Global change will cause dense vegetation in temperate forests, resulting in higher competition for light, and soil resources among plants. Yellow birch (Betula alleghaniensis Britton) is a commercially important, and moderately shade-tolerant species in the temperate forests of eastern North America. To better understand interactive effects of multi-changing variables on the seedling development in the possible future, an experiment of completed randomized design for a total of 192 the seedlings was conducted with 2 CO₂ (350 and 700 S.E.<± 10 ppm) × 3 lights (2.9%, 7.7%, 26.1% of full sunlight) × 2 soils (non- and sterile soil) with 4 replications in a phytotron. After 55 days of the experiment, all the seedlings were harvested to measure height, base diameter, and individual plant organ biomass.

Results/Conclusions

Elevated CO₂ increased root biomass ratio, and all the biomass except for branch biomass. Light also increased all the biomass as well as base diameter. Root to shoot ratio, stem biomass ratio, branch biomass ratio, and root biomass ratio decreased with increasing light. Leaf biomass ratio increased from low to middle light, and then decreased in high light. Sterile soil increased height, leaf biomass, and leaf biomass ratio.
Interaction of CO₂ × light did not influence the biomass and growth.
In the combinations of 2 CO₂ levels × 2 soils, total plant biomass increased in sterile soil with ambient CO₂, and both soils with elevated CO₂. Root biomass increased in non-sterile soil with elevated CO₂.
Interaction of light ×soil generally had complicated and positive effects on plant growth, organ biomass, and decreased branch biomass ratio.
In the combinations of 2 CO₂ levels × 3 lights × 2 soils, root biomass increased more in high light, followed by in middle light at both CO₂ levels. The increase was more in elevated CO₂ than that in ambient CO₂. In middle light with ambient CO₂, root biomass increased more in sterile soil than non-sterile soil, and vise versa in high light with elevated CO₂.
The study indicated that in the complicated changing environment, leaf, and root biomass increased much more than stem and branch biomass at both CO₂ levels and soils, and root biomass increased most in non-sterile soil with elevated CO₂, and high light, suggesting the seedlings may have a strongly competitive ability of light and soil resources in the denser vegetation, and the competition was stronger for soil resources than for light.