Scots pine and Norway spruce foliage biomass in Finland and Sweden — testing traditional models vs. the pipe model theory

The pipe model approach was compared with foliage biomass models by using the cross-sectional area at the tree crown base for predicting foliage biomass of Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.). We evaluated the impacts of site type, fertilization, and climate on the relationship between foliage biomass and cross-sectional area at the tree crown base, referred as to the pipe model ratio. Our hypotheses were that (i) the pipe model approach is a more precise and accurate method for foliage prediction than the traditional biomass models and (ii) the pipe model ratio for foliage does not explicitly depend on any single environmental driver. Data used here consisted of felled trees from Finnish and Swedish biomass studies. These data were analyzed by linear mixed models with different covariates, and the uncertainties of different modelling approaches were evaluated. The pipe model outperformed other models for Scots pine but not for Norway spruce. Results showed larger pipe model ratios for Scots pine in herb-rich forests compared with those of trees in subxeric heath forest. Results from fertilized trees indicated that the addition of nitrogen temporarily increased foliage biomass.