The word‘chipya’ is derived from the Bemba verb ‘kupya’ meaning ‘to burn’, the noun referring to a ‘burnt place’. Trapnell first introduced the term to the scientific literature in his pioneering ecological survey of Zambia, where he specifically applied the word to areas of wooded grassland dominated by fire-resistant woody species, tall grass and a characteristically luxuriant herbaceous layer defined by the key indicator species Aframomum alboviolaceum, Smilax anceps and Pteridium aquilinum. Trapnell also observed that chipya was often associated with relict patches of dry evergreen forest, and this factor together with the herbaceous floristic indicators led him to postulate that chipya occurs over areas formerly covered by evergreen forest and since degraded by fire. Trapnell’s theory of chipya’s evergreen origins has received support from a number of authors and Lawton has suggested a regression sequence of mateshe (evergreen forest) ® herb/grassland ® chipya, driven by fire and/or cultivation. In his original study, Trapnell also noted that chipya soils, particularly those of the lake basins, were characterised by a dark humic layer in their upper horizon, a factor he related to the luxuriant herbaceous characteristic of this vegetation type.

Lawton has since challenged the nature of the chipya humic horizon, the uniqueness of chipya soils, and the floristic integrity of chipya itself. Using data from nearly 400 vegetation-soil plots in north-eastern Zambia, he found it floristically impossible to categorise many of the plots into miombo, chipya or mateshe, encountering what he described as a continuum, with most plots intermediate between two or more vegetation types. He also found no significant differences between chipya and miombo soils, although no experimental evidence was published regarding this assertion. As a result of his findings, Lawton has proposed that a chipya-miombo-mateshe succession may occur, in reduced fire conditions.

Based on experimental evidence from Zambia’s Kasanka National Park (KNP), this paper attempts to answer some of the key questions raised in chipya ecology. Can mateshe, chipya and miombo vegetation types be clearly defined floristically and edaphically, and if so, how do these factors relate to mateshe-chipya-miombo dynamics?

The results from KNP suggest that chipya can be differentiated from miombo by the absence of the fire-sensitive genera Brachystegia, Julbernardia, Isoberlinia and Uapaca, and by its luxuriant herbaceous layer comprising the indicator species Pteridium aquilinum, Smilax anceps and Aframomum alboviolaceum. In addition, chipya soils were shown to typically possess high organic matter, low phosphorus and low pH compared to miombo soils. Finally, the evidence in KNP suggests that chipya arises from a mateshe-chipya regression driven by fire. There is no evidence that the fire-sensitive miombo genera Brachystegia, Julbernardia, Isoberlinia and Uapaca are part of the sequence.

1. CHIPYA IN ZAMBIA: A SHORT REVIEW

The term ‘chipya’ is derived from the Bemba verb ‘kupya’ meaning ‘to burn’, the noun referring to a ‘burnt place’. Trapnell (1943) first introduced the term to the scientific literature in his ecological survey of North Eastern Rhodesia, where he noted that the word was applied by the Bemba to areas of wooded grassland dominated by fire-resistant woody species, tall grass and a characteristically luxuriant herbaceous layer. Trapnell also observed that chipya is usually associated with pockets of evergreen forest or thicket, and this together with other floristic indicators led him to postulate that chipya occurs over areas formerly covered by evergreen forest and subsequently degraded by fire. Since Trapnell first used the term, it has been applied by many in a broader definition encompassing miombo-derived fire climax woodlands (e.g. Lawton, 1978; Celander, 1983; Rodgers, 1996). As we shall see below, the definition of chipya rests on the relative importance of factors as diverse as floristic composition, fire, soils, termites and climate. The respective roles of these elements is contentious and has led to these differing definitions.

In the regional vegetation treatments of Wild & Barbosa (1967) and White (1983) the distribution of chipya is centred around Zambia, with similar elements identified in Shaba province of Congo, Tete Province of Mozambique, and Malawi. Both of these studies are too broad in scale to map chipya separately. At least two authors (Kikula, 1986; Rodgers, 1996) have placed chipya in southern Tanzania. However, in both of these cases, the definition of chipya was taken from Lawton (1978), and no distribution maps were produced.

Trapnell et al. (1947) were the first to map the distribution of chipya in Zambia. The soil-vegetation map produced in this study was derived from a comprehensive series of ground traverses carried out by vehicle and on foot, mainly by C.G. Trapnell, over a period of many years (see Trapnell, 2000). Five chipya types were recognised: typical lake basin Erythrophleum-Pterocarpus chipya, mainly concentrated in the Chambezi-Bangweulu basin and Luapula valley, but with outliers in Mbala, Chinsali, Mpika and Serenje districts (B4); Chipya Forest mixtures with Brachystegia-Isoberlinia vegetation in the Copperbelt region (B4); a scrub chipya type on the Bangweulu sandbanks (S); a Kalahari or Bracken sand chipya type in Mwinilunga District (K11); and chipya on red earths near Mbala and on the Copperbelt (R).

Edmonds (1976) is the only other study which has attempted to map all of Zambia’s chipya. This work was based on aerial photographs of the whole country, combined with ground truthing carried out on foot and by vehicle. Fanshawe’s (1969) classification of the vegetation of Zambia (see below) was used as the basis for this study, and here only two types of chipya vegetation are recognised: lake basin chipya (equivalent to Trapnell’s B4 and R types), and Kalahari sand chipya (equivalent to Trapnell’s K11 type). Edmond’s map (scale 1: 500,000) is more detailed than that of Trapnell et al. (scale 1: 1,000,000) and identifies pockets of chipya in many areas not surveyed by Trapnell, particularly in Mpika and Serenje districts, and on the Copperbelt.

