Plant trait responses to variation in N and P availability

Global change drivers such as eutrophication and plant invasions will
create novel environments for many plant species. Through adaptive trait
plasticity plants may maintain their performance under these novel
conditions and may outcompete those showing low adaptive trait plasticity.
In a greenhouse study, we determined if plasticity in traits is adaptive
or maladaptive in endangered, non-endangered and invasive plant species in
response to variation of nitrogen (N) and phosphorus (P) availability (N:P
ratios 1.7, 15 and 135) and whether plastic trait responses are adaptive
and/or costly for fitness (i.e. biomass). Species choice comprised 17
species from three functional groups (legumes, non-legume forbs and
grasses), either classified as endangered, non-endangered or invasive.
After two months plants were harvested and nine traits related to carbon
assimilation and nutrient uptake were measured (leaf area, SLA, LDMC,
SPAD, RMR, root length, SRL, root surface area and PME activity). We found
more traits responding plastically to variation in P than in N. Plasticity
only created costs when P was varied. Plasticity in traits was mostly
adaptively neutral towards fitness, with plasticity in three traits being
similarly adaptive across all species groups: SPAD (as a measure of
chlorophyll content, adaptive to N and P limitation), leaf area and root
surface area (adaptive to P limitation). We found little differences in
trait plasticity between endangered, non-endangered and invasive species.
Synthesis. Along a gradient from N limitation, balanced N:P supply and P
limitation we found that the type of fluctuating nutrient (i.e. if N or P
is varied) is decisive for the adaptive value of a trait. Variation in P
availability (from balanced supply to P limitation) created both a
stronger reduction in fitness as well as created plasticity costs in more
traits than variation in N availability (from balanced supply to N
limitation). However, the patterns observed in our study may change if
nutrient availability is altered, either by nutrient inputs or by a shift
in nutrient availabilities, e.g. by decreasing N input as foreseen by
European Legislation, but without simultaneously decreasing P input.