International audiencePlant–plant chemical interactions in forests can have a strong impact on the biodiversity and dynamics of these ecosystems, particularly in Mediterranean forests where plants exhibit a high secondar

Chemical interactions in forested ecosystems play a role in driving biodiversity and ecosystem dynamics. Plant phenolics released by leaching can influence surrounding plants and soil organisms such as bacteria, fungi or arthropods. However, our knowledge about such chemically-mediated biotic interactions in Mediterranean oak forests is still limited, in particular whether they play a role in the limited forest regeneration. In this study, we analyzed how phenolics of Cotinus coggygria, a dominant shrub of Mediterranean downy oak (Quercus pubescens) forests, influence understory herbaceous plant species, downy oak regeneration and soil organisms in order to obtain a more integrative view of possible direct and indirect interactions triggered by this shrub species. We performed a series of experiments testing the effect of aqueous extracts of C. coggygria, mimicking natural leachates, on these organisms. Cotinus coggygria contained a high quantity of phenolics in green and senescent leaves but much less in leaf litter. Extracts from C. coggygria leaves stimulated bacterial communities, exhibited few effects on both saprophytic and symbiotic fungi, and negatively affected Collembola. Herbaceous species growth was particularly impaired by extracts from green and senescent leaves, although these effects were alleviated in the presence of soil microorganisms. In both greenhouse and field experiments, C. coggygria affected early oak seedling establishment in particular through a reduced root growth, but exhibited no effect on later seedling and sapling growth. We discussed the implication of these results for the balance between competition and facilitation in oak forests and concluded that C. coggygria has the potential to strongly alter biotic interactions, understory plant diversity and oak forest dynamics.

Aims: Soil temperature and moisture impact plants not only during growth and survival but also 9 during seed germination and interaction of seeds with the chemical environment. The quantitative 10 impacts of either temperature and moisture or plant specialized metabolites (PSM) on germination 11 are widely studied. However, the combined effect of PSM and moisture or temperature on 12 germination remains poorly understood. 13 Methods: We addressed this issue by studying the effect of PSM extracted from four Mediterranean 14 woody plants on germination speed and final percentages of a subordinate herbaceous plant, Linum 15 perenne. 16 Results: By using hydro-and thermal time threshold models, we show how PSM interact with 17 temperature and moisture levels to limit germination at dry and upper thermal limits, with the 18 magnitude of effects depending on the source plant. PSM effects on germination, also observed on 19 natural soils, persisted after their removal from the seed environment. 20 Conclusions: We conclude that the impact of climate change on reproduction of herbaceous plants 21 can be modulated by effects of PSM from woody plants, which might exacerbate the negative 22 impacts of global changes on biodiversity.