Local surveys which incorporate areas of chipya include studies carried out in the following areas: the Central African Rail Link corridor (Report, 1952); the Copperbelt (Report, 1956); Mukabi Forest Reserve (Lawton, 1964); Chishinga Ranch (Astle, 1968); Northern and Luapula Provinces (Mansfield et al., 1973); Kasanka National Park (Cassells, 1995).

Floristically, chipya is characterised by non-miombo (i.e. not Brachystegia, Julbernardia or Isoberlinia), fire-resistant trees, a luxuriant herbaceous component and tall grass. Typical tall tree species include Amblygonocarpus andongensis, Albizia atunesiana, Pericopsis angolensis, Pterocarpus angolensis, Burkea africana, Erythrophleum africanum and Parinari curatellifolia. Smaller trees are Combretum spp. (C. zeyheri, C. molle, C. collinum), Hymenocardia acida, Diplorhynchus condylocarpon, Oldfieldia dactylophylla, Pseudolachnostylis maprouneifolia, Maprounea africana and Terminalia sericea. The grass layer is dominated by Andropogon gayanus and Hyparrhenia spp. Also characteristic of the herbaceous layer, and of chipya itself, are the evergreen relict species Aframomum alboviolaceum, Smilax anceps and Pteridium aquilinum. Physiognomically chipya is usually a mosaic of woodland, wooded grassland, herb/grassland and small patches of dry evergreen thicket or forest, known in northern Zambia as ‘mateshe’. Mateshe may contain Landolphia spp., Chrysophyllum magalismontanum, Syzygium guineense subsp. afromontanum, Entandrophragma delevoyi, Marquesia acuminata, Parinari excelsa etc. Individual evergreen species may be found scattered throughout the mosaic.

It is difficult to define chipya in terms of its floristic composition alone for the simple reason that hardly any of the so-called chipya indicators are endemic or exclusive to chipya. With the exception of the evergreen relict components, all are important species in other habitats, and it is mainly their tolerance of fire which enables them to survive and flourish in chipya. For example, species such as Burkea africana, Erythrophleum africanum, Terminalia sericea and Amblygonocarpus andongensis are characteristic of sandy soils at varying altitudes throughout the Zambesian region, from the Nylsvley Reserve (altitude 1100 m) in South Africa’s Northern Province (Yeaton, 1988) to the Luangwa valley (altitude 600-700 m) in Zambia (Smith, 1998). Pterocarpus angolensis, Parinari curatellifolia, Combretum zeyheri and Maprounea africana all occur widely in miombo woodlands, while species such as Pericopsis angolensis, Diplorhynchus condylocarpon, Pseudolachnostylis maprouneifolia, Oldfieldia dactylophylla and Hymenocardia acida are ubiquitous species occuring primarily in fire-climax wooded grasslands over a huge climatic and altitudinal range on an enormous diversity of soils (e.g. Trapnell et al., 1947: Wild & Barbosa, 1967; White, 1983). As mentioned above, the herbaceous ‘indicators’ Smilax anceps, Aframomum alboviolaceum and Pteridium aquilinum are also found in evergreen forest or ‘mateshe’ and/or in upland riparian forest or ‘mushitu’ (Fanshawe, 1961; 1969), the latter two species also occurring locally in wetter plateau miombo woodland (Lawton, 1996). Astle (1965) reported the presence of all three of these herbaceous chipya ‘indicators’ in an alluvial savanna woodland on the Chambeshi flats.

In summary, chipya is defined floristically by the combination of its fire tolerant elements and its relict species. Physiognomically, chipya is recognised by its mosaic nature and perhaps most characteristically of all by the remarkable luxuriance of its herbaceous component.

Trapnell (1943) associated chipya mainly with lake basin soils, distinguished by a grey filmy surface comprising colloidal organic matter, and a darker, richly humic horizon to a depth of up to three feet. He also identified pockets of chipya on brown sandy and other soils of more variable characteristics in plateau regions (e.g. Mbala, Isoka, Kawambwa, Mansa, Mpika and Serenje Districts) and, where there is sufficient clay content, on microgranular soils or ‘red earths’ (e.g. near Mbala). This latter class falls within D’Hoore’s (1964) ferrallitic soils, ferruginous tropical soils and ferrisols (Trapnell & Webster, 1986), whereas the lake basin and allied soils are probably best described as humic ferrallitic soils, a nomenclature which has also been applied to evergreen forests (Young & Brown, 1962). Trapnell et al. (1947) suggest that the lake basin soils of the Chambezi-Bangweulu basin are of ancient alluvial origin. Cole (1963) takes this a step further by postulating that the chipya ‘outliers’ in the Copperbelt, Serenje, Mkushi and Mpika Districts also occur on old alluvial lake basin soils, remnants of a larger area of deposition than that apparent in the Bangweulu-Chambezi craton today. Cole suggests that in these outlying areas subsequent dissection has removed all but the vestiges of this once large area of alluvium. This may be so in some cases, but chipya, as described by Trapnell (1943) also occurs on residual and colluvial soils.

Physically the lake basin chipya soils have certain typical characteristics which Trapnell (1943: para 87) describes thus:

‘typically of a richly humic nature, dark, friable and decidedly crumb-structured when broken and more comparable to the deep mould of temperate woodlands. They are at once permeable and retentive of moisture until late in the dry season (a characteristic which is borne out in their local survivals of evergreen thicket) and pass downwards into a deep subsoil which shows a gradually increasing clay fraction but is easily penetrated by the soil augur. At the same time they are markedly acid soils, especially towards the lower horizons, the pH in two representative localities being 4.4 to 4.8 in the top foot and 4.2 to 4.6 in the second..’

