Some Effects of the 1981-87 Drought on Woody Plants in Botswana Author(s): David Parry Source: Botswana Notes and Records, Vol. 20 (1989), pp. 155-159 Published by: Botswana Society Stable URL: http://www.jstor.org/stable/40979823 . Accessed: 28/06/2014 10:36 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. . Botswana Society is collaborating with JSTOR to digitize, preserve and extend access to Botswana Notes and Records. http://www.jstor.org This content downloaded from 185.31.195.106 on Sat, 28 Jun 2014 10:36:29 AM All use subject to JSTOR Terms and Conditions
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Some Effects of the 1981-87 Drought on Woody Plants in BotswanaAuthor(s): David ParrySource: Botswana Notes and Records, Vol. 20 (1989), pp. 155-159Published by: Botswana SocietyStable URL: http://www.jstor.org/stable/40979823 .
Accessed: 28/06/2014 10:36
Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp
.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact [email protected].
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Some Effects of the 1981-87 Drought on Woody Plants ini Botswana
Introduction
A large number of tree and shrub die-offs have been observed in central and south-western Botswana. These die-offs are of such a magnitude that the landscape in many areas will change.
In ecology the role of these major events is important in shaping the environment patterns that we see today. These events are often a result of a number of unusual circumstances which coincide. A major yet sporadic happening of this magnitude is termed an * episodic event'.
The main determinants of arid savannas such as those occurring in Botswana are, in order of importance: moisture, nutrients, herbivore and fire (Walker, 1987). A drought of the magnitude just experienced effects all of the deter- minants, especially moisture.
The sandy nature of the Kalahari soils and the low and erratic rainfall force trees to develop some type of strat- egy to deal with dry periods.
Some trees have physiological attributes to deal with the conditions, while others are restricted to certain habitats where there is a localised increase in ground water (eg. drainage points of pans and perched aquifers on calerete).
Breyer (1983) assessed rainfall by plotting the accu- mulative variation from average annual rainfall. Her work indicated that drought years are the norm rather than the exception. In the drought under discussion, 198 1 to 1987, two events have combined to affect woody vegetation:
• the abnormally wet period of the late 1 970s probably raised ground water tables to such an extent that it allowed woody plant development in areas usually unsuitable for these plant species.
• the drought was continuous, i.e. although none of the years were incredibly dry, the result of the unbroken drought years had the cumulative effect of an episodic event.
This article contains a number of systematically re- corded and general observations of tree and shrub die- offs in the Central Kalahari and Makgadikgadi Pans. It does not represent more than a small percentage of the total plant deaths which have occurred in the country.
The aim of this article is to bring to the attention of environmental research workers the magnitude of drought effects in shaping the vegetation structures which we see today.
Methods
There was no systematic search for die-offs; rather obser- vations were incidental to the author's movements. When a distinct die-off was located, the circumstance of the trunk and the height of the tree were recorded. With shrubs, only the average heights were recorded. Plant densities were recorded for 400 m2 areas located 50m apart. The total population was estimated from a single cross-section through the two areas studied. As a result these data are only a rough estimation of the die-off.
Trees were considered dead if the bark was peeling off, while shrubs were classed as dead if the bark was peeling or the coppice from the base appeared dead. Most die-offs were observed more than once and in both wet and dry seasons.
At die-off locations, the species and size of plants which appeared unaffected were recorded.
Results
Tables 1 and 2 represent the systematically recorded die- offs which occurred in the Central Kalahari and the Makgadikgadi respectively. The minimum recorded number of trees and shrubs which have died is 4.25 million.
That area of the Kalahari with extensive drought effects is shown in Figure 1. In these area large numbers of trees, especially Terminalia sericea, died, with iso- lated deaths of Acacia luederitzii, Burkea africana, Combretum hereroense and a few Lonchocaropus nelsii.
In areas of extreme drought effects, the ríiajority of trees other than Acacia erioloba, L. nelsii and Boscia albitrunca have died. Almost total die-offs of shrubs such as T. sericea, Dichrostachys cinerea and Acacia mellifera occurred in some areas. On thepans many 'tree islands' of Acacia tortillis were seriously damaged.
Discussion and Conclusions
Systematically recorded die-offs in the Central Kalahari are reflected in Table 1, at Makgadikgadi in Table 2.
The observation of Acacia erioloba and Acacia leud- eritzii deaths occurred in an isolated depression where the soils were more calcareous and compact than the surroundings. Although all the plants of these species over 1.15m were dead, very few of the Acacia mellifera and Lonchocarpus nelsii trees were dead. This indicates a higher degree of drought tolerance in the latter two species under certain circumstances.
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A number of Acacia luederitzii die-offs were ob- served in the southern section of the Central Kalahari Game Reserve (CKGR) although only one was systematically atically recorded. The A. leuderitzii populations were typically associated with a slight rise, with the high point colonised by Albizia anthelmintica. In the recorded die- off, the acacias had an average trunk circumference of 57cm and were all dead. The albizias were up to 3m in circumference and all alive.
The difference in size and survival between the two species indicates that the albizia originally colonised the area and obtained the optimal habitat on top of the rise. During a wet period, the acacias colonised the sub- optimal habitat around the albizia. During extended drought periods the acacia die and only the albizia remain.
Lonchocarpus nelsii die-offs occurred in the rela- tively open plains where L. nelsii and Boscia albitrunca were the dominant trees. Some copicing of L. nelsii was observed. No deaths of B. albitrunca were observed in- dicating its drought tolerance.
