Intra- and inter-specific changes in leaf and root defence traits along an experimental plant diversity gradient.

The enhancement of biodiversity's positive impact on ecosystem functioning (BEF) over time is commonly observed and attributed to the accumulation of mutualists and dilution of antagonists in more diverse communities. If antagonists play a role in the BEF relationship, the reduction of plant antagonists in more diverse communities, could allow plants to reduce allocation to defence. This study aimed to assess the influence of plant diversity on the expression of defence traits in 16 plant species. Our hypotheses were: (1) increased plant diversity reduces allocation to defence, (2) this reduction is more pronounced in roots than in leaves, and (3) this effect varies among species.

We measured both physical and chemical defence traits in leaves and fine roots across communities with varying plant species richness in a 19-year-old biodiversity experiment. Using established methods and an innovative metabolome approach, we explored the interactive effects of plant diversity and species identity on defence traits through linear mixed models.

Our findings were mixed concerning the first hypothesis, with only some leaf defence traits (leaf mass per area, leaf dry matter content, and hair length) showing reduction along the diversity gradient. Unexpectedly, the values of some root traits, root tissue density and nitrogen content, suggested increased allocation to defence along the same gradient. This might be attributed to these traits serving other functions, e.g. in resource acquisition and competition, which potentially overruled the impact of declining antagonists on plant defences. Our results did not support the third hypothesis, suggesting an overall convergence responses to biotic and abiotic factors related to plant diversity after two decades.

While evidence for a consistent reduction in defence trait expression along the diversity gradient was limited, our findings underscore the complex nature of BEF relationships. Further experiments, possibly controlling confounding factors on trait expression or manipulating antagonist pressures along diversity gradients, are needed to elucidate the underlying mechanisms.