| Forests are a major component of the global carbon cycle and will likely be affected by rising atmospheric CO2. The growth response of different tree species varies and depends upon characteristics that enable response to CO2 concentration. The relationship between the sources (e.g., leaves) and sinks (e.g., new, developing leaves and roots) has been found to explain some of the variation in response. However, explanations for the responses of trees growing in nature are not well-known. The effects of elevated CO2 on tree growth were addressed in this research by monitoring growth rates, photosynthetic rates, and leaf lifespan of six hardwood species in a forest exposed to elevated CO2. I predicted that response to elevated CO2 would correspond to life-history characteristics such as the degree of synchrony in the date of leaf emergence, shade tolerance, and intrinsic rates of growth and photosynthesis. I made measurements of sapling growth, photosynthesis, and leaf demography over three years. There was a greater stimulation of relative growth rate (RGR) for Acer rubrum and Liquidambar styraciflua, both species that invest highly in leaf tissue, with no evidence that lower synchrony in leaf emergence predicts the responsiveness to elevated CO2. There was greater net photosynthesis in elevated CO2 across species. Species with lower relative growth rates at ambient CO2 tended to have a higher photosynthetic response to elevated CO2, but other life-history characteristics did not explain photosynthetic responses. Elevated CO2 increased the leaf lifespan of Quercus velutina and decreased it for Ulmus alata, with variation among species and years. Leaves in elevated CO2 generally had a higher probability of surviving to a given leaf lifespan. Inherent characteristics such as investment in leaf tissue and relative growth rate at ambient CO2 were more predictive of the responsiveness to elevated CO2 than the synchrony in leaf emergence. In the long term, species that have lower relative growth rates and photosynthetic rates, such as A. rubrum, may experience greater stimulations in overall sapling growth. Differences among species in the ability to adjust to changes in atmospheric CO2 may alter the relative abundance and successional trajectory of the future forest. |