 

There seems to be little dispute that chipya soils are generally of a deep, freely draining nature. The chipya types mapped and described by Trapnell et al. (1947) all occur on freely drained soils of a permeable and friable nature. Fanshawe (1969) observed that the dry evergreen forest types from which his three chipya types (lake basin, Kalahari and plateau) are derived ‘..although variable in texture are nearly all deep, permeable and well drained’. Other studies record the same trend (Schmitz, 1950; Lawton, 1964; Astle, 1968). Fanshawe (1969) goes on to suggest that specifically it is the water-holding characteristics of these soils that is important to the distribution of dry evergreen forest. White (1983, p.89) agrees with this view, saying:

‘In the Zambezian Region forest occurs, or formerly occurred, on deep, freely drained soils with an adequate supply of moisture in their lower horizons during the dry season.’

Huckabay (1989) cites the high water infiltration capacity of the sandy soils of parts of Central Province as the reason for the presence of dry evergreen forest pockets there, despite relatively low rainfall. A similar explanation is given for the distribution of dry evergreen Mavunda forest on Kalahari sands. For the same reasons the drainage characteristics of chipya soils may be important in chipya dynamics and species selection. As mentioned above, many of the canopy species characteristic of chipya (e.g. Erythrophleum africanum, Burkea, Amblygonocarpus) prefer deep, freely drained soils (Timberlake & Calvert, 1993; Nyamapfene, 1988), as do many of the evergreen species found in mateshe (Fanshawe, 1969; Timberlake & Calvert, 1993). In addition, the water-holding characteristics of chipya soils may be a factor in the characteristic luxuriance of the herbaceous vegetation in chipya (Trapnell, 1943: cited above).

The organic matter content and fertility of lake basin chipya soils has been disputed by Lawton (1963) who wrote:

‘Centuries of fire are probably responsible for the low soil fertility. Each fire destroys the accumulation of leaf litter and humus and this is a loss of the nutrient capital of the site.’

In the same paper (p.55), Lawton suggested that the black coloration of the upper horizon of lake basin chipya soil is not due to humic material, but instead to finely divided pieces of carbon derived from burnt plant material. This claim is reiterated by Lawton (1978) in his study of chipya/miombo/mateshe dynamics where he says:

‘The nature of this dark material in the upper horizon is controversial; in spite of its colour it is seldom rich in organic matter. During the preparation of soil samples for pollen analysis, Lawton (1963) found that much of the black material consisted of finely divided pieces of elemental carbon derived from burnt plant material, the product of a long history of fires.’

In fact, no figures for organic matter content of miombo or chipya soils were presented by Lawton in either of these studies. Indeed, other studies (e.g. Astle, 1968, and see Tables 2 & 3) have shown that organic carbon content is significantly higher in lake basin type chipya soils than in plateau soils. Nevertheless, Lawton’s assertion that the organic carbon content in chipya soils is elemental (inert) carbon, and therefore unavailable to plants is hard to disprove because chemical oxidation methods (e.g. Walkley & Black, 1934) and combustion/ignition methods (e.g. Courtney & Trudgill, 1976) for measuring organic carbon in soil, are unable to reliably differentiate between organic and elemental carbon (Bremner & Jenkinson, 1960). In practice, organic matter content of soil, as determined by the ignition method, is invariably higher than that obtained through the (less severe) chemical oxidation methods (see Table 2), and part of the additional weight loss following combustion may be due to crystal water held in the clay fraction, and part due to greater oxidation of elemental carbon (Astle, 1968). However, the fact that the ratios of organic matter loss in chipya compared with non-chipya soils are high regardless of the oxidation method used (Table 2) suggests that the higher organic matter contents noted in chipya soils is a genuine trend applicable to readily oxidised organic matter.

Perhaps the most persuasive argument for the uniquely humic nature of lake basin chipya soils is the observation that other savanna vegetation types subjected to repeated fires do not develop the dark horizon characteristic of chipya soils. This despite the fact that they are presumably the recipients of the same types of fire product fall-out. No such layer has been reported in the late or early burnt Ndola miombo plots (Trapnell, 1959; Trapnell et al., 1976; Lawton, 1996) or in physiognomically similar, fire-climax woodland savannas elsewhere in the region (e.g. Scholes & Walker, 1993 ).

Table 2: % Organic carbon as measured by the Walkley & Black and ignition methods in Lake Basin type soil (Chichele Forest Reserve, Ndola -two sides of same pit) and plateau soil (Kambowa). From Muir (unpublished data).

If lake basin chipya soils do contain a highly humic upper horizon compared to miombo plateau soils, the next question raised is why is this the case?

Primary productivity is the main determinator for soil organic matter (Webster, 1970) and it is most likely that the high organic matter in chipya soils is either:

the product of the characteristically rich herbaceous layer of chipya, or;

a legacy of the evergreen forest from which chipya is derived

It is possible, for example, that the dense herbaceous component so characteristic of chipya may have arisen as a result of the fertilising effect produced by the burning of the dense evergreen forest that preceded this type (see Stent, 1933). Once established, the dense herbaceous component becomes a source of organic matter itself. Fires are not necessarily an annual event in chipya, and in fire-free years, the dense herbaceous layer will break down into a mulch or green manure, which will boost organic matter concentrations in the topsoil. During years in which the chipya does burn, charcoal and soot is produced as a result of the fire this is perhaps more accurately described as the product of incomplete (or retarded) combustion, rather than as elemental carbon. Chipya fires would be expected to produce a great deal of fine soot, due to the amount of green growth in the herbaceous layer. Regardless of the degree of combustion, soot and charcoal will further contribute to the un-decomposed organic reserves in the soil.

