Divergent trait responses to nitrogen addition in tall and short species

Asymmetrical light competition and direct detrimental effect of nitrogen
have been proposed as two main mechanisms driving species richness
declines following nitrogen (N) addition. N addition is also known to
alter functional trait composition towards increased dominance of tall and
fast-growing species. However, whether trait changes vary between species
and, in particular, how the traits of tall and short species respond to N
addition has rarely been studied. Understanding whether different trait
changes occur for tall and short species would provide insight into
mechanisms underlying N addition effects. Based on a long-term N addition
experiment, we measured the natural height of 44 plant species and the
photosynthetic active radiation (PAR) at seven heights in the vegetation
and identified a clear stratification of the plant community into tall
species (>30cm tall), which experienced high light conditions, and
short species that grew under the canopy. We also measured four functional
traits, including maximum plant height, specific leaf area, leaf dry
matter content and leaf thickness, on 35 species that occurred in
fertilized and unfertilized plots. Structural equation modelling revealed
that nitrogen (N) addition significantly reduced species richness by
enhancing light asymmetry and had no direct effects, suggesting that N
detrimental effects are negligible in our system. Consistent with this, we
found different responses of traits and diversity for the tall and short
species. Specifically, N addition reduced the number of short species but
increased the number of tall species. In addition, specific leaf area
increased, and leaf dry matter content decreased, for short species only,
suggesting that they shifted to a fast growth strategy to cope with lower
light levels. In contrast, tall species increased their height further to
capture more light at the top of the canopy.   Synthesis. The
divergent trait responses observed for tall and short species show that
although certain traits, like height and leaf traits, show correlated
responses at the whole community level, these correlations may not be
consistent across all species. Together, our results highlight that light
competition is the main mechanism driving species loss following N
addition gradient.