Soil carbon and nitrogen in a Great Basin pinyon–juniper woodland: Influence of vegetation,burning, and time

Much of the Great Basin, U.S. is currently dominated by sagebrush (Artemisia tridentate ssp. (Rydb.) Boivin) ecosystems. At intermediate elevations, sagebrush ecosystems are increasingly influenced by pinyon (Pinus monophylla Torr. & Frém.) and juniper (Juniperus osteosperma Torr.) expansion. Some scientists and policy makers believe that increasing woodland cover in the intermountain western US will create new carbon storage on the landscape; however, little is currently known about the distribution of carbon on these landscapes. This is especially true of below ground pools. Our objectives were to quantify the spatial distribution of soil carbon in expansion woodlands, and to determine prescribed fire's effect on soil C and N. We looked at two treatments (control and burn), three microsites (undertree, undershrub, and interspace), and four soil depths (0–8, 8–23, 23–38, and 38–52 cm). The study was conducted over a six year period with one year pre-fire and five years post-fire data. Results for both carbon and nitrogen were similar, indicating the close relationship between the two elements in this ecosystem. Undershrub microsites had higher soil C and N concentrations than interspace and undertree microsites; however, under tree microsites had higher C:N ratio than interspace and undershrub microsites. Carbon and nitrogen concentration tended to decrease with increasing depth at both control and burn sites. Prescribed burning caused immediate increases in surface soil C and N concentration, but over intermediate to longer periods of time no statistically detectable change in soil C or N content occurred from burning.     

Fallout radionuclide tracers identify a switch in sediment sources and transport-limited sediment yield following wildfire in a eucalypt forest

Fire can alter sediment sources and transport rates in river basins, changing landforms and aquatic habitats and degrading downstream water quality. Variability in the response between environments, between fires, and with time since fire makes predicting the catchment-scale effect of individual fires difficult. This study applies the fallout radionuclides ¹³⁷Cs and ²¹⁰Pb_xs to trace the sources and transport of fine sediment through a river network following a wildfire of moderate to extreme severity in the 629-km² eucalypt-forested Nattai River water-supply catchment near Sydney, Australia. The tracer analysis showed that post-fire erosion caused a switch in fine (< 10 µm) sediment sources from 80% subsoil derived from gully and river bank erosion to 86% topsoil derived from hillslope surface erosion. The fine sediment phosphorus content increased 4–10 fold over pre-fire levels. Annual post-fire sediment yields estimated from suspended solids rating curves were 109–250 times higher than they would have been without fire. A large additional amount of sediment remained stored within the river network for at least four years, particularly in lower-gradient reaches. Analysis of a sediment core showed that surface erosion following a previous fire had supplied at least 29% of total catchment sediment yield over the past 36 years. It is concluded that wildfire can alter catchment sediment budgets in two ways. Firstly, a spatially-diffuse pulse of elevated erosion is associated with moderate or intense rainfall events in post-fire years. Secondly, pulses of elevated catchment sediment yield are driven by the timing and river sediment transport capacity of runoff events. Severe post-fire erosion and high interannual hydrologic variability can result in large sediment stores persisting within the river network for many years. Fallout radionuclide tracers are shown to be useful in quantifying fine sediment sources and transport dynamics following wildfire, and the contribution of wildfire to catchment sediment yield.     

Exploring the relationships between post-fire vegetation regeneration dynamics,topography and burn severity: A case study from the Montane Cordillera Ecozones of Western Canada

