Are growth rings accurate fingerprints of plant age in a stem-succulent, drought-deciduous shrub growing in the Chihuahuan Desert?

- Age and lifespan of desert plants is often a puzzle because most lack growth rings.
- Ocotillo, an iconic North American desert shrub, produces persistent growth rings.
- Apparent equivalence between number of growth rings and age has been established.
- A century-plus species lifespan was supported by an ocotillo with 107 growth rings.

Ocotillo (Fouquieria splendens), an iconic plant of the Chihuahuan and Sonoran Deserts in the United States and Mexico, has the reputation of being a long-lived shrub capable of attaining ages of two centuries or more. However, evidence of this potential longevity is lacking given that 104 years has been the oldest confirmed age recorded. The recent finding that growth rings can be accurately counted in the woody trunks of ocotillo has provided an opportunity to determine whether these rings merely identify the punctuated, sporadic growth common in this plant, or whether they actually represent ocotillo age. In seedlings studied for 22 years in southern New Mexico, growth rings enumerated in their trunks equaled total plant age. This apparent equivalence between growth rings and age was in spite of the fact that no measurable stem elongation occurred in up to nine of the 22 years of study in an ocotillo that also had no stem elongation in four consecutive years. A mature ocotillo harvested at this site that had 107 growth rings further supports the reality of a century-plus lifespan for ocotillo. The puzzle of longevity in this distinctive shrub of the desert Southwest can now be addressed with future research, but the possibilities go beyond the determination of maximum lifespan. These possibilities include dendrochronological studies that may well allow important analyses of patterns related to climate change in deserts where biological monitors of change are not always present or obvious.