Abstract
The effects of tree mixture on stand productivity are usually examined using a substitutive approach whereby productivity in mixed stands is compared to productivity in monocultures, at comparable tree density. This approach has proved that mixed stands usually perform better than pure stands. The addition of a second tree species to an existing mono-specific stand has received less consideration. Yet, this approach may separate the facilitation effect from the complementarity effect. We compared the effect of tree species substitution vs. addition on the productivity of maritime pine and silver birch in a 7-year-old tree diversity experiment in south-western France. Given the very young age of the stands, the 2014 tree volumes were used as a proxy for tree productivity. Substituting pines with birches resulted in a significant increase in tree productivity at the stand level, beyond expectations from the weighted mean of monocultures (i.e. overyielding). In contrast, creating a mixture by adding birches to pine stands had no significant effect on the maritime pine productivity: the transgressive mixture effect was not significant. This absence of an effect was produced by two distinct density-dependence responses at the individual tree level. Our results suggest that the addition of a pioneer species with low demands in soil water and nutrients during young developmental stages can diversify stands and increase the provision of ecosystem services without altering the productivity of the target species.