 The study assessed the impacts of extreme late summer drought on carbon balance in a semi-arid forest region in Arizona, finding that drought shifted annual net ecosystem production, NEP, from a carbon sink to neutral at both an undisturbed ponderosa pine forest and a former forest converted to grassland by intense burning. The study also found that carbon fluxes were particularly sensitive to drought in August, with the reduction of GPP exceeding the reduction of TER at the undisturbed site, and the reduction of TER exceeding the reduction of GPP at the burn site. These results illustrate the high vulnerability of forest carbon sink strength in the southwest U.S. to predicted increases in intense burning and precipitation variability. This article was authored by Thomas Kolb, Sabina Dore, and Mario Montes Helu.