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International audience; For forest ecosystems, studies dealing with diversity-productivity relationships are often based on diameter increment observations. Studying how height growth is affected by species interactions can provide new insights on this issue. We studied the mixture effect on dominant height growth in order to answer two questions. Do species interactions in mixed forest modify the dominant height growth of species? Does the diversity effect on diameter found in previous studies correspond to actual overyielding, or rather to an effect on allocation of growth between diameter and height? We used the French National Forest Inventory (NFI) data to model the mixture effect on dominant height. We included biophysical factors in the models to compare the dominant height of mixed and monospecific stands, all other parameters being equal. We studied five target species – Quercus petraea (Matt.) Liebl., Fagus sylvatica L., Picea abies (L.) Karst., Abies alba Mill., and Pinus sylvestris L. – in association with sixteen other species. Mixture effects on dominant height were weak, though often significant. They were either positive or negative according to species association. We showed that mixture effect on dominant height corresponds to a leveling process between species: the taller one limits its growth while the smaller one’s growth increases. Furthermore, most of the time, mixture effects on dominant height are in the same direction as those found on diameter, though with a lower magnitude. Our results confirm that tree diversity results in overyielding rather than in a different allocation of volume between the parts of the tree.

The IPCC previsions for the upcoming decades include an increase in frequency and intensity of drought events in several regions worldwide, including Northern Europe. Drought significantly affects forest ecosystems through decreased productivity, increased vulnerability to biotic disturbances and increased subsequent mortality. How forest ecosystems maintain resistance and resilience to drought events are important questions. Our study aimed to assess whether species mixture or an individual tree size within a stand alters a given tree’s resilience and resistance to drought. A retrospective study of tree-ring widths allowed us to calculate resistance, resilience and recovery indices for five recent drought events: 1976, the 1990–1992 period, 2003, 2006 and 2010. These drought events were selected based on the SPEI (Standardized Precipitation Evapotranspiration Index) drought index. Our study sample consisted of 108 individual sessile oak ( Quercus petraea (Matt.)) or Scots pine ( Pinus sylvestris L.) trees sampled in 2012 and 2013 (Orleans forest, central France) in pure and mixed stands, divided into three diameter classes corresponding to three sizes: large, medium and small trees. Scots pine performed better than sessile oak during the 1990–1992 and 2010 droughts while the contrary was observed for the 2003 and 2006 droughts. They performed equally in 1976. We suggest that the differing sensitivity of the two species to spring and summer drought explained this result. In our study, stand composition had no effect on resilience or resistance for either species. The size effect in oaks was unclear as small oaks displayed either a better performance or a worse performance than large oaks. Small pines displayed better resistance and resilience than pines of a larger size. This work stressed the importance of taking into account stand composition and trees size as well as soil and climatic conditions for each drought events to achieve a better understanding of the diversity of responses to climatic variations among forest ecosystems.