The Ecology of Black Cotton Glades

1. Brush only Clearing only Dung only Dung + clear Brush + dung None None Brush, clear, dung a) Fert + clear =dung =N fertilizer =Acacia brush =trees/uncleared Figure 4. Mean (+1 S.E.) of Cynodon and Pennisetum cover in 5 new (<45 yrs) and 5 old (>45 yrs) glades. b) Clearing only Dung + clear c) Figure 5. Herbivore dung count data inside and outside of new (<45 yrs) and old (>45 yrs) glades. Figure 2. Successional sequence of abandoned black cotton boma vegetation: (a) a newly abandoned boma, (b) a new glade (~7 years) covered with Star grass, Cynodon plectostachyus, and (c) an old glade (>45 years) covered with Pennisetum stramineum. Kari E. Veblen University of California-Davis, USA (keveblen@ucdavis.edu) The Ecology of Black Cotton Glades Research overview My research explores the ecology of abandoned boma sites in the black cotton ecosystem. In Laikipia, old boma sites, referred to as “glades,” are recognized as large treeless patches (approx. 0.5-1.0 ha) of nutrient-rich grass that dot the savanna landscape (Figures 1 and 2). Abandoned boma sites such as these occur throughout much of eastern and southern Africa in various forms (Stelfox 1986, Blackmore et al. 1990, Reid and Ellis 1995). In Turkana, Kenya, old boma sites are the mirror image of Laikipia’s old bomas: dense Acacia patches in an otherwise treeless landscape (Reid and Ellis 1995). Glades can remain on the landscape for many decades—at least 45 years in Laikipia. Understanding how glades develop over time and how plants and animals fit into their development are integral to a scientific understanding of the black cotton ecosystem as a whole. Additionally, better understanding glades, which attract both wild and domestic animals, may be useful to livestock and wildlife managers. Here I present two components of my research: 1) description of the plants, animals, and soil associated with black cotton glades as they develop over time and 2) experimental boma-building that explores how dung addition, tree-clearing and Acacia brush-addition— three major components of bomas—influence glade development. • Making glades • Introduction • The objective of this experiment is to test methods of creating glade-like structures—areas that are nutrient rich and attractive to wildlife and livestock. I am testing the effects of three factors on glade development: • fertilization with dung or commercial fertilizer • clearing of trees to make open areas • addition of Acacia brush, which may create safe sites for the establishment and growth of certain plant species preferred by herbivores (Figure 6). • Fertilization may increase the nutritive value of the plants in the area or encourage the establishment of lawns of Cynodon, thereby attracting wildlife or cattle. If cleared or brush-addition areas attract cattle or wildlife, increased defecation and/or grazing may stimulate grass production to produce more attractive forage. Each of these three factors or some combination of them could perpetuate an escalating cycle of increased nutrients and increased herbivore use, leading to glade-like structures. • Methods • I am using a combination of 50x50 m. plots (shared with C. Riginos’ tree density experiment) and 10x10 m. plots to test the influence of dung and fertilizer addition, tree clearing and Acacia brush addition on glade development in the Mpala and Jessel black cotton soil (Figure 7). In 50x50 m. plots (with a 10 m. buffer on each side), I will compare five replicates each of the following three treatments: 1) trees cleared, 2) trees cleared with 10x10 m. patch of dung at center, and 3) no tree clearing. In 20x20 m. plots, I will compare five replicates each of the following treatments: 1) dung addition in inner 10x10 m. area, 2) tree clearing in full 20x20 m. area, 3) Acacia drepanolobium brush addition around inner 10x10 m. area, 4) brush addition+dung addition, 5) clearing +dung addition, 6) brush addition +clearing+dung addition, 7) fertilizer addition in inner 10x10m. + clearing, and 8) control (no manipulations). In addition to making comparisons among plots of the same size, I will be able to compare similar treatments of different sizes. I am regularly sampling vegetation, dung (animal use), termite presence, and soils to test which factors are most responsible for the functional traits of glades. • Significance/application • Understanding how old bomas develop into glades is of significance for understanding dynamics of the ecosystem and is also potentially of interest to livestock and wildlife managers. The results of this study will complement my descriptive study by lending insight into the mechanisms behind glade development. Furthermore, my results may aid interested land managers in creating glade-like structures to provide nutrient-rich grazing areas for cattle or wildlife. Figure 1. Quickbird satellite image (June 2003) displayed in false color infrared, with red indicating higher plant productivity. Image covers portions of Mpala, Jessel and Segera black cotton soil. Glades appear as small red patches across the landscape. Recently abandoned boma • Description of black cotton glades • Introduction • Although the vegetation, soil and animals associated with glades in red soil have been described (Young et al. 1995, Augustine 2003), black cotton glades have not yet been formally described. Because the two ecosystems differ so dramatically, we would expect black cotton glades to be characterized by very different communities of plants and animals, and by unique soil characteristics. I surveyed plants, animals and soil on new and old glades to better understand how glades develop over time in the black cotton soil. • Methods • I surveyed five young glades (<45 years old, Figure 2b) and five old glades (>45 years old, Figure 2c) on Mpala and Jessel properties. In these glades, I measured plant cover inside and outside of the glades, using a series of pin-frame measurements (Figure 3). I also assessed animal use of these glades by identifying and counting different piles of animal dung found inside two 20x20 meter blocks inside each glade and along two 100m-long (and 4m-wide) transects located at 100m and at 200m away from each glade. Finally, I collected soil (at 0-30cm depth) inside and outside each glade and had the soils analyzed for nitrogen, phosphorus, potassium, calcium, magnesium, and sand/silt/clay content. I tested the data for differences in vegetation, dung, and soil inside and outside of new versus old glades. • Results and conclusions • New glades in the black cotton soil were dominated by the mat-forming Cynodon plectostachyus (Star grass), followed by dominance by the taller bunch grass, Pennisetum stramineum, in old glades (Figure 4). • Cynodon plectostachyus almost never occurred outside of glades. • Bracharia, Lintonia, Themeda, and Mezianum grass species, which dominate outside of glades, were almost never found inside glades. • Total dung density of all herbivores was higher inside new glades than outside (Figure 5). Zebra dung density was highest outside of glades, while Grant’s gazelle dung was highest inside glades. Elephants appear to strongly prefer new glades, and cattle show a slight preference for new glades. • Relative to non-glade soils, glade soils were high in phosphorus and potassium, but low in magnesium, calcium, sodium and clay content. Nitrogen was high in all glades, but highest in new glades. • Because cattle and wildlife are important players in the black cotton ecosystem and also appear to use glades especially heavily, herbivore activity may influence the rate at which glades make the transition from Cynodon- to P.stramineum-dominance. For example: • A plant that has been damaged by herbivory may be at a competitive disadvantage for resources such as water or light. In such a case, heavier grazing of Cynodon would lead to a faster takeover by P. stramineum. • Herbivores may export nutrients out of glades when they graze on high-nutrient glade grasses. If Cynodon competes better in high-nutrient environments, then herbivores exporting nutrients may speed the transition from Cynodon to P. stramineum. • I am using the KLEE experiment (Young et al. 1998) and conducting additional experiments on Mpala, Jessel and Segera properties to try to understand how the effects of herbivores influence the nature of competition and interactions between the two glade grass species. Glade ~7 years old Glade > 45 years old Figure 3. Ten-point pin-frame used to assess plant cover. Observers count how many times each plant species is hit by a vertical pin (maximum 10 hits for each species). Figure 7. Schematic diagram of treatments for glade-making experiment testing the effects of dung and fertilizer addition, brush addition, and tree clearing on glade development. Large plots are 50x50m., and small plots are 20x20m. Small focal plots included within larger plots are 10x10m. References:  Augustine, D. J. 2003. Long-term, livestock-mediated redistribution of nitrogen and phosphorus in an East African savanna. Journal of Applied Ecology 40:137-149. Blackmore, A. C., M. T. Mentis, and R. J. Scholes. 1990. The origin and extent of nutrient-enriched patches within a nutrient-poor savanna in South Africa. Journal of Biogeography 17:463-470. Reid, R. S., and J. E. Ellis. 1995. Impacts of pastoralists on woodlands in South Turkana, Kenya: livestock-mediated tree recruitment. Ecological Applications 5:978-992. Stelfox, J. B. 1986. Effects of livestock enclosures (bomas) on the vegetation of the Athi Plains, Kenya. African Journal of Ecology 24:41-45. Young, T. P., B. D. Okello, D. Kinyua, and T. M. Palmer. 1998. KLEE: a long-term multi-species herbivore exclusion experiment in Laikipia, Kenya. African Journal of Range and Forage Science 14:94-102. Young, T. P., N. Patridge, and A. Macrae. 1995. Long-term glades in acacia bushland and their edge effects in Laikipia, Kenya. Ecological Applications 5:97-108. Figure 6. Experimental Acacia drepanolobium brush addition to simulate boma fence. Acknowledgements: Many thanks are owed to the following for their help in the field: Frederick Erii, John Lochukuya, Jackson Ekadeli, Patrick Etelej, Corinna Riginos, James Ekiru, Jake Goheen, Ali Hassan, Dan Kelly, John Lemboi and Simon Lima. Thank you to Truman Young for advice, support and field assistance. Thank you to Mpala Research Centre, Mpala Ranch and Peter Jessel for permission to use field sites and for logistical support. Funded by NSF and U. California-Davis Jastro Shields grant.