To what extent do time, species identity and selected plant response variables influence woody plant interactions?
Better knowledge of plant interactions is essential to understanding plant community dynamics and has broad applications in restoration and forest operations. Yet studies investigating positive and negative interactions, and indirect interactions, are scarce. We quantified the nature and intensity of plant interactions between target species, neighbours and ground vegetation in an experimental plantation in southern France to detect competition, direct and indirect facilitation and shifts in interactions with time. Mediterranean oak seedlings (Quercus pubescens and Quercus ilex) were planted as target species within the following neighbourhoods: pine trees Pinus halepensis at two densities, N-fixing shrubs (Coronilla valentina), mixed pines and shrubs, and controls. The ground vegetation was either weeded or unweeded. We computed the relative interaction index (RII) for survival, growth and elongation of the target species in different treatments over 3 years. Light and soil water content were also monitored. Results showed that the outcome of plant interactions depended on the variable selected to evaluate plant response. Decrease in Quercus pubescens survival and in diameter growth of both Quercus species indicated competition, whereas enhanced Quercus elongation and higher Quercus pubescens height growth in the first year indicated facilitation. Indirect facilitative interactions also occurred, resulting in the alleviation of competition by neighbours in the unweeded treatment. This result was explained by the reduction of herb competition caused by shade from neighbours. With time, competitive interactions became dominant, but were strongly modulated by the type of neighbours. Competition was more severe with shrub neighbours than with pine neighbours due to much greater light interception and also higher water uptake. Synthesis and applications. We show that the outcome of plant interactions is species specific and varies with time and the indicator selected. Facilitation was detected with tree neighbour treatment, but over time interactions clearly shifted to competition. Hence, forest planting operations can benefit from the interactions induced by neighbouring woody vegetation provided that neighbour species are carefully selected and active post-planting management is performed.