## 2 Materials and Methods
### Analysis of the Impact of SLA Variation on Gross Primary Productivity  
We conducted six model experiments to examine the impacts of SLA variation on gross primary productivity at the QYZ (temperate NET), CBS (temperate BDT), DHS (temperate BET), and HBG (C3 grassland) sites. Different SLA values were used in the six experiments. The default SLA values in the CLM4.5 model (\(p_{0}\)) were used in experiment S1. In experiment S2, we used the mean value of the observed SLA data collected in this study for each PPT (\(p_{obs}\)). In experiments S3 and S4, we added the observed SLA variation to the mean SLA value used in experiment S2 (i.e., \(p_{obs}-SD_{obs}\) and \(p_{obs}+SD_{obs}\), respectively). In experiments S5 and S6, the variation in default SLA values across the models was added to the mean default SLA values (\(p_{mod}\)) of the seven terrestrial biosphere models (i.e., \(p_{mod}-SD_{mod}\) and \(p_{mod}+SD_{mod}\), respectively).  
For each model experiment, we first ran the CLM4.5 model to obtain the initial values of the state variables under equilibria using a two-stage spin-up approach. The first stage of spin-up followed the accelerated decomposition for 600 years; then, a normal decomposition was implemented for 200 years, with a repeating cycle of 6 years (2003-2008) with dynamic meteorological forcing and constant land use types, CO\({}_{2}\) concentration (284 ppmv), and N deposition (0.5 g N m\({}^{-2}\) yr\({}^{-1}\)) at the pre-industrial level. A transient run was operated from 1850 to 2008 after reaching equilibrium. The CO2 concentration and N deposition data for the four sites were downloaded from the global dataset described in [PERSON] et al. (2015).  
The relative changes in gross primary productivity (GPP), RuBP-limited photosynthesis rate (Ac), and leaf area index (LAI) in model experiments S3 and S4 were compared with the results in experiment S2 (Equation (7)). The relative changes in GPP, Ac, and LAI in model experiments S4 and S5 were compared with the results in experiment S6 (Equation (8)).  
\[\rm R1=MAX(\frac{\left|M_{S_{3}}-M_{S_{2}}\right|}{M_{S_{2}}},\frac{\left|M_{S _{4}}-M_{S_{2}}\right|}{M_{S_{2}}})\times 100\% \tag{7}\]  
\[\rm R2=MAX(\frac{\left|M_{S_{5}}-M_{S_{1}}\right|}{M_{S_{1}}},\frac{\left|M_{S _{6}}-M_{S_{1}}\right|}{M_{S_{1}}})\times 100\%\] (in temperate NET. By contrast, the CLM4.5 model underestimated the SLA values by 0.002 m\({}^{2}\)/g in tropical BET and temperate BET and by 0.005 m\({}^{2}\)/g in temperate BES.  
We also evaluated the vertical gradients in simulated SLA varying with light from the bottom to top of the canopy for temperate BDT, temperate NET, and temperate BET in CLM4.5 (Figure 3). Compared with the observations, the CLM4.5 model overvalued canopy gradients of SLA in temperate BDT (Figure 3a) and temperate NET (Figure 3b), mainly because of the overestimation of SLA values at the top canopy. Specifically, the decrease of 0.016 m\({}^{2}\)/g in simulated SLA in temperate BDT was larger than the mean reduction of 0.014 m\({}^{2}\)/g observed for _Phellodend amurense_ [PERSON], _Frazius mandschurica_ [PERSON], and _Juglans mandshurica_ [PERSON]. when light intensity increased from 15% to 100%. For temperate NET, the simulated SLA decreased by 0.005 m\({}^{2}\)/g as light intensity increased from 15% to 100%, which was the same as the observed change in _Picea asperata_ [PERSON], but larger than the slight reduction of 0.002 m\({}^{2}\)/g for _Pinus koraiensis_ [PERSON] & Zucc. In addition, the simulated vertical gradients in SLA in temperate BET were within the range of SLA values observed for the three species (Figure 3c). However, the response of simulated SLA value was underestimated by 0.006 m\({}^{2}\)/g compared with the observed variation for _Ficus titoria_ [PERSON] and _Serianthes nelsonii_ [PERSON]. when light intensity varied from 0 to 100%. Moreover, the CLM4.5 model ignored the interspecific diversity of plants in the response of SLA to light gradients, especially for the temperate BDT (Figure 3a) and BET (Figure 3c).