Webster’s work at Katuta (Table 3) suggests that mateshe soils, have a high organic matter content in their upper horizons, although less than than that seen in chipya. A study carried out by the same author in the evergreen forests of Malawi (Webster, 1970) revealed high organic matter in the upper horizons and an apparent regression to grassland in which some of the soil organic matter content is retained. These findings, while providing some support for the legacy theory, need further corroboration in the chipya context. If it is found that mateshe soils consistently have high organic matter content, comparable in its chemical constituents to that found in chipya, this is a strong argument for the organic matter content of chipya soils being at least partially derived from the evergreen forest from which the chipya arose. If this is found not to be the case, or that chipya soil organic matter content is significantly higher than that of mateshe, then it is more probable that the organic matter is derived from chipya itself, as described above.

Table 3: organic C (%) figures from Katuta area – top 6 in. only, Lake Basin type soils. From Webster (unpublished data).

Clearly, if the organic matter in chipya soil is a relict of the previous covering of evergreen forest, then some factor or combination of factors is causing that organic matter to persist, perhaps over hundreds of years. If, however, that organic matter is derived from the primary productivity of chipya itself, in the form of its luxuriant herbaceous layer, persistence is probably not the key issue, rather primary production.

In attempting to answer these questions it is perhaps useful to examine the factors which are influential in recycling organic carbon in Central African soils. The organisms responsible for mineralisation of soil organic matter can be conveniently separated into the soil fauna (earthworms, termites etc.) and flora (micro-organisms).

The presence of earthworms and hence their role in organic matter degradation in sub-tropical African soils appears to be dependent to a certain extent on soil texture, but more particularly on the water table, which apparently should be no lower than 40-50 cm below the surface (Ljungstrom & Reinecke, 1969). This factor alone excludes earthworms from most Central African soils, where water tables are usually considerably lower than this, and which are completely dry for up to seven months of the year. The only soils which could conceivably support earthworms in Zambia are some alluvial soils and the illuvial soils of dambo margins, but even these are seasonally waterlogged and therefore unsuitable for part of the year. The theoretical absence of earthworms from savanna or woodland soils is supported by field observations by the authors who have rarely encountered earthworms in soil pits dug in these habitats.

Of much more significance in organic matter breakdown in sub-tropical African soils is the role of termites (Trapnell et al., 1976; Ferrar, 1982). Trapnell et al. (1976) found that fire-protected miombo woodland did not show significantly higher organic carbon than that detected in miombo woodland subjected to early or late burning over a period of 23 years. They suggested that the reason for this was that in the protected plots, the lignivorous litter feeding Macrotermitineae kept surface organic matter low and humus feeding termites (e.g. Cubitermes) kept subsoil organic matter concentrations low. These conclusions were supported by analyses of Macrotermitineae mounds and Cubitermes mounds, which were found to contain three to four times the concentration of organic matter compared to adjacent topsoil. In the burnt woodlands, Trapnell and his co-workers suggested that the role of the Macrotermitineae was assumed by the fire, which consumed the surface litter, and soil organic matter continued to be eaten by the humus feeding termites, thus organic matter was kept low in all the soils regardless of burning regime. The absence of Macrotermitineae in vegetation types subject to frequent fires has been noted by a number of researchers. For example, according to Schmitz (1962), high termitaria are present in evergreen forest but not where subject to fire. Morison et al. (1948) also suggested that large mound-forming species can only colonise shaded ground under evergreen woody plants. In the chipya context, a fire-induced reduction in the occurrence of Macrotermitineae will clearly cause an increase in the amount of ligniferous organic material on the soil surface, and thereby increase organic matter concentrations and persistence in chipya soils.

The role of Cubitermes and other grass and humus-feeders may also be critical. Trapnell et al. (1976) found that the number of Cubitermes mounds present in miombo woodland increased significantly in late burnt plots (160 mounds per 0.4 ha) compared to early burnt (120) and completely protected plots (80). This finding suggests that, in miombo habitats at least, fire actually increases the numbers of Cubitermes, species known to be grass and humus feeders. If a similar trend were found in chipya soils, and the active species were confirmed as humus feeders, this would argue against the long term persistence of humic matter in chipya and infer that the high organic matter content of chipya soils is the result of the high primary productivity of chipya itself. Alternatively, future studies may show that in chipya, where fires are frequent, very fierce and all surface organic matter is consumed, the grass-eating and soil-feeding humivorous termites are excluded or their activities curtailed. Ferrar (1982) showed that humus feeding termites foraging in a recently burnt broadleaf savanna was greatly reduced for two months after the fire. Interestingly, the fire itself was relatively mild and shortlived, burning only to a height of 15 cm, and for a duration of 40 seconds or so, and it was thought that the decrease in soil termite activity may have been due to the effects of subsequent insolation on the bare soil, reducing termite activity there. Ferrar (1982) also observed that the grass-humus feeding termites active in broadleaf savanna were much less efficient foragers than those species found in fine leaf savanna; in addition, seasonal patterns of foragers differed greatly. Other factors unique to chipya soils may also play a role in the termite fauna present. For example, studies in Nylsvley and elsewhere (Josens, 1983; Scholes & Walker, 1993) have shown that different termite species are sensitive to factors such as soil texture, with some soil-feeding termites unable to establish their tunnel systems in sandy soils. Reduced activity of humus-feeding termites in chipya soils would perhaps lend support to the theory that chipya organic matter is the legacy of an evergreen past and has persisted due to chipya conditions.

Clearly, a great deal of work remains to be done on the role and activity of termites in chipya. In fact, the extent to which termites are responsible for degrading humic matter in tropical soils is by no means clear. It has been suggested (Scholes and Walker, 1993) that the higher concentrations of organic matter found in the walls of termite mounds compared to that in the surrounding soils (e.g. Trapnell et al., 1976) implies that most of the energy requirements of these termites are met by small plant fragments in the soil rather than by humified soil organic matter. If this is so, the role of humus–feeding microbes may well be more significant.

