State and change in carbon pools in the forests of tropical Africa

To improve estimates of the state and change in C pools due to changes in land use in tropical forests of Africa, we combined spatially explicit estimates of biomass C density, obtained by modelling in a geographical information system (GIS), with new data on the area of forests (woody formations with a minimum of 10% crown cover) reported at subnational units for 1980 and 1990 by the Food and Agriculture Organization (FAO). Estimates of the biomass C densities for grass/shrub savannas were also included using a simple model based on precipitation. The total C pool in above– and below–ground forests and grass/shrub savannas of Africa for 1980 was 50.8 Pg (10¹⁵g), with aboveground forest biomass accounting for 75% of the total, below–ground forest biomass for 21%, and grass/shrub savannas for 4%. Area weighted mean biomass C densities were about 180 Mg ha^–1 for lowland moist forests, 82 Mg ha^–1 for all forests, and 6 Mg ha^–1 for grass savannas. The total change in the aboveground forest C pool for the decade 1980–90 due to changes in land cover and use was estimated to be a decrease of 6.6 Pg C. Of this total, 43% was due to deforestation and 57% due to biomass reduction by other human activities. Six countries, mostly in central Africa, accounted for more than 73% of the total change in the C pool. The difference between state and change of C pool estimates made at the subnational scale and those made at the national scale proved to be insignificant across the region as a whole (2% for pools and – 1% for change in pool) but potentially important to individual countries (from + 36% to – 39% for pools and from + 43% to – 57% for change in pool). The differences between the two approaches may reflect a better match of the areas being deforested with the biomass C density of forests being cleared.     

The utilization of bicarbonate ions by the macroalga Ascophyllum nodosum (L.) Le Jolis

Abstract A comparison of some of the methods used to determine whether aquatic plants have the ability to utilize bicarbonate ions as a source of inorganic carbon for photosynthesis has been applied to the intertidal macroalga Ascophyllum nodosum. These include: observing photosynthesis at a high pH (below the alga's CO₂ compensation point), pH compensation point determinations, comparing the photosynthetic characteristics at low pH (5.20) and at high pH (7.95), estimating the maximal rates at which CO₂ can diffuse through the unstirred layer and the rate at which CO₂ can be produced from bicarbonate dehydration in the unstirred layer. All indicated that Ascophyllum nodosum can use bicarbonate ions for photosynthesis, though some were not always consistent. Calculating the total inorganic carbon concentration from pH measurements and acidification CO₂ determinations revealed that the assumption that the alkalinity remains constant during pH drift experiments is not always valid.