Why trees?

Go to any deciduous woodland at this time of year and it’s not difficult to see plants competing for light.  The bluebells on the floor of High Wood in Durham have one strategy – flower and set seed early, before the leaves come on the trees above them.

Bluebells, late April (left) and July, 2016

But what of the trees which shade them? How did they evolve to be so tall?  A recent review by Kevin Boyce and colleagues in New Phytologist  asks why we assume that the evolution of trees was driven by competition for light when the early vascular plants, from which they evolved, would have been much less productive than today’s flowering plants and so unlikely to have been limited by light levels.  Instead, the authors suggest, the ability of trees to disperse their reproductive propagules more widely, carry a larger leaf area and produce deeper roots might have been more significant in driving their evolution.

We are bound to look at plant history through the lens of what we see around us today but it’s worth remembering that the angiosperms (flowering plants) have only been abundant for a relatively short period of Earth’s history – the last 100 million years, or so.  Looking at fossils, we can see that their predecessors, the ferns, seed ferns and conifers of the Carboniferous period, had a much lower density of veins in their leaves, making it difficult for them to take up water rapidly enough to compensate for that lost in photosynthesis. This would limit the productivity of such plants, despite CO2 levels being around twice what they are today (see How did plants evolve stomata?).  Long-lived, slow-growing plants would be favoured over ‘flash in the pan’ species, needing to grow fast and reproduce quickly.

Similarly, to be able to make use of higher light levels, plants would need the higher leaf vein density which only flowering plants have.  So whilst, today, it may be perfectly reasonable to assume that angiosperm trees are often, though not always, competing for light it seems unlikely that this could have been the driving force for the evolution of tree ferns, for example.

Glossopteris_Restoration

Reconstruction of a Glossopteris plant: a) whole tree b) pollen-producing organ c) seed-bearing organ d-g) different types of fruit

Image: http://earthphysicsteaching.homestead.com/Glossopteris.html

The indeterminate growth form of plants means that, in order to survive, they need to grow – either upwards or outwards (by rhizomes).  New leaves are needed to replace old and damaged ones – prolonged exposure to solar radiation is as damaging to the photosynthetic apparatus, in the end, as it is to everything else.  New leaves have to be produced on new stem material, particularly in long-lived, less productive plants. Growing larger also gives plants the opportunity to carry more leaves and more spore producing organs.

Growing upwards rather than outwards means plants can release their spores from a greater height so they get carried further away from the parent plant, increasing their chances of successful growth. Think about sycamore ‘helicopters’ – the greater the height from which they are shed, the further they are likely to be carried by the wind. The same would be true of the spores of much earlier plants.

Sycamore ‘helicopter’

Growing upward also keeps the biomass of the plant in one place for longer, allowing roots to grow deeper – this would have been important in allowing plants to creep from the margins of wetland onto drier ground where they have to access the water table for both water and minerals.  Even amongst the angiosperms, trees don’t always grow where they need to compete for light.  In arid areas, for example, the development of a shrub or tree growth form might almost be a secondary effect of producing slow-growing, deep roots able to anchor the plant and secure the necessary water and minerals.

Velvet_mesquite

Velvet mesquite tree, Arizona desert – image via Wikimedia commons

As usual, it seems there are multiple reasons why trees are an important part of so many landscapes today, whether they are angiosperms or their older cousins, the gymnosperms (conifers).

Boyce C. K., Fan Y. & Zwieniecki M.A. (2017) 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. New Phytologist,  doi: 10.1111/nph.14387

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