Elevated CO 2 changes the moderate shade tolerance of yellow birch seedlings

Abstract

To demonstrate the existence of light thresholds in plant growth and to examine the effects of elevated CO 2 on the shade tolerance of a tree species an experiment consisting of a completely randomized design for a total of 96 yellow birch ( Betula alleghaniensis Britton) seedlings was conducted with 3 light levels (2.9% 7.7% 26.1% of full sunlight) × 2CO 2 levels (350 and 700±10 ppm) with 4 replications in a phytotron. The study proved that thresholds exist and they vary in different plant organs. In ambient CO 2 the thresholds were 13.3% 18.7% 15.0% 15.2% and 15.6% of full sunlight for stem leaf root total plant biomass and the averaged value respectively. In 700 ppm CO 2 the corresponding thresholds were 16.7% 21.3% 18.1% 21.7% and 19.5% for stem leaf root total plant biomass and the averaged value respectively. The lowest threshold in the stem is an indicator of the minimal light intensity for regular growth for seedlings of this species. Below this threshold light-stressful growth occurs. The result of a paired t-test indicated that the thresholds in elevated CO 2 were significantly higher than in ambient CO 2. This suggests that yellow birch will lose its moderate shade tolerance evolutionally becoming a shade-intolerant species and that it may become more difficult to naturally regenerate in the future.