Data from: Intraspecific trait changes have large impacts on community functional composition but do not affect ecosystem function

1.    Plant functional traits can provide a
mechanistic understanding of community responses to global change and of
community effects on ecosystem functions. Nitrogen enrichment typically
shifts trait composition by promoting the dominance of acquisitive plants
(high specific leaf area [SLA] and low leaf dry matter content [LDMC]),
translating into high biomass production. Changes in mean trait values can
be due to shifts in species identity, relative abundances and/or
intraspecific trait values. However, we do not know the relative
importance of these shifts in determining trait responses to environmental
changes, or trait effects on ecosystem functioning, such as biomass
production.  2.    We quantified the
relative importance of species composition, abundance and intraspecific
shifts in driving variation in SLA and LDMC, and how these shifts affected
above- and belowground biomass. We measured traits in a grassland
experiment manipulating nitrogen fertilisation, plant species richness,
foliar fungal pathogen removal and sown functional composition (slow vs.
fast species). We fitted structural equation models to test the importance
of abundance and intraspecific shifts in determining 1) responses of
functional composition to treatments and 2) effects on above and
belowground biomass.   3.    We found
that species intraspecific shifts were as important as abundance shifts in
determining change in functional composition (community weighted mean
trait values), and even had large effects compared to substantial initial
variation in sown trait composition. Intraspecific trait shifts resulted
in convergence towards intermediate SLA in diverse communities; although
convergence was reduced by nitrogen addition and enhanced by pathogen
removal. In contrast, large intraspecific shifts in LDMC were not
influenced by the treatments. However, despite large responses,
intraspecific trait shifts had no effect on above or belowground biomass.
Only interspecific trait variation affected functioning: belowground
biomass was reduced by SLA and increased by LDMC, while aboveground
biomass was increased by SLA.  4.  
 Our results add to a growing body of literature showing large
species intraspecific trait variation and emphasise the importance of
using field sampled data to determine community functional composition.
However, they also show that intraspecific variation does not necessarily
affect ecosystem functioning and therefore response-effect trait
relationships may differ between vs. within species.