Layers of diversity

Leaves emerge through the fog as we drive through cloud forest. Visibility is no more than a few meters, and the road is mud beneath our wheels. The change is remarkable – just a few hours earlier we were in the sunny and sticky warmth of a moist tropical forest. It’s amazing how quickly plant communities change with environmental conditions. The species that can coexist at one elevation are almost completely different from those 500 meters higher. A notable counterexample to this pattern, however, are the spiny palms in the genus Bactris. We found them everywhere we went – tall, unpleasant organisms covered in long sharp spines on the trunk and the leaves, with smaller, sharper spines covering the entire surface of the leaves. Dangerous to walk around, or into them!
A major goal of our trip was to survey how plant diversity changes with varying environmental conditions. We sampled tree species at elevations ranging from sea level to 3200 meters. A standardized way to do this is a Gentry plot, named after the late tropical biologist Alwyn Gentry (whose small plane crashed into a mountain ridge in Ecuador some years ago). At each site, lay out a 100 x 100 m grid, and walk 10 evenly spaced parallel 100 m lines. Down each line, look for any stem rooting within 1m of the line, and ‘count’ it if it has a diameter of at least 2.5 cm. Then, identify the species, collect material, and move on. It sounds simple, but gets tricky very quickly, when there are hundreds of species to potentially distinguish, and most of them are exceedingly tall. Let’s not mention the overgrown vegetation on the ground, assorted spiny plants, and the likelihood of pouring rain. We were lucky to be working with an expert botanist, who was able to identify most species to family or genus level within seconds or minutes. He was able to work so effectively because of his ability to quickly assess plant characters that quickly narrowed down the potential pool of species. Most of the things you might notice looking at a tree – size of its leaves, color of its bark, overall size, and so on, turn out to not be very useful – but things like the way leaves branch from a stem, or the color of the sap, or the smell of the bark, turn out to mean quite a lot. These characters are highly conserved, meaning they don’t vary much at fine taxonomic scales. Anything that has evolved to have a certain combination of characters – say, leaves that grow opposite to each other, that are not compound, and that grow on a trunk that oozes yellow latex – point to a single group of plants, that all share a common evolutionary ancestor. Very cool, and lucky for biologists that some characters don’t vary much!
One thing I was surprised to learn is that a lot of leaf characters are highly conserved – for example, having compound or simple leaves. A simple leaf is one like you’d imagine – just a single flat lamina surface attached to a petiole, which is itself a branch. A compound leaf looks like many leaves attached together, usually in a regular arrangement, on a much smaller branch-like thing (called a rachis), which is itself attached to a larger branch by a petiole. It’s thought that compound leaves have better airflow around them (more leaf perimeter per area) which might help keep them cool in hot conditions – but on the other hand, compound leaves need more structural investment to be deployed. This tradeoff seems rather important to a plant’s growth, so you might expect plants to adapt between compound and simple leaves quite easily under selection from variable environments. But it turns out that if a plant’s evolutionary ancestors had compound leaves, its descendants will also have compound leaves – or the same situation for simple ones. Compound/simple is a highly conserved character for whatever reason – it may be difficult to evolve away from that state. Yet plants find ways around this evolutionary constraint, by evolving many small simple leaves on small branches to mimic a compound leaf, or developing very thick and strong rachises with large leaves to mimic many simple leaves. It’s actually quite difficult to tell, on first glance, if a plant truly has compound or simple leaves. Evolution is full of examples of these work-arounds, where convergent functional solutions are found despite strong evolutionary constraints on the actual tissues and developmental processes available to an organism.

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