In a nutshell: exploring single tree parameters and above-ground carbon sequestration potential of common walnut (Juglans regia L.) in agroforestry systems

Abstract

Although agroforestry systems (AFS) provide numerous ecosystem services and are a recognized strategy for climate change mitigation and adaptation, knowledge on the woody component is lacking. Single tree data could improve planning, management and optimization of AFS. One tree species which is of great interest due to its valuable timber and non-timber products is walnut (Juglans regia L.). We used terrestrial laser scanning data to fit quantitative structure models (QSMs) for 65 walnut trees in AFS with diameter at breast height (DBH) ranging from 1 to 77 cm. Based on the QSMs, volumetric information as well as height and crown parameters were derived. By combining the volumetric data with bark and wood density followed by carbon and nutrient concentration, whole tree biomass, nutrient and carbon content were derived. To enable the application of our results, we modeled allometric relationships based on the DBH. The maximum crown projection area of a tree was more than 340 m2, the maximum leafless above-ground dry biomass was 7.4 t and the maximum amount of stored carbon was 3.6 t (in metric tons). A modelled AFS comprising 15 trees per hectare with a target DBH of 60 cm projects at the end of its 60-year rotation period an above-ground tree volume of more than 100 m3, about 60 t of dry biomass and roughly 30 t of sequestered carbon. By producing allometric functions, we provide much needed information for small-scale modelling of AFS.