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 7 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, narrowe...