Abstract
• Determining whether the terrestrial biosphere will be a source or sink of carbon (C) under
a future climate of elevated CO2 (eCO2) and warming requires accurate quantification of
gross primary production (GPP), the largest flux of C in the global C cycle.
• We evaluated six years (2007-2012) of flux-derived GPP data from the Prairie Heating
and CO2 Enrichment (PHACE) experiment, situated in a mixed prairie grassland in
Wyoming (USA). The GPP data were fitted to a mixed effects model that extended a
light response model to include the effects of environmental (soil water content,
vegetation greenness, nitrogen) and meteorological data (air temperature, vapor pressure
deficit, photosynthetically active radiation) at current and past times.
• The stimulation of the cumulative six-year GPP by warming (20%, P=0.06) and eCO2
(19%, P=0.14) were primarily driven by enhanced C uptake during spring (96%,
P=0.003) and fall (115%, P=0.001), respectively. These enhancements were consistent
across each year, suggesting mechanisms for extending the growing season.
• Vapor pressure deficit from 1-3 days prior was the most significant predictor of temporalvariability in GPP and for explaining treatment differences in GPP, suggesting that atmospheric drought plays an important role for predicting GPP now and under future climate conditions.
