Mary C. Scholes

Fuel biomass and combustion factors associated with fires in savanna ecosystems of South Africa and Zambia

Fires are dominant factors in shaping the structure and composition of vegetation in African savanna ecosystems. Emissions such as CO 2 , NO x , CH 4 , and other compounds originating from these fires are suspected to contribute substantially to changes in global biogeochemical processes. Limited quantitative data exist detailing characteristics of biomass, burning conditions, and the postfire environment in African savannas. Fourteen test sites, differentiated by distinct burn frequency histories and land- use patterns, were established and burned during August and September 1992 in savanna parklands of South Africa and savanna woodlands of Zambia. Vegetation physiognomy, available fuel loads, the levels of biomass consumed by fire, environmental conditions, and fire behavior are described. In the South African sites, total aboveground fuel loads ranged from 2218 to 5492 kg ha -1 where fire return intervals were 1-4 years and exceeded 7000 kg ha -1 at a site subjected to 38 years of fire exclusion. However, fireline intensity was only 1419 kW m -1 at the fire exclusion site, while ranging from 480 to 6130 kW m -1 among the frequent fire sites. In Zambia, total aboveground fuel loads ranged from 3164 kg ha -1 in a hydromorphic grassland to 7343 kg ha -1 in a fallow shifting cultivation site. Dormant grass and litter constituted 70-98% of the total fuel load among all sites. Although downed woody debris was a relatively minor fuel component at most sites, it constituted 43-57% of the total fuel load in the fire exclusion and shifting cultivation sites. Fire line intensity ranged between 1734 and 4061 kW m -1 among all Zambian sites. Mean grass consumption generally exceeded 95%, while downed woody debris consumption ranged from 3 to 73% at all sites. In tropical savannas and savanna woodlands of southern Africa, differences in environmental conditions, land- use patterns, and fire regimes influence vegetation characteristics and thus influence fire behavior and biomass consumption.

NO and N2O emissions from savanna soils following the first simulated rains of the season

Data on the emissions of oxides of nitrogen from the soil during the early part of the wet season are reported for nutrient-rich and nutrient-poor sandy soils at Nylsvley, South Africa. The emissions of NOx and N2O following the first wetting event of the season are elevated relative to subsequent events. The observed high emission rates (76 ng N-NO m-2 s-1) are partially attributed to the sandiness of the soil, which permits NO to diffuse out of the soil rapidly. The pulse of high emissions following wetting is maintained for approximately 72 hours, thereafter continuing at around 20 ng NO m-2 s-1 while the soil remains moist. The initial pulse is suggested to be due to the accumulation of a substrate pool during the dry period, coupled with an inability of plants and microbes to use it effectively during the first few days after wetting. There were no significant differences in the peak or subsequent emission rates for either NO or N2O between two sites of differing nitrogen mineralisation potentials. N2O emissions averaged 8% of NOx emissions. The enhanced emissions of NOx which follow the first wetting after a prolonged dry period do not make a very large contribution to the annual gaseous N emission budget, but could be a significant contributor to the high tropospheric ozone levels observed over southern Africa in springtime.

Biogenic and pyrogenic emissions from Africa and their impact on the global atmosphere.

Abstract Tropical regions, with their high biological activity, have the potential to emit large amounts of trace gases and aerosols to the atmosphere. This can take the form of trace gas fluxes from soils and vegetation, where gaseous species are produced and consumed by living organisms, or of smoke emissions from vegetation fires. In the last decade, considerable scientific effort has gone into quantifying these fluxes from the African continent. We find that both biogenic and pyrogenic emissions have a powerful impact on regional and global atmospheric chemistry, particularly on photooxidation processes and tropospheric ozone. The emissions of radiatively active gases and aerosols from the African continent are likely to have a significant climatic effect, but presently available data are not sufficient for reliable quantitative estimates of this effect.

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.

