Dominant woody species can determine the structure and composition of a plant community by affecting environmental conditions experienced by other species. We explored how dominant tree species affect the tree species richness, diversity, evenness and vertical structural heterogeneity of non-dominant species in wet and dry miombo woodlands of Tanzania. We sampled 146 plots from eight districts with miombo woodlands, covering a wide range of topographic and climatic conditions. We recorded 217 woody plant species belonging to 48 families and 122 genera. Regression analysis showed significant negative linear associations between tree species richness, relative species profile index of the non-dominant and the relative abundance of the dominant tree species (Brachystegia spiciformis and Brachystegia microphylla in wet, and Brachystegia spiciformis and Julbernardia globiflora in dry miombo woodlands). Shannon diversity and evenness had strong non-linear negative relationships with relative abundance of dominant tree species. A large number of small individual stems from dominant and non-dominant tree species suggesting good regeneration conditions, and intensive competition affecting survival. We suggest that dominant miombo tree species are suppressing the non-dominant miombo tree species, especially in areas with high recruitments, perhaps because of their important adaptive features (extensive root systems and ectomycorrhizal associations), which enhance their ability to access limited nutrients.

Decrease in productivity of pastureland is a common problem in dry areas and needs to be enhanced through conservation and management of soil biodiversity and available plant resources. Diversity and population dynamics of soil arthropods, and soil nutrients were studied in different silvopastoral (tree-integrated grassland) systems for effective management and enhancement of grassland productivity. The most prominent combinations of trees and grasses in silvopastoral systems of the selected sites were Prosopis cineraria with Cenchrus ciliaris and C. biflorus (PC), Acacia nilotica with Elusine compressa and C. ciliaris (AN), Zizyphus nummularia with C. ciliaris and E. compressa (ZN), Capparis decidua with C. biflorus and Digitaria marginatus (CD) and A. senegal with C. ciliaris and D. marginatus (AS). Pure grass blocks outside tree canopy were selected as control plots. Acari, Myriapoda, Coleoptera, Isoptera, Collembola and other soil arthropods were the major soil faunal groups. Relative densities of Acari, Myriapoda and other arthropods were highest in silvopastoral systems and those of Coleoptera, Isoptera and Collembola were highest in pure grass plots. Variations in soil arthropod populations in response to rainfall, soil water content and soil temperature indicated greater sensitivity of these groups to environmental factors. The highest densities and negative relative tree effect (RTE) values of Acari, Myriapoda and other soil arthropods in the ZN, Coleoptera in the AS, Isoptera in the CD and Collembola in the AN systems indicated that these soil arthropods preferred the silvopastoral systems involved. Seasonal variations in soil organic matter and available NH4-N, NO3-N and PO4-P were similar to those of soil arthropods. The concentrations of soil nutrients were highest in the ZN system, which had the highest densities of soil arthropods. The other systems with high populations and soil nutrient status were the CD and AS. Silvopastoral systems based on Z. nummularia, C. decidua and A. senegal promoted soil arthropod populations and enhanced soil nutrient status, highlighting the positive role of tree presence and the need for proper management to promote soil biodiversity, nutrient cycling and sustained production in a fragile environment.

Damage caused by elephants was monitored in two woodland stands of Zakouma National Park (Chad) between February and March 1998. The Acacia seyal savanna was more severely damaged (29.8% of trees damaged of which 13.2% severely) than the Combretaceae savanna (26.5% of trees damaged of which only 4.2% severely). Nearly all severely damaged trees showed resprouts (respectively 86.8% and 88.5% in Combretaceae and A. seyal savannas). Both low damage rate and low mortality rate indicate that no serious ‘elephant problem’ occurred in Zakouma National Park. Elephants selected trees to damage according to species and height. Such selection also occurred for severe damage in A. seyal savanna, but only height affected severity of damage in Combretaceae savanna. In both savannas, the plots close to a water point were the most frequently damaged. In Combretaceae savanna, the distribution of damaged trees was clumped. On the other hand, spatial auto-correlation was not significant for the severity or the frequency of damage in A. seyal savanna. Neither tree density nor diversity of woody species affected the spatial patterns of damage or severe damage.

Roost selection by tree roosting noctules Nyctalus noctula and Daubenton's bats Myotis daubentonii was determined by comparing roost characteristics with the characteristics of randomly chosen tree cavities in the area. The following parameters were determined for 81 noctule roosts and 27 Daubenton's bat roosts: tree species, type of cavity (woodpecker or natural), entrance height, compass direction of entrance and distance to the edge of the wood. Roost choice of both bat species was similar except that Daubenton's bats used woodpecker cavities less than noctules. Both bat species were selective in their roost choice. Daubenton's bats preferred oaks over beeches, and noctules preferred woodpecker cavities over natural cavities, indicating dependence on woodpeckers. Both bat species preferred to roost close to the edge of the wood. Roosting close to the edge of the wood may reduce commuting flight costs for both species or may be preferable for thermoregulatory reasons.

Changes in density, structure and species composition of the woody component of the two predominant savanna types in Tarangire National Park, northern Tanzania, during a period of 25 y were investigated. The park is known for its large, increasing elephant numbers and high frequency of fires. In 1996 a study on woody species density, composition and age structure, which was first performed in 1971, was repeated, using the same transects and method. Access to the original data of 1971 allowed for a full comparison of the changes and an investigation whether these could be related to changes in elephant numbers and fire frequency. The total tree density declined during the 25 y, but the decline was not evenly distributed over the different height classes. Although the density of trees taller than 5 m declined significantly, the greatest decline occurred in the density of trees shorter than 1 m. The density of trees in the intermediate height class of 1–5 m did not decline. Although damage to trees by elephants increased during the 25-y period, c. 25% showed no browse damage and, except for some severely damaged trees, elephant damage was not found to reduce tree vigour. Elephants affected the size distribution of the savanna woody component much more than the density, while the data suggest no significant effect of fire on changes in tree density. The large decline in density of small trees was attributed to a severe drought in 1993. Based on large numbers of elephants during the past decades and on relatively low elephant impact on the total tree density, the present study suggests that the current elephant number of 2300 can be sustained in the park without causing detrimental effects, provided that their current range is maintained.