As the CO2 content of the atmosphere continues to rise, many plants will respond by reducing the stomatal apertures of their leaves and needles. This phenomenon commonly occurs at elevated CO2 concentrations, because plants don't need to open their stomates as wide as they do at lower atmospheric CO2 concentrations to allow for the inward diffusion of sufficient CO2 to accommodate the potential for photosynthetic utilization. As a consequence of this phenomenon, plants typically exhibit greater water-use efficiencies at higher levels of atmospheric CO2, which subject has been reviewed elsewhere on our website (see Water Use Efficiency - Summary). Hence, we here review some of the recent literature pertaining to elevated CO2 effects on foliar stomatal conductance in various tree species.
In a three-month growth chamber study performed on five boreal tree seedlings (quaking aspen, paper birch, tamarack, black spruce and jack pine), Tjoelker et al. (1998) reported that a 210-ppm increase in the air's CO2 concentration reduced stomatal conductance in all species by 10 to 25%. Comparable CO2-induced reductions in stomatal conductance of 25 and 21% were observed in seedlings of Alnus glutinosa (Poole et al., 2000) and young birch trees (Rey and Jarvis, 1998) exposed to twice-ambient levels of atmospheric CO2, while much greater reductions of 40 and 42% were manifest in Pinus radiata (Griffin et al., 2000) and scrub oak seedlings (Lodge et al., 2001), respectively, grown at approximately 700 ppm CO2.
In many studies, the effects of elevated CO2 on stomatal conductance were not explicitly reported. Nonetheless, atmospheric CO2 enrichment caused measurable reductions in this parameter, which helped contribute to substantial increases in plant water-use efficiency in oak (Broadmeadow et al., 1999; Anderson and Tomlinson, 1998; Tognetti et al., 1998a; Tognetti et al., 1998b), beech (Egli et al., 1998), longleaf pine (Runion et al., 1999), cherry (Centrittio et al., 1999), and the woody chaparral shrub Adenostoma fassciculatum (Roberts et al., 1998).
In a few reported cases, elevated CO2 did not affect foliar stomatal conductance, as was observed in mature Arbutus unedo (Bartak et al., 1999) and spruce (Roberntz and Stockfors, 1998) trees. In each of these studies, however, elevated CO2 enhanced rates of net photosynthesis, which thus still led to in large CO2-induced increases in plant water-use efficiency.
In summary, as the air's CO2 concentration increases, the stomatal conductances of the leaves and needles of many tree species will decrease, thus reducing transpirational water loss and enhancing plant water-use efficiency. In addition, in the few reported cases where elevated CO2 did not reduce foliar stomatal conductance, enhanced water-use efficiencies were still sustained, in this case by CO2-induced increases in photosynthesis, conferring upon the trees the ability to grow in places that were too dry for them in the past. In a future high-CO2-world, therefore, there should be both more trees and more robust trees, sequestering ever greater quantities of carbon in their tissues and the soils in which they are rooted.
References
Anderson, P.D. and Tomlinson, P.T. 1998. Ontogeny affects response of northern red oak seedlings to elevated CO2 and water stress. I. Carbon assimilation and biomass production. New Phytologist 140: 477-491.
Bartak, M., Raschi, A. and Tognetti, R. 1999. Photosynthetic characteristics of sun and shade leaves in the canopy of Arbutus unedo L. trees exposed to in situ long-term elevated CO2. Photosynthetica 37: 1-16.
Broadmeadow, M.S.J., Heath, J. and Randle, T.J. 1999. Environmental limitations to O3 uptake - Some key results from young trees growing at elevated CO2 concentrations. Water, Air, and Soil Pollution 116: 299-310.
Centritto, M., Magnani, F., Lee, H.S.J. and Jarvis, P.G. 1999. Interactive effects of elevated [CO2] and drought on cherry (Prunus avium) seedlings. II. Photosynthetic capacity and water relations. New Phytologist 141: 141-153.
Egli, P., Maurer, S., Gunthardt-Goerg, M.S. and Korner, C. 1998. Effects of elevated CO2 and soil quality on leaf gas exchange and aboveground growth in beech-spruce model ecosystems. New Phytologist 140: 185-196.
Griffin, K.L., Tissue, D.T., Turnbull, M.H. and Whitehead, D. 2000. The onset of photosynthetic acclimation to elevated CO2 partial pressure in field-grown Pinus radiata D. Don. after 4 years. Plant, Cell and Environment 23: 1089-1098.
Lodge, R.J., Dijkstra, P., Drake, B.G. and Morison, J.I.L. 2001. Stomatal acclimation to increased CO2 concentration in a Florida scrub oak species Quercus myrtifolia Willd. Plant, Cell and Environment 24: 77-88.
Poole, I., Lawson, T., Weyers, J.D.B. and Raven, J.A. 2000. Effect of elevated CO2 on the stomatal distribution and leaf physiology of Alnus glutinosa. New Phytologist 145: 511-521.
Rey, A. and Jarvis, P.G. 1998. Long-term photosynthetic acclimation to increased atmospheric CO2 concentration in young birch (Betula pendula) trees. Tree Physiology 18: 441-450.
Roberntz, P. and Stockfors, J. 1998. Effects of elevated CO2 concentration and nutrition on net photosynthesis, stomatal conductance and needle respiration of field-grown Norway spruce trees. Tree Physiology 18: 233-241.
Roberts, S.W., Oechel, W.C., Bryant, P.J., Hastings, S.J., Major, J. and Nosov, V. 1998. A field fumigation system for elevated carbon dioxide exposure in chaparral shrubs. Functional Ecology 12: 708-719.
Runion, G.B., Mitchell, R.J., Green, T.H., Prior, S.A., Rogers, H.H. and Gjerstad, D.H. 1999. Longleaf pine photosynthetic response to soil resource availability and elevated atmospheric carbon dioxide. Journal of Environmental Quality 28: 880-887.
Tjoelker, M.G., Oleksyn, J. and Reich, P.B. 1998. Seedlings of five boreal tree species differ in acclimation of net photosynthesis to elevated CO2 and temperature. Tree Physiology 18: 715-726.
Tognetti, R., Longobucco, A., Miglietta, F. and Raschi, A. 1998a. Transpiration and stomatal behaviour of Quercus ilex plants during the summer in a Mediterranean carbon dioxide spring. Plant, Cell and Environment 21: 613-622.
Tognetti, R., Johnson, J.D., Michelozzi, M. and Raschi, A. 1998b. Response of foliar metabolism in mature trees of Quercus pubescens and Quercus ilex to long-term elevated CO2. Environmental and Experimental Botany 39: 233-245.