Humus is part of the insoluble organic matter component of soil; its chemical composition has been described as ‘an amorphous, polymeric material resulting from the decomposition of organic matter, especially lignin.’ (Campbell, 1983). Humus is very difficult to degrade, and in temperate soils microbial mineralisation only occurs at a rate of 1-2% per year in the best agricultural soils, and residence times of 250-1500 years have been quoted for different humus fractions in various other temperate soils (Campbell, 1983). Humus biodegradation will also depend on factors such as temperature, pH, pO₂, redox potential and the presence of interfaces, co-metabolites, toxins, essential nutrients and appropriate organisms (Fewson, 1988). In chipya soils, which are intrinsically very acidic, have little available P and K, high (possibly toxic) concentrations of aluminium and few reactive surfaces, microbial activity may be greatly reduced, and as a consequence, humus residence times significantly longer. The most obvious factors likely to impact on biodegradation which are peculiar to chipya, as opposed to mateshe, are the fierce fires and their effects, notably on soil structure and insolation. These factors will certainly affect soil water availability, and may have more direct consequences on the micro-organisms themselves. Trapnell et al. (1976) found no significant long term differences in soil respiration rates in protected, early burnt or late burnt miombo soils, suggesting that fire doesn’t reduce microbial populations significantly. However, respiration rates were very low in miombo soils, and no comparable work has been carried out in chipya soils.

The dark ‘humic’ horizon is not present in all chipya soils – some plateau soils carrying chipya, for example, may have a reduced organic horizon or none at all. Until more is known about the origin and nature of the dark layer, it is impossible to say why this is the case. The question of organic matter in chipya soils has important agricultural consequences. Trapnell (1943) first drew attention to the agricultural potential of chipya soils, noting their high humic content, water-holding characteristics and good texture. Schmitz (1950) stated that the Aframomum sous-association offered average to good conditions for agriculture, saying that near the local villages most such land was already cultivated (p.4). In Zambia this is not the case and Trapnell (1943) refers to ‘their numerous unoccupied belts in the Northern Province’. The apparent reason for this is that as a source of woody biomass for ash-fertilisation, as practised in traditional chitemene agriculture, chipya is inferior to the much woodier miombo. High acidity and aluminium concentrations may also be important. Some areas of chipya woodland, such as the National Forest Reserve near Samfya, have been successfully planted with Pinus spp., but on the whole chipya soils have not proved suitable for most arable crops.

Clearly, a great deal of work remains to be done on chipya soils. Chemical analysis data from chipya soils is almost completely lacking in the literature. Of particular importance is the nature and origin of the organic matter in chipya soils, and how it relates to the herbaceous layer. Is the organic matter in chipya soils different from that in mateshe soils? Is it largely made up of elemental carbon? How long does organic matter persist in chipya soils, and which factors are important in its degradation and recycling? How is soil organic matter related to the luxuriance of the herbaceous layer? What other soil factors are important to herbaceous layer biomass? These and other questions can only be answered by new studies involving field work and laboratory analysis.

Chipya dynamics are controversial, and intimately wrapped up in the chipya definition debate. It is perhaps most useful to try and follow the arguments chronologically.

As stated above, Trapnell (1943) recorded that his Lake Basin chipya soils were characterised by a dark humic upper horizon of great depth, and were commonly associated with relict patches of ‘mateshe’ or evergreen forest. This, together with the evergreen component of the herbaceous flora (Aframomum, Pteridium and Smilax) led him to postulate (op cit. para 58) that chipya may represent areas formerly covered by evergreen forest.

Schmitz (1950), working in Shaba Province in Congo, clearly identified chipya in the form of what he referred to as ‘La sous-association à Aframomum’ a sub-type of his wet miombo woodland, found on loams to very sandy loams. This miombo sub-type, according to Schmitz, is an edaphic association and although not explicitly part of a regression sequence, is regarded by him as a form of degraded evergreen forest or ‘muhulu’. Schmitz suggests that muhulu is the natural climax vegetation in Shaba, and that Marquesia-Brachystegia transitional woodland and miombo woodland types, including the Aframomum sub-type, are secondary, brought about by human disturbance in the form of cultivation and fire.

In the Ndola miombo burning experiments set up by Colin Duff in 1933 (Trapnell, 1959), regression and succession were investigated under conditions of complete protection from fire, and early and late burning. Although there was a chipya/mateshe area adjacent to the miombo woodland plots, which could act as a source of evergreen species for colonisation of fire-protected miombo, this was not included in the experiment. The Ndola plots did show, however, that burning of miombo does not produce ‘chipya’ vegetation, as characterised by the luxuriant herbaceous layer of Aframomum alboviolaceum, Smilax anceps, Pteridium aquifolium and Andropogon gayanus (C.G. Trapnell, pers. comm.).

Lawton (1964), working in Mukabi Forest Reserve in Zambia’s Kawambwa district, was one of the first to consider chipya dynamics and succession. In this study he looked at the relationship between Marquesia acuminata evergreen forest, chipya, and miombo woodland. Lawton postulated that human exploitation of Marquesia acuminata had led to regression of evergreen forest to the Aframomum-Pteridium-Hyparrhenia association which, subject to frequent fires, was over a period of time colonised by fire-tolerant species from adjacent miombo woodland to form chipya. Lawton also found evidence to suggest that fire-intolerant species from the evergreen forest could recolonise chipya under the right (fire-excluded) conditions.

Lawton postulated edaphic differences between the miombo woodlands and the other three vegetation types at the Mukabi F.R. site, noting that the miombo occurred on clayey soils, and the other types on more sandy soils. His observations supported Trapnell’s theory that chipya occurred on areas formerly occupied by evergreen forest, and that the two types were often associated (p. 476, op. cit.). No attempt was therefore made to include miombo in the evergreen forest-chipya regression, except as a source of fire-tolerant species.

