Another riff on losing leaves.

Here's a shot from Lyons Woods FP this morning:

White Oak (Quercus alba) and Red Oak (Quercus rubra),
Lyons Woods FP, Lake Co, IL  1/16/14

The tree in front is a White Oak (Quercus alba) and the one in back is a Red Oak (Quercus rubra). Both species are deciduous, yet here we are halfway through the winter and neither one has lost it's leaves -- what's up with that?

To answer this question, first we need to look at how trees drop their leaves. When the right conditions occur (temperature, water levels, photoperiod), the tree starts to develop a waterproof leaf scar at the base of the leaf. Once that is complete, the leaf dries up, and in most species the leaf scar develops to the point where the leaf itself is no longer held to the branch. At that point, gravity (or the wind) takes over and the leaf falls.

Looking again at these leaves, they're all brown, and if you were to handle them, you'd find that they're pretty dried out. So the tree isn't retaining them to photosynthesize. Why, then?

People have proposed a number of reasons why this might be adaptive -- the most successful of those proposals argue that the nutrients are better retained in leaves that remain on the tree, so losing them in the spring when you can use those nutrients makes more sense than dropping them in the fall and letting them lose those nutrients when they can't be used. (1) On the other hand, leaves tend to catch both snow and wind, either one of which can cause a tree to lose limbs -- which is definitely not adaptive!

However, in 1983 Hans-Erik Wanntorp pointed out that most oaks are tropical, and they're nearly all evergreen. In fact, the ancestral state in oaks is evergreen, which means that the adaptation of interest here is not the retention of the leaves, but the fact that they don't keep them green all year round. (2) So in fact, the best answer we have to that initial question is that they simply haven't fully adapted to their current environment -- they let the leaves dry out, but they haven't (yet?) developed an easy way for them to drop.

Wanntorp developed this idea more generally, pointing out that if we want to make evolutionary inferences about modern critters, it's awfully important that we first figure out what we can about the pathways those critters took. (3) Otherwise, we end up asking the wrong questions -- "why did our Oaks evolve to hang onto their leaves?" instead of "why haven't they evolved the ability to drop their leaves?" Since the questions we ask channel the data we collect and the methods we use to do so, this is a rather important point.

Why study evolution? Because without it, biology becomes a form of "stamp-collecting." (Leaf-collecting?) With it, biology becomes a powerful tool for understanding how our world comes to be. Even when we're just looking at a couple of young oak trees on a snowy January afternoon.

(1) Otto, Christian, and Lars M. Nilsson. "Why do beech and oak trees retain leaves until spring?." Oikos (1981): 387-390.

 

(2) Wanntorp, Hans-Erik. "Historical constraints in adaptation theory: traits and non-traits." Oikos 41.1 (1983): 157-160.

 

(3) Wanntorp, Hans-Erik, et al. "Phylogenetic approaches in ecology." Oikos (1990): 119-132.