Carbon allocation to wood formation in an unmanaged deciduous forest in Thuringia (Germany)-a case study of biomass production efficiency

Carbon allocation to wood formation is the key process that drives biomass accumulation in forest ecosystems. Particularly important from a carbon balance perspective is the fraction of carbon taken up through photosynthesis (i.e. gross primary productivity) that is allocated to and sequestered in long-lasting wood tissues. This fraction is known as “biomass production efficiency” and a comprehensive understanding of its inter- and intra-annual variability in response to climatic fluctuations and ecosystem dynamics is still lacking. In this study, we assessed and reconstructed the above-ground biomass increment of three deciduous tree species, European beech (Fagus sylvatica), Sessile oak (Quercus petraea) and European hornbeam (Carpinus betulus) in Hainich National Park, Thuringia (Germany). Trees were sampled in a fixed plot design within the footprint area of a long-term eddy-covariance site (DE-Hai). We applied allometric equations to estimate tree volume and combined them with tree-ring width and wood density measurement to quantify and reconstruct the carbon stored as above-ground biomass in wood tissues. We scaled these measurements from the tree to the plot level and integrated the annual biomass increment with the carbon fluxes from the tower to quantify biomass production efficiency. Finally, we correlated species-specific growth with carbon fluxes and various climate parameters at daily, monthly, seasonal, and annual resolution to better understand, how climate variations affect carbon allocation to wood growth at this site. Our study represents a well-constrained observational framework to provide both quantitative and qualitative information on forest carbon cycling that can be used, e.g., to better parameterize tree-centered mechanistic vegetation models.