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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,351:57-66.

Document type: Journal Article
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IRMA ID 84376995xPUB212
Title Modelling the growth of young rainforest trees for biomass estimates and carbon sequestration accounting
Author Preece, Noel D.
Lawes, Michael J.
Rossman, Allison K.
Curran, Timothy J.
van Oosterzee, Penny
Journal Name Forest Ecology and Management
Publication Date 2015
Volume Number 351
ISSN 0378-1127   (check CDU catalogue open catalogue search in new window)
Scopus ID 2-s2.0-84937220093
Start Page 57
End Page 66
Total Pages 10
Place of Publication Netherlands
Publisher Elsevier BV
HERDC Category C1 - Journal Article (DIISR)
Abstract 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.
Keywords Wood density
Root:shoot ratio
Biomass expansion factor
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