WTF does deciduous mean?

Welcome to my WTF Gardening series, where I take common gardening terms and explain them for those who are new to the world of plants. Find the entire WTF Gardening series here.

A deciduous tree or shrub is one which sheds its leaves seasonally. This occurs at different times of the year depending on where you are in the world: areas with temperate climates will usually see leaf loss on deciduous trees in autumn and winter, while tropical climates are likely to experience this during their dry season.

The process of a tree or shrub dropping its leaves is called ‘abscission’. 

Why do deciduous trees and shrubs shed their leaves?

 

Abscission of the leaves on a deciduous tree is actually a method of protection from the effects of winter. Unlike evergreen trees with their robust, wax-coated leaves, deciduous trees grow thinner, more delicate leaves which are prone to freezing. If these trees didn’t shed their leaves before the temperature fell to freezing, the cells in the leaves would actually rupture and the leaves would become useless anyway. 

How do deciduous trees know when to shed their leaves?

You might think that leaves are blown off in the wind, but actually, this form of abscission is a very complex process. 

In temperate regions, as the days get shorter, chlorophyll production ceases and the leaves begin to change colour – this is when we start to see those fantastic red, orange and yellow shades. 

Then, the vessels that carry nutrients and water to the leaves are closed off and a layer of cells (the abscission layer) begin to grow between the leaf stalk and the twig. This layer eventually parts the leaf from the tree without leaving an open wound. 

Why do some deciduous trees forget to shed their leaves?

Well, remember that word I mentioned earlier: ‘abscission’? There’s an opposite to this, and it’s called marcescence. This is where a tree or plant keeps the parts of it that are normally shed during a particular season.

Some deciduous trees intentionally undergo marcescence, such as pin oak (Quercus palustris) which completes its abscission in spring – much later than many other deciduous trees.

Pin oak in autumn – By Famartin CC BY-SA 4.0

Some trees, however, experience marcescence accidentally. This can occur after an early frost – the leaves die, but the abscission layer never forms, and therefore the tree keeps its rather ugly dead leaves.

So wtf does ‘semi-deciduous’ mean?

Let’s make this a little more complex by throwing semi-deciduous trees into the mix. These are trees that lose old leaves as new leaves begin to appear.

You may also hear the term ‘semi-evergreen’. This is usually when leaves are shed before the next growing season, but some are retained during the winter or dry periods (again, depending on climate), giving the appearance of being an evergreen.

Some examples of deciduous trees

What would you like to learn about next in the WTF Gardening series? Let me know in the comments below!