In this study the relationships between vegetation regeneration dynamics to topography and burn severity for a Canadian landscape were investigated using freely available Earth Observation (EO) imagery from Landsat TM sensor. The Okanagan Mountain Park, located in the Montane Cordillera Ecozones of Western Canada at which a fire occurred in 2003, was used as a case study. First, vegetation regeneration dynamics were quantified for a period of 8 years following the fire event based on a chronosequence analysis of the Normalized Difference Vegetation Index (NDVI) and the Regeneration Index (RI). The spatio-temporal patterns of post-fire NDVI from each image date were statistically compared to the pre-fire pattern to determine the extent to which the pre-fire spatial pattern was re-established and also the rate of recovery. Subsequently, the relationships of vegetation regrowth to both topography and burn severity was quantified using a series of additional statistical metrics. Burn severity was derived from the differenced Normalized Burn Ratio (dNBR) index computed from the Landsat TM images. Information on topography properties of the region was obtained from the ASTER global operational product.NDVI and RI analysis indicated a moderate vegetation recovery to pre-fire patterns, with regeneration to over 60% of the pre-fire levels 8-years after the fire. Regression analysis of pre- and post-fire mean NDVI exhibited significant re-growth in the first 3 years after the fire with a more gradual return in later years (an increase of 0.400 in R² by 2006 compared to only an increase of 0.129 for the subsequent 5 years). Re-growth rates appeared to be somewhat higher in north-facing slopes in comparison to south facing ones. As expected, NDVI decline due to fire was positively correlated with burn severity class, whereas negative correlation was found between damage and regeneration ability (recovery after 3 years = low severity 64%/high severity 58%, recovery after 8 years = low severity 72%/high severity 70%). To our knowledge, this study is one of the few attempting to explore the interrelationships of post-fire vegetation regrowth, topography and burn severity, especially in the case of a single large fire. RI based on control plots provides a valuable tool to quantify fire impact and subsequent vegetation regrowth. Furthermore, indication of burn severity is useful for strategically rehabilitating areas of slow or unsuitable post-fire vegetation recovery. This study corroborates the significance of EO technology as a successful and cost-effective solution in providing information related to economic and environmental post-fire regeneration assessment.     

Seedbank responses to spring and fall prescribed fire in mountain big sagebrush ecosystems of differing ecological condition at Lava Beds National Monument,California

Land use change in the American West has altered sagebrush dominated ecosystems, facilitating exotic grass invasion, increased woody species, altered fire regimes, and native species losses. Restoring fire regimes may be integral to the restoration of sagebrush ecosystems. However, little is known about how the timing of prescribed fire differentially impacts the soil seedbank. Further, sites of different ecological condition may variably respond to fire. To address these uncertainties, we quantified germination of invasive cheatgrass and plant functional groups (native dicots, exotic dicots, native bunchgrasses, and native shrubs) following spring and fall prescribed burns in three mountain big sagebrush communities at Lava Beds National Monument, California. At cheatgrass and native-dominated sites, there were fewer cheatgrass seedlings (91% and 86%, respectively) immediately following spring fire than in unburned controls, but this reduction did not persist one year later. Fall burns did not significantly impact cheatgrass or exotic dicot germination. At a native dominated site, native dicots increased one year following spring (1620%) and fall burns (67%), suggesting adaptation to a natural fire regime. This research shows that the prefire condition of an ecosystem greatly influences the postfire germination response, and that fire-adapted ecosystems can benefit from the natural disturbance regime.     

Occurrence of soil erosion after repeated experimental fires in a Mediterranean environment

In the Mediterranean area, forest fires have become a first-order environmental problem. Increased fire frequency progressively reduces ecosystem recovery periods. The fire season, usually followed by torrential rains in autumn, intensifies erosion processes and increases desertification risk. In this work, the effect of repeated experimental fires on soil response to water erosion is studied in the Permanent Field Station of La Concordia, Valencia, Spain. In nine 80 m² plots (20 m long × 4 m wide), all runoff and sediment produced were measured after each rainfall event. In 1995, two fire treatments with the addition of different biomass amounts were applied. Three plots were burned with high fire intensity, three with moderate intensity, and three were unburned to be used as control. In 2003, the plots with the fire treatments were burned again with low fire intensities. During the 8-year interval between fires, plots remained undisturbed, allowing regeneration of the vegetation–soil system. Results obtained during the first 5 months after both fire experiments show the high vulnerability of the soil to erosion after a repeated fire. For the burned plots, runoff rates increased three times more than those of 1995, and soil losses increased almost twice. The highest sediment yield (514 g m^− 2) was measured in 2003, in the plots of the moderate fire intensity treatment, which yielded only 231 g m^− 2 of sediment during the corresponding period in 1995. Runoff yield from the control plots did not show significant temporal changes, while soil losses decreased from 5 g m^− 2 in the first post-fire period to 0.7 g m^− 2 in the second one.     

