Long-Term Exposure to Global Changes in Grassland Ecosystems: Whole-Season and Temporal Trends in Changing Productivity in Response to Increased Carbon Dioxide Level and Air Temperature

Six commercial grass cultivars were exposed to elevated CO2 level (±700 ppm), increased air temperature (+4°C) or a combination of both during an entire growing season, using a "field tracking system" for continuous simulation of outside field temperatures (or ambient 44C), inside sunlit greenhouse units. On a whole-season basis, elevated CO2 positively affected above-ground productivity, with a range of 11-30%, depending on species, although early-season stimulation was smaller than late spring CO2 gain. Effects of increased temperature were positive in spring, but decreased towards summer and became negative for almost all species, yielding a "no effect" seasonal balance. Almost no significant interactions were found between elevated CO2 and increased temperature using 2-way ANOVA, in spite of previously determined CO2 x temperature synergism on the process level. Possible explanations are presented.