Experimental cation exchange capacities (CEC) of kaolinites were determined and compared to theoretical calculations of CEC. The comparison reveals that the exchangeable cations occur mostly on the edges and on the basal (OH) surfaces of the mineral. It also shows that permanent negative charge from isomorphous substitution of Al3+ for Si4+ is insignificant. The CEC of kaolinite strongly depends on the particle size (both thickness and diameter in the (00l plane) and pH value. Particle size is more important than crystallinity in affecting kaolinite CEC. This study shows that the hydroxyls on the exposed basal surfaces may be ionizable in aqueous solutions. The amount of negative charge on the edges and the exposed basal hydroxyls depends on pH and other ion concentrations. A higher pH value gives rise to more negative charges, which lead to a higher CEC value. This study indicates that charge from broken edges and exposed OH planes rather than charge from Al/Si substitution determines the kaolinite CEC, even at zero point charge. A high CEC in some kaolinites is found to be due to smectite layers on the surface of the kaolinite crystals.

Effective management of anthropogenic threats is key to sustaining biological diversity in protected areas. Types and distribution of threats to Bwindi Impenetrable National Park, Uganda were investigated to assess the Park's status 12 years after it was upgraded from a forest reserve to a national park. Bwindi, like many tropical forested parks, is surrounded by dense human populations. Threats were quantified in 104 1-km edge-interior transects set around the Park. The distribution of threats was patchy and was most common within 300–350 m of the edge. The commonest threat was harvesting of wood and poles. Other threats included occurrence of exotic species, degradation of adjacent habitat fragments and high impact of problem animals on some of the neighbouring communities. The fact that threats were primarily associated with the edges of the Park, when previously they were widespread throughout the Park, suggests that illegal resource harvesting has been reduced since the forest was upgraded to a national park. Park legislation, enforcement and related conservation efforts have been effective, and there should be increased effort to manage the people-park interface. Edge-based assessments appear to be useful for quantifying threats to protected areas and identifying areas in which they are concentrated.

Forest edges bordering on pasture were created by cutting and burning the surrounding Amazonian lowland rain forest in the dry season (June) of 1990. Litterfall was measured for 3.5 y along transects 10, 50, 100 and 250-m into the forest from the forest edge. Litterfall along the 10-m transects increased by up to 2.5 times that on spatial controls (250-m transects) in the dry season in which the edge was created. In the second dry season after edge creation litterfall at 10-m was lower than on controls, after which it returned to control rates in the second wet season, 1.5 y after edge creation. Litterfall 50-m into the forest was less affected; there was a smaller rise in the dry season in which the edge was cut, and no significant effects after that. At 100-m there was no effect of edge creation on litterfall. Phosphorus concentrations in litterfall were elevated along 10-m transects, 10–12 wk after edge creation, possibly because of reduced retranslocation prior to abscission. The changes in litterfall described here are inextricably linked to the biomass collapse, which occurs near forest-fragment edges in the same area.