Vegetation changes in Lake Manyara National Park.

Woody vegetation was surveyed in the five stands employing the point-centred quarter method. Plants were identified, recorded and various quantitative measurements taken. Absolute and relative values of density, frequency and dominance; as well as importance values and diversity indices were calculated. The number of plant species ranged from 15 to 33. The density of both trees and shrubs was highest in the ground water forest; it was lowest in the Acacia tortlis woodland for trees and in the Acacia xanthophloea Phoeniz reclinata forest for shrubs. The tree species composition varied considerably in the five stands, and only the Endabash mixed woodland and the Magara extension showed some similarity in this respect. The shrub composition was about the same throughout the Park although different shrubs dominated different stands. It was greatest in the Endabash mixed woodland and lowest in the A. xanthophloea P. reclinata forest. However, tree diversity easy highest in the ground water forest and least in the A. tortilis woodland. Shrub diversity was highest in the ground water forest also but lowest in the A. xanthophloea P. reclinata forest. Plant succession was measured by studying the species composition, relative characteristics (density and frequency) of the important tree species and their age class-distribution in the various stands and the results compared with previous data available on similar stands and species. The age-class distribution of the various plant species was also a measure of regeneration potential of the woodlands was considered fair and depended on rainfall and edaphic factors including soil erosion. Climatic changes seemed to affect plant succession in the park, in that the species which were favoured during the wet period of 1968-1970 were being replaced by others during the subsequent drier period. An example of this are the shrubs Osimum suave and Solanum incanum which were very common during the wet period but are now rare species. Fluctuations in the lake level (presently at a low level) and in the ground water flow from above the escarpment also affected plant succession leading to a rapid conversion of woodland to grassland. A comparison of animal population from present figures and previous ones showed a dramatic increase in Hippopotamus (Hippopotamus amphibius) and most of the plains game species. The results also suggested that the elephants were making use of the southern extension and the Marang forest reserve especially now that access to it has been improved by extending the Park. In general, animal (elephant) impact on the vegetation in the Park (especially trees) in the Acacia tortilis woodland had declined since 1970. Younger-age classes of A. tortilis appeared to suffer a higher mortality than the older ones, but elephant attack on these trees did not appear to be selective. Explanations for these observations have been discussed. A study of growth rate of A. tortilis saplings suggested that growth was less vigorous during dry periods when elephant impact was minimal and vice versa. Animal damage to Trichilia emetica in the ground water forest had also declined considerably. Elephant damage was heavier on young Tabernaecrotana ventricosa and Croton macrostachys trees which formed the majority of the understory species. In the A. xanthophloea P. reclinata forest damage to A. xanthophloea trees was still high but the trees appeared to have developed a mechanism of protecting their stems from further elephant. In the remaining stands elephant damage to trees were dying of a unidentified disease. Animal impact on the shrub layer was very severe throughout the Park except in the extension, especially during such a dry spell when browse was the only available diet. It was concluded that the dynamics of the vegetation in the Park was controlled mainly by variations in rainfall patterns and both edaphic factors and elephant impact were secondary depending on rainfall. It was suggested that although diversity was initially increasing, it was now decreasing, and that if such a trend continued, reversible conditions of the habitat would be very slow if they occurred.