The Age of a Joshua Tree

A Beautifully Bizarre Joshua Tree
-Photo by Matthew T. Ross-

One of the most common ways to determine a tree's age is to look at the rings that form as a tree grows. A great example of this is the trunk of a Douglas-fir tree on display at Mt. Rainier National Park.

You can see how the park rangers marked specific historical events on different rings.

This particular tree started growing around 1293 and lived for almost 700 years. In 1963 it was chopped down to make paper. 
In an earlier post we talked about the layers that make up tree bark. You can find that here. The rings that form as the tree grows, however, are the result of the tissue layers that lie below the bark. As the cell's of the cambium layer divide, the outermost cells become part of the phloem (the innermost layer of the bark, discussed here, which carries food produced in the leaves down the trunk), while the inner most cells of the cambium become part of the xylem (which carries water and nitrogen from the roots up the trunk to the leaves). The phloem and the xylem create a two-way street for nutrients to flow through the tree.
During the springtime the tree grows quickly resulting in large cells in the xylem. This gives them a lighter color. Towards the end of summer the tree’s growth starts to slow down and the xylem cells become smaller. This increased density results in a darker appearance. This produces the alternating light and dark rings that you see in the trunk. Taking the species and climate into account, this is a great way to determine a tree's age.

The Joshua tree is bizarre for many reasons, one of which is that its xylem and phloem are combined, making up fiber bundles (rather than large rings) that carry nutrients throughout the plant. Given that the plant doesn’t produce tree rings,  this makes age difficult to determine.

Nevertheless, science is on the case! In 2006 Kimberly D. Gilliland, Nancy J. Huntly, and J.E. Anderson published a study in which they used a variety of measures to estimate the ages of a group of Joshua trees in Utah. Starting back in 1987, the lab tagged a sampling of Joshua trees and subsequently measured and remeasured their growth over a 14 year period. This allowed them to develop a growth curve that they could use to estimate the age of other Joshua trees. The majority of the trees in their population were estimated to be between 25 and 100 years old. However, they estimated some to be near 300 years old! While not as long lived as some plant species, that’s a long time to exist on the planet.

Gilliland, K. D., Huntly, N. J., & Anderson, J. E. (2006). Age and Population Structure of Joshua Trees (Yucca Brevifolia) in the Northwestern Mojave Desert. Western North American Naturalist66(2), 202–208. doi:10.3398/1527-0904(2006)66[202:AAPSOJ]2.0.CO;2