Air temperature optima of vegetation productivity across global biomes

Abstract

The global distribution of the optimum air temperature for ecosystem-level gross primary productivity ([Formula: see text]) 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. [Formula: see text] is consistently lower than the physiological optimum temperature of leaf-level photosynthetic capacity, which typically exceeds 30 °C. The global average [Formula: see text] is estimated to be 23 ± 6 °C, with warmer regions having higher [Formula: see text] values than colder regions. In tropical forests in particular, [Formula: see text] 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.

Fig. 1 |. Distribution of ecosystem-scale optimal temperature (Topteco) for vegetation productivity derived from flux tower sites and satellite-based data for near-infrared reflectance of vegetation (NIR_V).

a, Relationship between mean annual daily maximum air temperature during the growing season $\left(T_{max gs}^{air}\right)$ and $T_{opt}^{eco}$ derived from daily measurements of photosynthesis across eddy-covariance sites. Flux-derived $T_{max gs}^{air}$ and $T_{opt}^{eco}$ were both obtained using observations from flux towers. Error bars indicate ±SD. The dotted gray line represents y=x and the dot line in red is y=0.61x+10.65, which is derived by linear regression with the statistical significance of the slope, or its p-value, given by Student’s t test. b, Relationship between $T_{opt}^{eco}$ derived from flux data and $T_{opt}^{eco}$ derived from NIR_V data. For each site, we extracted and averaged $T_{opt}^{eco}$ values within a 3×3 pixel window around the site from NIR_V-derived $T_{opt}^{eco}$ map, and calculated the SD of the nine $T_{opt}^{eco}$ values within the window. Error bars indicate ±SD. The dotted gray line represents y=x and the dot line in red is y=0.74x+7.10, which is derived by linear regression with the statistical significance of the slope, or its p-value, given by Student’s t test. c, Spatial distribution of $T_{opt}^{eco}$ for vegetation productivity (left panel), and $T_{opt}^{eco}$ averaged by latitude (right panel). $T_{opt}^{eco}$ is determined using NIR_V data calculated based on satellite observations from Moderate Resolution Imaging Spectroradiometer (MODIS). Note that only gridded pixels with annual mean NDVI value larger than 0.1 and detectable $T_{opt}^{eco}$ are shown here. Areas of tropical forests based on current vegetation distribution are indicated by hatching. The circles on the map are colored according to the local value of $T_{opt}^{eco}$ retrieved from GPP at the location of each flux site. The solid line and shaded area in the right panel indicate the mean and SD, respectively, of $T_{opt}^{eco}$ summarized by latitude. d, $T_{opt}^{eco}$ in the climate space (left panel) and the temperature sensitivity of $T_{opt}^{eco}$ along the precipitation gradient (right panel). Each climate bin is defined by 1-°C intervals of $T_{max gs}^{air}$ and 100-mm intervals of mean annual precipitation, based on current climate conditions averaged over 2001–2013. The solid line in the right panel represents the temperature sensitivity of $T_{opt}^{eco}$ along the precipitation gradient, calculated as the slope of the linear regression between $T_{opt}^{eco}$ and $T_{max gs}^{air}$ for a given precipitation level. The shaded area indicates the SD of temperature sensitivity of $T_{opt}^{eco}$ estimated by bootstrapping.

Fig. 2 |. Relationship between mean annual daily maximum air temperature during the growing season (Tmax gsair) and ecosystem-scale optimum temperature for vegetation productivity (Topteco) across vegetation types.

The error bars indicate the SDs of $T_{opt}^{eco}/T_{max gs}^{air}$ for each vegetation type: ENF, evergreen needle-leaved forest; EBF, evergreen broad-leaved forest; DNF, deciduous needle-leaved forest; DBF, deciduous broad-leaved forest; MF, mixed forest; Shrub, closed and open shrublands. The light-gray dotted line represents y=x. The dark-gray dotted line is y=0.76x+6.48 derived by linear regression with the slope value (estimated using Student’s t test) shown in the bottom right. The red dotted line is the flux tower derived slope (0.61) from Fig. 1a. The size of each symbol corresponds to the three categories (< 3%, 3%−10% and > 10%) of occupied vegetated area on land. Error bars indicate ±SD.

Fig. 3 |. Change with latitude in ecosystem-scale optimal temperature for vegetation productivity (Topteco) and daily maximum air temperature averaged over the growing season (Tmax gsair).

a, Current $T_{opt}^{eco}$ versus current $T_{max gs}^{air}$; b, Future $T_{opt}^{eco}$ versus future $T_{max gs}^{air}$. Current $T_{opt}^{eco}$ and $T_{max gs}^{air}$ are calculated using current temperature for 2001–2013, whereas acclimated $T_{opt}^{eco}$ and future $T_{max gs}^{air}$ are first calculated pixel by pixel using temperature for 2091–2100 projected by General Circulation Models (GCMs) under the RCP4.5 scenario and then averaged by latitude. Acclimated $T_{opt}^{eco}$ is determined based on the projected temperature and temperature sensitivity of $T_{opt}^{eco}$ using the annual precipitation level predicted for 2091–2100. The solid line and shaded area in each panel indicate the mean and SD, respectively, of $T_{opt}^{eco}$ or $T_{max gs}^{air}$ summarized by latitude. c, Future $T_{opt}^{eco}$ versus future $T_{max gs}^{air}$ for tropical evergreen forests. ** indicates that $T_{opt}^{eco}$ is significantly lower than $T_{max gs}^{air}$ at P<0.01 in a paired t-test. Error bars indicate ±SD.