Global Change Biology (1999) 5, 255–267 Monoterpene emission from coniferous trees in response to elevated CO 2 concentration and climate warming JOHN V. H. CONSTABLE*, MARCY E. LITVAK*, JAMES P. GREENBERG† and RUSSELL K. MONSON* *Department of EPO Biology, University of Colorado, Boulder, CO 80309–0334, USA, †Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO 80307, USA Abstract It was hypothesized that high CO 2 availability would increase monoterpene emission to the atmosphere. This hypothesis was based on resource allocation theory which predicts increased production of plant secondary compounds when carbon is in excess of that required for growth. Monoterpene emission rates were measured from needles of (a) Ponderosa pine grown at different CO 2 concentrations and soil nitrogen levels, and (b) Douglas fir grown at different CO 2 concentrations. Ponderosa pine grown at 700 μmol mol –1 CO 2 exhibited increased photosynthetic rates and needle starch to nitrogen (N) ratios when compared to trees grown at 350 μmol mol –1 CO 2 . Nitrogen availability had no consistent effect on photosynthesis. Douglas fir grown at 550 μmol mol –1 CO 2 exhibited increased photosynthetic rates as compared to growth at 350 μmol mol –1 CO 2 in old, but not young needles, and there was no influence on the starch/N ratio. In neither species was there a significant effect of elevated growth CO 2 on needle monoterpene concentration or emission rate. The influence of climate warming and leaf area index (LAI) on monoterpene emission were also investigated. Douglas fir grown at elevated CO 2 plus a 4 °C increase in growth temperature exhibited no change in needle monoterpene concentration, despite a predicted 50% increase in emission rate. At elevated CO 2 concentration the LAI increased in Ponderosa pine, but not Douglas fir. The combination of increased LAI and climate warming are predicted to cause an 80% increase in monoterpene emissions from Ponderosa pine forests and a 50% increase in emissions from Douglas fir forests. This study demonstrates that although growth at elevated CO 2 may not affect the rate of monoterpene emission per unit biomass, the effect of elevated CO 2 on LAI, and the effect of climate warming on monoterpene biosynthesis and volatilization, could increase canopy monoterpene emission rate. Keywords: atmospheric chemistry, carbon dioxide, climate change, nitrogen, Pinus ponderosa, Pseudotsuga menziesii Received 27 October 1997; revised version received 27 January and accepted 7 February 1998 Introduction The emission of volatile organic compounds (VOCs) from vegetation exerts the dominant control over regional oxidative chemistry of the atmosphere in many terrestrial ecosystems (Monson et al. 1995; Lerdau et al. 1997). In the case of coniferous forests, monoterpene compounds represent the dominant biogenic VOC (Lerdau 1991). Monoterpenes are 10-carbon compounds used primarily for herbivore defence, and volatilization to the atmo- Correspondence: John V.H. Constable, Department of Biological Sciences, University of Illinois at Chicago, 845 West Taylor St., Chicago, IL 60607, USA, fax +1/312-996-2805, e-mail constabj@tigger.cc.ulc.edu © 1999 Blackwell Science Ltd. 255 sphere occurs from storage reservoirs in the needles and bark of conifers. Once in the atmosphere, monoterpenes react readily with oxidative species (e.g. hydroxyl radical, nitrate radical, and ozone) to form a variety of secondary products that influence atmospheric chemistry and cli- mate change (Wuebbles et al. 1989). Among the products of monoterpene oxidation are CO, organic acids, organic nitrates, and reactive organic peroxy compounds (Fehsenfeld et al. 1992). Thus, monoterpene emissions from coniferous forests represent a primary contribution to the broader discipline of biosphere–atmosphere inter- actions. Theoretical models of resource allocation suggest that