Effects of site characteristics and stand management on biomass and carbon sequestration in spotted gum (Corymbia citriodora subsp. variegata) plantations

Understanding influences of site characteristics and stand management on biomass and carbon in spotted gum plantations, over time, is needed for sustainable land management and to encourage the successful implementation of climate change strategies. However, a lack of reliable allometric equations and growth data has limited accurate determination of the biomass and carbon stocks in long-rotation spotted gum (Corymbia citriodora subsp. variegata) hardwood plantations. In this study, we used allometric equations developed specifically for spotted gum to convert forest inventory data into estimates of total tree and stand biomass and carbon. Inventory data was available for two sites with 210 plots and more than 10,000 repeated tree records from planting to an age of 18–20 years. We used this data to test whether biomass and carbon sequestration differ between sites and slope positions over time. We also evaluated the effect of thinning on biomass and carbon at both the individual level and stand level over time. For stands at ages 20, 14-, 9-, 6- and 3-year, biomass was 538.9>393.7>206.9>60.3>18.4 kg tree−1, which equated to 264.0>192.9>101.3>27.8>8.5 kg tree−1 of stored carbon. Biomass and carbon stocks on the lower slope were higher than those on the upper slope (p<0.05). At 19 years (stocking presumed 240 tree ha−1), the average biomass and carbon on the lower slope were 173.3 and 84.9 Mg ha−1 respectively, whereas the upper slope position accumulated about 122.1 and 59.8 Mg ha−1. Thinning had a large effect on the growth, biomass and carbon storage of individual trees (p<0.05); the low stocking (240 tree ha−1) treatment had an estimated biomass and carbon content of 644.1 and 315.6 kg tree−1 respectively, which was higher than the high stocking treatment (440 tree ha−1) of 371.3 and 181.9 kg tree−1, respectively. Although high stocking plots had 8.8 Mg ha−1 more biomass and 4.3 Mg ha−1 more carbon than the low stocking treatment, the mean biomass and carbon sequestration at a stand level was not significantly different at the age of 18 years (p>0.05) and some small trees may not reach commercial size in the high stocking treatment. Therefore, choosing suitable landscape positions and selecting appropriate stocking rates will have a significant influence on biomass and carbon accumulation. This has important implications for balancing timber products and carbon accumulation, especially if carbon credits are involved. Management strategies, such as thinning regimes, might differ depending on the objective of the plantation (i.e. timber production or carbon credits).