Physical and chemical properties of soils under some piñon-juniper-oak canopies in a semi-arid ecosystem in New Mexico

Piñon (Pinus edulis)-juniper (Juniperus monosperma)-ecosystems increased substantially in the western USA during the 20th century. Sustainability of these ecosystems primarily depends on soil quality and water availability. This study was undertaken with the objective of assessing the effect of tree species on soil physical quality in a semi-arid region in the western part of Sugarite Canyon, northeast of Raton, Colfax County, NM (37°56′32″N and 104°23′00″W) USA. Three cores and three bulk soil samples were obtained from the site under the canopy of three juniper, Gambel oak (Quercus gambelii) and piñon trees for 0–10 and 10–20 cm depths. These samples were analyzed for particle size distribution, soil bulk density (ρ_b), water stable aggregation (WSA), mean weight diameter (MWD) of aggregates, pH, electrical conductivity (EC) and soil organic carbon (SOC) and total nitrogen (TN) concentrations and stocks. Sand content was greater under juniper (48%) than oak (32%), whereas clay content followed the opposite trend. The ρ_b, WSA, MWD, pH and EC were similar under juniper, piñon, oak canopies for both depths. Estimated (from Philip and Green and Ampt infiltration models) and measured water infiltration parameters did not vary among these sites and were in accord with the values for ρ_b, WSA and MWD. The SOC concentrations and stocks were greater under oak (43.1 Mg ha⁻¹ for 0–10 and 37.5 Mg ha⁻¹ for 10–20 cm depths) than piñon (23.3 Mg ha⁻¹ for 0–10 and 18.5 Mg ha⁻¹ for 10–20 cm depths). The TN concentrations were greater under oak (3.4 g kg⁻¹) than piñon (1.7 g kg⁻¹) for the 0–10 cm depth only. Accumulation of detritus material under tree canopies reduced soil compaction and crusting caused by raindrop impact and increased SOC, and TN concentrations, and water infiltration. Coefficients of variation ranged from low to moderate for most soil properties except infiltration rate at 2.5 h, which was highly variable. Overall, soil quality for each site was good and soil aggregation, water infiltration and SOC concentrations were high, and soil ρ_b was low.     

A large daylight geodesic dome for quantification of whole-ecosystem CO2 and water vapour fluxes in arid shrublands

We designed, constructed, calibrated and field-tested a lightweight (30 kg), 4.2 m diameter, 16.4 m³ polyethylene-covered dome static chamber ecosystem gas exchange cuvette that can quantify ecosystem CO₂ and water vapour fluxes as low as 0.1 μmol CO₂ m⁻² s⁻¹ and 0.1 mmol H₂O m⁻² s⁻¹ with little impact on environmental conditions. Fluxes measured in May 2001 in an intact Great Basin sagebrush ecosystem at midday were significantly higher than in an adjacent post-wildfire successional ecosystem, with observed ranges from –0.71 to 1.49 μmol CO₂ m⁻² s⁻¹ for CO₂ and from –0.09 to 0.53 mmol H₂O m⁻² s⁻¹ for water vapour.     

Deriving hillslope sediment budgets in wildfire-affected forests using fallout radionuclide tracers

Information on post-fire sediment and nutrient redistribution is required to underpin post-fire catchment management decisions. Fallout radionuclide budgets (²¹⁰Pb_xs, ¹³⁷Cs and ⁷Be) were derived to quantify soil redistribution and sediment yield in forested terrain following a moderately severe wildfire in a small (89 ha) water supply catchment in SE Australia. Application of these techniques in burnt terrain requires careful consideration of the partitioning of radionuclides between organic and mineral soil components. Beryllium-7 and ²¹⁰Pb_xs were shown to be closely associated with ash, litter and soil organic matter whereas ¹³⁷Cs was more closely associated with subsurface coarse mineral soil. Comparison of the three tracer budgets indicated that the dominant sediment source areas were ridgetops and steep valley sideslopes, from which burnt surface material was conveyed to the stream network via pre-existing gullies. Erosion was predominantly driven by sheetwash, enhanced by soil water repellency, and modified by bioturbation which both supplies subsurface sediment and provides sinks for erosive overland flow. Footslope and riparian zones were not important sediment source areas. The estimated event-based (wildfire and subsequent rainfall) sediment yield is 58 ± 25 t km^− 2, based on fallout ⁷Be measurements. The upper estimate of total particulate phosphorus yield (0.70 kg ha^− 1) is more than 10 times that at equivalent unburnt sites. This illustrates that, soon after fire, burnt eucalypt forest can produce nutrient loads similar to those of agricultural catchments. The tracer budgets indicate that wildfire is an important control on sediment and phosphorus inputs to the stream network over the decadal timeframe and the pulsed nature of this release is an important concern for water quality management.     

