DISCUSSION
This is the first study that tested how above- and belowground
functional traits of grassland species relate to their success across
multiple spatial scales. Among the 242 study species, low root tissue
density was identified as important for all success metrics. However,
the effects of the other traits were frequently dependent on the success
metric considered. For example, while seed weight was negatively related
to the occurrence frequency in Germany, it was positively related to the
abundance in German grasslands. This illustrates that some traits may
have opposite effects on different dimensions of species rarity and
commonness. Moreover, we found that many of the effects had significant
non-linear effects, in most cases with the highest success-metric value
at intermediate trait values. While most previous studies on
relationships between functional traits and species distributions
focussed on easy-to-measure aboveground traits , our study shows that
belowground traits can also explain a significant amount of variation in
species success.
The spatial scale of success ranged from local abundance in 16 m²
grassland plots in Germany (GPs and EPs) to the global occurrence
outside the native range (number of GloNAF regions). While the local
scale abundance data are restricted to a single habitat type, the
occurrence at larger spatial scales also covers other habitat types
(e.g. 86% of the area in Germany is not used as grassland; BMEL 2017).
Each habitat type might select for different values of a trait ,
resulting in no clear relationship between the trait and the success
metric overall. This context specificity could explain why the models of
the success metrics at larger spatial scales had overall fewer traits
with significant effects than the models on abundance in the grassland
plots.
Accordingly, we found varying degrees of consistency in the trait values
of successful species across spatial scales. Root-tissue density was the
only trait with a consistent effect on all success metrics. Probably, a
low root-tissue density, which is indicative of a high
resource-acquisition-rate strategy , is beneficial in nutrient-rich
habitats, which are locally and globally widespread as a consequences of
agriculture. On the other hand, the effect of specific leaf area, an
aboveground trait associated with the resource-acquisition strategy
(Onoda & Wright, 2018), depended on the spatial scale of the success
metric. Occurrence frequencies at all spatial scales tended to
asymptotically increase with specific leaf area, which is in line with
the frequent observation that high specific leaf area promotes invasion
success . However, abundance in the grassland plots was negatively
related to specific leaf area, possibly reflecting that persistence
under highly competitive pressures in dense grasslands could require a
more conservative growth strategy.
Bud-bank size had significant effects on all success metrics, but the
direction and shape of the relationship varied a lot. Species with a
large bud-bank had the highest abundance and occurrence frequency in the
grassland plots. A large bud-bank is essential for regrowth of
long-lived perennials after e.g. grazing or mowing . Species with small
bud-banks, on the other hand, had the largest naturalized ranges. This
dual effect of bud-bank size on success was also evident from the fact
that the highest occurrence frequencies in the native range and in
Germany were found for both species with high and low bud-bank sizes.
Although buds themselves are not very costly , they require bud-bearing
organs and nutrient reserves, which may trade-off with seed production .
Thus, species with smaller bud-banks may invest more in seed production,
resulting in a higher dispersal ability and larger native and
naturalized distributions.
The effect of seed weight on species success metrics ranged from
positive at the plot scale to negative or absent at larger spatial
scales. The finding that species with heavy seeds tended to be more
frequent and abundant in the grassland plots, most likely reflects that
large amounts of resources stored in seeds increase seedling survival
under the strong competition in grasslands . Species with light seeds,
on the other hand, might have a higher reproductive output , could
potentially disperse over longer distances , but see Thomson et
al. , 2011), and could persist longer in the seed bank . At the larger
spatial scales, this benefit of small seeds could have compensated or
overcompensated the reduced seedling survival chances.
Maximum rooting depth was related to success metrics at the largest
spatial scales, where it had positive effects. Deep roots allow a plant
to take up water with nutrients from deeper soil layers, increasing
survival and growth, particularly when the surface soil regularly dries
out . As most of the agriculturally used grasslands in Germany are mesic
, this could explain why rooting depth was not significantly associated
with occurrence frequency and abundance in the grassland plots. At the
larger spatial scales, which also cover other habitat types, species
with deep roots might be more persistent. For naturalization success,
however, there was a significant non-linear effect of rooting depth as
both deep-rooting and superficially rooting species were most
successful. This could indicate that the alternative strategy of lateral
spread to acquire resources and avoid competition with deeper rooting
species might also be beneficial at the global scale.
Height of the plants was not related to success of the species at larger
spatial scales. This is surprising, particularly for naturalization
success, as numerous studies on naturalization and invasion success
found that tall species were more successful . Most of those studies,
however, considered woody species that are overall much taller than the
grassland herbs in our study. Interestingly, while plant height
increased abundance in the German grassland plots (at least in the GPs),
it decreased the occurrence frequency in those grasslands. On the one
hand, tall plants, when they occur somewhere, might be competitively
superior and become dominant, whereas, on the other hand, small plants
might be less at risk of losing reproductive organs due to mowing or
grazing.
We found that species that are abundant in grasslands are typically
characterized by thin low-density roots, which promote the uptake of
belowground resources. The patterns for success metrics at larger
spatial scales, at least those in the native ranges, were less clear.
Indeed, plant traits explained large proportions of the variation in
local abundance and occurrence frequency in the grasslands
(>25%; Table 1), whereas the proportion of variation in
occurrence frequency in the native range and in Germany was very low
(2% and 11%, respectively; Table 1). This suggests that plant traits
could be good predictors of species success if one considers a single
habitat type, but that this is less the case for success metrics at
large spatial scales that are not habitat specific. However, a notable
exception is the global naturalization success of the species, as 41%
of the variation in occurrence frequency outside the native range was
explained by the plant functional traits. Possibly, this reflects that
most naturalizations happen in anthropogenic environments , and thus
largely in a single habitat type.
The plant economics spectrum postulates that the high specific leaf area
typical for “acquisitive” plants should be mirrored belowground by a
high specific root length, low root-tissue density and a low root
diameter . Indeed, specific leaf area was negatively correlated with
root-tissue density, but it was not significantly correlated with
specific root length and diameter of the first-order roots (Appendix
S3). This decoupling from the plant economic spectrum has previously
been found for grassland plants as well as tree seedlings . Seed weight
and plant height, the other aboveground traits frequently used in
studies on functional ecology of plants, were also not very strongly
correlated with the belowground traits in our study. These belowground
traits explained a considerable proportion of variation in the success
metrics, in addition to the variation explained by the three aboveground
traits (Table 1). Indeed, for all success metrics, except occurrence
frequency in the EPs, the belowground traits explained at least as much
of the variation in success metrics as the three aboveground traits did.
Therefore, our results show that aboveground traits cannot substitute
for belowground traits in studies on plant functional ecology.