Shifts in dominance and complementarity between sessile oak and beech along ecological gradients

Whether tree species benefit from growing in a mixed forest depends on the relative importance of positive versus negative interactions, which varies according to abiotic conditions. In mixture with sessile oak Quercus petraea [Matt.] Liebl., European beech Fagus sylvatica L. is often competitively dominant. Sessile oak, however, is more resistant to water deficit stress. Under water-limiting conditions, facilitation or even competition exerted by oak may gain in importance. We investigated the relative importance of dominance and complementarity in seven triplets of pure and mixed forests in Belgium's temperate oceanic climate. We analysed ring width data of 145 oaks and 138 beech trees along three distinct gradients: a spatial (i) and a temporal (ii) water availability gradient and a temporal gradient in growing conditions (iii). Gradient (ii) was represented by a precipitation variable, obtained by defining a large set of candidate climate variables over variable time frames, narrowed down using a species and site-specific lasso model. Growing conditions on a temporal gradient (iii) were represented by standardized tree growth. Two sets of linear mixed-effects models were used. Growth models assess mixing effects on ring width along gradients (i) and (ii). Mixing index models test for all three gradients how they affect the relative difference in average ring width of trees in mixed and pure stands. Beech trees grew faster than oak trees, and mixing further increased beech growth while decreasing oak growth, except on drier sites where the negative effect on oak growth disappeared. Low precipitation years, in contrast, reduced the beneficial effect of mixing on beech. In years of low growth, the positive mixing effect on beech growth was reinforced, and the negative mixing effect on oak growth decreased. Synthesis. Mixing buffered against growth limitations in general, though this depends on the nature of the limitation and the gradient over which it is measured. On dry sites, competitive dominance of beech was replaced by higher complementarity between species. During dry years, however, oak did not profit from a decreased mixing benefit of beech.