Mixed stands of spruce and beech under drought stress and stress recovery. From pattern to process.

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Norway spruce (Picea abies (L.) Karst.) and European beech (Fagus sylvatica L.) dominate Central European forests. The effects of drought stress on the growth of these two species are significant, while the mechanisms behind are not yet fully understood, particularly when they grow in mixed stands. Our aim within the joint project KROOF is to understand the processes of tree and stand growth in monocultures and mixtures of Norway spruce and European beech under drought stress and stress recovery. The common base of all partners of the KROOF project is a throughfall-exclusion-experiment (TEE) and a series of plots along a precipitation gradient across Bavaria (PGR), which both offer the unique opportunity to study the interactions between soil processes (e.g. soil water, nutrient availability), resource availability (e.g. light, water, nutrients) and tree and stand growth. After analyzing the drought experiments and their effects on tree and stand growth within the first funding term our null hypotheses for the second funding term are: (1) Norway spruce and European beech react similarly to annual and extended drought periods, (2) both, Norway spruce and European beech do not show different growth responses to drought in intra- and inter-specific neighbourhood, (3) the water use efficiency of Norway spruce and European does neither differ in annual and extended drought periods nor in intra- and inter-specific neighbourhood, (4) drought does not modify stem and crown allometry, (5) growth recovery is similar on dry and moist sites along the PGR for Norway spruce and European beech, and in intra- and interspecific environments, (6) growth recovery is independent of tree size and competitive status, similarly for Norway spruce and European beech, in intra- and interspecific environments, (7) drought stress is independent of the relative size of trees within the population, (8) wood density as well as the relationship between tree ring width and wood density is equal under drought and control conditions and does not differ between mixed-species stands and monocultures and (9) overyielding in mixed stands compared to monocultures is equal along the PGR and independent of the respective growth limiting resource (water, nutrient, light). Consequently, our work plan aims at six objectives, i.e. the retrospective analyses of the drought reactions documented in the time series of growth and resource measurements on TEE and PGR (O1), the analysis of the growth recovery from the 2015 drought stress along the PGR (O2), the analysis of the growth recovery from extreme drought stress at the TEE site (O3), the modification of growth partitioning among trees of different sizes within a population under drought stress (O4), the analysis of the effects of drought and mixing both tree species on their wood density (O5), and the scrutiny of light, water, and nutrient supply as potential causes of overyielding in mixed-species stands versus monocultures (O6).