How dominant height responds to mixing species: Effect of traits and height difference between species

Adapting forests to climate change is a major challenge for forest ecology and forestry. Among the management options available, encouraging the use of mixtures is a promising way forward. However, this practice requires a thorough understanding of how species respond to mixing. In this article, we analyzed species dominant height responds to mixing and how species ontogeny and traits drive this response. We compared species observed dominant height in mixed even-aged stands with the expected dominant height of the same species in a monospecific stand under the same environmental conditions. We then related this dominant height variation due to mixing to between-species dominant height difference and to species traits linked to competition (shade tolerance, wood density, specific leaf area).

We focused our analyses on 76 pairs of forest tree species. We used data from the French National Forest Inventory to calculate species dominant height in 1368 mixed stands. We then used previously developed models to estimate the expected dominant height in virtual monospecific stands with the same environmental conditions.

We found that mixture had a significant impact on species dominant height for 15 out of 50 species-combination considered. Dominant height of a given species was higher in mixture than in pure stands when this species had a lower dominant height in pure stands, a lower shade tolerance, a lower specific leaf area or a higher wood density than its companion species.

Our results suggest that species dominant height response to mixing depends on how mixture influences the competition for light. Our results will help inform strategies aiming to diversify species in forests, and will be especially useful in anticipating a given species’ behavior in response to competition for light when it is mixed with other species.