Monoterpene emission from coniferous trees in response to elevated CO2 concentration and climate warming

It was hypothesized that high CO2 availability would increase monoterpene e mission to the atmosphere. This hypothesis was based on resource allocation theory which predicts increased production of plant secondary compounds wh en carbon is in excess of that required for growth. Monoterpene emission ra tes were measured from needles of (a) Ponderosa pine grown at different CO2 concentrations and soil nitrogen levels, and (b) Douglas fir grown at diff erent CO2 concentrations. Ponderosa pine grown at 700 mu mol mol(-1) CO2 ex hibited increased photosynthetic rates and needle starch to nitrogen (N) ra tios when compared to trees grown at 350 mu mol mol(-1) CO2. Nitrogen avail ability had no consistent effect on photosynthesis. Douglas fir grown at 55 0 mu mol mol(-1) CO2 exhibited increased photosynthetic rates as compared t o growth at 350 mu mol mol(-1) CO2 in old, but not young needles, and there was no influence on the starch/N ratio. In neither species was there a sig nificant effect of elevated growth CO2 on needle monoterpene concentration or emission rate. The influence of climate warming and leaf area index LAD on monoterpene emission were also investigated. Douglas fir grown at elevat ed CO2 plus a 4 degrees C increase in growth temperature exhibited no chang e in needle monoterpene concentration, despite a predicted 50% increase in emission rate. At elevated CO2 concentration the LAI increased in Ponderosa pine, but not Douglas fir. The combination of increased LAI and climate wa rming are predicted to cause an 80% increase in monoterpene emissions from Ponderosa pine forests and a 50% increase in emissions from Douglas fir for ests. This study demonstrates that although growth at elevated CO2 may not affect the rate of monoterpene emission per unit biomass, the effect of ele vated CO2 on LAI, and the effect of climate warming on monoterpene biosynth esis and volatilization, could increase canopy monoterpene emission rate.