Phytolith evidence for Last Glacial Maximum grass-rich landscapes and postglacial forest development in the highlands of Tenerife

Transitions from glacial to interglacial periods are well documented across many regions through paleoenvironmental records, indicating that most forests suffered transformations linked to global climatic shifts through species migration or decline. On mountainous islands, forest vegetation is often organised into distinct altitudinal belts, driven by elevational gradients in temperature and moisture. As climatic conditions change through time, these vegetation belts can shift upslope or downslope, leading to changes in forest composition and distribution. However, the long-term dynamics of island forests, which often harbour endemic species with small populations, remain in many cases poorly understood. In particular, it is unclear whether ecosystems of the Canary Islands tracked regional or global climatic trends, or were buffered from major environmental changes by surrounding oceanic conditions. Here, we present a new palaeoenvironmental record from the Caldera de La Orotava, Tenerife (c. 1200 m asl), based on siliceous microremains and geochemical data. Using this record, we analysed vegetation responses to climatic transitions since the Last Glacial Maximum (LGM), as well as the impacts of human activities and volcanism. Vegetation changes were reconstructed from 23 cal ka BP to the present using phytolith analysis, as pollen was not preserved in the record. Siliceous algal remains and tephra shards were used to reconstruct changes in palustrine habitats and identify episodes of volcanic ash deposition. Comparisons with modern soil samples from the main Canary Island ecosystems provided a basis for interpreting ancient microfossil assemblages. Our results show a unique phytolith assemblage during the LGM, dominated by grasses of the Pooideae subfamily, and lacking a modern analogue in present-day ecosystems. Forests developed and shifted upslope during deglaciation after 14.5 cal ka BP, a pattern consistent with records from the western Mediterranean. Increases in the thermophilous Canarian palm ( Phoenix canariensis ) occurred during several phases of the African Humid Period, while humid pine forests ( Pinus canariensis ) expanded after c. 5 cal ka BP. We also document the ecological impacts of human settlement over the last two millennia, and the influence of volcanism on ecosystem dynamics. Our findings highlight the value of island palaeoecological records, particularly siliceous microremains, for reconstructing ecological responses to climatic changes, volcanism, and human impacts.