CO2 enrichment does not entirely ameliorate Vachellia karroo drought inhibition: A missing mechanism explaining savanna bush encroachment

Abstract

There is theoretical, empirical and modeled evidence that atmospheric enrichment of carbon dioxide (CO2) is driving the conversion of open savannas to woodlands (i.e., woody thickening or bush encroachment). This study measured the impacts of available CO2 and water limitation on seedling growth and physiology to determine how these two environmental conditions may be influencing savanna seedling establishment. We hypothesized that a model savanna tree, Vachellia karroo, would experience positive growth responses to increasing CO2 and that CO2 enrichment would partially offset slowed growth and physiological inhibition imposed by drought. To test this hypothesis, we grew V. karroo in open-top chambers at the Kirstenbosch Research Center (Cape Town, South Africa). The seedlings were divided equally into four CO2 treatments: 180 ppm, 270 ppm, 400 ppm, and 800 ppm. Seedlings were grown in their CO2 treatments for 35 days before drought was initiated. The drought was initiated by removing the irrigation from half of the pots. Leaf gas-exchange, stem water potential, leaf non-structural carbohydrate concentrations, and plant morphology were measured at drought day-30 and drought day-60. The results showed significant benefits from CO2 enrichment to aboveground plant structures, including larger leaves and stems, when water was abundant (p < 0.05). CO2-related increases to photosynthetic parameters, such as instantaneous water-use efficiency, were significantly diminished by water limitation (p < 0.05). Our results present the first experimental evidence of the relationship between the two driving factors structuring the large-scale spatial patterns of savanna vegetation productivity and cover. These results suggest that atmospheric enrichment of CO2 will significantly affect aspects of plant growth and physiology, yet prolonged drought will remain an overarching determinant of savanna plant community dynamics.