Diversity and ecology of tropical African fungal spores from a 25,000-year palaeoenvironmental record in southeastern Kenya

Abstract

Fossil fungal spores and other non-pollen palynomorphs (NPPs) are powerful environmental proxies in European palaeoecological and archaeological contexts. However, their application on other continents, and particularly in the tropics, is hampered by uncertain equivalence with morphologically similar taxa in Europe, and incomplete knowledge of their ecology in the new local contexts. Here we use fossil NPP assemblages in a 25,000-year sediment record from Lake Challa, a steep-sided crater lake near Mt. Kilimanjaro in southeasternKenya, to assess NPP diversity in atropical-African context and the equivalence of African taxa with their European counterparts. We recovered a total of 65 well-defined NPP types, of which 61 are fungal spores, and 42 could be linked to known taxa. We provide diagnoses and illustrations of 61 recovered taxa, 58 of which have not been documented before.

Using the Challa pollen record of past regional vegetation dynamics and two independent proxies of past temperature and rainfall, we also assessed the association of individual fungal taxa with particular species and biomes of tropical-African vegetation, and with the history of regional climate change. We often found strong correspondence between the stratigraphic distribution of individual fungal spore taxa and the occurrence of specific vegetation types. Changing climate conditions appear to have had a strong impact on the ability of fungi to play a role in the decomposition of dead plants. For fungal spore assemblages, the most prominent change in regional palaeoenvironments over the past 25,000 years occurred at the start of the wet early Holocene, following Younger Dryas drought. Epicoccum purpurascens is common in the Glacial and Late-Glacial parts of the sequence, but shows a strong decline during the early Holocene. Coniochaeta cf. ligniaria occurs throughout the record but shows dramatic fluctuations that appear to relate to major changes in humidity. Correlation between fungal abundance and humidity is also observed in taxa for which the Challa region provided suitable habitat from ca. 16,500 cal. yr BP (e.g., Curvularia) or from the Late-Glacial to Holocene transition (e.g., Tetraploa aristata, Dictyoarthrinium cf. sacchari, cf. Byssothecium, types HdV-1032 and HdV-1033, cf. Alternaria, cf. Brachysporium, cf. Helminthosporium, Spegazzinia tessarthra and cf. Lasiodiplodia theobromae). Many of these taxa did not occur, or were rare, during both wet and dry phases of the Glacial period, suggesting an additional temperature effect on their occurrence in tropical African environments. A possibly dominant role of temperature is revealed in the stratigraphic distribution of Acrodictys, which appears at the onset of deglacial climate warming ca. 17,500 cal. yr BP and remains common throughout both wet and dry phases of the Holocene. Spores of the dung-inhabiting fungus Sporormiella occur throughout the 25,000-year record without notable fluctuations, suggesting little changes in the overall population density of large herbivores in the region.