Species diversity,population structure and regeneration of woody species in fallows and mature stands of tropical woodlands of southeast Angola

This study in the dry tropical woodlands of SE Angola in Cuando-Cubango Province assessed the diversity and composition of woody species in fallows compared to those in mature woodlands. We assessed the population structure of the most harvested woody species by calculating size class distribution and evaluated their regeneration potential based on the density of saplings. The vegetation was surveyed in 20 plots of 20 × 50 m(1000 m2). In each plot, we measured the diameter at breast height(DBH) of all woody species with DBH ≥ 5 cm. The saplings were counted, identified and recorded; 718 individuals, corresponding to 34 species in 32 genera and 15 families were recorded. The size class distribution of target woody species showed three different patterns in fallows and mature woodlands. In general,most of the smaller diameter classes had more individuals than the larger ones did, showing that the regeneration may take place. However, in some diameter classes, the absence of larger stems indicated selective logging of larger trees. Few saplings were recorded in the fallows or mature woodlands; fire frequency and intensity is probably the main obstacle for seed germination and seedling survival rates in the studied area.

Patterns of forest composition and their long term environmental drivers in the tropical dry forest transition zone of southern Africa

Background： Tropical dry forests cover less than 13 % of the world＇s tropical forests and their area and biodiversity are declining. In southern Africa, the major threat is increasing population pressure, while drought caused by climate change is a potential threat in the drier transition zones to shrub land. Monitoring climate change impacts in these transition zones is difficult as there is inadequate information on forest composition to allow disentanglement from other environmental drivers. Methods： This study combined historical and modern forest inventories covering an area of 21,000 km2 in a transition zone in Namibia and Angola to distinguish late succession tree communities, to understand their dependence on site factors, and to detect trends in the forest composition over the last 40 years. Results： The woodlands were dominated by six tree species that represented 84 % of the total basal area and can be referred to as Bdikioea - Pterocarpus woodlands. A boosted regression tree analysis revealed that late succession tree communities are primarily determined by climate and topography. The Schinziophyton rautanenfi and Baikiaea plurijuga communities are common on slightly inclined dune or valley slopes and had the highest basal area （5.5 - 6.2 m^2 ha＆-1）. The Burkea africana - Guibourtia coleosperma and Pterocarpus angolensis - Diafium englerianum communities are typical for the sandy plateaux and have a higher proportion of smaller stems caused by a higher fire frequency. A decrease in overall basal area or a trend of increasing domination by the more drought and cold resilient B. africana community was not confirmed by the historical data, but there were significant decreases in basal area for Ochna pulchra and the valuable fruit tree D. englerianum. Conclusions： The slope communities are more sheltered from fire, frost and drought but are more susceptible to human expansion. The community with the important timber tree P. angolensis can best withstand high fire frequency but shows signs of a higher vulnerability to climate change. Conservation and climate adaptation strategies should include protection of the slope communities through refuges. Follow-up studies are needed on short term dynamics, especially near the edges of the transition zone towards shrub land.