Carbon Storage in Old-Growth Homestead Windbreaks of Small Islands in Okinawa: Toward the Sustainable Management and Conservation

Research Highlights: This study contributes to the improvement of the understanding of ecosystem functions of trees growing outside the forest, by quantifying the carbon sequestration function of a homestead windbreak, for example, a linear forest belt planted bordering a farmhouse in small islands. Background and objectives: Carbon storage in small-scale stands of forests have been less studied compared to that in large-scale forests. The aims of the present study were to clarify the ecological functions of carbon storage and the economic value of homestead windbreaks to propose effective conservation strategies for old-growth homestead windbreaks in the face of climate change. Materials and Methods: On the small islands of Okinawa Prefecture, the dominant tree species used for the homestead windbreaks is fukugi (Garcinia subelliptica Merr.). We collected data on the diameter at breast height (DBH) and the height of 23,518 fukugi trees in 10 villages from 2009 to 2018. Results: The total amount of carbon stored in the remnant fukugi homestead trees of the 10 surveyed hamlets was 6089 t-CO₂. The amount is equivalent to the carbon amount that is stored in a 40-year-old Japanese cedar (Cryptomeria japonica) forest, a representative tree species in Japan, of 20.9 ha area. Furthermore, the estimated economic value of the homestead trees was equivalent to USD 235,433, in terms of the plantation and management costs of 40-year-old Japanese cedar forests. This study revealed that homestead trees planted in an orderly line usually have a high density; hence, they have a high potential for biomass accumulation, carbon sequestration, and climate change mitigation. Moreover, homestead trees could contribute to a reduction in carbon diffusion, by cooling the house and reducing potential energy consumption. The findings related to homestead trees are consistent with those of other types of trees outside forests or small patches of trees: not adding to future land use competition and highly effective at carbon sequestration. Conclusions: The finding related to the carbon storage of homestead trees will provide basic information, as well as a new perspective on future local conservation and its contribution to climate change mitigation. This study suggests the necessity of the existing trees being properly managed, recruiting trees to be planted to replace old-growth trees, and replanting trees near newly established houses or old homesteads where trees have been cut.