# Creating a Regional MODIS Satellite-Driven Net Primary Production Dataset for European Forests
## 4 Discussion
The NFI NPP on the one hand represents the conditions of the (small) area covered by an inventory plot, while MODIS NPP provides a smoothed average NPP of a 1-km pixel. A consistent stand density map at 1-km resolution would be needed to test this hypothesis. But NFI NPP estimates capture not only differences in stand density and forest management, they are also strongly influenced by local tree allometries and local carbon estimation methods [38]. For Germany, stand density cannot explain the observed discrepancies satisfactory in Figure 7b. In fact the results are quite different compared to whole Europe (Figure 6), France (Figure 7a) or our pilot study [15]. Germany is planning to modify the currently used tree biomass estimation methodology [39] which is used in this study, for future carbon assessments. Following reanalysis of existing data [40] and collection and analysis of new sample data [41], improved biomass functions were developed for Germany [42]. This new updated methodology results in approx. 5% lower aboveground biomass estimates. Thus future German NFI NPP estimates will be lower as well, which will most likely reduce the gap between MODIS and NFI NPP observed for this country in this study. This suggests that, interpretation of discrepancies between NPP estimates needs consideration of the tree carbon estimation methods, since they directly affect increment estimates. However, there might be other potential drivers leading to inconsistencies both in MODIS EURO and NFI NPP, that could be analyzed in future studies. Concerning NFI NPP, few countries do not consider adequately the contribution of small trees to the NPP of a forest, either by not considering the ingrowth of small trees [33] or a particular large diameter threshold in some countries (Table S1). This could explain, why in Spain and France MODIS EURO is higher than NFI NPP, as we were not able to include ingrowth here and thus the French and the Spanish NFI NPP estimates might not represent the NPP of their forests sufficiently. The accuracy of the litter fall and fine root estimates for NFI NPP (Equations (3)-(5) need further research as well. The litter fall models used in this study were derived in a meta-analysis using Eurasian litter fall data [31]. They have substantial variation in the used input data and might contain potential inaccuracy, when applied in certain regions. In addition, the estimates for litter fall and fine roots are driven by the same climate data than MODIS EURO. Although the specific climate input differs (periodic average climate used in Equations (4) and (5) for NFI NPP versus daily maximum, minimum temperature and precipitation used in MOD17), it cannot be ruled out yet that the climate source explains the better match of MODIS EURO and NFI NPP. Thus, the performance of the currently used approach and alternative options for instance by using Foliage mass and Leaf longevity [43] needs to be tested using European litter fall data. Potential errors in the MODIS EURO product could involve wrong classification of forest biomes by MODIS Land cover [44], limitations of the global parameters of the MOD17 algorithm capturing European forest conditions (see discrepancies in NPP for evergreen broadleaf forests in Figure S3), mismatches in LAI and FPAR by region or forest fragmentation [45].