Tree tissue nitrogen content, respiration and net primary production in boreal and temperate forests (NicerNPP)

• Applicant: Dr. Martin Thurner
• Subject Area: Forestry
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 554263399

Plant respiration (or autotrophic respiration; Ra) is, in addition to gross primary production (GPP; photosynthesis at large scales), an integral part of net primary production (NPP = GPP - Ra). The NPP of trees determines the amount of carbon (C) taken up from the atmosphere that is not respired and can be used for tissue growth and reproduction. Despite being key C fluxes for quantifying forest growth and C uptake, it is highly uncertain how Ra and NPP respond to changes in climate and tree species distribution at large spatial scales. Since Ra increases with temperature over a wide range of temperatures, increased Ra might offset enhanced productivity that has been observed in recent decades as temperatures keep increasing. Up to now, regional to global estimates of Ra are not available, because this C flux is hardly measureable at such spatial scales. For this reason, also estimates of NPP from satellite data like MODIS NPP or by dynamic global vegetation models (DGVMs) involve strong modelling assumptions and simplifications and thus a high uncertainty. To address this need for research, within the NicerNPP project recent spatial products of forest biomass will be used together with information on tree tissue nitrogen (N) concentrations and the tree species distribution to calculate Ra. In doing so, the relationship between respiratory costs and vegetation N content will be exploited. This work becomes possible now because of the recent availability of remote sensing based biomass products and important preliminary work that the applicant has carried out, involving the differentiation of tree living tissue (leaf, branch, stem sapwood, root) biomass from satellite biomass data, the compilation of a tree genera map covering northern boreal and temperate forests and the identification of the controls of tree tissue N concentrations based on an unprecedented data collection. The first data-driven spatially explicit estimates of Ra and NPP that are proposed to be derived will cover the entire northern hemisphere boreal and temperate forests. We will focus on these ecosystems because their composition and functioning is fundamentally different from tropical forests (distinct spatial patterns in tree species distribution). The environmental controls underlying the spatial patterns in tissue N content, Ra and NPP will be identified. The derived novel NPP product will also be used to evaluate NPP estimates by MODIS and the LPJ-GUESS DGVM. In this way, the NicerNPP project will be an important step forward towards monitoring the response of Ra and NPP to changes in climate and species distribution and improving their implementation in DGVMs. This will facilitate optimizing both climate change mitigation by C sequestration and wood productivity.

• DFG Programme: Research Grants