上海滨岸潮滩水沉积物中无机氮的季节性变化

对长江口南翼上海滨岸带3个站点潮滩上覆水，沉积物和间隙水中的3态无机氮的含量分布的年度季节性监测研究表明：潮滩上覆水中溶解无机氮以NO3－N为主：表层沉积物中可交换态无机氮以NH4－N为主，约占70％－85％，沉积物间隙水中主要无机氮为NH4－N和NO3－N。潮滩水体中NH4－N的季节性变化幅度不大，而NO3－N和NO2－N的季节性变化明显，在冬季含量明显降低；但沉积物和间隙水中氮氨和硝态氮的浓度在冬季则有较大增加。初步探讨了潮滩水和沉积物中无机氮分布季节性变化的主要影响因素，估算了潮滩表层沉积物－水界面无机氮的扩散通量，指出NH4－N的扩散释放对滨岸水环境质量影响较大。     

Fire in Chihuahuan Desert grassland: Short-term effects on vegetation, small mammal populations, and faunal pedoturbation

A prescribed burn resulted in significant decreases in canopy cover of the grasses: Bouteloua eriopoda, Sporobolus flexuosus, and Aristida purpurea. One year post-burn, basal cover of B. eriopoda remained significantly lower in burned patches than in unburned areas but there were no differences in basal cover of the other perennial grasses. Only one species of the 14 summer annual species occurred in both burned and unburned plots. There were six species of spring annuals in burned patches but no spring annuals in the unburned grassland ten months post-burn. Fire killed 100% of the snakeweed shrubs (Gutierrezia sarothrae), 77% of the Ephedra torreyana shrubs, and 36% of the Yucca elata. All mesquite shrubs that were top-killed by fire, resprouted one month post-burn. Fire had no effect on abundance and species richness of rodents. There were fewer wolf spider, Geolycosa spp. burrows in burned areas than in unburned grassland. The area and volume of soil in termite galleries and sheeting were significantly larger in the unburned grassland than in the burned areas.     

Carbon exchange rates in Polytrichum juniperinum moss of burned black spruce forest in interior Alaska

The Boreal black spruce forest is highly susceptible to wildfire, and postfire changes in soil temperature and substrates have the potential to shift large areas of such an ecosystem from a net sink to a net source of carbon. In this paper, we examine CO₂ exchange rates (e.g., NPP and Re) in juniper haircap moss (Polytrichum juniperinum) and microbial respiration in no-vegetation conditions using an automated chamber system in a five-year burned black spruce forest in interior Alaska during the fall season of 2009. Mean ± standard deviation microbial respiration and NEP (net ecosystem productivity) of juniper haircap moss were 0.27 ± 0.13 and 0.28 ± 0.38 gCO₂/m²/hr, respectively. CO₂ exchange rates and microbial respiration showed temporal variations following fluctuation in air temperature during the fall season, suggesting the temperature sensitivity of juniper haircap moss and soil microbes after fire. During the 45-day fall period, mean NEP of P. juniperinum moss was 0.49 ± 0.28 MgC/ha following the five-year-old forest fire. On the other hand, simulated microbial respiration normalized to a 10 °C temperature might be stimulated by as much as 0.40 ± 0.23 MgC/ha. These findings demonstrate that the fire-pioneer species juniper haircap moss is a net C sink in the burned black spruce forest of interior Alaska.     

长江口潮滩沉积物－水界面无机氮交换通量

对长江口滨岸潮滩7个典型断面三态氮的界面交换通量进行了三年多的季节性连续观测,结果表明无机氮的界面交换行为存在复杂的空间分异和季节变化。NO^-₃-N和NH⁺₄-N的界面交换通量正负变化范围较大,分别介于-32.82~24.13 mmol.m^-2.d^-1和-18.45~10.65mmol.m^-2.d^-1之间;而NOsup>-₂-N的界面交换通量很小,仅为-1.15~2.82 mmol.m^-2.d^-1。NO^-₃-N的界面交换具有明显的上下游季节性时空分异特征,而NH⁺₄-N的界面交换则表现为南北岸季节性时空分异现象。盐度是控制长江口滨岸潮滩NH⁺₄-N界面交换行为的主要因素,而沉积物粒度、水体 NO^-₃-N浓度、沉积物有机质含量、水温和溶解氧含量则以不同的组合方式,共同制约着 NO^-₃-N在潮滩界面交换的时空分异格局。     

