Understanding the Cenozoic vegetation history of what is now the Qinghai-Tibetan Plateau is crucial for elucidating the co-evolutionary dynamics between plateau development, its environment, and the organisms it hosts. In this study, we conduct a comprehensive analysis of phytoliths within the late Oligocene-Early Miocene lacustrine sedimentary section of the Lunpola Basin, central Qinghai-Tibetan Plateau. The diverse phytolith morphotype assemblages indicate that the vegetation of the central Tibetan region mainly comprised a mixed coniferous and broad-leaved forest. Grasses in the understory primarily consisted of Pooideae, distinguished by phytolith morphotypes such as rondel, crenate and Stipa-type bilobate forms. Combined with previous work, we infer that riparian vegetation of the central Tibetan region transitioned from a humid subtropical forest, dominated by broad-leaved woody plants during the middle Eocene, to a more seasonally arid open woodland containing abundant woody and herbaceous plants during the late Eocene, before developing into a cooler mixed coniferous and broad-leaved forest during the late Oligocene-Early Miocene. The growth of the central Tibetan region and retreat of the Tethys Ocean, together with the uplift of the Himalaya, contributed to this vegetation change. This study provides new evidence from the phytolith perspective for the evolutionary history of Qinghai-Tibetan Plateau vegetation being tied to plateau formation and regional climate change.

Within the ongoing controversy regarding the orogeny of the Tibetan Plateau region, two directly-conflicting endmember frameworks have emerged, where either: 1) a high central ‘proto-plateau’ existed prior to the onset of India-Asia continental collision, or 2) the early Paleogene central Tibet comprised a wide E-W oriented lowland ∼1-2 km above sea level, bounded by high (> 4.5 km) mountain systems. Reconstructing plateau development correctly is fundamental to running realistic Earth system models that explore monsoon and biodiversity evolution in the region. Understanding the interplay between monsoon dynamics, landscape and biodiversity are critical for future resource management. We explore the strengths and weaknesses of different palaeoaltimetric methodologies as applied across the Tibetan region. Combining methodologies, appreciating the vulnerabilities arising from their underlying assumptions and testing them using numerical climate models, produces consilience (agreement) allowing further refinement of both models and proxies. We argue that an east-west oriented Paleogene Central Tibetan Valley was a cradle and conduit for thermophilic biota seeding the modern regional biodiversity. The rise of eastern Tibet intensified regional rainfall and erosion, which increased topographic relief and biodiversification. Gradual monsoon development reflected the evolving topography, but modern-like Asian monsoons developed only after a plateau formed in the Miocene.