging followed by medium-intensity silvicultural treatment (MT), and logging followed by high-intensity silvicultural treatment (HT). Trees above 10 cm diameter at breast height (DBH) in all treatments were inventoried in 1967 (before logging), 1969 (after logging), September 1969 (after silvicultural treatments), and in 1973, 1981, 1989, 1997 and 2015.In Chapter 3, I investigated the effects of the different intensity silvicultural treatments on species composition over 48 years. The results show that the intensity of silvicultural treatment had an important impact on the recovery time of species composition. The number of species, species abundance distribution and species composition of trees (≥ 10 cm DBH) under selective logging followed by LT and MT treatments recovered to their pre-logging levels within 46 years following silvicultural treatments. However, the number of species, species abundance distribution, and species composition in the HT treatment only partially recovered to the post-logging level due to much greater initial species loss compared to the L, LT and MT treatments.In Chapter 4, I examined how AGB recovered over time in the four treatments. The AGB in the LT and MT treatments recovered to their pre-logging levels over 48 years and HT required more time to recover to its pre-logging levels because of the greater removal of biomass during silvicultural treatments. Over the 46 years after treatment, three distinct stages of biomass recovery were evident. Initially, biomass recovery from 1969 to 1973 was slow and was largely concentrated on an increase in biomass of residual stems. From 1973 to 1997, AGB increased at a near linear rate with a similar slope for all sites. Following the 1997 measure, the rate of biomass accumulation slowed, especially for the L site. The AGB recovery over time can be explained through the number of trees that remained after treatment, along with the growth rates, recruitment, and mortality of trees. All silvicultural treatments stimulated more recruitment, especially from 1973 to 1981, which contributed strongly to biomass increment. Growth rates of trees were stimulated by reduced competition caused by silvicultural treatments. In the final measurement period, biomass recovery rates decreased mainly due to increased mortality, particularly in the small- and medium-sized trees from pioneer and early secondary species.In Chapter 5, it was found that compared to selective logging, silvicultural treatments substantially enhanced the harvestable timber volume of the five most valuable commercial species, with MT increasing most, up to the double of its pre-logging timber stocking after 46 years. The commercial timber volume recovery rates of the most desirable species in LT and HT were similar. The reason why the timber volume increase in HT was not as great as in MT might be because a large number of small- and medium-sized remnants (10 cm to 40cm in DBH) and fewer large remnants (> 40 cm in DBH) in MT following treatments accelerated the growth of remaining medium-sized trees into large trees. In addition, the MT and HT treatments shifted the community towards a greater proportion of pioneer and early-successional trees, thus leading to the community having a lower community-weighted mean wood density.In Chapter 6, the effects of crown size and class, initial tree size and neighbourhood competition on individual growth rates of three Flindersia species were discussed. The MT and HT treatments facilitated the recruitment of the three Flindersia species. Additionally, MT but not HT facilitated the mean growth rates of the three Flindersia species. As expected, growth rates were influenced by initial tree size, neighbourhood competition indices, crown size and classes, and silvicultural treatment intensities. Overall, my thesis highlighted that a trade-off occurred between facilitating commercial timber production and maintaining conservation values of tropical forests both under different intensity silvicultural treatments and over time. For better management of tropical production forests, the intensities of silvicultural treatments, changes in the silvicultural effects over time, and the effects of silvicultural treatments on both timber production and conservations values should all be taken into consideration.