Growth rates in four cacti:
Echinocactus horizonthalonius (Nichol's Turk's Head Cactus)
Ferocactus wislizeni (barrel cactus)
Ferocactus cylindraceus (barrel cactus)
Carnegiea gigantea (saguaro)

The Carnegiea data are from:
Pierson, E. A. & Turner, R. M. 1998. An 85-year study of saguaro (Carnegiea gigantea) demography. Ecology 79(8): 2676-2693.

The rest of the data are unpublished, collected by M. E. McIntosh, L. A. McDade, A. E. Boyd, P. A. Jenkins, and others.

Inspired by the Carnegiea paper, I have created some figures for looking at growth and death in all four cacti side by side.

First, the basic growth rates:

Figures illustrate growth as a function of height (a surrogate for age), mortality as a function of height, and distribution of heights.

Distribution of heights:

• E. horizonthalonius
• F. cylindraceus
• F. wislizeni
• C. gigantea

Growth rates:

• E. horizonthalonius
• F. cylindraceus
• F. wislizeni
• C. gigantea

Mortality as a function of size:

• E. horizonthalonius
• F. cylindraceus
• F. wislizeni
• C. gigantea

The most interesting points from these data:

• The two barrel cacti (F. wislizeni and F. cylindraceus) grow much faster than E. horizonthalonius (2.6 and 2.7 cm per year, versus 0.4 cm per year).
• The two barrel cacti have similar growth rates.
• In saguaros, growth rates are highly age/size dependent. Onset of flowering reduces growth rates.
• Flowering in E. horizonthalonius does not appear to affect growth rates.
• For F. cylindraceus, F. wislizeni, and E. horizonthalonius, growth rates are relatively constant, regardless of height. For the two barrel cacti, however, growth rates might increase slightly in the tallest individuals.
  
  
• In saguaros, mortality is highest in the smallest height classes and in the largest height classes. I expect that the taller the saguaro, the more likely it is to fall over.
• Mortality in E. horizonthalonius is not obviously related to height.
• For F. cylindraceus, mortality increases with height for the largest height classes. Again this is probably due to falling over. These cacti are relatively slender, and the population studied is on steep rocky slopes.
• F. wislizeni barrels are more barrel-shaped and probably have a lower center of gravity. The population studied is on level ground. Hence their mortality is not as correlated with height as that of F. cylindraceus, although it is still the case that for the tallest plants, mortality is greater than expected.