## 3 Results
### Plant Traits Influencing GPP  
Our global sensitivity analysis of [PERSON]'s GPP model showed that LAI (m\({}^{2}\) leaves m\({}^{-2}\) ground) and leaf CO\({}_{2}\) uptake (A\({}_{\text{leaf}}\); the CO\({}_{2}\) uptake m\({}^{-2}\) leaf) are the main determinants of short-term GPP (CO\({}_{2}\) uptake m\({}^{-2}\) ground) during well-illuminated, unstressed conditions (Figure 3a). Both aspects have a similar contribution to GPP (LAI accounts for 48%, and A\({}_{\text{leaf}}\) accounts for 42%, of overall GPP variation). All of the other attributes, including mean leaf angle, have a relatively minor contribution. GPP modeled with these two traits closely matched the observed variation of GPP among sites (Supplementary Material 4).  
We further explored how LAI and A\({}_{\text{leaf}}\) influence GPP (Figure 3b). GPP increases with increasing LAI and the fraction of light intercepted by leaves before saturating and reaching 80% of its maximum response around 2.5-3 m\({}^{2}\) m\({}^{-2}\). Conversely, increasing A\({}_{\text{leaf}}\) caused a comparatively steady increase  
Figure 2: Relationship between vegetation indices and gross primary production (GPP) across the 10 study sites based on eddy covariance and Moderate resolution Imaging Spectrometer (MODIS) MCD43A4.006 observations. Subplots show the relationship of GPP with (**a**) NDVI, (**b**) EVI and (**c**) NIR\({}_{\text{v}}\). Colored lines indicate linear regressions for each site (temporal or within-site relationships), the dashed line represents the linear regression across all sites. The dots are a representative sample of individual observations at the coastal sage shrubland site.  
in GPP, with only minor saturation. LAI and A\({}_{\text{leaf}}\) have a multiplicative effect over GPP, such that the sensitivity of GPP to A\({}_{\text{leaf}}\) increases with LAI.  
Our analysis of leaf traits at the study sites showed that light-saturated leaf photosynthesis (A\({}_{\text{leaf}}\)) was negatively correlated to LMA (log-log relationship, r\({}^{2}\) = 0.32, p = 0.002), with relatively thick, high LMA leaves showing a lower A\({}_{\text{leaf}}\). A similar pattern occurred across sites, where community weighted A\({}_{\text{leaf}}\) decreased with increasing LMA (Figure 4). Previous analyses have also reported a correlation between LMA and A\({}_{\text{leaf}}\)[56; 57]. The relationship between LMA and leaf photosynthesis reflects a series of tradeoffs and convergent evolutionary strategies along the leaf economics spectrum: thicker leaves are longer lived, with lower nutrient content and slower rates of photosynthesis [31; 32].  
Figure 4: Relationship between leaf mass per area and light-saturated leaf photosynthesis observed at the field sites, and also obtained from the TRY database for species that are found at the Sierra Nevada sites. Crosses indicate the mean values for each of the 4-7 most dominant species at each site. The solid line is the linear regression across all species values. Filled circles indicate the community-weighted mean for each site, estimated by weighting mean species values by species abundance. See Supplementary Material 2 for more details about species and site data.  
Figure 3: (**a**) Global sensitivity analyses of [PERSON]’s model for Gross Primary Production (GPP) and (**b**) modeling analysis of the effect of Leaf Area Index (LAI) and light-saturated leaf photosynthesis rate (A\({}_{\text{leaf}}\)) on GPP. We ran the Thornley GPP model varying LAI from 0–5 m\({}^{2}\) m\({}^{-2}\) and A\({}_{\text{leaf}}\) across the range found at the field sites (see Figure 4) at an irradiance of 2000 μmol m\({}^{-2}\) s\({}^{-1}\) of photosynthetically active radiation (a typical midday illumination).