Below- and aboveground traits explain local abundance, and regional, continental and global occurrence frequencies of grassland plants

1. Plants vary widely in how common or rare they are, but whether
commonness of species is associated with functional traits is still
debated. This might partly be because commonness can be measured at
different spatial scales, and because most studies focus solely on
aboveground functional traits. 2. We measured five root traits and seed
mass on 241 Central European grassland species, and extracted their
specific leaf area, height, mycorrhizal status and bud-bank size from
databases. Then we tested if trait values are associated with commonness
at seven spatial scales, ranging from abundance in 16-m² grassland plots,
via regional and European-wide occurrence frequencies, to worldwide
naturalization success. 3. At every spatial scale, commonness was
associated with at least three traits. The traits explained the greatest
proportions of variance for abundance in grassland plots (42%) and
naturalization success (41%), and the least for occurrence frequencies in
Europe and the Mediterranean (2%). Low root tissue density characterized
common species at every scale, whereas other traits showed directional
changes depending on the scale. We also found that many of the effects had
significant non-linear effects, in most cases with the highest
commonness-metric value at intermediate trait values. Across scales,
belowground traits explained overall more variance in species commonness
(19.4%) than aboveground traits (12.6%). 4. The changes we found in the
relationships between traits and commonness, when going from one spatial
scale to another, could at least partly explain the maintenance of trait
variation in nature. Most importantly, our study shows that within
grasslands, belowground traits are at least as important as aboveground
traits for species commonness. Therefore, belowground traits should be
more frequently considered in studies on plant functional ecology.