Needle and branch biomass turnover rates of Norway spruce (Picea abies)

Turnover rates of needle and branch biomass, number of needle cohorts, and needle-shed dynamics were modelled for Norway spruce (Picea abies (L.) Karst.) in southern Finland. Biomass turnover rates, vertical distribution, and biomass of the branches were modelled simultaneously. The rate of needle turnover was determined from needle-shed dynamics. The potential litterfall of branches was modelled by combining the vertical distribution of branch biomass and the annual change in height of the crown base. The mean annual turnover rates for needle and branch biomass are 0.10 and 0.0125, respectively. At the age of 5.5 years, 50% of the needles in the needle cohort have been shed. In addition, at the age of 12 years, all needles of the needle cohort have been shed. Turnover of branch biomass was dependent on stand density and tree size. The modelled rates of biomass turnover agreed with measurements of needle and branch litterfall. Many process- or inventory-based models use a single turnover rate for branch litterfall based on literature, and some of the models are fully ignoring the litterfall of branches. Speciesspecific turnover rates or dynamic litterfall models should be applied when carbon flows in forest stands are modelled.