The Burkea africana die-offs affect large areas (Fig- ure 2). The tree height and circumference indicate that the trees have been in the area for a long time. The effect of the die-off will be extreme, changing the area from a savanna to an open grassland. The survival of burkea increases eastward, indicating the trees are at the extreme west of their range and have invaded the area during wet periods.
The Terminalia sericea die-off of the size class inter- mediate between shrub and tree indicated a stage sensi- tive to drought.
Extensive stands of Terminalia sericea died in the Makgadikgadi fossil lake basin. These stands were usu- ally on slight sand rises with relatively open grasslands between. All the die-offs recorded in the Makgadikgadi and Kalahari indicate that the plants which died were in the 4 - 6m tall group. Isolated trees usually survived and the trees along the Nata-Maun road which received runoff from the road were better developed and had a higher percentage of survival.
The tall dense shrub west of the Gidikwe Ridge near the Nata-Maun road is made up of Terminalia sericea, Combretum collinum and Lonchocarpus nelsii, with Dichrostachys cinerea in patches. In some areas only the L. nelsii has survived the drought and open grasslands could develop.
General observations
In the area around Serube borehole (used by Basarwa livestock) there was a 100% per cent die-off of Termi- nalia sericea and Lonchocarpus nelsii trees with only Boscia albitrunca surviving above the shrub layer. The drought in northern Khutse was very severe, killing entire clumps oí Acacia mellifera. From Khutse to Gope borehole and north up the eastern boundary of the CKGR, most Burkea africana and L. nelsii trees were dead (Figure 3), with B. albitrunca showing greatest survival.
The entire area will open up into a shrub/grassland with the occasional boscia in the upper story.
In the shrub layer, varying percentages of T. sericea, Dichrostachys cinerea and Grewia retinervis died. Drought tolerance in the woody shrub layer was found to be, in order of tolerance; Commiphora angolensis, B. albitrunca, G. flava, L. nelsii, G. retinervis, T. sericea and D. cinerea.
Effect of the die-off
Although approximately 4.25 million tree and shrub deaths were recorded, the total number of deaths are much higher. It is predicted that in areas most affected by drought, fires will destroy much of the present tree and shrub cover, especially acacia and burkea species. The result will be a move towards an open, broad-leafed savanna with burkea and acacia invading during wet periods.
Circumstances after the drought will to a large extent dictate the overall future situation; i.e. high rainfall in the post-drought period together with the low herbivore biomass will result in high fire fuel loads in the dry season. Fires could transform the shrub dominated areas into open grasslands.
The size of the die-off and the predicted changes in vegetation indicate an 'episodic event1 which will affect the landscape long after the drought is forgotten.
Summary
In this first recording of tree die-off in Botswana, the following conclusions are made:
• There will be long term changes to the vegetation communities.
• Various tree and shrub species exhibit different tolerances to drought.
The transitional stage from shrub to tree in Terminalia sericea is susceptible to drought.
• The drought is considered to have been an 'episodic event' due to the size and extent of the woody plant die-off and the probable long term effect on the landscape.
• The 'episodic event' is probably a combination of the abnormally wet years preceding the dry period and the unbroken continuity of the drought.
REFERENCES
Breyer, J.I.E. (1983) Soils of the Lower Boteti Region, Central District. N.I.R. Working Paper 47.
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Caughley, G. and Walker, B.H. (1983) "Working with Ecological Ideas", in Guidelines for the Management of Large Mammals in African Conservation Areas, ed. Ferrar, A.A. (S.A. National Scientific Programmes No 69, Ch 2).
Walker, B.H. (ed.) (1987) Determinants of Tropical Savannas; Presentations made by Savanna Researchers at a workshop in Harare, Zimbabwe. IUBS monograph 3. (IRL Press, UK) p. 156.
By David Parry Kalahari Conservation Society
TABLE 1
NO SPECIES LOCATION CIRCUMFR SAMPLE DEAD% HEIGHT (M) DENSITY/ha AREA (km) TOTAL (cm) NO DEAD
1 A.erioloba East of Nojane 46.5 +/- 3.7 36 72 - 25-30 2 5 000 - 6 000 A. leuderitzii (6/4/87) 68.7 +/- 8 36 56
2 A. leuderitzii 20km north. 57.5 +/- 4.4 30 100 5.75 -f/- 0.37 250-350 2 50 000-75 000 west of Kamaga pan, CKGR (12/4/87)
3 T. sericea 15km north of - 15 100 3.08 +/- 0.23 529 +/- 69 1.3 59 800-77 700 Xaka (14/4/87)
4 L. nelsii South-east of 100 3 75 4 30 000 ca Molopo CKGR (18/4/87)
5 T. sericea Upper Okwa - - 100 2.5 200 0.05 1 000 ca (19/4/87)
6 B. africana 20-67km south of - 20 90 3-7 11+/- 9.5 96+1 1 600 - 177 100 Metsesmonong (20/4/87)
7 B. africana 10-27km south 92.47 +/- 8 32 99 88 +/- 0.5 80-100 289+ 2 312 000- of Metsesmonong 2 890 000 (20/4/87)
8 L. nelsii Cutline east of - - 100 4-6 25 64+ 160 000 ca Gope (21/4/87)
TOTAL 2 629 400 3 416 800
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