Shading and root-shoot relations in saplings of silver birch, pedunculate oak and beech

Silver birch (Betula pendula Roth), pedunculate oak (Quercus robur L.) and beech (Fagus sylvatica L.) can regenerate successfully under a canopy of Scots pine (Pinus sylvestris L.). Shading reduces plant growth and modifies plant form, two related aspects. This study focuses on the effects of shading on root-shoot relations of these species. Saplings were grown for 2 years under full light (light availability of 100%) and shaded conditions (light availability of 30%) and then harvested and their root system excavated. Shading had no effect on the biomass distribution within the root system. Shading reduced the root-shoot ratio of the species studied. Only for silver birch the fine root biomass-leaf biomass ratio decreased by shading, indicating a reduced partitioning to the fine roots in comparison to the leaves. Shading reduced the fine root length-leaf area ratio of the three species studied. However the magnitude of reduction differed between the species; small in pedunculate oak, intermediate in beech and large in silver birch. For pedunculate oak and beech this effect must be primarily attributed to an increase in specific leaf area under shaded conditions, while for silver birch this effect can be attributed to both the morphological adjustment in the leaves and the increased partitioning to the leaves. As the capacity of leaves and fine roots to acquire resources has to balance we postulate that under the experimental conditions an increase the efficiency of carbon uptake relative to the efficiency of nutrient uptake is the dominant acclimation of pedunculate oak. The response of silver birch indicates that the dominant acclimation to shading is based on a shift in a biomass partitioning between the leaves and the fine roots. Beech holds an intermediate position, as there is evidence for an increase in the efficiency of carbon uptake relative to the efficiency of nutrient uptake and an increase of leaf area relative to fine root length. (C) 2002 Elsevier Science B.V. All rights reserved.