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

Poaceae (the grass family) is highly diverse, geographically widespread, and an important component of many terrestrial ecosystems. 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 unclear if this variable can be used at broader spatial and deep-time scales. Here we set out to perform a comprehensive assessment and to test the robustness of this proxy. We generated pollen size data, as well as associated measurements of the pollen wall and pore, using 127 plant specimens from 58 species across the Poaceae phylogeny from the Amazon drainage basin (ADB). We tested the relationship between pollen size and a range of abiotic and biotic variables (vegetation type, soil composition, climate conditions, photosynthetic pathway and genome size), and fitted models using phylogenetic generalized least squares (P-GLS) and linear mixed models (LMM). Our measurement data shows that Poaceae pollen size presents a very wide range (18.77–71.62 μm), and varies not only among genera and species but also within species. There is no obvious relationship between pollen size and the explanatory variables considered here. We suggest that pollen size cannot be used as generally applicable proxy to reconstruct past vegetation and climate.