Drought and soil macrofauna functional diversity effects on Mediterranean tree growth in an Ecotron ecosystem experiment

The intensity, frequency and severity of drought events are projected to increase in the Mediterranean region under ongoing climate change. This may impact plant growth and survival in Mediterranean forests leading to altered ecosystem dynamics. The functional diversity of soil macrofauna may have a critical role in mitigating negative climate change effects as it determines soil structure and regulates biogeochemical cycling as well as water retention. However, very little is known on how changes in climate and soil biodiversity conjointly influence plant growth. In a 5‐year experiment, we tested two levels of soil macrofauna functional diversity (Low and High), using combinations of endogeic and anecic earthworms, diplopods and isopods, and measured growth and biomass allocation in three tree species ( Acer monspessulanum , Arbutus unedo and Quercus pubescens ) as well as total tree biomass production. Our results showed that tree growth was overall only moderately affected by stronger drought and functional diversity of soil macrofauna. The effects of functional diversity of soil macrofauna differed among tree species. High functional diversity was associated with greater stem diameter in Q. pubescens . In contrast, low functional diversity was associated with greater stem diameter in A. unedo and proportionally greater biomass allocation to roots in A. monspessulanum . Individual macrofauna groups had stronger effects than overall functional diversity. Endogeic earthworm presence was associated with greater ecosystem‐level fine root and total plant biomass, but with lower fine root and total biomass in A. unedo . Diplopod presence was associated with greater stem diameter and higher total tree biomass in Q. pubescens . Overall, four consecutive summers of intensified drought had few significant effects on tree growth and biomass production, possibly because of high variability among individual trees. Most macrofauna effects were independent of drought, although drought modified the effect of diplopods on ecosystem‐level shoot biomass. These results show that macrofauna effects on tree growth depend on both macrofauna group and tree species identity. Read the free Plain Language Summary for this article on the Journal blog.