A dissertation submitted in fulfilment of the academic requirements for the degree of Master of Science in the School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Johannesburg.
June 2016 / The African baobab (Adansonia digitata) is a charismatic and iconic tree. This
keystone species has highly specific moisture requirements for recruitment, a very
slow life history, low dispersal capabilities, and is exposed to heavy, often
destructive utilization by baboons, elephants and humans. These characteristics
result in the reduced ability for baobabs to accommodate and respond to changing
environmental conditions. During periods of unfavourable conditions, baobabs
rely on the persistence of established individuals for the survival of the
population.
The Kruger National Park (KNP), South Africa falls within the southern-most
distribution of the baobab. A perceived lack of recruitment together with high
numbers of dead large baobabs recorded in aerial surveys prompted concern over
local population decline in the early 1990s. Long-term monitoring is necessary in
order to quantify changes occurring within the KNP baobab population. For this
reason Dr Ian Whyte sampled 424 baobab trees in northern KNP in 1995/1996 to
describe population size-class distributions and elephant damage. Nearly all of
these baobabs, with the addition of 486 individuals were resampled in 2001 by
Michele Hofmeyr. This study resampled the majority of these 910 baobabs plus
126 individuals within 50m of those previously sampled, culminated in a data set
spanning 18 years and most of the dominant vegetation and climatic zones within
the baobab’s KNP range. Photographs taken in 1995/6 and repeated in 2013
allowed for simultaneous direct visual comparisons of elephant damage to baobab
stems. This study aimed to record the temporal and spatial changes in the
demographics of, and damage to, this sample of 1036 baobabs in order to define
and evaluate the factors threatening KNP baobab persistence.
The overall population stem diameter structure of baobabs in the KNP showed a
healthy inverse J-shaped distribution, with a high proportion of smaller trees and a
decreasing proportion of larger trees at each sampling snapshot. However no
seedling recruitment was observed during intensive searches within 50m of
sampled trees, accumulating in a total area of 4km2. Growth was only noticeable
in sub-adult trees >0.15m and <1m in stem diameter. Annualised mortality
increased three-fold between the periods 1995/6-2001 (0.25%) and 2001-2013
(0.79%). It is speculated that this increase in mortality is due to the cumulative
effect of increased elephant damage, together with below average rainfall years of
2002 to 2013. Trees that had previously sustained severe and very severe damage
contributed the highest proportion (32%) of mortality. When comparing sizeclasses,
the majority of mortality (61%) was recorded within the <1m stem
diameter size-class. However, a high proportion of very large trees (21% of the 4-
4.5m and 29% of the 5-5.5m stem-diameter baobabs sampled) had died from
unknown causes. With no recruitment of seedlings evident and little recruitment
between size classes, mortality alone was not enough to significantly change the
baobab population structure between 1995/6, 2001 and 2013. This might change
should more very large (>4m stem-diameter) trees die, as these size classes make
up less than 2% of the population.
At a spatial scale, maximum fire return interval, level of elephant damage and
mean annual temperature all had significant impacts on the size-class distribution
of baobabs. Longer (>30 years) maximum fire return intervals and higher mean
annual temperatures (23°C) supported an inverse J-shaped baobab population
structure whilst shorter (<30 years) maximum fire return intervals and lower mean
annual temperatures (21-22°C) supported a bell-shaped population structure.
Increases in bark damage over time correlated with increased elephant density
(primarily attributed to mature male elephant density). Smaller baobabs – those
less than <1m in stem diameter, generally escaped elephant utilization altogether,
or suffered very severe damage by elephants.
Despite a 3-fold increase in mortality and no recruitment over the 18 year study
period, the KNP baobab population has maintained its inverse J-shaped
population structure. The high proportion of mortality in the >4m stem diameter is
however concerning as the persistence of these large baobabs is essential for the
survival of the population during unfavourable conditions for recruitment.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/21026 |
| Date | January 2016 |
| Creators | Taylor, Robert William |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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