Estimation of carbon stock in young sweet chestnut forest and agroforest plantations

Removing CO₂ from the atmosphere and storing carbon in soil and vegetation are important ecosystem services provided by sweet chestnut (Castanea sativa Mill.) plantations, both in forest and agroforest systems, for which producers may be paid under certain circumstances. Diversifying chestnut owner’s revenues beyond timber or nut, turning storage of carbon into a passive income stream, could be an opportunity to make these areas more profitable. Estimates of the carbon sequestration for the species in Portugal are based on data from national inventory which do not distinguish between agroforestry (orchards or groves) and forest systems (within the latter, between coppice or high forest regime), or site quality. Considering that the capacity for carbon sequestration is strongly dependent on the cultural system, management and the quality of the site, the objective of this study is to evaluate and compare the carbon storage capacity in young stands of chestnut (up to 24 years old) in forest system (high forest and coppice regime) and agroforestry, considering low, medium and superior site qualities. For this purpose, dendrometric data from combinations of local-inventory date selected from a universe of 18 permanent plots were considered, and also 4 coppice permanent plots, with different management systems, measured over time. Biomass equations referenced in the literature were selected to estimate biomass and after converted to carbon using the conversion factor of 48.4%. The estimated carbon sequestration or conservation: high forest 3.0-91.7 Mg C ha^‑1 for 7 and 24 years, accumulation rate 1.6-14.1 Mg CO₂ ha^‑1 year^‑1 (above biomass and roots); coppice 21.2-89.2 Mg C ha^‑1 for 7 and 24 years, accumulation rate 10.7-16.2 Mg CO₂ ha^‑1 year^‑1 (above biomass); orchards 1.5-40.8 Mg C ha^‑1 for 7 and 24 years, accumulation rate 0.3-6.2 Mg CO₂ ha^‑1 year^‑1 (above biomass and roots). Coppice systems show the highest carbon storage capacity, followed by high forest and orchards. Site quality and management regimes significantly influence carbon storage capacity.