One era's cactus boom is another's bust.

The way-too-short life of a human biologist doesn't allow much perspective on the saguaro cactus. The stately giants routinely live 125 to 175 years, and some for almost 300 years.

Combining data from three generations of botanists studying a single population of plants, however, has shown that what one scientist sees as a dramatic population boom or a bust may just be a small part of a long-term cycle.

The current generation of scientists, Elizabeth A. Pierson and Raymond M. Turner of the U.S. Geological Survey and the University of Arizona in Tucson, went back to records of saguaro surveys on Tucson's Tumamoc Hill beginning in 1908. The long stretch of data allowed them to verify their method of estimating growth rates and ages, the researchers report in the December 1998 ECOLOGY.

Taking the long perspective also allowed them to see slow changes in cactus numbers. They concluded that the hill's population probably went into a decline around the late 1870s but began to recover in the 1920s. That surge in population peaked about 1970, then numbers began to dwindle again. A scientist inferring the health of a saguaro population just from data during his or her lifetime would have been misled, they say.