Why did trees start growing up? (or did they just start growing down?)

One pop-biology book I like is What Does a Martian Look Like?* by Ian Stewart and Jack Cohen. They look at how organisms expand in variety fill out the phase-space of what they can possibly do. Phase-space, in this case, all the potential forms they can evolve into. A problem is that the phase-space of evolution isn’t constant. For example, you can’t fill a niche as a flower foraging insect if flowers haven’t evolved. It means that you can’t always apply the rules we see today back to the past. There’s a paper out this month in New Phytologist that demonstrates this nicely.

“Did trees grow up to the light, up to the wind, or down to the water? How modern high productivity colors perception of early plant evolution” by Boyce, Fan and Zwieniecki point out that when looking at early trees, there’s a major difference we need to remember. Angiosperms didn’t evolve till roughly 200 million years ago. It’s easy to forget the forests of the Carboniferous period wouldn’t have had a single angiosperm among them. So did the same pressures that apply to trees today also apply back then?

In a rain forest today there might be a race to the canopy and the light. Boyce et al. argue that angiosperms are such a radical innovation that you can’t assume this was true in the past. In particular, they look at leaf physiology and the ability to get water to enable photosynthesis. If the water doesn’t arrive faster enough stomata have to close, shutting down photosynthesis. This means the non-angiosperms would have been less sensitive to CO₂ concentrations than angiosperms as they would have had more problems with their internal plumbing than gas availability.

If you accept that Devonian plants were less productive explaining trees as a race to the light becomes a problem. Boyce et al. compare the vein densities of plants in this period with modern angiosperms, and find that the comparison is between Devonian plants and obligate shade-requiring angiosperms. They argue this pretty much knocks out the idea that plants were desperate for light and fighting to avoid being shaded. If that’s the case why would you need to grow up?

Boyce et al. look over a few explanations, and point out given trees independently evolved at least seven times so it probably doesn’t make sense to look for one answer. However, there are a few recurring themes.

One is water. Boyce et al. note that the first vascular plants evolved at waterway margins, so there was always water on hand. Evolving to vertical growth doesn’t just mean a plant grows up, it also grows down, so it’s able to tap deeper supplies of water. If this is right then asking why trees grow up is getting the question exactly the wrong way round.

Another theme is sex. Getting taller is an excellent way to spread your spores. Boyce et al. also point out that simply being bigger means you can produce more quantity of reproductive material.

The idea in the paper that appeals to me most, is that trees got bigger because they could. The earliest vascular plants were at the small end of their possible size range so they might have drifted into becoming larger. As appealing as that is, the authors note that a tree is not simply a big herb – so they don’t think that works as a simple answer, but they do argue that the cell biology of plants was a possible driving for the need to grow.

The only thing I don’t see – which indicates what I’m about to suggest is Very Wrong – is protection. Are there advantages to having some branches protecting lower branches from ice or sun? It’s easy to see the reduced self-shading by angiosperms as an advantage in a system where access to light is important, but if you take that pressure away could self-shading help defend against the sun or the wind? Answers telling me why that’s not likely are welcome in the comment box below.

The appeal to me isn’t just palaeobotany. A simple demonstration of how life, as it is now, isn’t how it always was is important. As Boyce et al. say in their paper:

“When considering the fossil record, analogy to the modern world is unavoidable. However, multiple competing analogies will always be available.”

That, to me, has importance for Astrobiology where the quest for an Earth-like planet changes depending on what you think of as Earth-like.

Even if you’re not as much of a space cadet as me, it’s still definitely a paper worth your time. It not only has a clear question and argument, but it’s also well laid out. The writing is very accessible as follows a trail from here’s the problem to here’s why it matters to here’s why the usual explanation doesn’t work before going on to suggest some solutions. The article also is currently free to access.

* Also known as Evolving the Alien, because publishers love to change titles for books between the US and the UK. It gets a slightly higher review score on Goodreads under this title.