# Soil conditions modify species diversity effects on tree functional trait expression

## TL;DR

This study shows how soil conditions and species diversity together influence tree traits, affecting how trees use resources.

## Contribution

The study reveals distinct effects of soil microbiota and phosphorus on tree trait expression under species diversity.

## Key findings

- Phosphorus fertilization partly strengthens diversity effects on leaf traits.
- Soil microbiota inoculation leads to counter effects on trait expression.
- Soil conditions influence adaptability and resource-use strategies of tree species.

## Abstract

Examples of positive effects of biodiversity on ecosystem functions have kept accumulating in the last two decades, and functional traits are considered suitable tools to explain their underlying mechanisms. However, traits are rarely studied at the scale where these mechanisms (e.g., complementarity) are likely to originate, that is, between two interacting individuals. In an 18-month greenhouse experiment, we investigated how species diversity (i.e., monospecific or heterospecific tree pairs) affects within-individual leaf traits expression and variation and how this effect is modified by soil conditions. While resource addition through phosphorus fertilization partly strengthened the diversity effects, inoculation of soil microbiota (potentially leading to increased resource accessibility) resulted in counter effects. Hence, in contrast to our expectations, we did not find synergistic effects of the two soil treatments, but we found distinct effects on species following an acquisitive or conservative growth strategy. Overall, our study showed that the effect of species diversity on young trees’ adaptability and resource-use strategy needs to be considered alongside soil biotic and abiotic aspects. The influence of soil conditions on species diversity effects is essential to understand mechanisms behind complementarity at the individual level, which ultimately translate to the community scale.