The charcoal trap: Miombo forests and the energy needs of people

<p>Abstract</p> <p>Background</p> <p>This study evaluates the carbon dioxide and other greenhouse gas fluxes to the atmosphere resulting from charcoal production in Zambia. It combines new biomass and flux data from a study, that was conducted in a <it>miombo </it>woodland within the Kataba Forest Reserve in the Western Province of Zambia, with data from other studies.</p> <p>Results</p> <p>The measurements at Kataba compared protected area (3 plots) with a highly disturbed plot outside the forest reserve and showed considerably reduced biomass after logging for charcoal production. The average aboveground biomass content of the reserve (Plots 2-4) was around 150 t ha^-1, while the disturbed plot only contained 24 t ha^-1. Soil carbon was not reduced significantly in the disturbed plot. Two years of eddy covariance measurements resulted in net ecosystem exchange values of -17 ± 31 g C m^-2y^-1, in the first and 90 ± 16 g C m^-2in the second year. Thus, on the basis of these two years of measurement, there is no evidence that the <it>miombo </it>woodland at Kataba represents a present-day carbon sink. At the country level, it is likely that deforestation for charcoal production currently leads to a per capita emission rate of 2 - 3 t CO₂y^-1. This is due to poor forest regeneration, although the resilience of <it>miombo </it>woodlands is high. Better post-harvest management could change this situation.</p> <p>Conclusions</p> <p>We argue that protection of <it>miombo </it>woodlands has to account for the energy demands of the population. The production at national scale that we estimated converts into 10,000 - 15,000 GWh y^-1of energy in the charcoal. The term "Charcoal Trap" we introduce, describes the fact that this energy supply has to be substituted when woodlands are protected. One possible solution, a shift in energy supply from charcoal to electricity, would reduce the pressure of forests but requires high investments into grid and power generation. Since Zambia currently cannot generate this money by itself, the country will remain locked in the charcoal trap such as many other of its African neighbours. The question arises whether and how money and technology transfer to increase regenerative electrical power generation should become part of a post-Kyoto process. Furthermore, better inventory data are urgently required to improve knowledge about the current state of the woodland usage and recovery. Net greenhouse gas emissions could be reduced substantially by improving the post-harvest management, charcoal production technology and/or providing alternative energy supply.</p

Climate control of terrestrial carbon exchange across biomes and continents

Peer reviewe

Cambial growth season of brevi-deciduous Brachystegia spiciformis trees from south central Africa restricted to less than four months.

We investigate cambial growth periodicity in Brachystegia spiciformis, a dominant tree species in the seasonally dry miombo woodland of southern Africa. To better understand how the brevi-deciduous (experiencing a short, drought-induced leaf fall period) leaf phenology of this species can be linked to a distinct period of cambial activity, we applied a bi-weekly pinning to six trees in western Zambia over the course of one year. Our results show that the onset and end of cambial growth was synchronous between trees, but was not concurrent with the onset and end of the rainy season. The relatively short (three to four months maximum) cambial growth season corresponded to the core of the rainy season, when 75% of the annual precipitation fell, and to the period when the trees were at full photosynthetic capacity. Tree-ring studies of this species have found a significant relationship between annual tree growth and precipitation, but we did not observe such a correlation at intra-annual resolution in this study. Furthermore, a substantial rainfall event occurring after the end of the cambial growth season did not induce xylem initiation or false ring formation. Low sample replication should be taken into account when interpreting the results of this study, but our findings can be used to refine the carbon allocation component of process-based terrestrial ecosystem models and can thus contribute to a more detailed estimation of the role of the miombo woodland in the terrestrial carbon cycle. Furthermore, we provide a physiological foundation for the use of Brachystegia spiciformis tree-ring records in paleoclimate research

Spatial and temporal variation of CO2 efflux along a disturbance gradient in a miombo woodland in Western Zambia

Carbon dioxide efflux from the soil surface was measured over a period of several weeks within a heterogeneous Brachystegia spp. dominated miombo woodland in Western Zambia. The objectives were to examine spatial and temporal variation of soil respiration along a disturbance gradient from a protected forest reserve to a cut, burned, and grazed area outside, and to relate the flux to various abiotic and biotic drivers. The highest daily mean fluxes (around 12 μmol CO2 m−2 s−1) were measured in the protected forest in the wet season and lowest daily mean fluxes (around 1 μmol CO2 m−2 s−1) in the most disturbed area during the dry season. Diurnal variation of soil respiration was closely correlated with soil temperature. The combination of soil water content and soil temperature was found to be the main driving factor at seasonal time scale. There was a 75% decrease in soil CO2 efflux during the dry season and a 20% difference in peak soil respiratory flux measured in 2008 and 2009. Spatial variation of CO2 efflux was positively related to total soil carbon content in the undisturbed area but not at the disturbed site. Coefficients of variation of efflux rates between plots decreased towards the core zone of the protected forest reserve. Normalized soil respiration values did not vary significantly along the disturbance gradient. Spatial variation of respiration did not show a clear distinction between the disturbed and undisturbed sites and could not be explained by variables such as leaf area index. In contrast, within plot variability of soil respiration was explained by soil organic carbon content. Three different approaches to calculate total ecosystem respiration (Reco) from eddy covariance measurements were compared to two bottom-up estimates of Reco obtained from chambers measurements of soil- and leaf respiration which differed in the consideration of spatial heterogeneity. The consideration of spatial variability resulted only in small changes of Reco when compared to simple averaging. Total ecosystem respiration at the plot scale, obtained by eddy covariance differed by up to 25% in relation to values calculated from the soil- and leaf chamber efflux measurements but without showing a clear trend.ISSN:1726-4170ISSN:1726-417

