Development of tools to estimate the contribution of young sweet chestnut plantations to climate-change mitigation

Sweet chestnut plantations, irrespective of their main productive orientation (nut or timber production), are key elements of the landscape as well as the cultural heritage of the areas where they are found and provide important functions and services. Hence, recent initiatives have been aimed at extending the area occupied by chestnut trees through forest plantations. Nevertheless, the role of these young chestnut plantations as carbon sinks has been often ignored. The National Inventory of Greenhouse Gas Emissions (GHG) must include estimates of the so-called 'transition forests' during the 20 years following their plantation. In this study, new tools for estimating the total amount of above and belowground biomass stored in young plantations of chestnut were developed to quantify the carbon storage capacity of these plantations. A new set of aboveground biomass and root-shoot ratio models were fitted for individual-tree level based on four different independent variables - root collar diameter, total height, diameter at breast height and crown projection area - and their combinations. The expansion to stand level was based on age, plantation density, productive orientation of the plantation (nut or timber), site index and climate covariates as possible independent variables. At tree level, the best aboveground biomass models were those that include the product of root-collar or breast height diameters and tree height, whereas for root-shoot ratio the best results were obtained when only diameter at breast height is included. At stand level, the most accurate models included age, plantation density and site index for aboveground biomass and only age for root-shoot ratio. The fitted models provided accurate and unbiased predictions of aboveground biomass in the first years of reforestations. The different fitted equations can be used to estimate carbon stocks in young plantations depending on the available data and the objective of the prediction. Individual tree-level equations are recommended when accurate estimates are needed and detailed inventory measurements are available. Stand level equations, only using plantation age, can be an appropriate alternative for use with forest statistics at national scale, although the inclusion of additional covariates can greatly improve the accuracy of the age-based stand level equations. Our results indicated that even low-density nut-oriented chestnut plantations can play a relevant role as C sinks.