Air temperature optima of vegetation productivity across global biomes

U nderstanding how photosynthesis responds to warming has been a focus in plant research in recent decades, and most of the existing knowledge comes from leaf-scale measurements 1-4. Most leaf-scale temperature response curves show that photosyn-thetic capacity increases with temperature up to an optimum temperature (T opt leaf), which typically occurs in the 30-40 °C temperature range 5,6. Above this optimum temperature, foliar photosynthetic capacity sharply declines as electron-transport and Rubisco enzy-matic capacities become impaired 7. Field et al. 8 suggested that ecosystem-scale optimum temperature T opt eco may differ from T opt leaf. At the ecosystem scale, elevated air temperatures do limit canopy photosynthesis by processes other than leaf carboxylation rates. For instance, elevated air temperatures may accelerate leaf ageing and increase leaf thickness (phenology; for example, ref. 9) and control stomatal closure because a higher temperature usually comes with a higher vapour pressure deficit (VPD) 10. In a more extreme case, warming-induced water stress could suppress canopy photosyn-thesis through partial hydraulic failure (hydraulics) by cavitation (for example, ref. 11). Empirical leaf-scale photosynthesis-temperature relationships 12 have been directly incorporated into global ecosystem models, with variants to account for acclimation, that is, a temporal adjustment of optimum photosynthetic temperature to air temperature during growth 5,13,14. This direct scaling of temperature responses from leaves to ecosystems partly determines model projections of gross primary productivity (GPP) and CO 2 uptake by terrestrial ecosystems in climatic scenarios. Verifying the existence of T opt eco in real-world ecosystems, defining its spatial distribution across and within biomes, and understanding the relationships between T opt eco , prevailing air temperature and T opt leaf are important for evaluating models and understanding the impacts of various climatic warming targets on ecosystem productivity. In this study, we formulate and test the following hypotheses: (1) T opt eco is higher for biomes when air temperature during growth is warmer, (2) T opt eco is lower than T opt leaf for any given ecosystem because the limitations mentioned earlier of stomatal conductance and phenology emerge before temperature begins to impair foliar pho-tosynthetic capacity, and (3) tropical forests already operate near a high T opt eco , above which canopy photosynthesis may decrease with even moderate air temperature warming 15,16. Here, we defined T opt eco as the daytime air temperature at which GPP is highest over a period of several years, and thus T opt eco can be empirically determined from productivity observations and proxies (see Methods). Results and discussion We first applied this approach on time series of daily GPP derived from CO 2 flux measurements at 153 globally distributed eddy cova-riance sites and found that a robust estimate of T opt eco could be derived The global distribution of the optimum air temperature for ecosystem-level gross primary productivity (T opt eco) is poorly understood , despite its importance for ecosystem carbon uptake under future warming. We provide empirical evidence for the existence of such an optimum, using measurements of in situ eddy covariance and satellite-derived proxies, and report its global distribution. T opt eco is consistently lower than the physiological optimum temperature of leaf-level photosynthetic capacity, which typically exceeds 30 °C. The global average T opt eco is estimated to be 23 ± 6 °C, with warmer regions having higher T opt eco values than colder regions. In tropical forests in particular, T opt eco is close to growing-season air temperature and is projected to fall below it under all scenarios of future climate, suggesting a limited safe operating space for these ecosystems under future warming.