The Changing Saguaro Population in Saguaro National Park
Sequence of images showing the Cactus Forest over time.
NPS
These three photos taken from the same location in 1935, 1998, and 2010 show an amazing story of change in Saguaro National Park’s Cactus Forest.
But is there more here than meets the eye?
This famous photo sequence was created for the book The Changing Mile Revisited by two great Arizona scientists, Rodney Hastings and Raymond Turner, who were fascinated by how the desert Southwest has changed over time, and why1. Look carefully at the photos through the years. What do you see? The most obvious change is a huge decline in saguaros, the giant cactus that Saguaro National Park was created to celebrate and protect. Do you notice anything else that changes from photo to photo? Some people point out the development in the Santa Catalina mountains in the background, and that the dirt road in 1935 was paved sometime before the next photo. Many also notice the striking absence of trees and shrubs in the first photo, whereas today they dominate the landscape. At first glance, we might focus just on the saguaros, but these trees are an important part of the story.
What we now know is most of the older saguaros died of natural causes, and young ones were delayed in re-establishing due to an absence of nurse trees. Once the nurse trees came back after the Monument and then Park were established, so did the baby saguaros. These young saguaros are just now beginning to rise above the trees, just as the saguaros did before them well over a hundred years ago.
Approximate age of saguaros of different heights, based on average growth(2).
NPS
Background
When determining what may have caused the changes we see in the repeat photos, it is useful to know a few facts about saguaros:
Saguaros live a long time, 150 to 200 years2
They grow very slowly – after 10 years, they are only about 1 ½ to 2 ¾ inches tall2
After 50 years, a typical saguaro is still only 7–12 feet tall2
When they are young, saguaros are extremely vulnerable to drought, grazing animals, winter cold, and summer heat3.
Protection from these threats is typically provided by “nurse trees” such as mesquite and palo verde. Without them, young saguaros cannot survive3.
Typical saguaro under a palo verde “nurse tree” in Saguaro National Park.
NPS
It is also helpful to know that the reason there are so few trees in the first photo is that from the late 1800s through the 1930s, many were cut down for domestic and industrial use4. A huge amount of wood was needed for nearby kilns (furnaces) that created lime for plaster. The area was also heavily grazed by cattle4,5.
When Saguaro National Monument was created in 1933 (it became a National Park in the 1990s) the wood-cutting stopped, and cattle were excluded from the prime Cactus Forest habitat in the photos by 19585. As a result of these policies, after many years, the nurse trees started to grow back.
What happened to the saguaros?
People shocked by these photos have offered many possible explanations for what caused the disappearance of so many giant saguaros: climate change, the removal of cattle, poaching or “cactus rustling”, the loss of bat pollinators, competition with trees, and many others6. But the evidence favors a simpler answer: most of the older saguaros died of natural causes associated with age such as freezes, windstorms, lightening, drought, and disease. As these older and larger saguaros died, they were not immediately replaced because the population of nurse trees in the area was still recovering6,7.
Panorama of Cactus Forest in 1931
NPS
Panorama of Cactus Forest in 1961
NPS
Panorama of Cactus Forest in 1992
NPS
The hidden re-growth of the Cactus Forest
However, as the largest saguaros continued to die and disappear from the photos, a subtle change was occurring closer to the ground. In the 1970s Ray Turner, Stan Alcorn, and other scientists studying the cactus forest started finding baby saguaros under the new and growing nurse trees1,7. By the 1980s the population was surging during an extended wet period, and today scientists have documented literally thousands of young saguaros in the area displayed in the repeat photos7,8.
So why can’t we see them in the 2010 photo? The answer is that they are not tall enough yet. The oldest of the saguaro “cohort” from the 1960s are about 15 feet tall feet and are only just now starting to peek through their protective nurse trees.
But the good news is that these young saguaros are out there under the trees, growing every year, and starting to reach reproductive age. Saguaros begin to start flowering when they are 35–65 years old and can continue to reproduce for more than 100 years2.
Up close of Saguaro flowers.
NPS
What is the future of the saguaro?
Recent studies at Saguaro National Park confirm that the population in the cactus forest has nearly doubled since the 1960s7,8,9. But what about the future?
The surge in baby saguaros in the Cactus Forest ended in the early 1990s, and since then few have survived8,9. This time, the absence of seedlings has coincided not with wood-cutting, but with a long and deep drought that appears to be exacerbated by the higher temperatures experienced in Tucson and throughout the Southwest7,9. The intensity and duration of the drought will most likely determine the long-term future of the saguaros in the park.
For the short-term, however, the population is healthy and thriving. The long lifespan of saguaros is an adaptation to the hot and dry desert conditions, and the population is positioned to produce another large cohort during the next cool and wet climatic period. We may never see a scene again like the one from 1935. Yet, during the next 50 to 100 years we can expect to see many more of them rising above the trees, blooming every spring, and being enjoyed by the many people who visit Saguaro National Park.
References
Turner, R. M., R.H. Webb, J.E. Bowers, and J.R. Hastings. 2003. The Changing Mile Revisited. University of Arizona Press, Tucson.
Steenbergh, W. F. and C. H. Lowe. 1983. Ecology of the saguaro: III. Growth and demography. Scientific Monograph Series 17. NPS, Washington, D.C.
Turner, R.M., Alcorn, S.M., Olin, G., Booth, J.A., 1966. The influence of shade, soil, and water on saguaro seedling establishment. Botanical Gazette 127.
Clemensen, A.B. 1987. Cattle, copper, and cactus: the history of Saguaro National Monument. NPS Service Center, Denver, CO.
Pinto, R.L, 2014. Cattle Grazing in the National Parks: Historical Development and History of Management in Three Southern Arizona Parks. Dissertation: The University of Arizona, Tucson.
McAuliffe, J.R., 1996. Saguaro Cactus Dynamics. In: Halvorson, W.L., Davis, G.E. (eds.), Science and Ecosystem Management in the National Parks. University of Arizona Press, Tucson.
Orum, T.V., N. Ferguson, and J.D. Mihail. 2016. Saguaro (Carnegiea gigantea) mortality and population regeneration in the Cactus Forest of Saguaro National Park: seventy-five years and counting. PLos ONE 11:e0160-899.
Conver, J. L., T. Foley, D. E. Winkler, and D. E. Swann. 2017. Demographic changes over> 70 yr in a population of saguaro cacti (Carnegiea gigantea) in the northern Sonoran Desert. Journal of Arid Environments 139:41–48.
Winkler, D. E., J. L. Conver, T. E. Huxman, and D. E. Swann. 2018. The interaction of drought and habitat explain time-space patterns of establishment of saguaro (Carnegiea gigantea). Ecology 99:621-631.