Biogenic soil emissions of nitric oxide (NO) and nitrous oxide (N2O) from savannas in South Africa: The impact of wetting and burning

In this paper we report on the first measurements of microbial soil emissions of nitric oxide (NO) and nitrous oxide (N2O) from the savannas in South Africa. In addition to natural, unperturbed emission measurements, we investigated the impact of natural rainfall, artificial irrigation, and fire on these emissions. Wetting and burning resulted in a significant enhancement in the emissions of NO. Mean background NO emissions from the dry sites ranged from 0.4 to 6.2 ng N m−2 s−1 and from the wetted sites ranged from 4.7 to 34.0 ng N m−2 s−1. After burning, the mean NO emissions from the dry sites increased and ranged from 13.3 to 15.2 ng N m−2 s−1 and from the wetted sites increased, exceeding 60 ng N m−2 s−1. Measurements of biogenic emissions of N2O were attempted, but emissions were not detected throughout the measurement period, indicating emissions below the minimum delectability of the instrumentation (2 ng N m−2 s−1).

Biogenic NO emissions from savanna soils as a function of fire regime, soil type, soil nitrogen, and water status

A study of NOx emissions from soils representative of nutrient-poor and nutrient-rich savannas and their response to burning and soil water content was carried out in the southern Kruger National Park, South Africa. The study spanned the end of the dry season and the beginning of the wet season (September–December 1992). Nitrogen mineralization rates were measured using an in situ technique simultaneously with measurements of NOx emissions. NOx emissions were almost entirely as NO. The relationship between NO emission rate and soil moisture was parabolic regardless of soil type and management practice, with the lowest NO emission rates being measured at low ( 0.542) water-filled pore space values. The initial increase in NO emission rates with increasing soil moisture are paralleled by increases in the nitrate concentration in the soil. The highest NO emission rates (20 ng N-NO m−2 s−1—excluding the brief initial peak) were measured on plots from which fire had been excluded for 35 years. The next highest rates (8 ng N-NO m−2 s−1) were measured on the more fertile soils. Infertile soils, burned every second year, had rates of 3.5 ng N-NO m−2 s−1. The NO emission rates show a positive correlation with soil total N content and N nitrification rate. The effect of excluding fire from a savanna is to increase the soil nitrogen content through increased litter inputs, which in turn increases nitrification rates and soil NO emissions.

Effects of moisture and burning on soil-atmosphere exchange of trace carbon gases in a southern African savanna

Soil fluxes of carbon dioxide (CO2), methane (CH4), and carbon monoxide (CO) were measured during a period of extreme drought at semi-arid savanna sites located in the Kruger National Park (KNP), South Africa, as part of the SAFARI-92 experiments (Sept., 1992). Soil respiration in this savanna was little affected by burning, but was strongly stimulated by addition of moisture. Mean soil respiration from the dry soil was 0.4 g C m−2 d−1 in open savanna plots that had been burned biennially and 0.5 g C m−2 d−1 in woody savanna plots. A light natural rain (about 0.6 mm) increased the CO2 flux in the open savanna sites by 5-fold but the effect was short-lived. A simulated heavy rain (25 mm of added distilled water) increased CO2 fluxes by over an order of magnitude in both burned and control sites and the emissions remained over 5 times pre-wetting values during a week of drying. Over 65% of our measurements indicated no significant soil-atmosphere methane exchange; most of the few non-zero measurements indicated a small (<1 mg CH4-C m−2 d−1) flux of methane to the atmosphere. Soil-atmosphere CH4 exchange was not significantly affected by either burning the grass layer or by the addition of distilled water to the soil. The net soil CO fluxes, which generally increased with increasing soil temperature, were positive up to 356 × 109 molecules cm−2 s−1 with an average of 8.8 × 1010 molecules cm−2 s−1 for the untreated open savanna plots. After burning, the fluxes rose by over an order of magnitude but dropped back to preburn levels within a few days. Observed CO fluxes were higher than those previously reported for southern Africa savannas during non-drought conditions. Added moisture had little effect on CO fluxes during the 3-week period of SAFARI-92.

