The effects of fire on the regeneration of acacia species in Lake Mburo National Park, Uganda

Abstract

The thesis investigated the effect of fire on the regeneration of Acacia species in Lake Mburo National Park, Uganda. The mechanism that cause the effects and the manner in which the effects are influenced by fire are also discussed. The study was carried out in Lake Mburo National Park between 1997 and 1998. To explain the relationship between the incidence of herbivory and the height and position of the sprouts, the effects of fire and cutting were investigated. All sprounts were monitored for incidence of herbivory. Results of fire experiments in five sites in Lake Mburo National Park showed that the quantity and quality of fuels available for fire varied widely in relation to weather but also in relation to the degree of use by man and herbivores. The amount of phytomass available was an important factor in determining the fire intensity, scorch height and char height. Furthermore, the intensity of fire also depended on the season and frequency of burning. Wet periods which foster shrub establishment also promoted abundant grass growth. However, frequent burning prevented the building up of fuel load sufficient to support a high intensity fire. Although fire induced the germination of all Acacia species in the study, the ability of seeds to germinate depended on the fire intensity and the seed size. Low fire intensities induced the germination of Acacia hockii seeds. However, these positive effects of fire on seed germination were short lived and only 20% of the seedlings got established. The population structure of Acacia species was investigated and non- uniform population structure was found. The results indicated that population structures differed widely among stands and changes in population structures after fire varied from site to site depending on the fire intensity and Acacia species. Although Acacia hockii was found to be dominant, most individuals were trapped in the lower height size classes. While fire intensity was the prime factor influencing the effects of fire on Acacia species, increase populations of species can also be attributed to frequent burning and increased human activities in and around Lake Mburo National Park. The results of this study indicated that frequent burning and the subsequent low intensity fires experienced in Lake Mburo National Park favoured the proliferation of Acacia hockii. The factors that enhanced the ability of Acacia species to escape being forced back by fire were also investigated. In all instances, the escape height varied among species with Acacia hockii escaping at the lowest height and Acacia polyacantha had the heighest escape height. In this study, the capacity for Acacia species to escape fire depended on the fire intensity, bark thickness, height and diameter of individual plants at the time of burning. Sprouts from Acacia sieberriana and Acacia gerrardii were most preferred by the browsers. Although the incidence of herbivory decreased with increase in height of the sprouts, the position of the sprouts did not determine palatability of all Acacia species. Only sprouts from the base of Acacia sieberriana were browsed whereas for Acacia gerrardii all sprouts were browsed irrespective of the position. The study also showed that cutting down of Acacia species for charcoal burning and building poles was a major factor in determining the tree structure and the sprouts density. All individuals of the different species in the study area sprouted after cutting irrespective of the size class and the position of the sprouts was correlated to the size of the original tree. Small individuals sprouted at the base and as the size increased, the position of the sprouts changed. Burning of the stumps after cutting had more negative effects on Acacia gerrardii compared to other Acacia species where only individuals in 4.00- 4.99m height size class retained the capacity to sprout.

Fire was also seen to increase the flowering capacity of the Acacia species. Acacia hockii had the shortest individuals flowering whereas Acacia polyacantha started flowering after attaining a height of 4m. There was a positive correlation (r = 0.974, n = 4, P < 0.05) between the minimum height of flowering and the maximum height an individual could attain. Sprouts from burning and or cutting treatments were also monitored for flowering and only sprouts of Acacia hockii were able to flower the next flowering season after treatment and produced more seeds.