Fanshawe’s (1969) definition of chipya is derived from that of Trapnell et al. (1947), and identifies ‘Lake Basin Chipya’ and ‘Kalahari Sand Chipya’, both types being characterised as fire-climax wooded grasslands with a luxuriant herbaceous layer including bracken and Aframomum. He also recognised a plateau chipya ‘identical with that found in the Bangweulu basin’, the result of a regression from Parinari forest, present in ‘miniature lake basins, e.g. Chichele and limestone troughs like Jiwundu swamp in Solwezi district’ and a ‘group found only on granite sands in Luano Forest Reserve.’ In the Bangweulu lake basin and associated river valleys, Fanshawe identified the evergreen climax as ‘Marquesia forest’. The regression sequence put forward by Fanshawe was as follows:

‘Partial destruction of Marquesia forest results in a gradual regression to miombo woodland…There is a marked intermediate stage in the regression where the forest is invaded by Brachystegia and Isoberlinia species, chiefly Brachystegia spiciformis, just as happens in the regression of Parinari forest to miombo woodland.

Total destruction, and this means largely the destruction of the canopy, favours replacement of the canopy species by fire-hardy chipya elements along with a population explosion of one or more of the chipya indicators – Aframomum, Smilax and Pteridium – and the final result is lake basin chipya.’

Fanshawe was therefore close to Lawton (1964) in that he believed that miombo species were invading the degraded evergreen habitat, and that continued fire pressure selected the fire-tolerant miombo species that eventually make up the woody component of chipya. Unlike Lawton (1964), however, he suggested that Brachystegia and Isoberlinia species may establish themselves as part of the regression sequence, although he stops short of postulating that chipya may derive directly from miombo woodland. Instead, he proposes that the current distribution of evergreen forest, and hence chipya, is edaphically controlled, the lake basin, Kalahari and certain plateau sites possessing water-holding characteristics which enable evergreen species to survive the long dry season of today’s climate.

In 1978 Lawton revised his earlier opinions about the uniqueness of chipya soils. Using data from nearly 400 vegetation-soil plots in north-eastern Zambia, he found it floristically impossible to categorise many of the plots into miombo, chipya or mateshe, encountering what he described as a continuum, with most plots intermediate between two or more vegetation types. Lawton also stated that:

‘No significant differences were determined between soils under chipya or miombo vegetation, although the depths of the dark, almost black, upper horizon may vary from 50-80 cm in the Lake Basin or chipya soils to 5-20cm in plateau or miombo soils.’

This observation is crucial to Lawton’s revised succession hypothesis, which states that there is a chipya-Uapaca-miombo-mateshe succession. In this sequence, Uapaca kirkiana and other members of the same genus colonise chipya, producing coppice colonies up to 20 m in diameter, shading out the grasses and allowing miombo species to develop free from the effects of fire (Lawton, 1978; 1996). The difference between this sequence and Lawton’s observations in the Mukabi FR is that in the latter study, Lawton’s definition of ‘chipya’, is not dependent on soil type or even the presence of Trapnell’s chipya indicators, Aframomum, Smilax etc. Instead, Lawton applies the term ‘chipya’ to fire-tolerant species in fire-degraded miombo woodland, on plateau soils. His contention that there are no significant differences between chipya and miombo soils arises because his floristic definition of chipya is broader than Trapnell’s, and because he attaches no significance to the high organic matter content of chipya soils. Even more controversially, Lawton (1978) is proposing a succession sequence for chipya, not regression, i.e. he is saying that even with fire present, evergreen forest may establish itself through the protection afforded by Uapaca species.

White (1983) is comparatively circumspect in his definition of chipya, edaphically and floristically, stating that:

‘It occurs locally on suitable soils on the Central Africa Plateau in parts of Zambia, Shaba and Malawi where rainfall exceeds 1000 mm per year, but is most extensively developed on the alluvial soils of lake basins, especially Lake Bangweulu, and their associated river systems…….It is now well established that chipya occurs on sites formerly occupied by forest or transition woodland and owes its existence to cultivation and fire. Three herbaceous species, namely Aframomum biauriculatum, Pteridium aquilinum and Smilax kraussiana, which are absent from most types of miombo woodland, are almost universally present in chipya.’

The ‘transition woodland’ that White refers to is a ‘secondary evergreen miombo woodland dominated by Marquesia macroura and Brachystegia taxifolia.’ Trapnell (1943; para 52) places this same association on transitional lake basin soils ‘at higher levels where there is a tendency to evergreen thicket’ Trapnell et al. (1947) also recognise the Marquesia macroura-Brachystegia type and associate it with chipya in lake basin areas (B1) and Northern Brachystegia woodlands on the plateau (P1). In contrast, according to Schmitz (1950), the Marquesia-Brachystegia association is not linked with any particular soil characteristics other than non-Kalahari sand types.

A summary of the main viewpoints on chipya distribution and dynamics is given in Table 1 below.

From the above, it is clear that the main arguments surrounding chipya dynamics are dependent on the influence of soil, climate and anthropogenic factors such as fire and cultivation.

As indicated above, Schmitz (1962), Lawton (1963, 1964, 1978) and Fanshawe (1969), in describing chipya-mateshe succession sequences, are proposing that dry evergreen forest is a climax vegetation type under today’s climatic conditions. As Lawton (1963 p. 52) says:

‘It is therefore suggested that with protection from fire the chipya woodland would be succeeded by a dry evergreen forest.’

Fanshawe (1969) is a little more cautious in his interpretation than Lawton, citing edaphic factors as being crucial, and suggesting that the current equilibrium is delicately balanced:

‘Dry evergreen forest is an edaphically-controlled vegetation type, surviving from a more pluvial regime only on specialised sites where ample soil water is available during the dry season. Under the present climatic regime it tends to be unstable and easily upset.’

