El Palo Amarillo Verde de Sonora

Explanation: Why does this small tree of the Sonoran Desert of western North America have a yellow bark?

This is a yellow palo verde. It has the habit of dropping its leaves during periods of excessive stress of extended high heat and arid weather.

It then survives ... because the bark itself is, amazingly, photosynthetic, unlike most other trees on this planet.

Some sources suggest that native peoples of northwest Mexico used the seeds for flour and consumed the green seed pots.

Growing as a low, branching shrub, yellow palo verdes sometimes provide protection and shade for saguaro cacti to become established, thus serving as a nurse plant.

Although yellow palo verdes grow commonly in the Sonoran Desert -- and in the Mexican state of Sonora -- the tree can hybridize with the palo brea tree, also called Sonoran palo verde, Parkinsonia praecox). But the hybrid is also referred to as Sonoran palo verde (P. x sonorae), confusingly.

The name palo verde derives from the Spanish for green stick or green wood. Fitting.

A few other trees with photosynthetic bark include aspen (Populus tremuloides, family Salicadeae), beech (Fagus sylvatica, family Fagaceae), and birch (Betula pendula, family Betulaceae), which are trees associated more with colder-weather climates. Along with this week's star, here are trees of at least four different botanical families with this same odd trait. Why bark-photosynthesis has evolved in trees among different taxonomic groups and in such disparate environments -- arid deserts and seasonal temperate habitats -- is a most interesting pattern that may deserve further research.

Information:
Wittmann, C., and H. Pfanz. 2007. Temperature dependency of bark photosynthesis in beech (Fagus sylvatica L.) and birch (Betula pendula Roth.) trees. Journal of Experimental Botany 58(15-16):4293-4306.

EPOW - Ecology Picture of the Week