Leaf and root chemical and physical defence traits mediate monoculture
yield decline in a grassland experiment
Abstract
Plant monocultures growing for extended periods face severe losses of
productivity. This phenomenon, known as ‘yield decline’, is often caused
by the accumulation of above- and belowground plant antagonists. The
effectiveness of plant defences against antagonists might help
explaining differences in yield decline among species. Using a
trait-based approach, we studied the role of 20 physical and chemical
defence traits of leaves and fine roots on yield decline of 18-year old
monocultures of 27 grassland species. We hypothesized that yield decline
is lower for species with high defences, that root defences are better
predictors of yield decline than leaf defences, and that in roots,
physical defences better predict yield decline than chemical defences,
while the reverse is true for leaves. We additionally hypothesized that
species increasing the expression of defence traits after long-term
monoculture growth would suffer less yield decline. We summarized leaf
and fine root defence traits using principal component analysis and
analysed the relationship between defence traits mean as a measure of
defence strenght and defence traits temporal changes of the most
informative components and monoculture yield decline. The only
significant predictors of yield decline were the mean and temporal
changes of the component related to specific root length and root
diameter (e.g. the so called collaboration gradient of the root
economics space). The principal component analysis of the remaining
traits showed strong trade-offs between defences suggesting that
different plant species deploy a variety of strategies to defend
themselves. This diversity of strategies could preclude the detection of
a generalized correlation between the strength and temporal changes of
defence gradients and yield decline. Our results show that yield decline
is strongly linked to belowground processes particularly to root traits.
Further studies are needed to understand the mechanism driving the
effect of the collaboration gradient on yield decline.