Cassava about‐<scp>FACE</scp>: Greater than expected yield stimulation of cassava (<i><scp>M</scp>anihot esculenta</i>) by future <scp>CO</scp>₂ levels

Abstract Globally, cassava is the second most important root crop after potatoes and the fifth most important crop overall in terms of human caloric intake. In addition to its growing global importance for feed, fuel, and starch, cassava has long been vital to food security in Sub‐ S aharan A frica. Climate change is expected to have its most severe impact on crops in food insecure regions, yet little is known about how cassava productivity will respond to climate change. The most important driver of climate change is globally increasing atmospheric CO 2 concentration ([ CO 2 ]). However, the potential for cassava to enhance food security in an elevated [ CO 2 ] world is uncertain as greenhouse and open top chamber ( OTC ) study reports are ambiguous. Studies have yielded misleading results in the past regarding the effect of elevated [ CO 2 ] on crop productivity, particularly in cases where pots restricted sink growth. To resolve these conflicting results, we compare the response of cassava to growth at ambient (ca. 385 ppm) and elevated [ CO 2 ] (585 ppm) under field conditions and fully open air [ CO 2 ] elevation. After three and half months of growth at elevated [ CO 2 ], above ground biomass was 30% greater and cassava root tuber dry mass increased over 100% (fresh weight increased 89%). High photosynthetic rates and photosynthetic stimulation by elevated [ CO 2 ], larger canopies, and a large sink capacity all contributed to cassava's growth and yield stimulation. Cassava exhibited photosynthetic acclimation via decreased Rubisco capacity early in the season prior to root tuber initiation when sink capacity was smaller. Importantly, and in contrast to a greenhouse study, we found no evidence of increased leaf N or total cyanide concentration in elevated [ CO 2 ]. All of our results are consistent with theoretical expectations; however, the magnitude of the yield increase reported here surpasses all other C 3 crops and thus exceeds expectations.