Biomass allometric equation and expansion factor for a mountain moist evergreen forest in Mozambique

Abstract

BackgroundWorldwide, forests are an important carbon sink and thus are key to mitigate the effects of climate change. Mountain moist evergreen forests in Mozambique are threatened by agricultural expansion, uncontrolled logging, and firewood collection, thus compromising their role in carbon sequestration. There is lack of local tools for above-ground biomass (AGB) estimation of mountain moist evergreen forest, hence carbon emissions from deforestation and forest degradation are not adequately known. This study aimed to develop biomass allometric equations (BAE) and biomass expansion factor (BEF) for the estimation of total above-ground carbon stock in mountain moist evergreen forest.MethodsThe destructive method was used, whereby 39 trees were felled and measured for diameter at breast height (DBH), total height and the commercial height. We determined the wood basic density, the total dry weight and merchantable timber volume by Smalian’s formula. Six biomass allometric models were fitted using non-linear least square regression. The BEF was determined based on the relationship between bole stem dry weight and total dry weight of the tree. To estimate the mean AGB of the forest, a forest inventory was conducted using 27 temporary square plots. The applicability of Marzoli’s volume equation was compared with Smalian’s volume equation in order to check whether Marzoli’s volume from national forest inventory can be used to predict AGB using BEF.ResultsThe best model was the power model with only DBH as predictor variable, which provided an estimated mean AGB of 291 ± 141 Mg ha−1 (mean ± 95% confidence level). The mean wood basic density of sampled trees was 0.715 ± 0.182 g cm−3. The average BEF was of 2.05 ± 0.15 and the estimated mean AGB of 387 ± 126 Mg ha−1. The BAE from miombo woodland within the vicinity of the study area underestimates the AGB for all sampled trees. Chave et al.’s pantropical equation of moist forest did not fit to the Moribane Forest Reserve, while Brown’s equation of moist forest had a good fit to the Moribane Forest Reserve, having generated 1.2% of bias, very close to that generated by the selected model of this study. BEF showed to be reliable when combined with stand mean volume from Marzoli’s National Forestry Inventory equation.ConclusionThe BAE and the BEF function developed in this study can be used to estimate the AGB of the mountain moist evergreen forests at Moribane Forest Reserve in Mozambique. However, the use of the biomass allometric model should be preferable when DBH information is available.

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“…However, it did not improve the performance of models and were not significantly different in the biomass estimation (p=0.0091) in all tested models. This is supported by findings from other researches such as that of Lisboa, Guedes [18] and Paul, Roxburgh [67].…”

“…For example, pruning and coppicing can affect the rate of biomass accumulation [11] as well as the change in biomass without changing the diameter at breast height (DBH) [12,13]. The general model has shown a large prediction error based on the finding of Mugasha, Eid [14] and Kachamba, Eid [15] in miombo woodlands, Tesfaye, Bravo-Oviedo [16] and Ubuy, Eid [17] in dry forests, and Lisboa, Guedes [18] in moist forests. Therefore, such generic equations may show a systematic error of up to 400 % at the site level [13].…”

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“…It also has a strong correlation with other tree attributes, such as height, volume, and. Therefore, some studies also reported the diameter is the best variable to represent tree growth biomass (Puri et al, 2013;Lisboa et al, 2018;Abrantes et al, 2019). According to the results, the mean diameter of 25 sample trees was 16 cm with a minimum of 6.80 cm and a maximum of 25.89 cm (Table 1).…”

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“…The allometric equation for biomass estimation was also reported in Olea europaea, L. Subsp. cuspidate in Mana Angetu forest (Kebede et al, 2018) and mountain moist evergreen forest in Mozambique (Lisboa et al, 2018). Using the per cent contribution of biomass to the components (trunk, branches and leaf) of the uprooted trees, the corresponding biomass of the tree components in standing trees were estimated.…”