Past and present vegetation ecology of Laetoli, Tanzania

We are attempting to set up a new protocol for palaeoecological reconstruction in relation to the fossil hominin site Laetoli, Tanzania. This is based on the premise that habitat variability in the past was at least as great as at present; that this variability at the landscape level is a function of variations in geology, soils, and topography rather than climate; and that vegetation type at the landscape level can be reconstructed from these environmental variables. Measurable variation in climate in tropical Africa today occurs over distances of at least 100 km, so that ranges of habitat variation within the limited area of Laetoli today can be reconstructed in relation to soils and topography, and the effects of climate changes are then estimated in relation to these other factors. In order to document the modern vegetation, we have made voucher collections of plants in the Laetoli region, recorded distributions of plants by habitat, climate, soil, and topography, and mapped the vegetation distributions. Results show that areas of low relief have soils with impeded drainage and dense Acacia drepanolobium woodland, having low canopies when disturbed by human action, higher when not; shallow brown soils on volcanic lavas have four woodland associations, two dominated by Acacia species, two by Combretum-Albizia species; shallow volcanic soils to the east have a woodland association with Croton-Dombeya-Albizia species; elevated land to the east on volcanic soils has two associations of montane-edge species, one with Croton-Celtis-Lepidotrichilia, and the other with Acacia lahai; the eastern highlands above 2,750 m have montane forest; seasonal water channels flowing from east to west have three Acacia riverine woodland associations; three deep valleys to the north of the area have dense riverine woodland with Celtis, Albizia, Euclea, Combretum, Acacia spp.; emergence of springs at Endulen feed a perennial stream with closed gallery forest with Ficus-Croton-Lepidotrichilia; and, finally, recent ash falls have produced immature alkaline soils with calcrete formation and short grass vegetation. All of these vegetation associations have been modified by human disturbance to greater or lesser degrees, and we have attempted to allow for this both by basing the associations on the least modified areas and by predicting how the associations, or parts of associations, have been altered by human action. Past land forms at Laetoli have been based on the geology and geomorphology of the area. Past vegetation patterns were estimated by superimposing present distributions of plant associations on equivalent landforms in the past, assuming similar climate to the present. This indicates the overall pattern of vegetation at Laetoli to have been a mosaic of low and tall deciduous woodlands and with riverine woodland and forest associations along water courses. Low woodlands would have been dominated by Acacia species, and tall woodlands by Combretum-Albizia species, with increasing increments of montane species, such as Croton species, to the east of the area. Riverine woodlands would have been dominated by Acacia-Euclea species, with wetter associations (downriver or linked with spring activity) supporting gallery forest with Ficus, Celtis, and Croton species. These are all species associations common in the area today, and with landforms little changed in the past, and assuming similar climate, there is every reason to predict that they would have been present in the past. Moreover, Pliocene environments lack the human disturbance that has destroyed much of the present day vegetation. Presence of woodlands is supported by fossil wood attributed to several of the tree species present in the area today and by similarities in the mammalian community structure between past and present. Having established the pattern for Pliocene vegetation based on climatic variables existing today, we then predict the effects of past variations in climate.