Abstract
A major challenge for ecologists has long been to develop a model to explain the coexistence of grasses and trees in the savanna. The recent shift in emphasis to non-equilibrium-based theories has resulted in a rethinking of this problem. As resource allocation models have been replaced by demographic ones, the focus has shifted to plant life histories. The tree/grass ratio is now conceptualized as a function of disturbance history. Empirical studies demonstrate that repeat fires trap tree sprouts in perpetual juvenile states. Ecologists suggest natural pathways for juveniles to escape, reach maturity, and maintain tree/grass ratios. This study documents how long-fallow agriculture serves as an anthropogenic pathway. The study compares tree cover on long-fallow and unfarmed savanna plots in southern Mali where burning is annual. Tree height, girth, and species were recorded for 29 quadrats. The results demonstrate a significant difference in the size, number, and species of trees; those on fallow plots were taller, more numerous, and more diverse.
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Notes
It is interesting to note that in South Africa and parts of East Africa, scientists were not concerned about savannization, rather they were concerned about the opposite effect—so called “shrub encroachment” because the predominant economic activity in the region was livestock grazing and thus increasing woodiness was viewed as degradation (e.g., Trollope and Trollope 2004; Ward 2005).
Due to the wide range of what constitutes a ‘savanna’ debates persist in the literature over how this vegetation should be classified, especially when it comes to distinguishing “savanna” from “woodland” (e.g., Lawesson 1994). Here I adopt the term “savanna landscape” from Bourliere and Hadley (1983) who recognized that savannas are complex landscapes that may be interspersed with patches of gallery forests, dense woodlands, marshes or other vegetation forms.
The “degradation” bias was sometimes recognized in later reports. For example, South African researchers reviewing early findings of one experiment concluded that, “a number of treatments were included to illustrate or prove the damaging impacts of natural and artificially ignited fires rather than to meet the needs of the original objectives.” Their report concluded that this “value-loaded” agenda resulted in the inclusion of unnecessary treatments and the exclusion of necessary ones (Biggs et al. 2003:2).
The term “savannization” has two meanings: in the first it concerns the processes through which tree cover declines as grass cover expands, and in the second, it concerns the processes through which forest species are replaced by savanna species. I use the term in reference to the former throughout the text.
It is possible, for example, that a shift from a regime of late to early fire caused by humans would not be detectable in the charcoal records.
While some scholars think of savannas as ever moving between what are called “multiple equilibrium states” such as open grassy savannas, wooded savannas, and woodlands (e.g., Dublin et al. 1990), others, including Stott (1991), are of a more radical disposition and regard savannas as being intrinsically non-equilibrium systems, in which every savanna organism responds individualistically to changes in the ecological determinants.
In some cases plots had been previously farmed, but in most cases details are absent from the published accounts.
It could be argued that our knowledge of the role of “natural” factors in savannas is far better developed than that of human ones in terms of disturbance and storage effect models. To give but one example, Higgins and colleagues (2000) (see above) fail to mention that the latter two factors in their model are a function of human practices because people set fires and cut trees.
See Mistry and Berardi (2006) for an exception.
For example, Menaut and colleagues’ (1991) found that 25–50% of the Sudan Savanna, and 60–80% of the Guinean Savanna in West Africa burned annually. Eva and Lambin (1998) found slightly lower values of 28.2% for the Sudan zone and 51.8% of the Guinea zone of central Africa. Barbosa and colleagues (1999) found higher levels of burning in the Sudan zone—70.1%, and lower values in the Guinea zone—57.7%.
Cut-off dates are problematic because the point in the dry season at which trees become susceptible to fire damage will vary from year to year depending upon rainfall and micro-environmental factors which play a key role in determining soil moisture content. As such, the drying point of grasses and trees tends to vary widely across the landscape.
Here I use Bowé as a generic term for all areas with similar upland soils on Ferricrete that support vegetation cover dominated primarily by annual grasses (Loudetia tongoensis and Andropogon pseudapricus) with few scattered trees.
Isoberlina doka are known locally as the “first tree.”
According to Nicholson (2000), mean rainfall in the Sahel decreased by 25–40% between 1931–1960 and 1968–1997; every year in the 1950s was wet, and nearly every year since 1970 has been anomalously dry.
Note that fires can burn annual grasses earlier during the dry season since these grasses tend to desiccate before perennials; however, when these early fires reach a boundary composed of perennial grasses, they often extinguish due to the higher moisture content of perennials. As such, patches of perennial grasses are commonly burned at later dates even if they are in close proximity to patches of annuals.
It should be noted that the northern most case, which still receives more rainfall than the Mali sites, experienced only minor increases in tree cover (Fairhead and Leach 1996).
One could make the case that farmed and burned fields are “intensively” managed. My point here is that compared with other sites, the Mali sites are relatively less intensively managed. The plots studied were on former hamlets and were typically farmed for 4 or 5 years and then completely abandoned.
Dauget and Menault (1992) also document a case in the Guinea savanna where trees have “naturally” (without the presence of agriculture or grazing) encroached on a savanna plot despite frequent burning.
It is also probable that the method of farming is equally important; as Nyerges (1989) has found, hoe farming tends to be far less disruptive of tree cover than farming with an ox plow.
In this study for example, the precise age of the fallow plots, the original land cover, the duration of farming, and the exact burning regime were unknown to the scientist who relied upon local knowledge for this information.
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Laris, P. An Anthropogenic Escape Route from the “Gulliver Syndrome” in the West African Savanna. Hum Ecol 36, 789–805 (2008). https://doi.org/10.1007/s10745-008-9203-4
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DOI: https://doi.org/10.1007/s10745-008-9203-4