Chemical variations in Quercus pollen as a tool for taxonomic identification: implications for long-term ecological and biogeographical research

Aim  Fossil pollen is an important tool for understanding
biogeographic patterns in the past, but the taxonomic resolution of the
fossil-pollen record may be limited to genus or even family level.
Chemical analysis of pollen grains has the potential to increase the
taxonomic resolution of pollen, but present-day chemical variability is
poorly understood. This study aims to investigate whether a phylogenetic
signal is present in the chemical variations
of Quercus L. pollen and to assess the prospects of
chemical techniques for identification in biogeographic research.
Location  Portugal Taxon  Six taxa (five species, one
subspecies) of Quercus L., Q. faginea, Q.
robur, Q. robur ssp. estremadurensis, Q. coccifera, Q.
rotundifolia and Q. suber belonging to three
sections: Cerris, Ilex, and Quercus (Denk,
Grimm, Manos, Deng, & Hipp, 2017) Methods  We collected
pollen samples from 297 individual Quercus trees across
a 4° (∼450 km) latitudinal gradient and determined chemical differences
using Fourier-transform infrared spectroscopy (FTIR). We used canonical
powered partial least-squares regression (CPPLS) and discriminant analysis
to describe within- and between-species chemical variability.
Results  We find clear differences in the FTIR spectra
from Quercus pollen at the section level (Cerris:
∼98%; Ilex: ∼100%; Quercus: ∼97%). Successful
discrimination is based on spectral signals related to lipids and
sporopollenins. However, discrimination of species within
individual Quercus sections is more difficult: overall,
species recall is ∼76% and species misidentifications within sections lie
between 18% and 31% of the test-set. Main Conclusions  Our
results demonstrate that subgenus level differentiation
of Quercus pollen is possible using FTIR methods, with
successful classification at the section level. This indicates that
operator-independent FTIR approaches can surpass traditional morphological
techniques using the light microscope. Our results have implications both
for providing new insights into past colonisation pathways
of Quercus, and likewise for forecasting future responses to
climate change. However, before FTIR techniques can be applied more
broadly across palaeoecology and biogeography, our results also highlight
a number of research challenges that still need to be addressed, including
developing sporopollenin-specific taxonomic discriminators and determining
a more complete understanding of the effects of environmental variation on
pollen-chemical signatures in Quercus.