Is Poaceae pollen size a useful proxy in palaeoecological studies? New insights from a Poaceae pollen morphological study in the Amazon

Aim: Grasslands occupy around 40% of the Earth's land surface and can be regarded as the most common vegetation type in the world, with Poaceae being the most widespread angiosperm family of all.   Poaceae pollen size has previously been suggested as a proxy to reconstruct the past vegetation and climates in the Amazon area, but it is still controversial if this variable indeed can be used in broader spatial and deep-time scales. Here we set out to perform a comprehensive assessment and test the robustness of this proxy.

Location: Amazon drainage basin (ADB).

Taxon: Poaceae.

Methods: One hundred and twenty-seven specimens from 58 species (non-crops) across the Poaceae phylogeny from the Amazon drainage basin (ADB) were prepared for pollen grain size analyses, in order to explore their relationship with abiotic and biotic variables (vegetation type, soil composition, climate conditions, photosynthetic pathway, and genome size). Phylogenetic generalized least squares (P-GLS) model and linear mixed models (LMM) were applied to assess the proxy.

Results: Our measurement data show that Poaceae pollen size presents a very wide range (18.77 - 71.62 μm), not only at the genus and species levels but also within species. There is no obvious relationship between pollen size and the explanatory variables considered here, however.

Main conclusion: Poaceae (non-crop) pollen size does not respond to explanatory variables, and therefore cannot as a useful proxy to reconstruct past vegetation and climate.