INTRODUCTION
Flowering plants are estimated to have diversified into an extant global
flora of about 369.000 species . Most of those species have small
spatial ranges and maintain low population densities, and are thus
considered rare . On the other side, a few species are considered common
or dominant, as they achieve high population densities and have
colonized large stretches of land . Though this pattern has been
recognized as early as the 19th century (Darwin,
1859), the reasons why most species are rare and a few are common or
dominant are still unclear .
Plant functional traits have been successfully used to explain species
occurrence patterns in relation to environmental conditions . For
example, leaf morphology is correlated with climate , and chemical
composition of plants is correlated with soil properties . It has also
been shown that species with particular trait values increase in
abundance when filtered by the environment . Surprisingly, however, a
basic description of trait differences between common and rare species,
irrespective of changes in environmental conditions, is still missing. A
reason for this might be that rarity (or commonness) has multiple
dimensions and can apply to different spatial scales. For example, while
a species may be locally abundant in a certain habitat type (i.e. be
common), it might have a restricted distribution globally (i.e. be
rare).
Most studies relating traits to ecological parameters are focussing on
aboveground traits, most likely because they are overrepresented in
trait databases . Although leaf traits, plant height and seed weight are
considered to capture most variation in plant form and function ,
inclusion of belowground traits can substantially increase predictive
power for species distributions . Belowground traits, however, are
usually not considered as their measurement is technically challenging
and labour intensive . Moreover, belowground traits are often assumed to
be correlated with aboveground trait . However, other studies show that
correlations between above and belowground traits vary tremendously
among clades and depend on the traits considered .
We tested if plant functional traits explain the success of species
across spatial scales from local abundance in grassland plots to their
worldwide distribution. To do so, we measured root traits and seed
weight on 242 grassland species grown in two large common-environment
experiments. In addition, we extracted aboveground and bud-bank traits
from databases. We then tested how the traits relate to the abundance
and occurrence in grassland plots of the German Biodiversity
Exploratories , the occurrence frequency across Germany, the occurrence
frequency across the European and Mediterranean region, and their global
occurrence as naturalized alien species. We aimed to answer the
following questions:
1) Do plant functional traits explain species success, i.e. abundance
and occurrence frequency across spatial scales?
2) Do the contributions of traits to species success vary with the
spatial scale considered?
3) Do above- and belowground traits contribute differently to explaining
species success?