Soil-Atmosphere Exchange of N<Subscript>2</Subscript>O and NO in Near-Natural Savanna and Agricultural Land in Burkina Faso (W. Africa)

In a combined field and laboratory study in the southwest of Burkina Faso, we quantified soil-atmosphere N₂O and NO exchange. N₂O emissions were measured during two field campaigns throughout the growing seasons 2005 and 2006 at five different experimental sites, that is, a natural savanna site and four agricultural sites planted with sorghum (n = 2), cotton and peanut. The agricultural fields were not irrigated and not fertilized. Although N₂O exchange mostly fluctuated between −2 and 8 μg N₂O–N m⁻² h⁻¹, peak N₂O emissions of 10–35 μg N₂O–N m⁻² h⁻¹ during the second half of June 2005, and up to 150 μg N₂O–N m⁻² h⁻¹ at the onset of the rainy season 2006, were observed at the native savanna site, whereas the effect of the first rain event on N₂O emissions at the crop sites was low or even not detectable. Additionally, a fertilizer experiment was conducted at a sorghum field that was divided into three plots receiving different amounts of N fertilizer (plot A: 140 kg N ha⁻¹; plot B: 52.5 kg N ha⁻¹; plot C: control). During the first 3 weeks after fertilization, only a minor increase in N₂O emissions at the two fertilized plots was detected. After 24 days, however, N₂O emission rates increased exponentially at plot A up to a mean of 80 μg N₂O–N m⁻² h⁻¹, whereas daily mean values at plot B reached only 19 μg N₂O–N m⁻² h⁻¹, whereas N₂O flux rates at plot C remained unchanged. The calculated annual N₂O emission of the nature reserve site amounted to 0.52 kg N₂O–N ha⁻¹ a⁻¹ in 2005 and to 0.67 kg N₂O–N ha⁻¹ a⁻¹ in 2006, whereas the calculated average annual N₂O release of the crop sites was only 0.19 kg N₂O–N ha⁻¹ a⁻¹ and 0.20 kg N₂O–N ha⁻¹ a⁻¹ in 2005 and 2006, respectively. In a laboratory study, potential N₂O and NO formation under different soil moisture regimes were determined. Single wetting of dry soil to medium soil water content with subsequent drying caused the highest increase in N₂O and NO emissions with maximum fluxes occurring 1 day after wetting. The stimulating effect lasted for 3–4 days. A weaker stimulation of N₂O and NO fluxes was detected during daily wetting of soil to medium water content, whereas no significant stimulating effect of single or daily wetting to high soil water content (>67% WHC_max) was observed. This study demonstrates that the impact of land-use change in West African savanna on N trace gas emissions is smaller—with the caveat that there could have been potentially higher N₂O and NO emissions during the initial conversion—than the effect of timing and distribution of rainfall and of the likely increase in nitrogen fertilization in the future.     

Detection of seasonal variability in microclimatic borders and ecotones between forest and savanna

Along eight forest-savanna transects, the borders, the width of associated ecotones, and the depth-of-edge influence (DEI) towards the forest interior were determined on the basis of microclimatic parameters. The analysis focused on the seasonal variability of microclimate at the forest-savanna ecotone that has so far rarely been investigated. The study was located in an intact mosaic of semi-deciduous forests and savanna in the Comoé National Park (north-eastern Ivory Coast). The microclimatic parameters air temperature, air humidity, and vapor pressure deficit were measured from the dry season (February) until the rainy season (September) during five measurement periods (MP). Borders and ecotones including their confidence intervals were determined by a border-and-ecotone detection analysis, which is based on non-linear regression analysis. The ecotone limits were interpreted as DEI towards the two habitats. During the dry season, the microclimatic border between forest and savanna was located further towards the forest interior than during the rainy season. This may be caused by different foliation patterns of tree species at the forest interior and the forest boundary, with the latter being completely defoliated during the dry season. In addition, the variability of microclimatic parameters was higher and differences between forest and savanna were less pronounced during the dry season. The minimum DEI towards the forest interior was 27.4±15.5 m for air humidity in the rainy season (MP-5). The maximum DEI of 137.3 ±138.3 m occurred for air temperature in the dry season (MP-1). The average DEI for all microclimate parameters and MPs was 50.5 m. These DEI values are similar to observations from temperate and tropical forest boundaries in the literature. As microclimate borders proved to shift over the seasons, detailed knowledge of species’ responses to this variability appears to be essential for predicting concomitant dynamics of forest core areas.     

