Walk through the Rocky Mountains in the summer, and you will be immersed in a sea of green, and will hear the soft rustling sound of millions of leaves. These are quaking aspen (Populus tremuloides) trees, and they dominate the landscape. When I first came to Colorado in late-fall 2009, I was captivated by the beauty of these trees and wanted to learn more about them. Three years later, my first study of aspens has just been published.
One of the key mysteries in functional ecology is how plant leaves use carbon – there is a universal tradeoff between ‘fast’ leaves that have high photosynthesis rates, low carbon costs, and short lifespans, and ‘slow’ leaves that have low photosynthesis rates, high carbon costs, and long lifespans. Globally it appears that none of these strategies is better than others – all leaves seem to return approximately the same amount of carbon to the plant over the course of their life. So why the tradeoff, and what causes it?
I thought that leaf veins might hold the secret – they form the main structural support and water/carbon transport network for every species. Perhaps variation in the geometry of this network could lead to tradeoffs in leaf functioning that match what is seen worldwide. Moreover, I thought that since water flow in leaves is linked to the dryness of the environment, precipitation should control how many veins a leaf has. If this were true, then we would have a theory linking climate to leaf form to leaf function, and ultimately to the explanation of a global pattern.
Testing these ideas was a multi-step process. The first step was with pen and paper – developing equations to test. Second was fieldwork – exploring all over Colorado in search of plants from different environments. Aspen were a perfect study system, because they grow in large clones, where genes are invariant but environments change. Since the theory was about physiological change, I wanted to disentangle changes caused by environment from those caused by genetic variation.
So it was time to go find aspen clones all over Colorado, then measure their leaves. Fieldwork is always the best part of any project, and this one was no exception. On many aspen clones, leaves are out of reach, and trees have to be climbed. Here Neill Prohaska is exploring different ways to reach the canopy. If you’ve never climbed an aspen before, you should know that they have a powdery sort of bark that is very slippery and that makes climbing very difficult!
After two summers’ of fieldwork, all the data were in. It turns out that most of these ideas about vein networks are correct – they are linked to climate, and closely predict how plants use carbon. I got some of the math in the model wrong, but a revised version seems to make more robust predictions. We’re off to test the revision in the Peruvian Andes next!