Biogenic hydrocarbon emissions from southern African savannas

Biogenic nonmethane hydrocarbon (NMHC) emissions were investigated at two field sites in the Republic of South Africa that include five important southern African savanna landscapes. Tropical savannas are a globally important biome with a high potential for biogenic emissions but no NMHC emission measurements in these regions or in any part of Africa have been reported. Landscape average hydrocarbon emissions were estimated by characterizing plant species composition and foliar density at each site, identifying and characterizing NMHC emissions of the most abundant plant species, and identifying and characterizing NMHC emissions of plant species with the highest NMHC emission rates. A hand-held portable analyzer proved to be a useful tool for identifying plants with high emission rates. A branch enclosure system, with gas chromatography and flame ionization detector, was used to quantify isoprene and monoterpene emission rates. Emission rates were species-specific and several genera had both high and low emitters. At least some species with high emission rates were identified in most savanna types. High and low emitters were found on both nutrient-rich and nutrient-poor soils. Landscape average emission capacities for the five savanna types range from 0.6 to 9 mg C m-2 h-1 for isoprene and about 0.05 to 3 mg C m-2 h-1 for monoterpenes. The savanna emission rates predicted by existing global models are within the range estimated for these five savanna types

Assessment of ozone impacts on vegetation in southern Africa and directions for future research: Commentary

Levels of background ozone in southern Africa are high enough to cause concern, as they frequently exceed the 40 ppb threshold currently adopted by the United Nations Economic Commission for Europe. They also surpass the exposure index of 3000 ppb.h, which is intended to protect crops and natural vegetation in Europe. Natural vegetation and crops in southern Africa may be tolerant of elevated ozone concentrations because of naturally high background levels, but additional anthropogenic inputs of ozone precursors may result in exceedances of ozone damage thresholds that affect vegetation. Current impact assessment policies in Europe are shifting from an exposure approach to one based on flux. If existing European methods are to be applied in southern Africa, the flux model would be the more appropriate of the two to assess likely impacts. Besides data requirements for flux modelling, the method would need to accommodate extended growing periods, locally appropriate crops such as maize, and the frequency and extent of drought periods. In southern Africa, crop production may be more greatly affected by drought, floods, and agronomic inputs but the possible deleterious effects of elevated ozone are sufficient to merit further investigation