Vegetation change in lerai forest Ngorongoro and its probable causes

In the 1970, Lerai forest, the only closed woodland community in the Ngorongoro Crater, was found to be faced with a threat of mortality among its mature tree population. The mortality of mature trees (Acacia xanthophloea) had increased and caused concern for the future of the forest. For a number of reasons the mature trees including also Rauvolf ia caffra were dying at a relatively rapid rate, and there was little regeneration to ensure adequate forest replacement. A research project was necessary, to establish the causes of both tree mortality and low levels of regeneration. The study served to establish baseline data as a basis for future monitoring. Multiple plot and systematic sampling were employed in the establishment of easily re-located permanent plots in the Lerai forest. A quantitative vegetation description of the forest was conducted. A. xanthophloea was found to be the most dominant species (Relative Dominance: RD=68.4%) and the species, together with R. caffra (RD=22.6%) constituted the canopy cover of the forest. The study of tree mortality identified A. xanthophloea as the species most threatened with mortality (35% of its live tree populations were damaged and it formed 95% of all dead trees), though R. caffra had also started to die. Trees of all age clesses were dying and very few died of old age (senescence). Flooding resulting from the rise in the crater water table appeared to be the primary cause of tree mortality. Tree mortality started soon after the onset of unusually high rains between 1962-1965 (Annual average 1200 mm). Though a few trees were dying from long term floods (suffocation), most trees were dying as a result of other factors identified as excess soil exchangeable sodium, high concentrations of salts in soils and high pH levels, as a consequence of flooding. Though elephant damage on trees was observed in Lerai, tree damage by elephants was not a significant element of tree mortality (only 8.4% of the trees were debarked by elephant). The debarking of trees by elephants, however, rendered tree trunks vulnerable to beetle infestation and wind snap. The saplings fell victims of elephant damage (cause of 100% sapling mortality) more than older trees through breaking, uprooting and pushing over. The study of regeneration covered seed dispersal, insect infestation and germination potential; seedling parameters of density, frequency and distribution, animal impact or regenerations; influence of old trees on seedling establishment and survival; and the influence of ground cover vegetation on seedling establishment and distribution. The seeds were observed to be wind dispersed within their light large surfaced pods. Seed insect infestation was found to be high (>70%) and in the majority of cases the individual seeds were severely damaged. Two insect predators of seeds were identified as Eurytoma sp. and Exechesops sp. Seed viability was found to be reasonably high (68%) in non-infested seeds; the germination potential was reduced by insect infestation to almost zero. Elephants were found to be of significance in the establishment, growth, and the recruitment of seedlings and saplings. Elephants supressed the growth, elongation and recruitment into higher size classes of seedlings and saplings, making young tree classes almost absent. Old A. xanthophloea trees were found to inhibit establishment and growth of seedling of their own species under their crown cover (autotoxicity). Since the study was carried out in a brief period (10 months), and covered many ecological aspects simultaneously, recommendations put forward involving the monitoring of rates of change of tree mortality; changes in precipitation, crater water table, seedling density, distribution and browse impact; sapling growth and recruitment with changes in elephant population should be implemented only after additional ecological monitoring studies. Fire in the Lerai should be avoided.