Competition-dependent modelling of foliage biomass in forest stands

Currently, foliage biomass is estimated based on stem diameter or basal area. However, it is questionable whether the relations between foliage and stem observed from plantations of a single tree species can be applied to stands of different structure or species composition. In this paper, a procedure is presented to simulate foliage and branch biomass of tree crowns relative to crown size and light competition. Crowns are divided into layers and segments and each segment is divided into a foliated and an unfoliated fraction. Depending on the competitive status of the segment, leaf area density, specific leaf area and foliated branch fraction are determined. Based on this information, foliage biomass is calculated. The procedure requires a crown shape function and a measure to characterise competition for light and space of each individual segment within the canopy. Simple solutions are suggested for both requirements to enable an application with data that can be measured non-destructively in the field; these were stem position, tree height, crown base height, crown radii and some general crown shape information. The model was parameterised from single trees of Norway spruce and European beech and partly evaluated with independent data close to the investigation plot. Evaluations showed that the model can attribute the ecology of the different crown forms. Modelled foliage distribution for beech and spruce as well as total needle biomass of spruce agreed well with measurements but foliage biomass of beech was underestimated. The results are discussed in the context of a general model application in structured forests.