Mesquite (Prosopis juliflora (Sw.) DC.), huisache (Acacia farnesiana (L.) Willd.) and catclaw (Mimosa biuncifera Benth.) and their effect on dynamics of carbon and nitrogen in soils of the semi-arid highlands of Durango Mexico

In the northern semiarid and arid part of Mexico, mesquite (Prosopis juliflora (SW.) DC.), huisache (Acacia farnesiana (L.) Willd.) and catclaw (Mimosa biuncifera Benth.), N₂-fixing trees or shrubs, dominate the landscape. It is unknown, however, how much the leaves of those shrubs contribute to dynamics of carbon (C) and nitrogen (N) in soil. We investigated this by adding leaves of each species to soil sampled under the canopy of mesquite, huisache, and catclaw and outside their canopy while monitoring production of carbon dioxide (CO₂), and dynamics of inorganic N (ammonium (NH₄⁺) and nitrate (NO₃⁻)) in an aerobic incubation. The (hemi)cellulose and N content of the catclaw leaves was lower and the lignin and polyphenol content was larger than in the mesquite and huisache leaves. If we considered no priming effect, then 41% of the C added with catclaw leaves, 47% with huisache leaves and 49% with mesquite leaves mineralized within 42 days. The addition of the leaves had little or no effect on N mineralized, and only 6% of organic N of the mesquite leaves was mineralized. It was found that catclaw, huisache and mesquite have a positive effect on the arid and semi-arid ecosystems as they increased soil organic matter and soil N content.     

Evaluation of annual radiation and windiness over a playa: possibility of harvesting the solar and wind energies

We set up an automatic weather station over a playa (the flat floor of an undrained desert basin that becomes at times a shallow lake), approximately 65 km east–west by 130 km north–south, located at the U.S. Army Dugway Proving Ground (40°08′N, 113°27′W, 1124 m above mean sea level) in north-western Utah, U.S.A., in 1999. This station measured the incoming (R_si) and outgoing (R_so) solar or shortwave radiation using two CM21 Kipp & Zonen pyranometers (one inverted), the incoming (R_li or atmospheric) and outgoing (R_lo or terrestrial) longwave radiation, using two CG1 KippZonen pyrgeometers (one inverted), and the net (R_n) radiation using a Q*7 net radiometer (Radiation Energy Balance System, REBS). We also measured the 10-m wind speed (U₁₀) and direction (R.M. Young wind monitor) and precipitation (Campbell Sci., Inc.). The measurements were taken every 2 s, averaged into 20-min, continuously, throughout the year. The annual (August 1999 – August 2000) comparisons of global or solar radiation and windiness with two other stations in central (Hunter) and northern (Logan) Utah, indicate higher solar radiation (R_si,Dugway=7797 MJ m⁻² period⁻¹vs. R_{si, Hunter}=7021 MJ m⁻² period⁻¹ and R_{si, Logan}=6865 MJ m⁻² period⁻¹) and much higher annual mean windiness (U_Dugway=387 km day⁻¹vs. U_Hunter=275 km day⁻¹ and U_Logan=174 km day⁻¹) throughout the period over the playa. These data reveal the possibility of simultaneously harvesting these two sources of clean energies at this vast and uniform playa.     

非点源污染研究中土壤溶解性无机氮的提取方法选择

用2M KCl溶液(5:1)、饱和CaSO4溶液(5:1)、0.01M CaCl₂溶液(20:1,100:1) 和去离子水(20:1,100:1)4种提取液、6种提取方式,采用Auto-analyzerⅢColorimeter (Bran Luebbe),水杨酸-次氯酸盐法和Griess-Ilosvay法测定并对比了北京地区4种天然土样的溶解性氨氮和硝态氮的含量。研究结果表明:溶解性硝态氮的测定结果不受提取液和水土比(小于100:1)的影响,而溶解性氨氮测定结果受影响较大。在非点源污染研究中,对于土壤中可向水体迁移的溶解性无机氮含量的测定,应根据来水水源特征和研究目的选择适宜的浸提剂和水土比。     

The Suburban Stream Syndrome: Evaluating Land Use and Stream Impairments in the Suburbs

