Refining current annual volume increment estimation for forest tree species in Spain

Aim of study: The aim of this study is to improve the estimation of current annual volume increment of forest tree species in Spain. Area of study: The volume increment of the main tree species was studied at national scale using the two most recent data sets of the Spanish National Forest Inventory (SNFI) available. Material and methods: We developed both a tree diameter and height growth models considering the two most recent cycles of the SNFI and used the estimations obtained to calculate the annual volume growth. Diameter, height and volume estimates were validated using Second and Third SNFI cycles that were not been used for modelling. Main results: The developed tree diameter and height growth models indicated a satisfactory model structure. In general, competition, expressed as basal area, negatively impacted tree growth. Although basal area of larger trees had a negative effect, it was modulated by site quality, where better site conditions positively affected growth. Furthermore, tree vitality was found to be an important factor modulating tree growth. Research highlights: The proposed methodology to estimate tree growth considers tree size, competition, site quality and tree vitality and improves the estimates compared to previous methods based on current annual volume increment equations.