Earthsky Interviews
Private: Soil Fertility
05-26-2002 - Earth
_DB:_ And this unnatural build-up of CO2 is thought by many scientists to be contributing to global warming. Meanwhile, trees absorb CO2 – it makes them grow faster. So people have suggested we might compensate for industrial CO2 emissions simply by planting more forests.
_JB:_ Scientists have been testing this possibility near Durham, North Carolina. Loblolly pine trees are grown here – not in a lab – but in the open air, under natural conditions. Tall towers support a system of pipes that deliver CO2 to the air around the trees. Researchers can simulate the atmospheric conditions predicted for 50 years from now, when CO2 levels are expected almost to double.
_DB:_ Loblolly pines that were exposed to high concentrations of CO2 did a great job of taking up the excess. At first, the trees grew faster than normal. But after three years, tree growth returned to normal despite the extra CO2. That’s because the trees had depleted the nutrients in the soil that are also essential for growth.
_JB:_ Natural levels of forest soil nutrients are low in the northern temperate zone. This research suggests that forests in this zone will not temper the effects of continued CO2 emissions.
_DB:_ Special thanks today to the “National Fish and Wildlife Foundation”:http://www.nfwf.org/ and to the “U.S. Forest Service”:http://www.fs.fed.us/. We’re Block and Byrd for Earth and Sky.
The following individual was interviewed for today’s program. Our thanks to:
David S. Ellsworth
Plant Physiol. Ecologist
School of Nat. Res. & Environment
University of Michigan
The following web sites and articles provided information relevant to this script:
Spotts, Peter N. “Trees no savior for global warming”:http://www.csmonitor.com/durable/2001/05/25/p3s1.htm. Christian Science Monitor, May 25, 2001, p. 3
“Soil fertility limits forests capacity to absorb excess CO2″:http://www.umich.edu/~newsinfo/Releases/2001/May01/r051801b.html University of Michigan Press Release, May 18, 2001.
Trees and warming. (editorial) Boston Globe, June 3, 2001, p. C 6.
Henderson, Mark. The thorny issue of forests. The Times (London), July 10, 2001.
Hendrey, George R., David Ellsworth, Keith Lewin, and John Nagy. A free-air enrichment system for exposing tall forest vegetation to elevated atmospheric CO2. Global Change Biology 5:293-309; 1999.
Hymus, Graham, David Ellsworth, Neil Baker, and Stephen Long. Does free-air carbon dioxide enrichment affect photochemical energy use by evergreen trees in different seasons? A chlorophyll fluorescence study of mature loblolly pine. Plant Physiology 120:1183-1191; 1999.
Saxe, Henrik, David Ellsworth, and James Heath. Tree and forest functioning in an enriched CO2 atmosphere. New Phytologist 139:395-436; 1998.
Author’s Notes:
Data from the North Carolina FACE (Free Air CO2 Enrichment) experiment on tree growth are seen as particularly reliable because of the experimental design. Earlier studies looked at tree seedlings growing in laboratory conditions. In the lab, it s easy to control such parameters as water input, light conditions, soil nutrients, and gas concentrations. But seedlings are not mature trees their metabolic processes are not the same. Plants growing in the controlled environment of the lab may not respond to stimuli the way trees growing in a natural environment do. Finally, trees are long-lived. A short-term experiment may not accurately reflect their overall effect on the environment in the long run.
The FACE experiment looks at real trees not seedlings in a natural setting, over the course of years instead of months. In the experimental plots, the wind blows, rain falls, insects nibble, and trees compete with one another for space, as they do in nature. The most innovative aspect of the facility is the elaborate system that delivers gases such as CO2 to the air around the trees. This system allows scientists to set up a realistic simulation of what trees in a forest might experience 50 years from now, if CO2 concentrations double, if predicted.
The discovery that tree growth is stimulated by extra CO2 but limited by soil nutrients raises another question for some observers: if soil nutrients are what ultimately limit tree growth, why don t we fertilize our forests?
This idea poses a number of obvious logistical complications. For one thing, forests are variously owned by governments, corporations, and private individuals. Getting these entities to agree on a fertilization plan would be an enormous and unlikely-to-be-successful enterprise. Then there would be the logistics of obtaining enough fertilizer, and delivering it in an appropriate form, at an appropriate time, to the forests. The cost would be prohibitive. Finally, these days the production of fertilizer is an industrial process that itself produces CO2 gas which might negate the carbon-scrubbing results of forest fertilization.