Climate-human-landscape interaction in the eastern foothills of Mt. Kilimanjaro (equatorial East Africa) during the last two millennia

Abstract

The Mt. Kilimanjaro region is known for its long history of intensive agriculture, but the temporal extent of human activity and its impact on the regional ecosystem are not well known. In this study, climate-human-landscape interactions during the past ~2200 years were examined using climate and vegetation proxies extracted from the continuous and high-resolution sediment record of Lake Chala. Ancient-to-modern regional human activity is documented against a backdrop of long-term vegetation dynamics in the low-elevation savanna woodland southeast of Mt. Kilimanjaro and riparian forest within Chala crater. During prolonged dry periods (~1170–1300 CE), succulent dry crater forest expanded relative to the moist lakeshore forest. The savanna landscape surrounding Chala crater was relatively stable through time, except that savanna grasses were stimulated by higher precipitation, consistent with the fuel-limited fire regime evidenced in the charcoal record. Expansion of subalpine ericaceous vegetation and a general decline in Afromontane forest taxa on Mt. Kilimanjaro after 550 CE may reflect a lowering of its upper forest line. The earliest robust signature of human influence on regional vegetation involves an increase in ruderal (weedy) plant taxa around 1100 CE, possibly associated with the development of Chagga homegardens and associated agroforestry in the submontane forest zone. A first hint of cereal agriculture (likely sorghum) is observed around 1550 CE, followed by a more robust signature from 1780 CE onwards which likely reflects the start of lowland irrigation agriculture. From 1780 CE we also find the first undisputed appearance of maize, introduced to East Africa about a century earlier.

Late Holocene environmental change and anthropogenic: Ecosystem interaction on the Laikipia Plateau, Kenya

“…The period around c. 2300 cal year BP to present generally coincides with archaeological evidence indicating that the Laikipia Plateau was already a site of food production involving domesticated animals (Taylor et al 2005;Lane, 2011). Major forest clearance appears to have occurred widely along the western part of Mount Kenya (Rucina et al 2009), although this was not unidirectional and there are documented examples of subsequent phases of forest regeneration, likely in response to periods of higher rainfall although with differences in associated species compositions, as recorded in sediment cores from Lake Bogoria (van der Plas et al 2019).…”

“…obs.). Such a shift is also supported by an increase in large Poaceae pollen grains in the record that could reflect an increase in cereal cultivation (Hamilton 1982), although this size-to-cereal relationship remains controversial (van der Plas et al 2020). More sustained, systematic archaeobotanical sampling of multiple sites, especially those associated with Pastoral Iron Age ceramics, is clearly needed to test this hypothesis.…”

“…Despite considerable and growing interest in the interrelationship between human activities and environmental change, equifinality makes palaeoenvironmental signals difficult to separate. Documenting the long-term history of climate-human-landscape interaction in East Africa, for example, is complicated by the challenge of discriminating between the palaeoecological signatures of human activities and the response of local vegetation ecotones (van der Plas et al 2019). Climate change events (for example a shift to a drier climate) may produce the same signal, such as an increase in grasses, as a shift to increased pastoral activity.…”

High-Resolution Multiproxy Record of Environmental Changes and Anthropogenic Activities at Unguja Ukuu, Zanzibar, Tanzania during the Last 5000 Years

“…This evidence indicated that the area was once again influenced by human activities associated with the human reoccupation phase at Unguja Ukuu during 14th-16th centuries CE, the arrival of the Portuguese on Zanzibar and continued to Omani colonisation and a sea-level fall period in the area (Figure 11). Although grass pollen larger than 40 µm is generally classified as domesticated grass pollen [113,114], grass pollen grain sizes between 60 and 80 µm are considered to be derived from east African cereals such as sorghum (Sorghum bicolor) [72]. Therefore, the presence of domesticated grass pollen in this study which is larger than 40 µm but smaller than 60 µm is unlikely to represent an east African cereal.…”

“…Similarly, Poaceae pollen is also periodically abundant in the sediment cores, especially towards the top of each core. Although several palaeoenvironmental studies show that grass pollen increases in dry conditions [67][68][69], there is a similar response attributed to human-induced changes, making it more difficult to attribute the change without additional information, e.g., [70][71][72][73]. C. nucifera is also abundant in sediment cores.…”