Spatial and temporal variation of CO2 efflux along a disturbance gradient in a miombo woodland in Western Zambia

Carbon dioxide efflux from the soil surface was measured over a period of several weeks within a heterogeneous Brachystegia spp. dominated miombo woodland in Western Zambia. The objectives were to examine spatial and temporal variation of soil respiration along a disturbance gradient from a protected forest reserve to a cut, burned, and grazed area outside, and to relate the flux to various abiotic and biotic drivers. The highest daily mean fluxes (around 12 μmol CO2 m−2 s−1) were measured in the protected forest in the wet season and lowest daily mean fluxes (around 1 μmol CO2 m−2 s−1) in the most disturbed area during the dry season. Diurnal variation of soil respiration was closely correlated with soil temperature. The combination of soil water content and soil temperature was found to be the main driving factor at seasonal time scale. There was a 75% decrease in soil CO2 efflux during the dry season and a 20% difference in peak soil respiratory flux measured in 2008 and 2009. Spatial variation of CO2 efflux was positively related to total soil carbon content in the undisturbed area but not at the disturbed site. Coefficients of variation of efflux rates between plots decreased towards the core zone of the protected forest reserve. Normalized soil respiration values did not vary significantly along the disturbance gradient. Spatial variation of respiration did not show a clear distinction between the disturbed and undisturbed sites and could not be explained by variables such as leaf area index. In contrast, within plot variability of soil respiration was explained by soil organic carbon content. Three different approaches to calculate total ecosystem respiration (Reco) from eddy covariance measurements were compared to two bottom-up estimates of Reco obtained from chambers measurements of soil- and leaf respiration which differed in the consideration of spatial heterogeneity. The consideration of spatial variability resulted only in small changes of Reco when compared to simple averaging. Total ecosystem respiration at the plot scale, obtained by eddy covariance differed by up to 25% in relation to values calculated from the soil- and leaf chamber efflux measurements but without showing a clear trend.ISSN:1810-6277ISSN:1810-628

Reaction to bi-weekly pinning in <i>Brachystegia spiciformis</i>.

<p>(a) Cross-section of a <i>Brachystegia spiciformis</i> tree that was pinned on a bi-weekly base between October (Oct) 27, 2001 and July (Jul) 20, 2002. The darker coloured compartmentalization zones around each inserted pin are clearly visible as well as the darker heartwood area in the center of the trunk, compared to the lighter coloured sapwood. Note that pinning continued after Jul 20,2002 (until Oct 17, 2002), but was conducted along a different cross-section of the same tree (see also Fig. 1c). (b) Close-up of the wound formed by the pinning on November (Nov) 10, 2001 and the darker coloured compartmentalization zone that resulted from it. (c) Detail of the wood zone surrounding the pinning on January (Jan) 19, 2002: 1–callus tissue formed as wound reaction to the pinning; 2–wound inflicted by the pinning, including the dark, died-off cambium layer; 3–parenchyma cells (between white arrows) formed as reaction to the pinning; 4–xylem formed after the pinning on Jan 19, 2002 but before the end of the growth season 2001–2002; 5–xylem formed between the start of the growth season 2001–2002 and the pinning on Jan 19, 2002; 6–xylem (growth ring) formed during the growth season 2000–2001; 7–compartmentalization zone. White scale-bar  = 25 mm (a), 5 mm (b), and 2 mm (c).</p

Sample-specific tree growth characteristics.

<p>Diameter at breast height (DBH), age, average tree-ring width (TRW), TRW in 2001–2002, and start and end date and maximum length of the cambial growth season for 6 <i>Brsp</i> trees in Kataba Forest Reserve.</p

Cumulative xylem formation, and precipitation throughout the growth season.

<p>Median percentage of maximum cell counts in 6 <i>Brachystegia spiciformis</i> trees (thin solid lines) and cumulative precipitation (gray bars) at Kataba Forest Reserve at bi-weekly intervals between October 27, 2001 and April 27, 2002. The apparently negative growth recorded in Tw56869 on February 16 can be explained by variation in the strength of cambial activity around the circumference of the tree.</p

Chronosequence of pinning reactions in <i>Brachystegia spiciformis</i> throughout the growth season.

<p>Chronosequence of pinning reactions in <i>Brachystegia spiciformis</i> between November (Nov) 24, 2001 and June (Jun) 22, 2002. White arrows indicate the parenchyma layer formed as a reaction to the pinning, black arrows indicate the marginal parenchyma band that forms the boundary of the 2000–2001 growth ring. On Nov 24, 2001, the wound parenchyma layer coincides with the 2000–2001 growth ring boundary, whereas on December (Dec) 7, 2001, a small number of xylem (fiber and vessel) cells is visible between the two parenchyma layers, indicating that the 2001–2002 growth season started between Nov 24 and Dec 7, 2001. The number of cells between the two parenchyma layers increases between Dec 7, 2001 and March (Mar) 2, 2002 as the tree continues its woody growth. No regular xylem cells are formed after the pinning on Mar 16, 2002, the wound parenchyma layer corresponds to the outside of the sample at this point in time, indicating the end of the 2001–2002 growth season. Note that callus tissue, as a reaction to the pinning, continues to be formed even after the end of the growth season and throughout the dry season (e.g., Jun 22). White scale-bars  = 2 mm in all photographs.</p