Jonas Jacobson asked:
In colder climates, many leaf trees drop their leaves during the autumn. Is there an average percentage amount of biomass that a tree drops through their leaves in a season? And related to that, some trees do not drop their leaves even in colder climates, so why is it evolutionary beneficial for some trees to drop their leaves (i.e. why didn't only the leave-keeping trees / evergreens come out on top)?
We asked resident plant scientist Howard Griffiths to answer this one...
Howard - Okay. Well thank you very much. That’s a great question because it’s all, basically, a question of economics. To answer your first question: a tree 10 metres high might have 30 kilograms of leaves that it might shed. Bigger trees will have more, up to 100 kilograms of leaves, but leaves are costly to make.
So we heard earlier about the nutrients you need to take out from the soil to build together with the carbon to make the structural material. And they’re also a risk in cold, snowy habitats because they interact with wind and they cause trees to blow over and they might cause water to be lost from the tree at a time when the ground is frozen. So, deciduous broadleaf trees, which we might recognise as Beech, and Oak, and Lime, they leaf out late in the year, at the end when the risk of frost has gone and, provided they’ve got plenty of water and nutrients, they can flush a new set of leaves every year and that allows them to grow faster than the evergreen conifers. So they outcompete the conifers when they grow side by side
So then, what about conifers? Well conifers tend to grow better in cold, waterlogged soils where nutrients are very infrequent, so they hold onto their leaves for two to three years and so the economics of building a leaf make more sense for a conifer to hold onto its leaves. Then they have a structure; they shed snow and so on in the cold winter and they’re very tolerant of freezing and so on, but they’re slower growing and so that’s the trade off. Soo conifers are generally restricted high up mountains or up into the northern latitudes.
I see that some trees that keep their leaves in snow season tend to have very spiky leaves. This small surface area (compared to wide, flat leaves) might reduce water loss in winter?
The reason trees loose leaves is to prevent loss of water during the winter. In the winter, the ground and the surface water is often frozen, while the air is very dry. The result is leaves provide too much surface area for water loss. This loss can exceed water the roots can find, so the tree loses it leaves.
Or consider the waxy coating of a holly leave, which is certainly not pine needle shape. That is another mechanism for controlling water loss.