Above-ground biomass recovery following logging and thinning over 46 years in an Australian tropical forest.

Managed tropical forests are a globally important carbon pool, but the effects of logging and thinning intensities on long-term biomass dynamics are poorly known. We investigated the demographic mechanisms of above-ground biomass recovery over 48 years in an Australian tropical forest following four silvicultural treatments: selective logging only as a control and selective logging followed by low-, medium- and high-intensity thinning. Initial biomass recovery rates following thinning were poor predictors of the long-term changes. Initial biomass recovery from 1969 to 1973 was slow and was largely concentrated on an increase in the biomass of residual stems. From 1973 to 1997, above ground biomass (AGB) increased almost linearly, with a similar slope for all sites. From 1997 to 2015, the rate of biomass accumulation slowed, especially for the L treatment. All thinning treatments stimulated more recruitment and regrowth of non-harvested remaining trees compared to the untreated control. Biomass at both the low and medium intensity treatments has almost fully recovered to 98% and 97% of pre-logging biomass levels respectively. The predicted times of complete above-ground biomass recovery for the logging only and high intensity treatments are 55 and 77 years respectively. The slower biomass recovery at the logging only site was largely due to increased mortality in the last measurement period. The slower recovery of the high intensity site was due to a combination of a higher initial reduction in biomass from thinning and the increased mortality in the last measurement period. The high mortality rates in the most recent measurement period are likely due to the impacts of two cyclones that impacted the study site. Our results suggest that it will take at least around 50 years for this site to recover to its pre-harvest biomass, much longer than many of the cutting cycles currently used in tropical forest management.

Drivers of Tree Growth, Mortality and Harvest Preferences in Species-Rich Plantations for Smallholders and Communities in the Tropics.

There is growing interest in multi-species tropical plantations but little information exists to guide their design and silviculture. The Rainforestation Farming system is the oldest tropical polyculture planting system in the Philippines and provides a unique opportunity to understand the underlying processes affecting tree performance within diverse plantings. Data collected from 85 plots distributed across the 18 mixed-species plantations in the Philippines was used to identify the factors influencing growth, probability of harvest, and death of trees in these complex plantings. The 18 sites (aged from 6 to 11 years at time of first measurement) were measured on three occasions over a 6-year period. We used data from the first period of data collection to develop models predicting harvesting probability and growth of trees in the second period. We found little evidence that tree species diversity had an effect on tree growth and tree loss at the community level, although a negative effect was found on tree growth of specific species such as Parashorea plicata and Swietenia macrophylla. While tree density of stands at age 10+ years (more than 1000 trees/ha with diameter > 5cm) did not have an impact on growth, growth rates were decreasing in stands with a high basal area. Tree size in the first period of measure was a good predictor for both tree growth and tree status in the next period, with larger trees tending to grow faster and having a greater chance of being harvested, and a lower possibility of mortality than smaller trees. Shade-intolerant trees were both more likely to be harvested, and had a higher probability of death, than shade-tolerant individuals. Native species and exotic species were equally likely to have been lost from the plots between measurement periods. However, shade-tolerant native trees were likely to grow faster than the others at age 10+ years. Our findings suggest that species traits (e.g. shade tolerance) could play an important role in optimizing species composition for this type of plantation. Shade-intolerant species with rapid early growth could contribute early income for farmers in mixed plantings where some products may take years to realize. We also suggest selective harvesting or thinning (for small shade-intolerant trees) applied at age 10+ years could reduce the competition for resources between individuals.

The balance between facilitation and competition in mixtures of Eucalyptus and Acacia changes as stands develop.

The balance between facilitation and competition is likely to change with age due to the dynamic nature of nutrient, water and carbon cycles, and light availability during stand development. These processes have received attention in harsh, arid, semiarid and alpine ecosystems but are rarely examined in more productive communities, in mixed-species forest ecosystems or in long-term experiments spanning more than a decade. The aim of this study was to examine how inter- and intraspecific interactions between Eucalyptus globulus Labill. mixed with Acacia mearnsii de Wildeman trees changed with age and productivity in a field experiment in temperate south-eastern Australia. Spatially explicit neighbourhood indices were calculated to quantify tree interactions and used to develop growth models to examine how the tree interactions changed with time and stand productivity. Interspecific influences were usually less negative than intraspecific influences, and their difference increased with time for E. globulus and decreased with time for A. mearnsii. As a result, the growth advantages of being in a mixture increased with time for E. globulus and decreased with time for A. mearnsii. The growth advantage of being in a mixture also decreased for E. globulus with increasing stand productivity, showing that spatial as well as temporal dynamics in resource availability influenced the magnitude and direction of plant interactions.