Contemplating the concept of evergreen plants can be head scratching, and often an opportunity to mess with your friends. I have often answered the question of “what is a live oak?” with “the opposite of a dead oak.” After the grimacing and occasional punch in the arm that follows, a basic answer emerges. A Live Oak is a tree that has leaves that live more than a year at a time, such that the tree always has leaves, even in winter.
But even that answer is only part of the story and leads to more questions. Why would a leaf need to last more than a year? How long does an individual leaf live? Are evergreens better than deciduous trees? The interesting thing for me as a scientist is that there are no straightforward answers to any of these. Science just hasn’t addressed these questions as much as you would think.
Starting with why a plant would evolve to be evergreen, the basic consensus is that many of these plants live or evolved in nutrient poor soils, where conservation of these limiting elements makes sense. If you are going to build a leaf, the longer it lives, the more you get out of the limited elements that were needed to build it in the first place. And yet there are many forests with a mixture of evergreen and deciduous trees and shrubs, and they all seem to thrive.
Another advantage of evergreens is that they do not go completely dormant in winter, especially in the temperate regions of the world. Photosynthesis may be slower, but it continues. Having functional leaves in late winter and early spring also helps jump start the growth of plants as the temperature rises. Deciduous plants must wait until leaves fully emerge.
As for the life span of any given leaf, the story can vary. For common evergreens like Southern Magnolia, the annual shedding and emergence of new leaves happen simultaneously and gives us the opportunity to see age classes of leaves. Beginning at the tips of each branch, the new leaves of the year emerge, with fully formed leaves just behind (last year’s leaves), and the dead leaves from the apparent year before that. That suggest a two-year life span for any given leaf.
The same pattern can be seen on other common evergreen plants like Red Bay, Wax Myrtle, and Yaupon Holly. In the case of pines, research for some species report leaf bundles living upwards of 3-5 years. As for our ubiquitous live oaks, patterns can vary greatly. For any given tree, one year may only see what appears to be a moderate number of lost leaves, but in another, nearly all fall to the ground. Best guess is that these leaves also live at least two years. Weather and other factors may well contribute to these patterns.
Without more so-called facts to go on, perhaps my basic answer to the question has some merit – or not (chuckle, chuckle). In any case, paying attention to what plants are evergreen or deciduous can be fun and gives us something to scratch our heads about.
Hope to see you in our great outdoors!
