The giant columnar cactus Pachycereus pringlei adaptively modifies its stem shape from the dry tropics into the arid mid-latitude deserts

- The diameter of adult cardons increases with latitude.
- The plant height when cardons begin to branch also increases with latitude.
- Populations in the tropical Cape Region had higher surface-to-volume ratios.
- Robust cardons in their northern limit are exposed to high temperature variation.
- The height-to-diameter pattern we report seems to occur in other monopodial plants.

Because of the lack of leaves, the distributions of columnar cacti are limited by their capacity to trade off their photosynthetic chlorenchyma and their non-photosynthetic storage parenchyma. For species with wide latitudinal ranges, variations in stem surface area:volume ratios could play an adaptive role. Based on the fact that Pachycereus pringlei spans more than a 1000 km along the Baja California Peninsula, we used this species to analyze changes in stem allometry between populations. We selected six sites, from latitude 23°-31° N, ranging from 518 to 55 mm of annual rainfall. We used an allometric model to analyze the diameter-to-height relationship, estimating the parameters through linear modeling. The height of the main stem when the first branch emerges was estimated by regressing the height of the plant against the number of lateral shoots. The solar radiation intercepted by an unbranched 6-m-tall cardon was estimated using an irradiance model. The diameter of adult plants, the stem height when cardons begin to branch, and the surface area:volume ratio, were all associated to latitude and to latitude-related environmental variables such as rainfall. For any given height, the surface area:volume ratio decreases from the wetter tropics into the arid mid-latitude deserts.