The idea of dry evergreen forest as a climax type receives some support from the Ndola plots (Trapnell, 1959) in which miombo woodland completely protected from fire has been invaded by evergreen species to form closed, evergreen thicket (Lawton, 1996). In addition, current distributions of evergreen forests indicate that evergreen species can at least survive under today’s climatic conditions in certain areas. Huckabay (1989) has examined the present extent of dry evergreen forest pockets in Zambia and concluded that it is related to rainfall and length of dry season (>1000 mm rainfall and <6½ month dry season), corresponding more or less to the distribution of White’s (1983) ‘wetter miombo’ type. Mavunda on Kalahari sands is an exception to this rule, surviving in areas with rainfall as low as 900 mm per annum, due to the unique water-retaining characteristics of the Kalahari sands. Huckabay (1989) suggests that it is no coincidence that the largest remaining patches of dry evergreen forest in Zambia are located in the highest rainfall areas around Solwezi and Lake Bangweulu. In fact, the current distribution of dry evergreen forest pockets doesn’t tell us a great deal about whether these are remnants or a potentially expanding vegetation type. For this it is necessary to compare the present day extent of dry evergreen forest with its distribution in the past.

 

Pollen records show that the modern biomes of southern and central Africa were well established by the beginning of the Pleistocene (Scott et al., 1997). However, the complex series of shifts in the phytochoria in response to changes in climate during the past two million years have been harder to chart. Over this period the vegetation has responded to the large scale cyclic fluctuations in temperature and precipitation brought about by glaciation (cold and dry) and interglacial pluvial periods (warm and wet), and the subtler, more localised weather conditions engendered by the changes in macroclimate (Sarnthein, 1978). There is good evidence to suggest that major fluctuations in temperature and precipitation have occurred in the southern-central Africa region in the past 20,000 years, and there is general agreement about maxima and minima within this time frame. Evidence from radio-carbon and Uranium-series dating (Sarnthein, 1978), lake level fluctuations (Stager, 1988; Street-Perrott et al., 1989), and optical dating (Stokes et al., 1997) seems to suggest a glacial maximum, corresponding to a cooler, drier climate than today at ca. 15,000-18,000 yr BP and a pluvial period, warmer and wetter than today ca. 6000-9000 yr BP (see Table 4 below).

Unfortunately, little work has been carried out on how these climate trends influenced the vegetation in south-central Africa. Fanshawe (1969) plotted hypothetical distributions of Zambia’s major vegetation types, including dry evergreen forest, under conditions of 500 mm higher and lower rainfall than today, but there is little evidence to go on as to when these conditions might have prevailed, and whether such distributions actually occurred. One of the few pollen analysis studies carried out in northern Zambia was that of Lawton (1963), at Lake Bangweulu. His findings suggested that the areas covered by sudd today once carried Syzygium swamp forest. However, no dating of the pollen-bearing peat deposits was possible.

Clark and van Zinderen Bakker (1964) carried out pollen analysis at Kalambo falls near Mbala and proposed a sequence of Pleistocene climatic regimes based on their findings. However, the utility of this work is constrained by the Kalambo river valley location, which has the disadvantage of being proximal to a huge altitudinal range (770-2400m) and consequently a number of very different vegetation types, regardless of prevailing climate. The authors themselves note the potential error presented by the possible transport of pollen by the river from higher, cooler altitudes to the sample site. In addition, the subsequent revision of Southern African radio-carbon dating chronology (Vogel & Beaumont, 1972) has shown that the dates proposed in this study are inaccurate.

Livingstone (1971) carried out pollen analysis of a lake sediment core taken from Lake Shiwa Ngandu in Chinsali District. The lowest part of the core sample was radio-carbon dated at ca. 22,000 years before present, but the only statistically significant difference in composition and concentration of pollen grains found throughout the sample was in the top 0.5m, extrapolated to ca. 3000 BP. Here, higher percentages of Combretaceae, Melastomataceae and Myrtaceae, and the absence or low abundance of forest indicators led the author to speculate that anthropogenic factors such as fire and cultivation may have influenced the vegetation and pollen record. Livingstone’s study is useful in highlighting the limitations of tropical pollen analysis for determining past climates, the main problems being the lack of good indicator species, producing enough pollen to provide a reliable indication of plant abundance and recognisable as belonging to a particular climate. Livingstone is careful to point out that his findings do not necessarily indicate a period of climatic stability over the past 20,000 years.

Stager (1988) carried out sediment analysis from Lake Cheshi near Mweru Wantipa covering the past 40,000 years. His results suggest a pluvial period from ca. 8000 to 4000 yr BP and maximum lake shrinkage at between 15,000 and 13,000 yr BP. Lake levels were low again around 3500 yr BP, an observation consistent with records from several other African lakes at this time. Core sediments were dated by radiocarbon methods, and lake levels related to diatom fauna. Unfortunately, no pollen analysis results were presented due to high proportions of unidentified grains. Stager noticed a dilution of the micro-fossil record at about 2000 yr BP which he suggested may have been the result of siltation caused by man’s land clearing activities.

Evidence from Mumbwa caves (Barham, personal comm.) where fireplace charcoal has been recovered from stone age dwellings is also inconclusive. Radio-carbon dated charcoal of 1000, 2000, 6500 and 7000 yr BP suggests that throughout this period the vegetation was little different from that of today, with firewood consisting mainly of Acacia, Brachystegia/Isoberlinia and Combretum species. A major limitation with charcoal analysis, as with pollen, is the inability to tie taxa down to species, and thus the lack of good indicators. In addition firewood selection is subjective, with certain trees selected ahead of others, for a number of reasons, including combustion characteristics, ease of extraction and distance from home.