Micrometeorological conditions and canopy energy exchanges of a neotropical rain forest (Surumoni-Crane Project,Venezuela)

Vertical profiles of air temperature, humidity, wind speed and photosynthetically active radiation were examined systematically within and above a primary Amazonian rain forest in Southern Venezuela. During daylight hours the observed gradients of temperature and humidity suggest that turbulent mixing between vegetation and the atmosphere is reasonably efficient in the top two-thirds of the forest, whereas the understorey remains partially isolated. At night, however, the canopy exhibits a significant decoupling from the overlying atmospheric layer due to substantial radiative cooling, causing a stable density stratification above. It is fairly clear that these variations in microclimate produce a spectrum of different living conditions for the flora and fauna.In addition, the collected data provide important information concerning the turbulent exchanges of heat and water vapour. Actual evapotranspiration loss from the forest was estimated using a single-layer version of the Penman-Monteith equation including a submodel of canopy conductance. The values computed for hourly and daily periods were found to agree well with the simultaneously recorded xylem sap flow of several tree species. Thus they prove that this application is suitable to describe the environmental impact of micrometeorological and physiological factors on the complex process of evapotranspiration.     

Biodiversity and vegetation of small-sized inselbergs in a West African rain forest (Taı, Ivory Coast)

The vegetation of small granitic rock outcrops (geomorphologically small-sized inselbergs) which do not reach the canopy was studied in the Taı rain forest (southwestern Ivory Coast) under aspects of species diversity and phytogeographical affinities. Rock outcrops form edaphically arid (due to absent or very sparse soil cover) and microclimatologically xeric (i.e. low air humidity, temperature regularly exceeding 50°C) islands with cryptogamic crusts, succulents and poikilohydric vascular plants as characteristic elements of their vegetation which differs totally from the surrounding forest. Altogether sixty-six species of vascular plants out of twenty-nine families occur, the number of species correlates positively with inselberg size. Compared with large inselbergs the microclimatic attributes of small-sized rock outcrops are less pronounced. This is accompanied by a decrease of typical inselberg taxa (i.e. species mainly occurring on inselbergs). Low beta diversity between inselbergs indicates deterministic influences as important regulators of species composition. Annual Poaceae and Cyperaceae are richly represented. It can be hypothesized that inselbergs may represent natural growing sites of widely distributed tropical weeds today. Inselbergs might provide habitat resources for savanna elements in rain forest zones.     

A torch in the rain forest: thermogenesis of the Titan arum (Amorphophallus titanum)

An outstanding flagship species in the plant kingdom is the Titan arum ( Amorphophallus titanum ), which produces a fountain-like bloom up to 3 m high. The unique appearance of three simultaneous inflorescences in May 2006 was a chance to analyse the flowering behaviour and thermogenesis of this giant. For the first time, the heating of the central column (spadix) could be documented using a high-performance thermographic camera. Time series analyses of the infrared image sequences revealed that the 3-m high spadix surface heats up in pulses emanating from the base of the inflorescence. The surface temperature reaches over 36 °C, compared to the ambient temperature of 27 °C. Waves of the carrion-like odour are synchronised with these heat pulses. The combination of heat pulses, the fountain-like shape plus the enormous size lead to a unique type of 'convection flower'. On the basis of our observations, we assume that Amorphophallus titanum is able to overcome thermodynamic decoupling by a self-produced convective process.