Why measuring trees and digging for roots help us understanding forests Net Primary Production and the limits of our planet?

Net Primary Production (NPP) limits the biomass produced by the Earth's surface and is the sum of carbon allocation in biomass and above- (litterfall) and belowground turnover (fine roots). Terrestrial ecosystems, including forests and woodlands, are sources for material and energy worldwide and are important for carbon (C) storage, livelihoods and biodiversity. We gained unprecedented insight in the magnitude and response of C accumulation by the Earth's vegetation by combining terrestrial observations of NPP with Earth Observing remote sensing data and biogeochemical ecosystem modelling. Data curation, data access and identifying knowledge gaps are needed to keep learning about terrestrial productivity.

The scientific community has accumulated large amounts of data on all components of NPP and with collaborative efforts and careful harmonization we are able to tap this source of information. We showcase the current state of available data on forest growth, litterfall and fine root production for European forests. We also report on the potential to use models to transfer information spatially - important for generating maps and substituting missing information.

Most information is available on C accumulation in woody vegetation, followed by data on aboveground litterfall. Least knowledge has been collected for fine roots and their turnover. Models can approximate litterfall and fine root production with some accuracy. Stand density appears to play a pivotal role in all of the analysed NPP components, which advocates for considering forest management, age effects and disturbances in more detail.

Maintaining and updating long-term earth observation records is as important as keep measuring our terrestrial research sites. Upcoming high-resolution remote sensing products and earth observation techniques (i.e. Lidar, SIF) will enrich our toolbox for quantifying NPP, yet we need observations to validate and further improve the next-generation of NPP models. We require more robust and accurate data in particular for understudied regions and NPP components. Coordinating international efforts on collating robust NPP data will likely open new viewpoints and benchmarks on the sustainable limits of growth.