San Francisco — If you have entertained the suspicion lately that the weeds in your garden are growing faster than they did a few years back, it might be more than a figment of your imagination.
Researchers at the University of Arizona (UA) Tree Ring Laboratory find that pine trees high in the mountains of Nevada and California are growing twice as fast as they did over a century ago. This enhanced growth does not appear to be the result of climatic effects, the scientists reported in an article in the Sept. 7 issue of the journal Science.
If not due to warmer temperatures or increased rainfall, this speeded-up tree growth may stem from the buildup of carbon dioxide (CO2) in the atmosphere since the beginning of the industrial age, suggest the authors.
Carbon dioxide is a colorless, odorless gas. Animals breathe in oxygen and breathe out CO2. Plants take up carbon dioxide and transpire oxygen. Together, these two processes form a closed and balanced cycle.
But burning wood and fossil fuels release carbon dioxide as well. As the fires powering the wheels of modern society have been stoked ever higher, CO2 has been pouring into the atmosphere at a quickening rate. For more than a century, it has been produced faster than the world's oceans can dissolve it or the world's plant life can incorporate it. So its concentration has been steadily rising: since 1850, the level of this gas in the atmosphere has increased by 0.06 percent. And the current concentration of 340 parts per million is expected to nearly double by the year 2020.
This trend has caused concern within scientific circles. Carbon dioxide in the atmosphere acts much like the panes of glass in a greenhouse, letting in sunlight but trapping heat. As a result, climatologists have been exploring the possibility that rising levels of CO2 will create a global warming trend. If such a trend were extreme enough, it could alter rainfall patterns and gradually melt the planet's polar ice caps.
Less attention has been paid to the possible impact on various ecosystems of increased CO2 levels. The Department of Energy is spending $12.5 million on carbon dioxide research this fiscal year. Out of this, $2.5 million is going to the study of plant response, primarly that of crops. Less than $1 million is going to ecological research.
Primarily agricultural experiments have shown that many, but not all, plant species grow faster when exposed to higher CO2 levels. This is known as the ''fertilization effect.''
Based on this, scientists have anticipated for some time that carbon-dioxide-enhanced growth would eventually be detected. Until the just-published tree-ring work, however, there was no concrete evidence that this was actually taking place.
''We've been wanting the first detection,'' acknowledges Boyd R. Strain of Duke University, a member of the scientific cadre working on this subject.
The natural place to look for the fertilization effect is at high altitudes. There, thinner air means less carbon dioxide, and less CO2 means a slower growth rate for subalpine trees. Consequently, their growth is most likely to be unequivocally affected by CO2 enrichment.
Nevertheless, the magnitude of the effect found by the UA dendrochronologists has come as a surprise to other scientists. At two of the long-lived bristlecone pines in California's White Mountains, the UA scientists measured growth-rate increases of 73 and 106 percent over that recorded in tree rings added in 1850.
''I'm very surprised. I would have only expected a 10 to 15 percent CO2 effect,'' says Robert W. Pearcy, professor of biology at the University of California at Davis. He and Dr. Strain caution that this evidence is still very preliminary. There is a chance, for instance, that some purely regional effect is involved. (One of the article's coauthors is now in Colorado collecting more tree-ring samples to see if this same effect will show up in another area.)
At this point, there are many more questions than answers regarding the ecological impact of rising CO2 levels.
''There is just so much that we do not know,'' sighs Dr. Strain. Each plant species responds differently to changes in CO2. For instance, a doubling of current levels causes soybeans to grow 58 percent faster, while corn gains only by 24 percent. Silver maples spring up 61 percent faster while sycamores add only 13 percent to their growth rate; jimson weed gains 74 percent faster while common ragweed grows only 10 percent more quickly.
In ecosystems with hundreds of competing plant species, the fertilization effect will undoubtedly shift the current balance, experts say. This will affect the communities of insects and animals that depend upon them. But how dramatic these effects will be remains an open question.
Some scientists, such as Fakhiri A. Bazzaz of Harvard University, worry about possibly catastrophic effects on especially fragile ecosystems such as tropical rain forests. Others, such as Dr. Strain, believe that the changes will come so gradually that they will be difficult to distinguish from some other effects of human activities - acid rain and overpopulation, for instance.
That does not mean that these changes won't have a serious impact on people, some scientists say. Take the case of cheat grass (bromus tectorum). This Mediterranean weed has been spreading vigorously throughout the Western United States.
Because cheat grass is only good forage for a short period each spring, its spread spells problems for the livestock industy. Also, the weed turns tinder dry and becomes a major fire hazard. Dr. Strain thinks that its spread is being hastened by the fertilizer effect and he is gathering data to test this hypothesis.
On the other hand, some agriculturists have actually welcomed this prospect. It will allow them to grow more food with less water, they argue. Most crops use water more efficiently when they have more carbon dioxide, Dr. Pearcy explains. But in so doing, agricultural processes are likely to be altered in a number of ways. The mix of crops will change, as will the areas where they will grow. Faster growth rates may also dictate increased usage of fertilizer, herbicides, and pesticides, experts say.
The next place ecologists expect to see evidence of the fertilization effect is in disturbed areas such as clearcuts, where all the trees have been removed for timber. More CO2 should mean that plants grow back faster and possibly with different plant communities than has been the case in the past.
Should Earth's biosphere begin to respond more vigorously to rising carbon dioxide levels than has been expected - as the tree-ring results hint - then the rate of the buildup of this gas, and its predicted climatic effects, could come more slowly than is currently expected.
''The fertilization effect may be buying us the time we need to study the carbon cycle . . . before we're hit with the greenhouse (effect),'' suggests Frank Telewski, a researcher at UA's Tree Ring Laboratory, who is involved in the studies following up on the newly reported results.