Belowground carbon varied with aboveground carbon along an elevation gradient at Southern Sierra Critical Zone Observatory in California

Terrestrial carbon storages are likely to change in the future as the frequency and severity of drought events and fire disturbances continue to increase in response to continued global warming. To understand how current climate controls below- and above-ground carbon storage, we synthesized existing measurements in four sites at Southern Sierra Critical Zone Observatory in California, USA. The four sites are located along an altitudinal gradient along the western slopes of the Sierra Nevada: oak savannah at 405 m elevation (San Joaquin Experimental Range), pine-oak forest at 1160 m (Soaproot Saddle), mixed-conifer forest at 2015 m (Providence Creek) and lodgepole pine forest at 2700 m (Shorthair Creek). Belowground carbon measurements included the whole soil and saprock profile. Aboveground measurements included the standing stock and production of living biomass from field monitoring and fluxes from eddy covariance. We found for both soils and saprock, carbon pools increased with higher elevations: from 4.5 kg m^-2 and 0.4 kg m^-2 in the oak savannah to 8.5 kg m^-2 and 3.2 kg m^-2 in the pine-oak forest, and to 19.4 kg m^-2 and 8.0 kg m^-2 in the mixed-conifer forests, respectively. The lodgepole pine forest had a soil carbon pool of 10.7 kg m^-2 but no saprock. The same pattern was found for aboveground carbon, where aboveground total aboveground biomass and net primary production increased from 405 to 2015 m elevation. Measured evapotranspiration also followed this pattern, even though mean annual temperature decreased and mean annual precipitation increased with elevation. With water limited at 405 m, and energy limited at 2700 m, the interaction of temperature and precipitation regulates both below and aboveground carbon storages.