Forests play an important role in mitigating and minimizing effects of climate change through absorption, sequestration and accumulation of Carbon Dioxide (CO2) (Brack 2019). Forests and woodlands absorb approximately one twelfth of the global CO2 atmospheric output, and store about 72% of the Earth's total carbon reservoirs (Malhi et al. 2002). Tropical forests and subtropical forests sequester half of a total amount of CO2 sequestered by all types of forests (Canadell & Raupach 2008). A study undertaken in 2010 by Food & Agriculture Organization (FAO) estimated a smaller total forest carbon stock of 652 Gt of carbon, with 44% in living biomass, 5% in dead wood, 6% in litter and 45% in soils (FAO 2010).
Over-exploitation and forest encroachment significantly degrade forest resources. The forest degradation results in reducing absorption of CO2 (Müller et al. 2014). Approximately 5,8 Gt/year of CO2, accounting for about 20% of the world's annual CO2 emissions, emitted due to forest loss in the 1990s (IPCC 2007). The current greenhouse gas concentration has doubled, ranging from 722 ppb in 1750 to 1803 ppb in 2011 (IPCC 2014).
Forest protection and new forest development are an effective mechanism that helps decrease carbon emission and increase capacities of carbon sequestration. Previous studies show that tree age and volume are important for carbon accumulation (Johnson &Abrams 2009). Trees grow with an increase in diameter and carbon accumulation over time (Canadell & Raupach 2008). Four concerns with respect to tree growth include (1) metabolism of trees which tends to increase mass productivity for the tree size (Purves et al. 2007; West 1999); (2) space competition among trees (Pretzsch 2009); (3) increased total leaf area and positive feedback of light for tree growth (Bloor & Grubb 2003) and (4) respiration of photosynthesis decreasing the adaptation process (Ishii et al. 2007).
1.1. Melaleuca forests and carbon sequestration
Previous studies show that Melaleuca forests, a genus of Myrtaceae family, store a substantial amount of carbon (Tran et al. 2013; Tran & Dargusch 2016). Melaleuca cajuputi Powell provides a large amount of woody biomass for human use such as charcoals, and construction materials, and plays an important role in carbon sequestration and supply of oxygen (Hong et al. 2015). Trees of Melaleuca cajuputi Powell with less than 10-year-old and more than ten-year-old trees store about 15.18 tons CO2/ha and 31.76 tons of CO2/ha respectively (Dan et al. 2014). Fragmented Melaleuca forests also play a crucial role in storing carbon (Tran et al. 2015).
1.2. Research gaps and the aims of this study
With the background discussed above, two questions remain, (1) how Melaleuca trees are planted in order to sequester highest levels of carbon, (2) what is the best growth stage of Melaleuca trees in storing carbon. The questions are urgent, particularly in the context in the Mekong Delta of Vietnam where Melaleuca forests are annually grown for developing new forest areas and improving local livelihoods under the Five Million Forest Ha Program issued by the Government of Vietnam (Vietnamese Prime Minister 1998), and revised in 2021 (Vietnamese Prime Minister 2021). Melaleuca seedlings are planted on bunds in inundated areas or in elevated areas. The planting is undertaken in protected areas and privately owned lands. Meanwhile, The Vietnamese Mekong Delta contains 176,296 hectares of Melaleuca cajuputi Powell (Quy 2010).
We select U Minh Ha National Park, Ca Mau, the Vietnamese Mekong Delta in search for answers to the questions identified in the literature because Melaleuca cajuputi Powell trees are well-protected in addition to new forest areas established with seedlings of Melaleuca cajuputi Powell planted annually. This study aims to explore the relationship between planting of Melaleuca forests and carbon sequestration. Our goals are to (1) identify how Melaleuca seedlings are planted in order to best sequester carbon and (2) determine at what tree growth process Melaleuca forests are best for sequestering carbon. To achieve these goals, we establish 12 standard plots where Melaleuca trees with 10 years of age and 15 years of age were planted on bunds and trenches. Then, we undertake field experiments in an attempt to analyze the correlation between carbon accumulation and the tree growth process. Finally, in response to the analysis, we provide recommendations in order to address the issues in relation to sustainable forest management and reduction of greenhouse gas emissions.