Abstract
High-elevation ecosystems, such as those on Mount Kenya are undergoing significant changes, with accelerated glacial ice losses over the twentieth century creating new space for alpine plants to establish. These ecosystems respond rapidly to climatic variability and within decades of glacial retreat, Afroalpine pioneering taxa stabilize barren land and facilitate soil development, promoting complex patches of alpine vegetation. Periglacial lake sediment records can be used to examine centennial and millennial scale variations in alpine and montane vegetation compositions. Here we present a 5300-year composite pollen record from an alpine tarn (4370 m asl) in the Hausberg Valley of Mount Kenya. Overall, the record shows little apparent variation in the pollen assemblage through time with abundant montane forest taxa derived and transported from mid elevations, notably high abundances of aerophilous Podocarpus pollen. Afroalpine taxa included Alchemilla, Helichrysum and Dendrosenecio-type, reflecting local vegetation cover. Pollen from the ericaceous zone was present throughout the record and Poaceae percentages were high, similar to other high elevation pollen records from eastern Africa. The Oblong Tarn record pollen assemblage composition and abundances of Podocarpus and Poaceae since the late Holocene (~4000 cal yr BP-present) are similar to pollen records from mid-to-high elevation sites of nearby high mountains such as Mount Elgon and Kilimanjaro. These results suggest a significant amount of uphill pollen transport with only minor apparent variation in local taxa. Slight decreasing trends in alpine and ericaceous taxonomic groups show a long-term response to global late Holocene cooling and a step decrease in rate of change estimated from the pollen assemblages at 3100 cal yr BP in response to regional hydroclimatic variability. Changes in the principal component axis scores of the pollen assemblage were coherent with an independent mid-elevation temperature reconstruction, which supported the strong influence of uphill pollen transport from montane forest vegetation and association between temperatures and montane vegetation dynamics. Pollen accumulation rates showed some variability related to minerogenic sediment input to the lake. The Oblong Tarn pollen record provides an indication of long term vegetation change atop Mount Kenya showing some decreases in local alpine and ericaceous taxa from 5300–3100 cal yr BP and minor centennial-scale variability of montane taxa from mid elevation forests. The record highlights potentials, challenges and opportunities for the use of proglacial lacustrine sediment to examine vegetation change on prominent mountain massifs.
Highlights
Evidence of glaciation on Mount Kenya (5199 m asl) suggests ice has likely persisted >100,000 years [1,2,3] and during the Last Glacial Maximum of the Late Pleistocene (~21000 yr BP), covered up to 200 km2, reaching as low as 3200–3300 m asl down the mountain [4,5,6]
The pollen assemblages of Oblong Tarn since 5300 cal yr BP were rather unresponsive to broad-scale environmental changes that have been documented for the Afrotropical region [81]
The Oblong Tarn pollen record consists primarily of montane forest and ericaceous pollen transported from lower elevations
Summary
Evidence of glaciation on Mount Kenya (5199 m asl) suggests ice has likely persisted >100,000 years [1,2,3] and during the Last Glacial Maximum of the Late Pleistocene (~21000 yr BP), covered up to 200 km, reaching as low as 3200–3300 m asl down the mountain [4,5,6]. Glacier retreat in the current century could potentially induce novel hydroseral transitions in the numerous proglacial tarn lakes [17] and change the alpine vegetation These are the headwaters for many montane streams flowing into lowland springs and river systems crucial to human populations, wildlife and socio-ecological resilience [18,19,20] the glaciers themselves are not significant regional water reservoirs [21]. Mount Kenya glaciers are controlled by a complex interaction between temperature, precipitation, and radiation, with cloud cover being an important influence [22] The impact of these environmental changes on to the unique vegetation assemblages of the alpine and ericaceous zone is little understood, and there is little data about the long-term response of these systems to Holocene climate variability. Given the strong spatial constraints on the vegetation zones around the mountain, understanding the long-term response of these plant assemblages, which contain many endemic taxa, to climate changes is a crucial question
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