The saguaro is the largest cactus in the United States, commonly reaching 40 feet (12 m) tall; a few have attained 60 feet (18 m) and one was measured at 78 feet (23.8 m).
The saguaro’s range is almost completely restricted to southern Arizona and western Sonora. A few plants grow just across the political borders in California and Sinaloa. Saguaros reach their greatest abundance in Arizona Upland. Plants grow from sea level to about 4000 feet (1200 m). In the northern part of their range they are most numerous on warmer south-facing slopes.
A tap root extends downward to more than 2 feet (60 cm). The rest of the extensive root system is shallow, as is the case for most succulents. Roots are rarely more than 4 inches (10 cm) deep and radiate horizontally about as far from the plant as the plant is tall.
In the Tucson Mountains, which averages 14 inches (355 mm) annual rainfall, a saguaro takes about 10 years to attain 1½ inches (3.8 cm) in height and 30 years to reach 2 feet (61 cm). Saguaros begin to flower at about 8 feet tall (2.4 m), which takes an average of 55 years. Compare this with 40 years to first flowering in the wetter eastern unit of Saguaro National Park (16 inches, 406 mm, average annual rainfall) and 75 years in the drier Organ Pipe Cactus National Monument (9 inches, 230 mm).
Saguaros may begin to grow arms when the plant is between 50 and 100 years of age (in the Tucson Mountains), usually just above the stem’s maximum girth at about 7 to 9 feet (2.1 to 2.7 m) above-ground. The number of arms and overall size of a plant seem to be correlated with soil and rainfall. Saguaros on bajadas with finer, more water-retentive soils tend to grow larger and produce more arms than do those on steep, rocky slopes. A few saguaros have been observed with as many as 50 arms; many never grow arms. Saguaro arms always grow upwards. The drooping arms seen on many old saguaros is a result of wilting after frost damage. The growing tips will turn upwards in time. There is a myth that arms are produced so as to balance the plants, but research shows arm-sprouting to be random. Many saguaros can be found with several arms all on the same side of the main stem.
The chief agent of mortality of mature saguaros in the Arizona Upland is freezing temperatures. The saguaro is a tropical cactus with limited frost tolerance, and it reaches the northern, coldest limit of its range in Arizona Upland….It is difficult to determine the lethal temperature for a saguaro or other plant. The seasonal timing and duration of freezing temperatures are at least as important as the minimum temperature. Healthy middle-aged saguaros have survived 10ºF (-12ºC) for a few hours in mid-winter, while 12 hours of 20ºF (-7ºC) in late fall have caused widespread damage and death.
The imminent demise of the saguaros is a recurring rumor dating back several decades. Its most recent incarnation began in the early 1990s and refuses to die, despite having been soundly refuted….The saguaro doom story first surfaced in the 1940s; at that time little was known about saguaro ecology. Saguaro National Monument was established in 1933 east of Tucson. That bottomland area was chosen because it had a tremendous population of giant old saguaros. (It had few young or middle-aged saguaros, due to the effects of livestock grazing and the cutting of potential nurse trees since the late 1800s.) But there was a catastrophic freeze in 1937, and during the next decade the giant forest was suffering massive mortality from bacterial necrosis. The Park Service bulldozed and buried thousands of rotting cacti in the hope of stopping what it mistook as a new, virulent disease. These efforts failed, and the alarm over the presumed fate of the saguaros became a factor in the establishment of the West unit of Saguaro National Monument on the other side of Tucson in 1961. This location in the Tucson Mountains did not have a cohort of giants so the impact of the freeze of 1937 was less evident, or according to the view of that time, the bacterial necrosis disease had not infected this population. (The western unit had mature stands of giants by the mid 1970s; this cohort was devastated by the freeze of 1978.) This first misinterpretation of the ecology of saguaros had a positive outcome—it engendered the preservation of another tract of splendid desert. Both units of the National Monument were designated Saguaro National Park in 1994.
So are saguaros declining? The answer is, yes, most of the time. So are most species of desert plants and animals—that’s the nature of this ecosystem. In most years there is slightly higher mortality than recruitment (the successful establishment of new individuals), so populations decrease. In years of severe droughts or freezes the mortality can be dramatic. In the occasional wet years mass recruitment reverses the trend of decline with a reproductive boom. In the case of saguaros these episodes of net recruitment seem to occur less than a half-dozen times per century in Saguaro National Park (west), and less often in the drier regions.