Woodland loss and restoration in a savannah park: a 20 year experiment

Summary

Study 1

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 and an experiment, summarised here, was set up to look at the effect of both invertebrate and vertebrate grazers on A.xanthophloea seedling growth. A second experiment was subsequently undertaken to assess A.xanthophloea seedling growth when soley elephants were excluded (see Case 244).

Study site: The study site was located in the centre of Amboseli National Park, Kenya, in an area between Ol Tukai Lodge and David Western's research house. The site was one of eighteen previously studied areas where investigations into causes of woodland decline had been undertaken in 1969 (Western & Van Praet 1973).

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 244 for second phase) were designed to establish the cause of the decline in acacia woodland, and to establish mangement techniques to enhance woodland restoration.

Establishment of study plots: In 1981, when this was initiated, all mature acacia trees had died in the study area and no successful recruitment had occurred for about 30 years. However, acacia seedlings were observed and an experiment was designed to investigate the potential problem of grazing, by both invertebrate and vertebrate grazers, upon seedling growth. In the study area, 32 plots were randomly selected. Plot sizes varied from 0.0025 to 0.4 ha, depending on seedlings density, which averaged 92 seedlings/ha.

Phase 1 experiments: The first phase of the study was conducted in 1981 to September 1984. Forty A.xanthophloea seedlings were tagged (to allow identification) in each study plot. Twenty were protected from large herbivores and the other 20 were left exposed. Both the protected and exposed seedlings were randomly assigned to four experimental treatments with five replicates. The treatments were aimed to detect:

i) Effect of invertebrate browsers - A light atomised mist spray of Roxion insecticide was applied every two weeks to deter invertebrate browsers.

ii) Effect of vertebrate browsers - Protection of the twenty seedlings in each plot from vertebrate browsers was undertaken by installation of a large 1.6 m wide x 1.3 m deep game barrier to exclude elephants Loxodonto africana, and an inner 1.5 m 5-strand wire fence to exclude other large ungulates including impala Aepyceros melampus, zebra Equus burchelli, wildebeest Connochaetes taurinus and buffalo Syncerus caffer. The remaining twenty seedlings outside the exclosure were exposed to large herbivores.

iii) Effect of of plant competitors - Above-ground plant competition was eliminated by regularly clipping the herb-layer to ground level within a metre radius of the experimental seedlings.

iv) Interactions between these three factors - Effects of varying the three treatments above on acacia seedlings growth. These observations continued twelve months after terminating the three above treatments.

The assumption was recruitment in the exclosure and the absence of recruitment in the control plot would highlight grazers as the main reason for lack of seedling recruitment and discount other theories such as climate change, soil salinity and pathogens as holding back woodland regeneration.

Seedling measurements: Height, stem diameter, leaf length, leafiness (an index of leaf area) and signs of herbivory were recorded monthly. Large herbivore browsing was distinguished from invertebrate browsing by the presence of stripped-off whole leaves and distinctly nibbled shoots.

 

Study 2

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.

References:

Western D. (1973) The structure, dynamics and changes of the Amboseli ecosystem. PhD dissertation, University of Nairobi, Nairobi, Kenya.


Note: If using or referring to this published study, please read and quote the original paper. Do not quote as a www.conservationevidence.com case as this is for previously unpublished work only.

The growth of Acacia seedlings under the different experimental treatments and inside vs. outside a large herbivore exclosure are shown in Tables 1 and 2, and Figures 1 and 2 (reproduced by kind permission of the authors, D.Western & D.Maitumo), results are summarised below.

Effect of invertebrates: No caterpillars or other invertebrate herbivores were observed on the sprayed plants, and leaf vigour was not affected by insecticide spraying. Seedling growth was slowed by invertebrate grazing in the early stages but these plants recovered.

Seedling growth inside & outside the exclosures: By October 1983, Acacia seedlings inside the exclosure were growing vigorously, the tallest plants had reached 2.8 m and were beginning to overshadow slower growing seedlings. In 1984 seedlings inside the exclosure were too tall to warrant further insect or plant competitor treatment so only stem diameter and height measurements of seedlings protected from and exposed to herbivores were monitored to see if they reached maturity (see Tables 1 and 2, attached).

Conclusions: The herbivory hypothesis predicted that the Acacia seedlings would grow in the absence of browsers but not in their presence. The experiment was consistent with this as seedlings grew rapidly when large vertebrate browsers were excluded but were stunted in the unprotected controls. To assess if elephants were the main cause of seedling failure as opposed to other large grazers, a second experiment was undertaken (see Case 344).


Note: If using or referring to this published study, please read and quote the original paper. Do not quote as a www.conservationevidence.com case as this is for previously unpublished work only.