In summary, although there is good evidence for, and some agreement about, climatic changes during the late Quaternary, there is an extreme paucity of information regarding past vegetation in Zambia. Clearly further research is required, not only within the paleo branches of palynology and wood anatomy, but also in developing new techniques aimed at elucidating vegetation history.

Huckabay (1989) provides a detailed summary of the evidence for Iron Age human activities in Zambia, and although he acknowledges that man has been using fire for much longer, he regards agriculture and forest clearing for charcoal as the major factors affecting vegetation changes in northern Zambia over the past few thousand years. Fanshawe (1969) is another proponent of tree-cutting as a major influence on habitat change, saying that on the plateau: ‘all, or nearly all, miombo woodland has been cultivated at one time or another…’ In compiling his evidence for past cultivation, Huckabay (1989) cites Livingstone (1971) who found that forest species were largely absent in 3000 year old pollen samples from Lake Shiwa Ngandu, and attributed this to Iron-Age farmers. In fact, sediment analysis from other central African lakes (see above) suggests that this was a climatically dry period, perhaps a more likely explanation for the change in flora. Cultivation and fuelwood clearing are undoubtedly important factors in vegetation changes today (Lawton, 1982; Kalapula, 1989), but how influential tree-cutting was in the past is not clear. Lake basin chipya soils, for example, have not been traditionally cultivated by Africans (Trapnell, 1943), apparently due to their low woody biomass, yet the evergreen forest which once occupied these sites has been degraded. Here, and perhaps elsewhere, the major instrument of change has been fire.

All of the authors who have written about the mateshe-chipya sequence are agreed that fire is an essential element driving these vegetation changes (Trapnell, 1943; Schmitz, 1950; Lawton, 1963; Fanshawe, 1969; White, 1983). Although all concur that fire is crucial to the creation of chipya, the main area of argument surrounding the chipya-evergreen sequence is the role of miombo (Brachystegia, Julbernardia) species in the presence or absence of fire. The Lawton (1964) evergreen to herb/grassland to chipya regression in which, following cultivation and in the presence of heavy fires, it has been shown that fire-resistant woody species, and herbaceous species, outcompete evergreen forest and Brachystegia/Julbernardia species, to produce chipya can be taken as the working model. Fanshawe (1969) deviates from this by suggesting that (but providing no evidence for) Brachystegia/Julbernardia species may also be part of the regression. In fact it is hard to imagine how fire-sensitive Brachystegia and Julbernardia species (Trapnell, 1959; Lawton, 1978) could possibly compete with fire-tolerant species in a fire-induced regression.

The idea of a chipya-mateshe succession (e.g. Lawton, 1963) is partly a question of climate (discussed above), i.e. whether a forest climax is possible in today’s climate. However, equally pertinent is the question of whether any area in Zambia is likely, in practice, to be spared from fire. It might be argued that even in the unlikely event that a pocket of chipya was protected from fire for a considerable period of time, the build up of litter would ensure that when the fire did come, it would be all the more fierce and would surely kill fire-sensitive seedlings, saplings or young trees. Lawton (1978) suggests a ‘chipya’-Uapaca-Brachystegia-dry evergreen succession in the presence of fire. However, his definition of chipya encompasses fire-degraded miombo, which is very different from the chipya of Trapnell in which the luxuriant herbaceous layer ensures fierce, hot fires. Evidence from the Ndola plots (Trapnell, 1959; Lawton, 1996) shows that an evergreen succession is possible in completely protected miombo but, ironically, even here the protected plots have latterly been accidentally burnt. Although Lawton’s proposed succession sequence may well occur in miombo, where the herbaceous layer is relatively sparse and fires consequently less severe, it is hard to envisage any succession sequence in chipya proper.

In summary, it is probable that fire has been more influential in mateshe-chipya dynamics than cultivation or wood-cutting, given the length of time that man has been using fire, and its comparatively devastating effects over large areas. Furthermore, on today’s evidence, a fire-induced regression sequence of mateshe to chipya is likely to dominate even if a chipya to mateshe succession engendered by protection from fire is possible. In any such regression sequence it is hard to see how fire-sensitive miombo species can compete with fire-tolerant chipya species, and it is therefore hard to envisage miombo woodland as an intermediate stage in the mateshe to chipya sequence.

On the face of it there is much to be done before we are able to fully understand chipya ecology and dynamics. However, many of the problems are interrelated, and a few key pieces of information may well make the picture much clearer. The first step is to produce a clear floristic and physiognomic definition of chipya. Trapnell (1943; para 59) has done this for lake basin chipya, and where subsequent authors have failed conspicuously is in relating their respective chipya definitions to the archetype. If we take lake basin chipya, as defined by Trapnell, as the model, then clearly the fundamental work which remains to be done on chipya soils, ecology and dynamics should be carried out in this habitat. How does the luxuriant herbaceous layer and evergreen component relate to the soil? Is the dark horizon made up of organic matter and, if so, is this the legacy of a previous covering of evergreen forest? Or is it the product of the luxuriant herbaceous layer? Do all chipya soils, as defined above, have the dark horizon? If not, why not? Has it disappeared, through long term burning perhaps, or was it never there? It is only through studying lake basin chipya and answering fundamental questions like these, that we can begin to extrapolate to the other chipya types. It is only through investigating the influence of soil, climate and fire on lake basin chipya dynamics that we can begin to understand the floristic and physiognomic variations evident in plateau, red earth or even Kalahari ‘chipya’ types.

In conclusion, by starting with the fundamentals – Trapnell’s (1943) definition of chipya, Lawton’s (1964) model of mateshe-chipya dynamics – a rational, focused approach, including relatively simple fieldwork, will go a long way towards answering the questions which remain in chipya ecology.