Carbon assimilation and water use efficiency increases in undisturbed core forest areas may not compensate carbon losses

Global heating is affecting the gross primary production (GPP; i.e., amount of CO₂ assimilated by plants) and evapotranspiration (ET; i.e., water loss from surface evaporation and transpiration) across forests in the northern hemisphere. Increasing temperatures have induced a prolonged growing season that has enhanced GPP during the spring and autumn seasons. In the summer season, it has resulted in higher ET. Although these findings are reported in multiple studies, we lack investigations that specifically analysed long-term regional scale changes in GPP and ET in undisturbed core forest areas, which play an important role in the carbon and water fluxes. Analyses of GPP and ET changes across undisturbed forest areas are essential to understand how these areas are adapting to new climate conditions and contribute to the mitigation of human greenhouse gas emissions. In our study, we used Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data to investigate the trends and changing rates of GPP, ET and water-use-efficiency (i.e., quantity of carbon assimilated by litre of water; WUE=GPP/ET) across undisturbed forest areas in Europe from 2000 to 2020 at monthly basis. We used the Mann-Kendal test to identify the significance of trends and the Theil–Sen estimator to quantify the monthly rate of change. The results indicated that during early spring and late autumn, approximately half of the total undisturbed core forest areas (3601.5 km²), mostly located in eastern Europe, showed an increase in GPP and ET. These areas also showed an increase in WUE because the increase in GPP was greater than the ET. However, most forest areas showed a decrease in GPP during summer, which was not compensated by the GPP increase during spring and autumn. These uncompensated forest core areas were spatially scattered across different forest types in Europe and were responsible for offsetting 20% of the total GPP increase in all European forest core areas. Our results provided evidence that certain forest core areas have limitations to act as carbon sinks, therefore, reducing the capacity to mitigate human carbon emissions. Moreover, by identifying the location of these forests, our results can support the application of management strategies that enhance carbon assimilation.