Résumé
Mixed plantations have been receiving increasing attention for their documented or supposed potential benefits over monospecific plantations. In particular, the use of neighbouring (or nurse) vegetation around target plants can enhance their performance through limitation of the competing herb layer, and can also improve their morphology. Here we examine the benefits and drawbacks of using neighbour treatments on the response of target trees in open plantations. We set up an experimental plantation in southern France, in which two co-occurring target oak species (the evergreen Quercus ilex and the deciduous Quercus pubescens) were introduced in different neighbour treatments using a tree (Pinus halepensis) and a shrub species (the nitrogen-fixing Coronilla glauca). Oaks were planted with pine neighbours at two densities, with shrub neighbours, in a mixture of pines and shrubs or without neighbours. The ground vegetation was either regularly weeded or left to grow in order to detect any indirect facilitation interactions. Target oak responses (survival, growth, and morphology) were monitored over 7 years. Soil water content and light availability were also measured throughout the experiment. We found competition to be the dominant process driving interactions between neighbours and target tree species. Growth was reduced for both species, but more in weeded than in unweeded treatments, showing an alleviation of competitive interactions by neighbours through limitation of herb layer development. However, in both ground vegetation treatments, growth was severely reduced with Coronilla shrubs. Survival was only significantly impaired for the less shade-tolerant Q. pubescens oak used in combination with shrubs. The negative influence of the neighbour treatments was mainly attributable to light interception, which was particularly high by shrub canopy. Soil moisture was also slightly reduced by shrub neighbours, but it remained high with pines in the unweeded treatment owing to a limited abundance of herbs. However, stem form was improved by the neighbour treatments: oaks developed narrower crowns and greater slenderness with neighbours, whereas oaks in the open showed a bushy morphology. These findings emphasize the need to clearly identify key objectives before implementing mixed plantations (e.g. maximising growth, survival, improving morphology, etc.) and to use active management to control potential undesirable effects of the neighbouring vegetation on target plants.