Expansion of gallery forests into central Brazilian savannas

Upland tropical forests have expanded and contracted in response to past climates, but it is not clear whether similar dynamics were exhibited by gallery (riparian) forests within savanna biomes. Because such forests generally have access to ample water, their extent may be buffered against changing climates. We tested the long‐term stability of gallery forest boundaries by characterizing the border between gallery forests and savannas and tracing the presence of gallery forest through isotopic analysis of organic carbon in the soil profile. We measured leaf area index, grass vs. shrub or tree coverage, the organic carbon, phosphorus, nitrogen and calcium concentrations in soils and the carbon isotope ratios of soil organic matter in two transitions spanning gallery forests and savanna in a Cerrado ecosystem. Gallery forests without grasses typically show a greater leaf area index in contrast to savannas, which show dense grass coverage. Soils of gallery forests have significantly greater concentrations of organic carbon, phosphorus, nitrogen and calcium than those of savannas. Soil organic carbon of savannas is significantly more enriched in ¹³C compared with that of gallery forests. This difference in enrichment is in part caused by the presence of C₄ grasses in savanna ecosystem and its absence in gallery forests. Using the ¹³C abundance as a signature for savanna and gallery forest ecosystems in 1 m soil cores, we show that the borders of gallery forests have expanded into the savanna and that this process initiated at least 3000–4000 bp based on ¹⁴C analysis. Gallery forests, however, may be still expanding as we found more recent transitions according to ¹⁴C activity measurements. We discuss the possible mechanisms of gallery forest expansion and the means by which nutrients required for the expansion of gallery forest might accumulate.     

The Formation of Alkaline and Acid Regions at the Surface of Chara corallina Cells

The magnitude of previously reported pH changes at the surfaceof Chara corallina cells has been measured with specially constructedpH electrodes. An experimental system was designed to reducethe dissipation of pH gradients by convection. When cells areilluminated their surfaces become partitioned into alkalineregions (pH 8.5–9.5) with sharp peaks and acid regions(pH about 5.5) which are more uniform. In the dark these regionsrapidly disappear and the pH of the cell surface becomes uniform(about pH 6.0). With cells initially free of any deposits ofCaCO₃, alkaline regions may appear temporarily, and may migratealong the cell surface. The presence of CaCO₃ stabilizes theregions. It is proposed that alkalinization results from HCO₃^–uptake, CO₂ fixation, and subsequent localized OH- efflux. Alkalinizationis prevented by photosynthetic inhibitors. The nature and magnitude of the HCO₃^–/OH- exchange processis discussed, and the possible role of H⁺ and OH- fluxes incontrolling the ionic and electrical properties of Characeancells is emphasized.     

哈萨克斯坦斋桑盆地新生代轮藻植物组合

斋桑盆地自晚白垩世以来即为淡水湖泊环境,其始新世至中新世湖相及湖边缘相沉积物产有丰富的轮藻化石。文中建立三个轮藻化石组合：(1)Harrisichara mitella—Peckisphaera zajsanica组合(中始新世早期);(2)Lychnothamnus formosus—Peckichara组合(中始新世晚期--渐新世早期);(3)Gyrogona aralica—sphaerochara clearus组合(中--晚渐新世)。斋桑盆地轮藻化石出现的最高层位为下中新统且化石稀少,推测系盆地环境日益干旱所致。