Mixing effects along ecological gradients in temperate oak-beech forests : radial growth, drought exposure and stability

Mixed forests often have higher productivity, stress resistance and temporal stability of productivity than comparable pure forests. Several mechanisms for this have been described: competition release, facilitation and competitive dominance of a highly productive species. We studied patterns in growth rings of sessile oak (Quercus petraea (Matt.) Liebl.) and European beech (Fagus sylvatica L.) on 8 study sites in Belgium, spanning a gradient in water availability. At each site, a mixed oak-beech forest stand is compared to a pure oak stand, and a pure beech stand. Using analysis of growth rings, we confirm prior literature results that this mixture is characterised by competitive dominance of beech over oak. A novel result is that, on drier sites, to which beech is less tolerant, dominance is replaced by higher complementarity among species. Mixing protected growth against stresses and disturbances, however, not when drought is the stressor, because the faster growth of beech is dependent on sufficient precipitation. This was also evident from an analysis of stable carbon isotopes on summer wood, used as indicator for exposure to the 2003 drought. Faster growth of beech trees in mixtures resulted in a higher drought exposure. Finally, we quantified the increase in temporal stability of growth of forest stands due to mixing at 21% on average over the study sites. Half of the total stabilisation was due to asynchrony among species, and half due to stabilisation at the population level.