## 5 Conclusions  
We explored the relationships and underlying linkages between Gross Primary Production (GPP) and three commonly used Vegetation Indices: NDVI, EVI, and NIR\({}_{\text{v}}\). We analyzed data from 10 eddy covariance sites located in the California analogues of six major terrestrial biomes, and found that GPP, EVI and NIR\({}_{\text{v}}\) have strong and convergent within- and between-site relationships, whereas NDVI and GPP showed weaker and more site-specific relationships. Our analyses showed that GPP is driven by the multiplicative interaction between leaf area index (LAI) and leaf-level CO\({}_{2}\) uptake (A\({}_{\text{leaf}}\)), with the importance of A\({}_{\text{leaf}}\) increasing with LAI. This analysis led us to conclude that a VI that is sensitive to the interacting effect of LAI and A\({}_{\text{leaf}}\) should provide better predictions of GPP than a VI that is sensitive to just one of these properties. Moreover, we found that leaf mass per area (LMA) provides a proxy for A\({}_{\text{leaf}}\), with thicker leaves showing lower rates of A\({}_{\text{leaf}}\). Finally, we found that EVI and NIR\({}_{\text{v}}\) are strongly sensitive to LMA and LAI, whereas NDVI is most strongly controlled by LAI over a range of realistic conditions. Hence, we conclude that the comparatively tight relationships between GPP, EVI, and NIR\({}_{\text{v}}\) are driven by the multiplicative sensitivity of these two VIs to LAI and LMA, which largely mimics the response of GPP to LAI and A\({}_{\text{leaf}}\). NDVI is comparatively less sensitive to a leaf trait that is tied to leaf photosynthesis, which explains its weaker correlation with GPP. Our analysis provides a roadmap for more mechanistically linking VIs, ecosystem biophysical properties, and GPP, and implies that VIs that better mimic GPP's sensitivity to LAI and A\({}_{\text{leaf}}\) may provide more general predictions of canopy photosynthesis. The following are available online at [[http://www.mdpi.com/2072-4292/12/9/1405/s1](http://www.mdpi.com/2072-4292/12/9/1405/s1)]([http://www.mdpi.com/2072-4292/12/9/1405/s1](http://www.mdpi.com/2072-4292/12/9/1405/s1)), Supplementary Material 1: Location of MODIS pixels used for each site; Supplementary Material 2: Plant traits at the study sites; Supplementary Material 3: Details about the relationships between vegetation indices and gross primary production; Supplementary Material 4: Analysis with [PERSON] (2002) and PROSAIL models; Supplementary Material 5: Leaf chlorophyll data from plant species from Southern California and Sierra Nevada; Supplementary Material 6: Correlations between vegetation indices: results from PROSAIL; Supplementary Material 7: Compilation of field measurements of mean canopy leaf angles. Conceptualization, C.H.-H. and M.L.G. ; Formal analysis, C.H.-H. and M.L.G. ; Investigation, C.H.-H.; Methodology, C.H.-H. and M.L.G. ; Resources, M.L.G. ; Supervision, M.L.G. ; Validation, C.H.-H.; Visualization, C.H.-H.; Writing--original draft, C.H.-H. and M.L.G. ; Writing--review & editing, C.H.-H. and M.L.G. All authors have read and agreed to the published version of the manuscript. The research was funded by a UC MEXUS-CONACYT Postdoctoral Fellowship to CHH and NASA grant NNX15 AU16A to MLG. This manuscript was improved by inputs from three anonymous reviewers. We thank [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] for helping with the field and laboratory measurements. The fieldwork was conducted at the University of California Natural Reserve System's Boyd Deep Canyon Desert Research Center and James San Jacinto Mountain Reserve; at the Southern Sierra Critical Zone Observatory sites in the Sierra National Forest, and at field sites managed by the Irvine Ranch Conservancy. We made use of data from the TRY initiative on plant traits ([[http://www.try-db.org](http://www.try-db.org)]([http://www.try-db.org](http://www.try-db.org))). The TRY initiative and database is hosted, developed and maintained by [PERSON] and [PERSON] (Max Planck Institute for Biogeochemistry, Jena, Germany).