A quantitative study of modern pollen–vegetation relationships in southern Brazil's Araucaria forest

Southern Brazil's highland Araucaria forest is ancient, diverse and unique, but its future is under significant threat from 20th Century habitat loss and 21st Century climate change. Paleoecological studies have revealed that it expanded rapidly over highland grasslands around 1000 years ago, but whether this expansion was caused by human land use or climate change has been a topic of considerable debate. Discriminating between these potential drivers has so far not been possible with fossil pollen, however, as the palynological representation of floristic and structural differences in Araucaria forest remains poorly understood. Here, we address this shortcoming using modern pollen rain from moss polsters and vegetation surveys in forest areas with minimal current human disturbance. We show that forest plots with evident structural differences lack consistent differences in their floristic composition and cannot be reliably distinguished by their pollen spectra. We quantify pollen–vegetation relationships for 27 key tree genera of Araucaria forest, showing that, despite significant intra-taxon variability, 22 of these are under-represented or absent in the pollen record. These palynologically under-represented and silent taxa include many of the forest's most ecologically important tree species, with only Araucaria, Lamanonia, Podocarpus, Myrsine and Clethra being more abundant in the pollen rain than vegetation. Our results suggest that subtle structural changes in Araucaria forest, as well as moderate to significant floristic changes, may not be clearly distinguished in fossil pollen records – an important limitation when attempting to identify past human and climatic impacts on Araucaria forest via pollen analysis.