FOREST RESPONSES TO CO2 ENRICHMENT AND CLIMATE WARMING

Two of the major uncertainties in forecasting future terrestrial sourc es and SinkS Of CO2 are the CO2-enhanced growth response of forests an d soil warming effects on net CO2 efflux from forests. Carbon dioxide enrichment of tree seedlings over time periods less than 1 yr has gene rally resulted in enhanced rates of photosynthesis, decreased respirat ion, and increased growth, with minor increases in leaf area and small changes in C allocation. Exposure of woody species to elevated CO2 ov er several years has shown that high rates of photosynthesis may be su stained, but net C accumulation may not necessarily increase if CO2 re lease from soil respiration increases. The impact of the 25% rise in a tmospheric CO2 with industrialization has been examined in tree ring c hronologies from a range of species and locations. In contrast to the seedling tree results, there is no convincing evidence for CO2-enhance d stem growth of mature trees during the last several decades. However , if mature trees show a preferential root growth response to CO2 enri chment, the gain in root mass for an oak-hickory forest in eastern Ten nessee is estimated to be only 9% over the last 40 years. Root data ba ses are inadequate for detecting such an effect. A very small shift in ecosystem nutrients from soil to vegetation could support CO2-enhance d growth. Climate warming and the accompanying increase in mean soil t emperature could have a greater effect than CO2 enrichment on terrestr ial sources and sinks Of CO2. Soil respiration and N mineralization ha ve been shown to increase with soil temperature. If plant growth incre ases with increased N availability, and more C is fixed in growth than is released by soil respiration, then a negative feedback on climate warming will occur. If warming results in a net increase in CO2 efflux from forests, then a positive feedback will follow. A 2 to 4-degrees- C increase in soil temperature could increase CO2 efflux from soil by 15 to 32% in eastern deciduous forests. Quantifying C budget responses of forests to future global change scenarios will be speculative unti l mature tree responses to CO2 enrichment and the effects of temperatu re on terrestrial sources and sinks of CO2 can be determined.