Unraveling another Mayan mystery - food production

While the maize ripened in the newly raised fields, Go mez-Pompawondered about ''the so-called virgin rain forests'' he had studied in Veracruz. As early as the 1930s archaeologists had noticed that many highland Maya sites were dotted with the tall, shady, fruit-producing ramon, which isn't normally abundant there, as well as other plant species that produced fruit, fuel, or building material. Were the Maya just lucky to inhabit such rich forests, Gomez-Pompa asked himself, or did they create them?

He joined forces with the late Alfredo Barrera-Marin, a Mexican botanist who was Mayan and whose father, Al fredo Barrera-Vazquez, had been a noted Maya scholar. By talking with modern descendants of the Maya in the Yu catan, they found that the Maya understood secondary succession -- the sequence of plant species that appear after virgin rain forest is cut down.

The critical variable in secondary succession, Gomez-Pompa knew,was the amount of light reaching the ground. The canopy of trees covering a rain forest is so thick that only occasional flashes of sunlight penetrate at a given spot. But when a gap in the canopy opens, light streams down. ''Seeds that have been accumulating in the ground germinate, and plants quickly cover the space,'' says Gomez-Pompa. ''Some of the weeds complete their life cycle in a few weeks. Then shrubs take over, then trees.''

The present-day Maya know secondary succession so well they evenhave names for its various stages. When an experienced farmer decides to thin out the ka'anal'k'aax (old forest), he carefully chooses which trees to cut, which to leave as stumps, and which to spare. He fells the fast-growing trees to burn their wood for fuel. But he doesn't touch those trees that provide food, medicine, building material, or other valuable commodities; the sapodilla, for example, produces chicle, the main ingredient in chewing gum, which the Maya sell for export. The farmer may also save certain flowering trees because they are beautiful or because mellipona bees depend on them for nectar, nd he depends on the bees for honey. In the space that's left he might plant maize, squash, and beans, which help restore nutrients to the soil.

The farmer calls this miniature ecosystem a sak'aab. He farms ituntil the soil wears out, then abandons it so the natural process of secondary succession can begin.

If the living Maya possess this knowledge, what more might theirancestors have known? Exploration and radar mapping of the Yucatan (see map, page 44) show broad areas of raised fields, many growing terraces sculptured into hillsides to prevent erosion, and numerous stone walls from pet kotoob.

To understand Mayan agriculture further, Gomez-Pompa plans new studies in an area of swamp and mud flat in the northwest Yucatan that was once heavily populated. ''If I were trying to survive on the Yucatan,'' he says, ''I wouldn't choose this area.''

Archaeologists hope such studies will answer the chief remainingmystery of the Maya: What brought them down? ''The central Maya lowlands have the most unusual population curve that we've found,'' says Turner. The Maya show a single rise, a single fall, and no recovery; most other ancient populations, like those in the Nile River Valley, rise and fall and rise and fall in a succession of peaks and valleys.