TCS Daily

The Forest and the Trees

By Robert C. Balling - June 18, 2003 12:00 AM

All of us engage in activities that involve burning fossil fuels, and according to some self-proclaimed environmentalists we should feel guilty doing so due to the emission of carbon dioxide (CO2) which can lead to ever-dreaded global warming. However, the botanical literature is full of 1,000s of articles showing that elevated atmospheric concentrations of CO2 will be beneficial for plants throughout the world.

Virtually every plant growing on the Earth evolved when CO2 levels were 10 times higher than modern concentrations. When we add CO2 to the atmosphere, plants feel a lot more at home, and they dramatically increase their rate of photosynthesis and they close their stomates (openings in the leaves) thereby reducing transpiration. This combination makes them grow faster and become more water-use efficient and more resistant to drought. The smaller stomatal pores in the leaves further protects the plants from other potential stresses floating around in the atmosphere such as elevated ozone and sulfur dioxide. The plants see elevated CO2 as a gift of the Industrial Revolution - it's like they're going home again.

Any trip to the science library will bring even more good news for crops, grasslands, and forests. If you're off to the woods this summer, feel good about the latest research published in leading scientific journals in the last few weeks:

(1) A large team of 31 scientists from throughout the world grew aspen trees at ambient (360 ppm) and elevated (560 ppm) atmospheric CO2 concentrations. Karnosky et al. concluded that after three years of the elevated CO2 treatment, "the above-ground volume of Aspen stands was 40% above those grown at ambient CO2, and there was no indication of a diminishing growth trend." How can the "environmentalists" be unhappy about elevated CO2 when the aspen seem so thrilled about a world with more CO2?

(2) A team of scientists from New Zealand grew pine trees at 360 ppm and 650 ppm levels of CO2. Greenep et al. reported that "photosynthetic rate in young needles during summer, autumn and spring was 34, 43 and 38% higher, respectively, in trees grown at elevated" CO2 compared to trees grown at ambient CO2. Furthermore, water use efficiency increased by 49% thanks to elevated CO2. These trees "pine" for higher levels of CO2.

(3) A pair of US scientists grew sweetgum trees at ambient and elevated atmospheric CO2 levels. Herrick and Thomas reported that "Overall, we found that leaf-level net photosynthetic rates were enhanced by atmospheric CO2 enrichment throughout the season until early November" and that "A simple shoot photosynthesis model indicated that elevated [CO2] stimulated photosynthesis by 60% in sun shoots". Sweetgums clearly have a sweet tooth for elevated CO2.

(4) A scientist from the University of Tasmania grew poplar trees for three years at ambient (360 ppm) and elevated (550 ppm) CO2 levels. Hovenden found that accelerated photosynthesis in the elevated CO2 trees increased carbon uptake by 33%. CO2 is obviously popular in the poplar community.

(5) A team of scientists from around the world analyzed the stomatal density of conifers grown at various levels of atmospheric CO2. Kouwenberg et al. found that "stomatal frequency, based on the number of stomata per millimeter of needle length, decreased significantly with increasing CO2." Their results re-confirmed that plants produce smaller stomates given elevated CO2, and the result is less transpiration, a greater water-use efficiency, greater drought tolerance, and less trouble dealing with a variety of stresses.

We are bombarded with a message that CO2 is somehow a pollutant that will degrade the global biosphere as concentrations continue to increase. In fact, experiments the world-over show just the opposite. As we've seen in these latest five articles in major peer-reviewed scientific journals, trees benefit enormously from elevated CO2. We see the forest and we see the trees, and they both thank us for the CO2 we're adding to the atmosphere.

So drive out to the forest and feel good about the CO2 coming out of your tailpipe!

Dr. Robert Balling is director of the office of climatology, Arizona State University.


Greenep, H. et al. 2003. Response of photosynthesis in second-generation Pinus radiata trees to long-term exposure to elevated carbon dioxide partial pressure. Tree Physiology, 23, 569-576.

Herrick, J.D. and R.B. Thomas. 2003. Leaf senescence and late-season net photosynthesis of sun and shade leaves of overstory sweetgum (Liquidambar styraciflua) grown in elevated and ambient carbon dioxide concentrations. Tree Physiology, 23, 109-118.

Hovenden, M.J. 2003. Photosynthesis of coppicing poplar clones in a free-air CO2 enrichment (FACE) experiment in a short-rotation forest. Functional Plant Biology, 30, 391-400.

Karnosky, D.F. et al. 2003. Tropospheric O3 moderates responses of temperate hardwood forests to elevated CO2: A synthesis of molecular to ecosystem results from the Aspen FACE project. Functional Ecology; 17, 289-304.

Kouwenberg et al. 2003. Stomatal frequency adjustment of four conifer species to historical changes in atmospheric CO2. American Journal of Botany, 90, 610-619.

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