Chemical Interaction Between Quercus Pubescens and Its Companion Species is Not Emphasized under Drought Stress

How plant–plant interactions will interact with global change drivers such as increased drought during the regeneration phase is a key question to forecast future vegetation dynamics. Chemical interaction and especially allelopathy and drought have been suggested to affect plant performance synergistically, i.e., that plant under drought stress would be more sensitive to allelochemicals and that exposure to allelopathic interactions could increase drought sensitivity through an inhibition of root development and mycorrhization. In this paper, we tested these hypotheses by using a controlled experiment with Quercus pubescens Mill. as a target species and three co-occurring species plus itself as source species. Allelopathic treatments consisted of annual provision of litter and monthly watering with green leaf aqueous extracts during two vegetation seasons starting from oak acorns. During the second vegetation season, a drought stress treatment was added on half of the seedlings. Allelopathy of co-occurring species reduced seedlings dimensions while Q. pubescens treatment increased it. During the second vegetation season, seedling growth rate and physiology were reduced by drought but poorly affected by allelopathic treatment. At the end of the experiment, drought stress and allelopathy from Cotinus coggygria and Pinus halepensis both reduced seedling biomass but had opposite effects on the root/shoot ratio. Drought and allelopathy did not interact significantly and, contrary to our hypothesis, there was a tendency of lower allelopathic effects under drought. Our results suggest that drought and allelopathy could additively alter seedling development, but the opposite effects of allelopathy and drought on the root/shoot ratio call for further experiments testing the interaction between these two factors.