# Exploring Ecosystem Functioning in Spain with Gross and Net Primary Production Time Series
## 6 Discussion
### Change Detection Analysis
Increase of productivity (positive GPP (NPP) trend) was observed in sites B (grasslands), E (irrigated crops), G (low-altitude DBF), and H (high-altitude DBF). While site B revealed a positive correlation between GPP and SPI and negative correlation between GPP and air temperature, site G provided a positive correlation between GPP (NPP) and both SPI and air temperature. Site B belongs to a Mediterranean savanna-like ecosystem with scattered trees (open holm oak woodland) over annual grassland, where the favorable condition of precipitation observed in Figure 6a may account for the increasing of GPP (reaching GPP values up to 5 g m\({}^{-2}\) d\({}^{-1}\)). Site G belongs to a broadleaved forest where favorable precipitation and air temperature conditions lead to the predominance of positive GPP (NPP) trends rather than negative ones (Figure 6 and Table 3). The smallest correlations between GPP (NPP) and SPI were found in sites E and H. Despite these low correlations, site E showed positive GPP trend mainly controlled by irrigation systems. Although this site exhibited productivity values up to 15 g m\({}^{-2}\) d\({}^{-1}\), it may be slightly affected by water restrictions due to water resources scarcity as it was observed in lower peaks over drought events (e.g., 2015). In this case, the response of the crop to this situation would depend on the drought magnitude and resilience of the affected area. Site H showed a higher correlation between GPP (NPP) and air temperature than between GPP (NPP) and SPI. It is probable that warming is amplified with elevation resulting in rapid changes in temperature, humidity, and water in mountains areas [68]. However, although these areas may be affected by drought periods, the development of the vegetation generally continues in spring and summer because enough water is still available. Thus, significant positive trends may be found (\(Q_{\text{GPP}}\)(10\({}^{-2}\)) = 0.161 \(\pm\) 0.006 d\({}^{-1}\); \(Q_{\text{NPP}}\)(10\({}^{-2}\)) = 0.111\(\pm\)0.005 d\({}^{-1}\)). To better understand the effect of these long-term changes, other magnitudes may be also examined regarding other significant information on carbon fluxes. This is the case of the carbon use efficiency (CUE). It is an interesting variable that describes how efficiently plants incorporate the carbon fixed during photosynthesis into biomass gain [69]. It can be calculated as the ratio between NPP and GPP. In a previous study [70], the CUE was analyzed for the Peninsula Spain study area throughout eight years, 2005-2012. Results showed that CUE exhibited a positive correlation with precipitation and a negative correlation with temperature in most ecosystems. In this study, GPP refers to the whole ecosystem, whereas NPP refers only to the trees and is only obtained for forest canopies; then, an analysis of CUE cannot be addressed to compare with previous results. However, considering the annual values of our NPP and GPP (Table 3) as a proxy of CUE, it is observed that, effectively, the correlation between CUE and precipitation is positive (for all the sites except for J), and the correlation between CUE and temperature is negative (for all sites except G), thus confirming a general tendency of CUE to decrease when the ecosystem conditions change towards aridity.