Modern and Future Forest Ecosystems
Richard J. Norby
Oak Ridge National Laboratory
Trees that are growing today started their life in an atmosphere with a lower CO2 concentration, and they will experience substantial increases in [CO2] in the future. Like other C3 plants, trees respond to higher [CO2] with an increased rate of photosynthesis and faster growth rates. While it is reasonable to assume that there should be a record of response to the CO2 increases that have occurred over the past few decades, tree-ring evidence is ambiguous, probably because of the myriad interactions between CO2, other environmental variables, and forest stand dynamics. These same issues make it difficult to predict the responses of future forests from the primary responses of individual trees to experimental CO2 enrichment. The very large increases in growth sometimes demonstrated in young trees in elevated CO2 experiments will not persist in a forest because other environmental factors will constrain leaf area and root volume. Nevertheless, it is reasonable to expect that net primary productivity (NPP) of many forests will be higher in a future, CO2-enriched atmosphere, and this premise is supported by evidence from ongoing experiments in which forest stands are exposed to elevated CO2 concentrations. However, the increased NPP is not resulting in higher biomass but rather in faster cycling of carbon through the ecosystem. The relationships between carbon cycling and the interacting cycles of nitrogen and water are particularly important for understanding the forest of the future.