## 4 Discussion  
### Characterizing Variatoin and Drivers of Structural and Physiological Phenotype  
Here, we discuss and characterize the phenology of a seasonally dry Mediterranean tree-grass ecosystems using high temporal resolution of VIs (i.e., daily) derived from PhenoCam and GPP from EC towers. We found large seasonal variations in both PhenoCam-based VIs (structure) and GPP (physiology). In general, seasonal variations in PhenoCam VIs were in phase with those of GPP (Figure 3 and Table 3). We argue that seasonal variations in VIs and GPP are driven by the herbaceous layer, which dominates the ecosystem dynamics in our study sites [59]. Our sites are characterized by relatively sparse evergreen broadleaf trees (~20%) and a larger fraction of annual grasses [59]. Foliage  
Figure 8: Growing season length (GSL\({}_{\text{RD-UD}}\)) derived from gross primary productivity (GPP), PhenoCam-based green chromatic coordinate (GCC), normalized difference vegetation index (CamNDVI), near-infrared reflectance of vegetation index (CamNIRv), and ratio vegetation index (CamRVI) for four Mediterranean tree–grass ecosystems (FLUXNET IDs are ES-LM1, ES-LM2, ES-LMa, and ES-Abr, respectively). The gold squares stand for mean GSL in all site-years, while the black points stand for the GSL derived from ES-Abr. Data from ES-LM1, ES-LM2, and ES-LMa falling outside the 10%–90% percentile range are plotted as cross. The statistically significant differences were tested between GPP and vegetation indexes—GCC, CamNDVI, CamNIRv, and CamRVI. \(p\)-values are as follows: \(\ast\) 0.01 \(<p\leq\) 0.05, ns for \(p>\) 0.05. Please see the definition of GSL\({}_{\text{RD-UD}}\) in the Table 2.  
amount in evergreen tree species remain relatively constant throughout the year, and they can utilize their vast root system to access the water deep within the soil [64; 65; 66]. By contrast, grasses are highly responsive to rainfall variations in rainy seasons as they tend to use water and nutrients in topsoil with dense shallow roots [11; 67]. The yellowing grasses in senescence lose their vigor during the dry and hot summer. Being that the VIs derived from the tree are relatively constant during the year, grasses contribute a large proportion of VIs (Figure A6) and GPP in rainy seasons, but trees contribute more during dry periods [49]. Meteorology plays an important role in governing seasonal variation of VI and GPP (Figures A2 and A3), though the role and importance of water availability and temperature varies across seasons (phenological stages). In autumn, after the dry season, the onset of greenness and GPP is initiated by the onset of the rainy season (Figure 3). In winter, water is not a limiting factor, due to the typical ample precipitation in late autumn and early winter. On the other hand, we observed that winter temperature is an important limiting factor of plant photosynthetic activity [68] (Figure 4). In spring, with the increase in incoming radiation and day length, temperature, and associated increase in atmospheric evaporative demand (i.e., VPD), we found precipitation strongly correlated with both GCC and GPP (Figure 4); this is consistent with previous findings over Mediterranean ecosystems [69; 70; 71]. In summer, which is the dry season, precipitation triggers an abrupt increase of VIs and GPP, as observed after the heavy rain that occurred in the June-2016 and July-2017 (Figure 3). The large rain pulses caused a decrease in temperature and an increase in water availability, further contributing to the regrowth of plants, in particular, at the early stage of the dry season (Figure 3). On the annual scale, growing season lengths (GSLs) derived from ES-Abr tower are significantly shorter than the GSL derived from the other sites in Majadas de Tietar (i.e., ES-LM1, ES-LM2, and ES-LMa; Figure 8), which might be attributed to a lower water availability at the ES-Abr tower more than the other sites. In fact, ES-Abr is characterized by about 200 mm of rain less than the sites located in Majadas de Tietar.