Woodland loss and restoration in a savannah park: a 20 year experiment
Published source details
Western D. & Maitumo D. (2004) Woodland loss and restoration in a savannah park: a 20 year experiment. African Journal of Ecology, 42, 111-121.
Published source details Western D. & Maitumo D. (2004) Woodland loss and restoration in a savannah park: a 20 year experiment. African Journal of Ecology, 42, 111-121.
Large scale loss of acacia savannah woodland, dominated by Acacia xanthophloea, has been recorded since at least 1961 in Amboseli National Park, Kenya. Several theories were proposed for the woodland loss and associated habitat changes, ranging from livestock overgrazing, game over-browsing, pathogens/pests, increasing soil salinity and climate change. Each theory assumed a different cause and by inference, remedy.
Of particular concern was collapse of elephant Loxodonto africana migration caused by heavy poaching in the 1970s in areas beyond the park boundaries. Elephants have a complex social structure that is sensitive to external abrupt changes e.g. if the alpha matriarch is taken out through poaching the group may alter dispersal behaviour, such as abandoning migration across dangerous territories. This led to a sharp rise in elepant numbers within the park resulting in increased grazing and browsing pressure. This was the main theory advanced for loss and lack of regeneration of the acia woodland. An initial series of experiment was set up to look at the effect of both invertebrate and vertebrate grazers on A.xanthophloea seedling growth (see Case 243). This was followed by a second phase looking at the effects of excluding elephants but allowing other large grazers access, on Acacia growth .
Study site: The study was located in the centre of Amboseli National Park, Kenya, in an area between Ol Tukai Lodge and David Western's research house.
At this site tree canopy cover in 1969 was estimated at 17%. At this time 8% of mature trees (average height 17 m) were dead or dying. There was also no evidence of any Acacia xanthopholea recruitment since about the 1950s, based on stem diameter measurements and aerial photographs dating back to this period (Western 1973).
A series of experiments in two phases (see Case 243 for the first phase) were designed to establish the cause of the decline in acacia woodland, and to establish mangement techniques to enhance woodland restoration. In 1981, when experiments were initiated, all mature acacia trees had died in the study area and no successful recruitment had occurred for about 30 years.
Phase 2 experiment: Conducted from April 1985 to December 1990, this second phase experiment was designed to distinguish the impact of elephants Loxodonto africana on A.xanthopholea growth from all other large vertebrate herbivores. Forty Acacia seedlings were tagged within a 100 x 100 m area.
Twenty seedlings were protected from elephants using an electric fence of single wire, suspended 2 m above ground. A solar electric energizer powered the wire with 5000v current pulsed at 1 second intervals. This fence design allowed all other herbivores e.g. impala Aepyceros melampus, zebra Equus burchelli, wildebeest Connochaetes taurinus and buffalo Syncerus caffer, to pass under, with the exception of giraffe Giraffa camelopardalis. No giraffe were infact recorded in the area during the experimental period. Twenty Acacia seedlings were left exposed to all grazers outside the enclosure.
Seedlings outside and inside the enclosure were matched for height at the start of experiment so that growth rate etc. could be monitored and compared.
Seedling measurements: Plant height, stem diameter and conditions (leaf growth vigour) were recorded monthly. Observations continued until December 1990 when a tangled growth of regenerating scrub in the elephant exclosures made further measurements impractical.
Phase 2: The Phase 2 experiments narrowed the candidate further to elephants as the main factor preventing acacia woodland recovery (see Table 3, attached). Over the 5-year duration of the experiment, in the area where elephants were prevented from entering but other large vertebrates could enter, seedlings grew from about 53 cm in 1985 to over 3 m by the end of 1989. In contrast in the area where elephants were able to enter, seedlings were severley stunted and reached on average, only 55 cm in height (an increase of only 2 cm in five years) by 1989.
Conclusion: This experiment narrowed the candidate for failure of of Acacia growth and recruitment to elephants as the main factor preventing acacia woodland recovery.
Ongoing restoration: The Ol Tukai restoration project was a test case for restoring other pockets of woodlands across Amboseli. As a direct outcome of the experiments, a woodland restoration project was launched in 1985. The restoration programme involved fencing off an area of 250 ha around Ol Tukai in 1985. Three additional elephant exclusion plots were set up around the lodges at Ol Tukai between 1990 and 1992. In 1995 all four plots were linked up to include an expanded contiguous woodland restoration plot of 1.6 km². As the restoration project encompassed the Phase 2 project area, observations of the original A.xanthophloea seedlings in the experimental plots are ongoing. In the long term it is hoped that these fenced plots can be successfully scaled up to provide refugia for woodland plants and animals that were declining or had been extirpated from the park, and also to restore seed production to further enhance the regeneration of Acacia xanthophloea woodland.
Western D. (1973) The structure, dynamics and changes of the Amboseli ecosystem. PhD dissertation, University of Nairobi, Nairobi, Kenya.
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