Modelling the growth of young rainforest trees for biomass estimates and carbon sequestration accounting

Preece, Noel D., Lawes, Michael J., Rossman, Allison K., Curran, Timothy J., and van Oosterzee, Penny (2015) Modelling the growth of young rainforest trees for biomass estimates and carbon sequestration accounting. Forest Ecology and Management, 351. pp. 57-66.

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Few measurements for carbon sequestration, ratio of above-ground to below-ground biomass and wood density exist for young trees. Current allometric models are mostly for mature trees, and few consider trees at the sapling stage. Over four years we monitored the growth rates, from seedling to the sapling stage, of 490 trees (five native species) in environmental plantings, in the Wet Tropics of north-eastern Australia. Our biomass estimates were greater by several orders of magnitude in the first year (6 × 10−3 Mg ha−1 cf. 4 × 10−6 Mg ha−1), and two orders of magnitude less at four years than those derived from the national carbon accounting model (5 × 10−1 Mg ha−1 cf. 13 Mg ha−1). We destructively sampled 37 young trees to accurately estimate the variation in below-ground and above-ground biomass (AGB) with stem size, and to derive a best fit model for predicting sapling biomass: ln AGB = −5.092 + 0.786 ln (Diam.base)2Height. Biomass expansion factors for young tree species ranged from 1.71 to 2.44, higher than average for tropical forests. Root:shoot ratios are consistent with mean estimates for mature rainforest. Stem wood densities ranged from 0.444 to 0.683 Mg m−3 for the five species measured, which was 6.5% lower than published estimates for three of the species, and 12% and 27% higher for two species. Relative growth rates were faster for species with lower wood density in the first four years, but these species also had the lowest survival over the same period. The findings are significant for a number of reasons. Ecologically, they indicate that young rainforest trees invest more in leaves and branches than in stem growth. From a survival perspective, in the context of rainforest restoration, it is best to invest in species with higher wood densities. From a carbon accounting point of view, refinements to the models used for national carbon accounting are required that include the contribution of the sapling stage. Sapling growth rates were significantly different from those assumed in the national model, requiring growth rates to be increased after four years (as opposed to after 2 years in the national model) before reaching an asymptote at some time in the future. This adjustment is essential to enable carbon farmers to judge the time it takes to receive returns from investment. Policies that encourage carbon plantings should take into account that young plantings grow slower than predicted by current national carbon accounting models.

Item ID: 38963
Item Type: Article (Research - C1)
ISSN: 1872-7042
Keywords: wood density, root:shoot ratio, biomass expansion factor
Funders: Australian Research Council (ARC), Biodiversity Fund, Biome5 Pty Ltd
Projects and Grants: ARC Linkage Project LP0989161
Date Deposited: 18 Jun 2015 04:02
FoR Codes: 05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050207 Environmental Rehabilitation (excl Bioremediation) @ 50%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060202 Community Ecology (excl Invasive Species Ecology) @ 30%
06 BIOLOGICAL SCIENCES > 0607 Plant Biology > 060703 Plant Developmental and Reproductive Biology @ 20%
SEO Codes: 82 PLANT PRODUCTION AND PLANT PRIMARY PRODUCTS > 8201 Forestry > 820103 Integration of Farm and Forestry @ 40%
96 ENVIRONMENT > 9612 Rehabilitation of Degraded Environments > 961202 Rehabilitation of Degraded Farmland, Arable Cropland and Permanent Cropland Environments @ 30%
96 ENVIRONMENT > 9603 Climate and Climate Change > 960302 Climate Change Mitigation Strategies @ 30%
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