Rising Atmospheric CO 2 and Photosynthesis of San Bernardino Mountain Plants

Photosynthesis of leafy shoots was determined under a range of CO2 concentrations that spanned past and likely future levels of atmospheric CO2. The temperature regime was also manipulated. Response of photosynthesis to CO2 was pronounced, and differed markedly among species. A substantial rise in atmospheric CO2 is likely to produce shifts in species composition of southern California mountain and foothill vegetation, with several conifer species being disadvantaged. INTRODUCTION How much has the well-advertised increase in atmospheric CO2 affected photosynthesis of land plants? What sorts of response can we expect from further increases in CO2? What might be the outcome in a forest? These questions are explored in this paper. In the late 1800's, concentration of atmospheric CO2 was ca. 285 ppm (Callendar, 1958). According to recent careful measurements (Keeling, 1970), the current mean world CO2 level is close to 324 ppm. Annual rate of CO2 increase is approaching 1 ppm and is rising. Levels of 400 ppm by the year 2000, and 500 ppm by 2020, are predicted (Peterson, 1969). Increased burning of fossil fuel is at the root of this increase. Carbon that was gradually withdrawn from global circulation over several hundred million years is suddenly being returned to the carbon cycle. It can be absorbed by sea water, or even contribute to deposition of new fossil fuel, but such processes take thousands to millions of years (Bolin, 1970). If the earth's fossil fuels are fully exploited over the next several hundred years, we face a precipitous rise in atmospheric C02, followed by an almost equally precipitous drop (Revelle, 1965). It is well known that CO2 is one of the principal limiting factors governing rates of photosynthesis on land. A number of studies have shown that plants respond to increased CO2 up to several thousand ppm (Hesketh, 1963; Madsen, 1968; Gordon and Gatherum, 1969). Enhancement of CO2 in greenhouses by artificial means is now accepted practice in many places. However, little is recorded concerning the response of native vegetation to small increments in atmospheric C02, such as have been experienced in the past 100 years or may be expected in the 50 years to come. A field study in the oak-pine forest on Long Island, New York, indicated that enhancement of CO2 up to about 100 ppm above ambient levels produced a sharp linear rise in photosynthesis of leafy twigs (Wright and Woodwell, 1970). The present study was designed to explore the response of a number of western species to modest CO2 enhancement, and to assess the relation of CO2 response to composition of the vegetation.