Development is known to impair stream water quality at moderate to high levels of urbanization, but the effects of low-density urban expansion, the kind occurring on the outskirts of many cities, remain unclear. We examined five suburban headwater streams in Duchess County, New York whose watersheds contained between 4.7% and 34% impervious surface cover. We measured Cl^- and nitrate-N (NO₃-N) concentrations in water samples taken at four to six sites on each stream in winter and summer. Even at low levels of population and impervious cover, concentrations of both Cl^- and NO₃-N exceeded reference levels found in cleaner streams in the region. Chloride levels were elevated in upper reaches and remained elevated or continued to increase downstream, with a linear response to impervious cover. Nitrate-N increased downstream in all watersheds, indicating that NO₃-N inputs exceeded natural denitrification and uptake in both winter and summer. Nitrate-N responded logarithmically to impervious surface cover, with steep increases at low levels of imperviousness. Per-capita inputs were also high in rural areas. Agricultural inputs were not sufficient to explain observed trends in NO₃-N; we interpret inputs to result chiefly from low-density exurban expansion. Widespread residential expansion has significant impacts on water quality that have not previously been acknowledged.     

Aeolian sediment transport following wildfire in sagebrush steppe

Wind erosion of soil is an appreciable but unstudied event following fires in cold desert. We examined aeolian transport of sediment for 1 year following fire in semi-arid shrub steppe on loess soils in southern Idaho, USA. Sediment collectors were used to determine horizontal mass transport of soil and saltation sensors and anemometers were used to determine saltation activity (fraction of time having saltation) and threshold wind speed in an area burned in August and an unburned control site. Horizontal mass transport (per 30-day period) was negligible in the unburned area, but in the burned area was 5.40 kg m^?1 in October and decreased to 2.80 kg m^?1 in November and 0.32 kg m^?1 in December. Saltation activity was high enough to determine threshold wind speeds only in the burn site during fall, when values ranged from 10.0 to 10.6 m s^?1. Sediment flux and saltation activity in the burned site became much less pronounced following the emergence of herbaceous vegetation in the spring. Post-fire sediment flux in the shrub steppe we examined was of greater magnitude but shorter duration than post-fire fluxes in warm deserts or sandier regions that experience more frequent wind erosion.     

Ecological effects of experimental drought and prescribed fire in a southern California coastal grassland

How drought and fire disturbance influence different levels of biological organization is poorly understood but essential for robust predictions of the effects of environmental change. During a year of severe drought, we conducted a prescribed fire in a Mediterranean-type coastal grassland near Irvine, California. In the weeks following the fire we experimentally manipulated rainfall in burned and unburned portions of the grassland to determine how fire and drought interact to influence leaf physiological performance, community composition, aboveground net primary productivity (ANPP) and component fluxes of ecosystem CO₂ exchange and evapotranspiration (ET). Fire increased leaf photosynthesis (A_net) and transpiration (T) of the native perennial bunchgrass, Nassella pulchra and the non-native annual grass, Bromus diandrus but did not influence ANPP or net ecosystem CO₂ exchange (NEE). Surprisingly, drought only weakly influenced A_net and T of both species but strongly influenced ANPP and NEE. We conclude that despite increasing experimental drought severity, prescribed fire influenced leaf CO₂ and H₂O exchange but had little effect on the component fluxes of ecosystem CO₂ exchange. The differential effects of prescribed fire on leaf and ecosystem processes with increasingly severe drought highlight the challenge of predicting the responses of biological systems to disturbance and resource limitation.     

Impact of prescribed burning on Acacia nilotica seed banks and seedlings in the Astrebla grasslands of northern Australia

Acacia nilotica is a tree of international significance both as a beneficial plant and as a species prone to thicket formation and negative impacts on savannas throughout much of its range. While fire has been identified as a useful tool for controlling negative impacts of some Acacia species in Africa, A. nilotica adult trees are apparently fire tolerant. The effects of fire on seed bank mortality, viability and germination, and on juvenile plants of this species are unknown. We undertook a study to investigate the impacts of fire on seeds and first and second year seedlings in Australian Astrebla savannas. The primary objective was to determine the effects of control burns applied during the early stages of invasion and thicket formation. Because of high rates of seedling dessication post fire in 1997, supplementary watering was undertaken until fire mortality could be measured the following year. High seed mortality occurred for seeds located either in cattle dung (100%) or on the soil surface (c. 80%). Germination of those seeds that survived was enhanced following exposure to fire. Very high fire mortality (99%) was observed among first year seedlings and moderate mortality (60%) for second year seedlings even after a low intensity burn. Negative impacts on major pasture species were minimal. The implications of these findings are discussed in relation to A. nilotica fire ecology and to the efficacy of fire as a tool for managing this species.