Stem growth is favored at expenses of root growth in mixed stands and humid conditions for Douglas-fir (Pseudotsuga menziesii) and European beech (Fagus sylvatica)

The study found an increased investment into stem growth (compared to root growth) if trees were surrounded by a complementary species. This response is consistent with known patterns about root-stem allometry under favorable conditions (humidity and stand density). The study investigated partitioning of resources between roots and stems in mono-species and mixed-species stands of Douglas-fir and European beech at four different sites. We combined tree ring analyses of stems and coarse roots to scrutinize root-stem allometry with a focus on how it is influenced by species mixture and humidity. The results show that allometry in mixed stands changed in favor of stem growth for both species. The greatest relative allocation into stem growth was observed for individual trees which were completely surrounded by trees of the other species. The data indicate that a decrease of stand density, which was used as a proxy for tree competition, has the same effect on allocation. To analyze the influence of humidity, we used a long- and short-term index. Based on these, we can show that allocation changes with general site conditions and annual humidity variations. We found that on both time scales, both species increase resource investment into stem growth if conditions are more humid. Under harsher conditions, allocation shifts into root growth. The findings contribute to understanding the overyielding in mixed stands. Mixing Douglas-fir and European beech leads to the same allocation patterns as an improvement of site conditions. We suggest that for both species, mixture is equivalent to growing on a better site.