Successional Status, Seed Size, and Responses of Tree Seedlings to CO^2, Light, and Nutrients

We studied how an enriched CO2 atmosphere, in a fully crossed design of light and nutrients, influenced 1 st-yr seedling growth in six New England deciduous forest tree species. The species, in the order of increasing shade tolerance, were gray birch (Betula populifolia), ash (Fraxinus americana L.), red maple (Acer rubrum L.), red oak (Quercus rubra L.), yellow birch (Betula alleghaniensis Britton), and striped maple (Acer pensylvan- icum). Elevated CO2 environments significantly stimulated the seedling growth of all six species. Generally this was more pronounced in low light. The greatest stimulation was found under the condition of low light and high nutrients. However, individual species responded differently to elevated CO2 levels. Among the three early-successional species, gray birch, ash, and red maple, a significant increase in seedling growth under elevated CO2 conditions was found only with high nutrients. The three late-successional species grown under elevated CO2 conditions (red oak, yellow birch, and striped maple) showed a greater percentage increase in seedling growth in low light than in high light. Thus, for the early-successional species, the degree of enhancement of seedling growth by elevated CO2 levels was more sensitive to nutrient levels, while in the late-successional species the enhancement was more sensitive to the level of light. Moreover, species with large seeds (e.g., red oak) exhibited a greater response to elevated CO2 levels under low light than species with small seeds (e.g., gray birch). The results emphasize the importance of plant species as well as other environmental resources in modifying the response of plants to elevated CO,. Considering the light and nutrient environment observed in forest gaps of various sizes, the results of the present experiment suggest seedling regeneration in New England deciduous forests may be altered in a future high CO2 environment.