BOSTON — Earth scientist Gifford Miller has a warning for destroyers of tropical jungles and other natural-plant communities: Spare that tree and safeguard your climate.
His research in Australia suggests that, if it weren't for human depredation, the dry outback might be a wetter, more hospitable place today. Widespread burning starting about 45,000 years ago may have replaced vegetation that favors rainfall with vegetation that discourages it.
A careful scientist and chairman of the University of Colorado's geological-sciences department at Boulder, Dr. Miller was restrained in his presentation of this ongoing research at a meeting of the American Geophysical Union in San Francisco last month. He noted that he and his colleagues do not yet have definitive proof of the human-climate-change connection.
In a recent telephone interview, however, he explained that their research has already made two important points. First, human occupation coincided with significant change of vegetation. Second, lake sediments show it also coincided with a significant increase in annual monsoon rainfall over a wide region, including Africa and India. Yet Australia's monsoon rains faltered.
Miller added that "the only thing [operational] on a large-enough scale" to account for this anomaly is vegetation change. Computer-based simulations that Miller and his colleagues have run indicate that a vegetated interior Australia would enjoy twice as much rain as it now receives in monsoon season.
The climate change that strengthened monsoons elsewhere probably was linked to known periodic changes in solar energy that are due to periodic changes in Earth's orbit around the sun. The computer studies indicate that the effect of vegetative change "is even stronger than" these well-known orbital effects.
A smoking gun
Miller concludes that "we've got a smoking gun" pointing to vegetation change as the factor that made interior Australia dry. It remains to be proved conclusively that the "smoke" came from humanly set fires.
Meanwhile, Miller says the findings so far "should be a warning" against wholesale forest destruction. Changes in vegetation cover can change the way land absorbs and reflects sunshine and radiates heat. They can change precipitation patterns.
Plants recycle moisture locally by absorbing rain water through their roots and transpiring it back into the air through their leaves. Half of the Amazon jungle precipitation is recycled in this way during the rainy season.
Such vegetation effects are part of a complex climate system involving land, sea, and air. Replacing forest with grass land or lush grassland with dryland scrub can have unintended long-term climate effects. "Without knowing what those consequences are, we should proceed with caution [in making changes]," Miller warns.
Trying to understand those consequences is a daunting task. Just pinning down the nature of Australia's prehuman vegetation is tough. The alkaline soil doesn't preserve fossil vegetation or pollen. However, fossil eggshells from Australia's flightless birds hold clues to what the birds were eating tens of thousands of years ago. Miller's study already shows that a more arid and mixed vegetation was replacing lush grasses by 35,000 years ago.
Meanwhile, scientists continue to find how little they know about tropical rain forests. For example, understanding the age distribution of trees is basic to understanding the ecology of a forest.
You can't count tree rings because the lack of seasonality means annual tree rings don't form. Jeffrey Chambers at the University of California, Santa Barbara, and colleagues report a study using radio-carbon dating in the current issue of Nature. They sampled 20 trees from 13 species near Manaus in Brazil.
They find, to their surprise, that ages range from 200 years to 1,400 years. They observe that the presence of ancient trees in significant numbers suggests that sustainable forest management will require either enormous areas of managed forest or long harvesting cycles.
As another example, an international team made a pilot study of what happens to the diversity of plants and animals when tropical jungle is disturbed. They worked in the Mbalmayo Forest Reserve in Cameroon. Ecologists often try to estimate biodiversity by studying a few "indicator" species. In the Jan. 1 issue of Nature, the team says this quick-and-dirty scheme can give "highly misleading" results. The team adds that to complete a comprehensive study for a single representative hectare (2.5 acres) "in a reasonable time" would "absorb 10 to 20 percent of the estimated entire global work force of 7,000 [qualified experts]."
Scientists have scarcely begun to understand the long-term impact of changes in vegetation. They know that gaining such understanding will take a far greater effort than now is being devoted to the job. As Miller said, unless and until that effort is made, we "should proceed with caution" in altering the vegetation landscape.