Desert Giants: Life and Death Among the Saguaros
Imagine the Sonoran Desert without saguaro cactus.
The saguaro is our desert's signature plant, a pivotal part of both natural and cultural life in the Sonoran borderland. It has a sacred place in Tohono O'odham society. It awed Spaniards who, in their 17th-century journeys near what now is I-10 and I-19 west and south of Tucson, dubbed this "the land of the marching giants."
Is the Sonoran Desert's saguaro population in decline?
In a unique study that began more than 90 years ago, Desert Laboratory botanists have documented saguaro booms and declines. Now current land-use practices may jeopardize saguaro survival, they say.
The explorer-botanists who first set up shop in the heart of saguaro country -- the men and women who traveled this land by fat-tired, early model cars and defined this desert according to its vegetation -- gave the saguaro its scientific name, Carnegiea gigantea.
They were Carnegie Institution scientists, and in 1903 they founded the Desert Laboratory on Tumamoc Hill. The 1,730-acre laboratory, the first facility of its kind devoted solely to arid lands research, has remained a permanent natural ecological preserve dedicated to studies of plants, animals and environment of North American deserts.
By 1906, Carnegie botanists began mapping saguaros growing over the Hill in all directions to discover how and why saguaro populations vary over the landscape. They measured the height of the saguaros and, using something called a plane table survey, plotted each individual from seedling to senescent over steep, prickly, rocky slopes of Tumamoc Hill, Sentinel Hill (A Mountain) and surrounding vicinity.
The map they published in 1908 revealed that saguaros clearly prefer southern and eastern slopes to northern slopes where, the scientists theorized, frost must be limiting numbers of this subtropical plant. And saguaros clearly prefer steep, rocky slopes -- all the better for root drainage, they also suggested.
Carnegie botanists were also the first to worry about saguaro extinction. In 1910, Forest Shreve published his study of the heights of saguaros at the Desert Lab, and based on those heights, their ages according to his growth-rate calculations. Shreve discovered a dearth of small saguaros, or no saguaro younger than 50 years. The initial evidence was that Tumamoc Hill likely was going to lose its majestic columnar cacti. Shreve noted this same disturbing pattern elsewhere around Tucson.
Shreve was at great disadvantage he didn't have longer-term data on growth rates, ages and saguaro population trends. Saguaros have an average life span of 125 - 175 years and a potential life span of almost 300 years. Further, they produce their offspring in periodic bursts, when a number of complex climate and microhabitat conditions are met. So, the only way to learn if new saguaros are replacing dying saguaros fast enough to maintain the population is to monitor the plant communities for decades.
By 1964, by which time the Desert Laboratory had become a joint facility of the University of Arizona and the U.S. Geological Survey, Raymond M. Turner, a botanist with the Desert Laboratory, created four, 25-acre permanent plots on north, south, east and west slopes within the original Carnegie study area.
Equipped with a telescoping aluminum rod for measuring plant height, Turner measured and mapped more than 4,000 saguaros within these plots. He discovered that the slopes of Tumamoc Hill carried from between two and three times as many saguaros in 1964 as they did in 1908. The new plants were born in a burst of saguaro production on the Hill from 1920 - 64.
Ironically, Turner's discovery of the saguaro population boom on Tumamoc coincided with heightened concern for a dramatic decline in saguaros at Saguaro National Monument East, now Saguaro National Park.
The magnificent groves of saguaro around the Rincon Mountains east of Tucson had prompted world renown botanist and UA President Homer Shantz to mount a campaign that resulted in the founding of the national monument in 1933. These saguaros were born in a pulse of saguaro production at around 1800 and had lived their average life expectancies. A more serious matter was the lack of new saguaro recruits at the monument, many of which were likely lost to grazing livestock, mining activity and other disturbances.
Turner repeated the saguaro census on the Desert Lab plots in 1970 using new measurements on individual plants to calculate growth rates. From this he developed a model for determining the age of saguaros on Tumamoc.
In 1993, Elizabeth (Betsy) Pierson tested Turner's model for determining Tumamoc saguaro ages. Using the 1908 Carnegie map of Tumamoc saguaros along with modern laser survey and radio communications equipment, she scoured the four permanent plots, revisiting every single saguaro ever documented by Desert Lab scientists. Pierson searched for any newcomers, measured the survivors and noted the condition of their health, recorded saguaros that had died and theorized possible reasons for their demise.
For future saguaro census takers, Pierson hammered L-shaped aluminum identification spikes into rocky ground for each plant in the survey. (Spikes, rather than aluminum tags are needed, for packrats stash aluminum tags in their middens.)
"It took me pretty much a year, every day, five days a week, a total 250 field days to revisit all of Ray's plots, Pierson said. She and her co-workers finished by wrapping each counted and mapped saguaro in biodegradeable toilet paper, marking their way through the forest of plants long enough to finish their field survey.
Pierson found the saguaros had actually grown to heights predicted by Turner's model. "It would be harder to find a nicer correlation in biology,'" she said.
Theirs is the first study in which a model for determining saguaro age using growth rates observed during one point in time has been verified with observations from a later, separate period.
Because they documented differences in saguaro recruitment, growth rates and mortality across environmentally dissimilar slopes of Tumamoc Hill, they believe their study is potentially useful for understanding long-term saguaro population trends in general.
The 1908, 1964-70 and 1993 censuses individually are a short-term "snapshot" of how saguaro populations fared on Tumamoc Hill at the time. But collectively, they give a truly unique, long-term view of saguaro demography.
So what have three generations of botanists at the Desert Lab collectively discovered in this remarkable nine-decades saguaro study? Some highlights:
- The saguaro populations on Tumamoc Hill have been in decline during much of the last two centuries, but they persist by successfully establishing new seedlings in episodic surges.
- Saguaro populations on all slopes have nearly doubled since 1908. Saguaros remain more plentiful on south- and east-facing slopes because more young saguaro have established there and survived.
- Saguaro ages show that saguaro regeneration fluctuates over several decades. Shreve and his colleagues in 1908 witnessed part of a saguaro population decline that began in the late 1860s. During the 1960s, Turner observed saguaros born in a 1920 regeneration boom that peaked in the 1970s. In the 1993 census, Pierson and Turner observed that saguaro populations on the Hill are in decline.
- In general, more saguaros germinate during wetter conditions and fewer germinate during drier conditions. However, other factors affect how many young saguaros survive. From the 1860s to 1920s, for example, a relatively favorable wet period, saguaro recruits didn't fare well, possibly because of colder winters, livestock grazing (the Desert Lab was fenced from livestock in 1907) and rock quarrying.
- In the future, the link between climate and successful saguaro regeneration may be weakened by the loss of native desert vegetation to expanding suburban areas and other land-use practices, Pierson and Turner noted. Saguaros reproduce in infrequent, periodic surges when a set of complex conditions are met. New land-use practices may increasingly upset the delicate process.
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University of Arizona in the News