There are not many places you will go and not encounter trees. Trees really are everywhere. Everybody recognizes their famous body plan of a thick stem, branching limbs and towering crown, often sketched out by people all over the world as characteristic ‘background’ features. It seems that their presence is very strongly felt by their impressive structure, and with trees structure is everything.
Trees didn’t just evolve as a monophyletic group all descended from a single common ancestor. The very word ‘tree’ actually describes a type of body plan, which many different species of plant have evolved over time. The body plan is so successful that different species of plant have all blindly evolved the same adaptation. It’s similar to the fact that although there are many different religions in the world, no matter how big their differences are, they all believe in some version of a god. I’m not purposely comparing trees with god here, but come to think of it they do have a certain awe-inspiring presence. It’s not that surprising that there are examples of nature ‘repeating’ itself in this way, as there are certain ‘limits’ to evolution which I will discuss later in the article. The technical term for this type of pattern is convergent evolution, where similar features are found in species with separate lineages.
So what is it about the body plan of a tree that is so iconic and successful that people and plants are reproducing them everywhere? First we must consider the 3 basic needs of a tree – light, nutrients and water. It is obvious when looking at the structure of a tree that it is adapted well to these 3 basic needs. It has a big crown of leaves to uptake light for photosynthesis and a complex network of roots to absorb nutrients and water from the soil. The size of the tree is a different matter. Not only our trees driven by these 3 basic needs, but they are also driven by the competition for these needs with other species of plant. Whereas fast-growing herbs die back every year, trees maintain a permanent woody structure, a permanent platform from where they can grow their leaves every year above everything else. This is all straightforward stuff, but what is interesting is that trees are products of a process that we call succession.
Ecological succession is the ‘change in species composition of an ecological community over time’ and it plays out with a fairly predictable course. Take a patch of bare ground. Studies of succession have told us that the first species to colonize the patch will generally be fast growing, well dispersed, so-called ‘r-selected’ species. This is because these are quick enough to take advantage of the light, nutrients and water available in the patch. However, over time there will be a transition to slower growing species that invest more energy into growth (like trees), the so-called ‘k-selected’ species. This is because, as they slowly grow taller than the other plants, they cast out shade over the patch and suppress the growth of the less shade-tolerant species that were present before. It’s a classic case of the tortoise and the hare, where the slowest eventually wins the race!
As I mentioned before, there are limits to evolution but these limits are roughly circumstantial. The ‘end’ type community of succession in the UK is predominantly woodland, a community of trees of various heights and shapes all adapted to life in different layers of the canopy and all driven by those 3 basic needs – light, nutrients and water. This community is referred to as a climax community because there is little despite stochastic effects that will change it any further. The community of trees would have altered the environment around them enough to make competition with them almost inevitable. That is until a great storm or disease breaks out and creates a gap in the canopy. The gaps would then flood with light, nutrient and water levels would increase in the soils and those r-selected species we talked of before would quickly carpet the floor with spectacular colours, desperate to pollinate before they suffer the consequence of living such a fast life.
However what I meant by ‘limits’ to evolution is that succession always seems to have a limit, a community or structure which cannot grow beyond what it is, it can only revert to earlier stages. This is because there are limits to an organisms ability to evolve new structures in competition with others. There is only so much light, nutrients and water available and only so much energy you can invest into growth until it compromises on survival in some way. Plants are slaves to their body size and their ability to reach light and reproduce before their time runs out. It’s no wonder we have terms for different plants based on their longevity – annuals, biennials, perennials! Plants are structures, different structures with different heights, blocking out different levels of light. Interestingly, no structure or species has been so successful without altering their environment enough to make competition from other species too tough. Just take a look at ourselves.