Biomass allocation to roots and shoots is more sensitive to shade and drought in European beech than in Norway spruce seedlings

We investigated the effect of light availability and soil moisture on growth and biomass partitioning of Norway spruce and European beech seedlings in a three (light availability levels) × two (soil moisture levels) factorial greenhouse experiment. The effects of factor levels on allocation to biomass compartments were analyzed using ANCOVA. As plant allocation patterns are size-dependent, tree size was used as a covariate. In both tree species, growth and biomass allocation to above and belowground plant components were affected by light availability. European beech showed a distinct increase in allocation to leaves, stem and branch biomass at the expense of fine and coarse roots with decreasing light availability. For Norway spruce, only allocation to stem biomass increased and allocation to fine root biomass decreased under low light. To drought a significant increase of the percentage of belowground compartments was found for European beech but not for Norway spruce. Overall, European beech seedlings were more plastic than Norway spruce seedlings. European beech seedlings appear better able to adjust biomass partitioning to resource availability. In contrast Norway spruce responded languidly. Our results indicate that biomass partitioning is not only driven by ontogeny, and thus tree size, but is environmentally determined to a substantial degree. A possible explanation for this divergence from other results on the role of ontogeny in biomass partitioning may be that seedling plasticity in response to limited resources declines with increasing age and/or time of exposure to the limited resources.

► Trees size and resource availability determine biomass partitioning of tree seedlings.
► Plasticity to reduced light and to moderate drought varied between species.
► The findings support optimal partitioning theory.
► Plasticity may decline with increasing age and/or time of exposure to limited resources.