# Exploring Ecosystem Functioning in Spain with Gross and Net Primary Production Time Series
## 5 Results
### Change Detection Analysis  
Figure 4 shows the annual GPP and NPP for the selected sites (Table 1) in order to illustrate general behaviors between land covers. A significant gradient is observed in the GPP values, where the highest GPP values are found in broadleaved forests, the intermediate ones in needle-leaved forests, and the lowest ones in shrublands and nonirrigated crops. Particularly, sites B, D, F, and J (much less pronounced) reproduce well the years characterized by low vegetation activity due to drought episodes (e.g., 2005, 2012, and 2015). In fact, site F provides the highest GPP and NPP decrease throughout the period considered (see \(Q_{\text{GPP}}\) and \(Q_{\text{NPP}}\) in Table 3). This effect is also identified in the annual NPP values for the evergreen site F, whereas a flat trend dominates in the rest of the forest sites. Figure 3: Average annual GPP (**a**) and NPP (**b**) for the period 2004–2018. The normalized trends of GPP (\(Q_{\text{GPP}}\)) and NPP (\(Q_{\text{NPP}}\)) and annual averages are presented in Table 3 together with their correlations between SPI and \(T\) annual components. The grassland (B), shrubland (C), rainfed crop (D), and low-altitude evergreen needle-leaved forest (I) sites are the most sensitive to annual precipitation. A strong negative correlation with temperature is also observed for sites B, C, D, and I, as opposed to the positive correlation found for deciduous broadleaved forest sites (G and H). The poorest correlations are obtained for the irrigated crop (site E) and the sparse vegetation (A). Site A is practically a desert and has the lowest mean annual GPP and the lowest interannual GPP variation. In case of NPP, a similar behavior is found for all forest sites with a notable decrease of biomass (\(Q_{\text{NPP}}<-0.28\times 10^{-2}\) d\({}^{-1}\)) shown by the low-altitude evergreen forest site (I) and evergreen broadleaved forest (F) site. A strong correlation between NPP and annual precipitation is observed for these sites, I and F.  
Figure 5a,b illustrate the GPP and NPP trend images for the period 2004-2018 (\(Q_{\text{GPP}}\) and \(Q_{\text{NPP}}\)). Red values refer to negative changes, whereas green values are indicative of positive variations. No significant variation during the 15-year period is observed in 28% (5%) of the GPP (NPP) values. Highly significant GPP (NPP) decrease is observed in 16% (25%) of the Spain pixels. Highly significant GPP (NPP) increase is observed in 56% (70%) of the Spain pixels. The associated errors with \(Q_{\text{GPP}}\) (Figure 5c) and \(Q_{\text{NPP}}\) (Figure 5d) are also computed. The images reveal that larger errors are observed for greater changes, and the opposite is also true. Therefore, there has not been a spatial pattern found in the relative image, and relative errors below 10% (absolute error \(<2\times 10^{-4}\)) are observed for the major part of the study area. Figure 4: Annual GPP (**a**) for all sites and NPP (**b**) only for forest sites described in Table 1.  
Figure 5a demonstrates a positive GPP trend in northeastern and western Spain. In this figure, increasing productivity is observed in areas 1 and 2. Conversely, the negative trends in the GPP come with an associated rainfall diminution along time (Figure 6a). As an example, in eastern Spain, croplands at the south of the _Teruel_ province (area 3 in Figure 5a) and north of the _Castilla y Leon_ region (area 5 in Figure 5a) can be observed. This behavior is also observed in site F (national park of _Los Alcornacales_ in _Cadiz_ province, area 6 in Figure 5a), with a decrease in GPP and NPP, as supported by results shown in Table 3.  
To better understand the nature of observed changes, Figure 7 identifies areas with positive and negative GPP trends based on different precipitation and temperature behavior.