Nitric oxide emissions from a southern African savanna

NO fluxes from soils of a periodically flooded tropical savanna in southern Africa were investigated and modeled. In the laboratory, NO production rates, NO consumption rate constants, NO mixing ratios, relationships between NO emissions and soil temperature and moisture were determined for nutrient-poor, nutrient-rich savanna soils and a floodplain soil. The NO production rate and consumption rate constants of the floodplain soil (1.96 ng N s−1 per kilogram of soil and 2.04×10−5 m3 s−1 per kilogram of soil, respectively) were significantly higher than those of the savanna soils (average of 1.28 ng N s−1 per kilogram of soil and 1.47×10−5 m3 s−1 per kilogram of soil, respectively), but there were no significant difference between the nutrient-rich and nutrient-poor soils. NO flux rates increased exponentially with soil temperature. NO flux rates increased with soil moisture reaching a maximum near the field capacity (7.5–10% and 31.2% gravimetric water content for savanna and floodplain soils, respectively), after which the NO flux rate declined. These laboratory data were used in a model to estimate field NO flux rates, which were compared with actual field NO emission measurements. The NO model was modified to incorporate NO “pulsing” after the first rains of the season. Correlation between the modeled and field NO fluxes from the nutrient-poor savanna, nutrient-rich savanna, and the floodplain soils showed r2 values of 0.91, 0.82, and 0.74, respectively. The NO model was linked with a soil moisture and temperature model to predict annual NO emission estimates from savannas. Annual NO flux from the nutrient-poor and nutrient-rich savannas was estimated to be 0.16×10−3 and 0.14×10−3 kg N m−2 yr−1, respectively, which agree well with estimates from other savanna studies.

DNA bar-coding reveals source and patterns of Thaumastocoris peregrinus invasions in South Africa and South America

Thaumastocoris peregrinus is a recently introduced invertebrate pest of non-native Eucalyptus plantations in the Southern Hemisphere. It was first reported from South Africa in 2003 and in Argentina in 2005. Since then, populations have grown explosively and it has attained an almost ubiquitous distribution over several regions in South Africa on 26 Eucalyptus species. Here we address three key questions regarding this invasion, namely whether only one species has been introduced, whether there were single or multiple introductions into South Africa and South America and what the source of the introduction might have been. To answer these questions, bar-coding using mitochondrial DNA (COI) sequence diversity was used to characterise the populations of this insect from Australia, Argentina, Brazil, South Africa and Uruguay. Analyses revealed three cryptic species in Australia, of which only T. peregrinus is represented in South Africa and South America. Thaumastocoris peregrinus populations contained eight haplotypes, with a pairwise nucleotide distance of 0.2–0.9% from seventeen locations in Australia. Three of these haplotypes are shared with populations in South America and South Africa, but the latter regions do not share haplotypes. These data, together with the current distribution of the haplotypes and the known direction of original spread in these regions, suggest that at least three distinct introductions of the insect occurred in South Africa and South America before 2005. The two most common haplotypes in Sydney, one of which was also found in Brisbane, are shared with the non-native regions. Sydney populations of T. peregrinus, which have regularly reached outbreak levels in recent years, might thus have served as source of these three distinct introductions into other regions of the Southern Hemisphere.

Input control of organic matter dynamics

The amount and quality of inputs into soil organic matter will be altered by both climate and landuse change. The increase in growth of plants caused by increasing CO2 concentration implies not only potential increases in yields but also increases in plant residues. Simulation models using doubled CO2 levels predict global net primary productivity (NPP) to increase by 16.3%, over half of which will occur in the tropics. For tropical ecosystems increases in NPP will be dominated by the effects of elevated CO2, with water and nitrogen availability and temperature playing a less significant role. Phosphorus limitation may determine whether the potential for increased plant growth will be realized. The distribution of C3 and C4 species in the tropics could be affected by landuse change and estimates of yield increases will be dependent on their proportions. The allocation of photosynthate to the root will increase under elevated CO2, resulting in increased fine root dry weight and root length. Root sink strength and the turnover of roots and associated symbionts are critical knowledge gaps. Carbon: nitrogen ratios in tissues will increase resulting in decreased decomposition rates. The concentration of secondary compounds will be affected more by nitrogen limitations than a direct CO2 effect. Changes in lignin, tannin and polyphenol levels are more important in the decomposability of tropical litters than changes in the C : N ratios. Decomposition models will have to be altered to take into account changes in plant composition. The role of models in predicting the effects of management practice on long-term fertility is addressed.