Light is an exceptionally important but often limited resource. Light availability determines seedling survival, establishment, and growth. Regardless of species identity, trees growing under high light availability produce more biomass and are generally larger than trees receiving less light. How stressed trees become under the conditions of limited light availability depends on species-specific factors like shade tolerance and plasticity as well as the competitive situation. Additionally, the taller individuals have the advantage to obtain more light since competition for the resource is asymmetric. In competitive environments, the niche complementarity of the coexisting species can reduce the competitive pressure and facilitate higher biomass production (i.e., positive mixing effect). We established a controlled pot experiment to study the effect of light availability and competition type on growth and its allocation, biomass production and allocation, and leaf morphology of European beech (Fagus sylvatica L.), Norway spruce (Picea abies L. Karst), and Douglas fir (Pseudotsuga menziesii Mirb. Franco) seedlings. The study site was located at the Experimental Botanical Garden of the University of Göttingen (51.55684392372871, 9.953489533796636). We planted four seedlings per pot, each pot being either monospecific or mixed (two seedlings per species) and exposed to one of three different light availability levels (10%, 20%, and 50%). We planted in a total of 576 pots – 6 species combinations (monocultures + mixtures) x 3 light treatment levels x 32 replicates. For planting, we used 1-year-old European beech and 2-year-old Norway spruce and Douglas fir seedlings that were not undercut or transplanted. The experiment lasted from April 2018 – November 2019. All seedlings received the same water treatment through an automatic dripping irrigation system. Nutrients were provided using a controlled release fertilizer (Osmocote Exact Hi.End with 12-14 month longevity (ICL SF)).

Relative Height Volume Growth (R) measures the share of the volume growth over one growing season that can be attributed to height growth alone. It is defined as $$ R = \frac{\Delta V_H}{\Delta V} = \frac{\pi \cdot (D_1 / 2)^2 \cdot H_2 - \pi \cdot (D_1 / 2)^2 \cdot H_1}{\pi \cdot (D_2 / 2)^2 \cdot H_2 - \pi \cdot (D_1 / 2)^2 \cdot H_1} = \frac{D_1^2 \cdot (H_2 - H_1)}{D_2^2 \cdot H_2 - D_1^2 \cdot H_1}, $$, where \Delta V_H is the volume growth due to height growth, and \Delta V is the total volume growth over one growing season. Furthermore, H_1 and H_2 are the height at the beginning and the end of the growing season, and D_1 and D_2 are the diameter at the beginning and the end of the growing season. Without measurement errors, the variable 'Relative Height Volume Growth' should take values in the interval [0,1]. All values outside of this interval have been removed.