# Exploring Ecosystem Functioning in Spain with Gross and Net Primary Production Time Series
## 6 Discussion
### Change Detection Analysis  
The temporal analysis based on daily TS offered an opportunity to infer a higher accuracy in the trend estimates since more significant pixels are obtained as compared to annual resolution studies. This can be checked with GPP: the combination of daily interannual time series (derived from the MRA-WT) with the Mann-Kendall and the Theil-Sen slope methods provided 72% of the considered pixels with statistically significant trend in this study, as opposed to only 17% found in a previous study over the same area using only annual series [58], even if three more years were used (18 instead of 15). Trend results are consistent with similar studies performed over different periods [8]. The associated errors with \(Q_{\rm GPP}\) and \(Q_{\rm NPP}\) reveal that the precision does not depend on the vegetation type cover, since a specific spatial pattern was not found. Furthermore, larger significant negative changes and associated errors are observed, especially in southeastern Spain, eastern Mediterranean coastal, and central Spain, as was already found in [14] for 1989-2002, where a remarkable loss of productivity in southeastern and eastern Spain was observed. Particularly, the sparse natural vegetation in _Almeria_ province (almost a desert) is the most affected land cover with a significant loss of production during this period. Positive GPP trends are generally coincident with positive SPI trends (see Figures 5a and 6a), but in some areas they may be partly assured by the abandonment of old cultivated fields in mountain areas that have been transformed to natural vegetation [59]. For example, positive patches in the _Ebro_ basin (Figures 5a and 7b) may be the consequence of the abandonment of old irrigated fields due to relocation and socioeconomic changes benefiting from the positive precipitation trend (\(Q_{\rm SPI}\) > 0 in Figure 6a) for the same period [59]. Most of these areas are classified as arable lands and grasslands ([[http://sigpac.aragon.es/visor/](http://sigpac.aragon.es/visor/)]([http://sigpac.aragon.es/visor/](http://sigpac.aragon.es/visor/)), accessed on 13 January 2022) with a high response to favorable meteorological conditions. Moreover, a high overlap with the conservative areas included in the Natura 2000 ecological network ([[https://www.miteco.gob.es/](https://www.miteco.gob.es/)]([https://www.miteco.gob.es/](https://www.miteco.gob.es/)), accessed on 13 January 2022) is found. It comprises up to40% of agricultural lands, predominantly marginal areas with low productivity. Conversely, the productivity increase in the _Guadalquivir_ basin (light green color in Figure 7b) coexists with unfavorable precipitation conditions (Figure 6a). This could be attributed to the remarkable expansion in irrigated areas and modernization of irrigation systems [60]. In other areas, vegetation recovery from forest fires that occurred in the beginning of the period may explain this behavior. This is the case of a 34,000 ha area affected by the 2004 forest fire in _Riotinto_ (_Huelva_ province) and 22,000 ha burnt in 2005 in _La Riba_ (_Guadalajara_ province), as well as croplands and needle-leaved forests in _Murcia_ and _Comunitat Valenciana_. Precipitation is one of the most important meteorological factors that drive GPP in natural vegetation. Negative trends in GPP demonstrate its close relationship with precipitation (Figure 5a). A similar pattern can be observed in the SPI trend (Figure 6a). These observations are particularly clear in arid and semiarid areas where cloudy skies are not frequent, and precipitation is a controlling factor of vegetation phenology and productivity [61]. In this case, the lack of water provision (Figure 6a) has a direct impact on carbon absorption (mainly controlled by PAR) and carbon assimilation after autotrophic respiration. Negative trends in agricultural areas (areas 7, 8, and 9 in Figure 7b) mainly belong to regions where average rainfall decreased and its seasonal distribution was altered.