# Constraints to Vegetation Growth Reduced by Region-Specific Changes in Seasonal Climate
## 3 Results
### Sensitivity Experiments for Climate and Vegetation  
Increased CO\({}_{2}\) affects LAI and GPP through climate change, through both the greenhouse effect and CO\({}_{2}\) fertilization effect, but the CMIP5 ensemble for the RCP 8.5 scenario cannot separate the importance of the two processes [19]. To isolate their varying importance, we generate an eight-model ensemble (highlighted in Table 1), comparing results from the sensitivity experiments. At first, we summarized the climate responses to increasing CO\({}_{2}\) concentration in these sensitivity experiments. The esmFdbk1 experiment shows that radiative effects clearly dominate projected warming, the signature feature of climate change (compare black and red lines in Figures 7a-c and 8a,d,g). Radiative effects also drive other key aspects of climate change, such as Arctic amplification and land-sea warming contrast (Figure 8a), equatorial and high-latitude increases in precipitation (Figure 8b), and high-latitude dimming from increases in fall/winter cloud cover (Figure 8c) [34]. Meanwhile, the esmFixClim experiment revealed that, except for increases in radiation for the temperature-limited region in JJA (Figure 7g), the vegetation physiological effect has little effect on region-averaged climate (e.g., near-zero changes in Figure 7a-f). Spatially, the physiological effects  
Figure 6: Easing of seasonal constraints to vegetation growth. Same was Figure 5, except for transpiration, **a–c**; GPP, **d–f**; and LAI, **g–i**. on climate are more consistent than the radiative effects, and include a slight drying (Figure 8e) and brightening because of reduced cloud cover (Figure 8f) over most land surfaces. Figure 7: Additive and counteracting impacts of CO\({}_{2}\) on climate. Simulated changes in climate and vegetation from the CMIP5 eight-model ensemble in response to increasing CO\({}_{2}\) from 280 ppm by 1%/year for 140 years: temperature, **a–c**; precipitation, **d–f**; and radiation, **g–i**. The changes are the mean of the last ten years minus the mean of the first ten years (see Figures 8 and 10 for a spatial representation of the changes). In order to assess the contribution of radiative and vegetation physiological effects on climate and vegetation, three experiments were carried out: (1) CO\({}_{2}\) has a radiative forcing on climate but no direct effect on vegetation; (2) CO\({}_{2}\) has a vegetation physiological impact, primarily on internal CO\({}_{2}\) concentration and stomatal conductance, but does not directly alter radiative forcing; and (3) CO\({}_{2}\) has a combined effect on both radiative forcing and physiological impacts. Shading is used to highlight changes in the limiting factor for each of the three regions (e.g., blue shading highlights temperature changes in the temperature-limited region). Circles indicate that the trend in the 140-year ensemble mean is significant at the 95% level from a Mann–Kendall trend test. On the other hand, the summary of the vegetation response to increasing CO\({}_{2}\) concentration shows the opposite of the climate response in the sensitivity experiment. In spite of inducing climate changes to temperature and precipitation, the CO\({}_{2}\) fertilization effect alone can account for much, and in some cases, almost all, of the simulated changes in GPP and LAI in the combined experiment (compare blue and red lines in Figure 9d-i). Increases in LAI and GPP in the FixClim1 experiment appear to be driven by more radiation (because of reduced cloud cover) and reduced transpiration (therefore increased soil water), both of which are consistent with stomatal down-regulation following CO\({}_{2}\) increases [35]. As shown in the esmFdbk1 experiment, in contrast, the climate change induced by radiative forcing has near-zero effects on simulated GPP for all three regions (black line in Figure 9d-f), except for a small positive effect on GPP in the boreal spring of the temperature-limited region.