Why Do Trees Have Different Shaped Leaves?

There are over 60,000 different species of trees in the world and hundreds of different shapes of leaves – but why the different shapes?

Broadleaf Leaf Types

Broadleaves can be either simple or compound.

Simple Leaves

There are some 25 different shapes of simple leaves. Simple leaves are whole and are not divided. Their edges may be different; smooth such as beech, toothed such as lime (see image), or lobed, such as hawthorn.

Compound Leaves

There are 4 different shapes of compound leaves. Compound leaves are divided right up to the central vein into separate leaflets. They are either pinnate, such as ash (see image), elder or rowan, or palmate, such horse chestnut. Pinnately compound leaves are feather-shaped where leaflets are attached in pairs along the central vein. Palmate leaves have three or more leaflets, or lobes, radiating from one point.

Conifer Leaf Types

Conifers have either scale like leaves or needles.

Needles can be different shapes and sizes and can be arranged differently on twigs. Needles can be arranged singly, such as Douglas fir, in clusters, such as larch, in pairs, such as Scot’s pine, in threes, such as Monterey pine, or in fives, such as White pine.

Arrangements of Tree Leaves

Tree leaves are either arranged as a ‘mono-layer’ or as a ‘multi-layer’.

In a mono-layer, the leaves are arrayed so that no leaf is above and therefore, no leaf is shading any other leaf on the tree. This arrangement is typical of trees that form the ‘shrub layer’ of a woodland i.e. trees that grow between 1.5m and 4.5m in height such as dogwood.

In a multi-layer, there are leaves above and below other leaves on the tree. This is typical of trees that form the light-rich ‘canopy layer’ in a woodland.

Why the Different Shapes?

The shape of a leaf is determined equally by its primary function of photosynthesis and by its immediate environment. A leaf must both absorb carbon dioxide and sunlight, whilst at the same time it must be able to restrict heat intake, minimise the risk of freezing and control water-loss – in other words, it is a balancing act.

The ‘Balancing Act’ and Leaf Shape

Trees growing in semi-arid (both hot and cold) and dry sub-humid environments typically have needle shaped leaves. These have a thick, outer cuticle coating and pit-shaped pores which restrict water loss, thus enabling conifers to grow in dryer soil conditions than trees with broad flat leaves. Smaller pores restrict water-loss but they also restrict intake of carbon dioxide – a balance.

Rounder leaves, such as beech or lime, are more efficient at absorbing sunlight but are also more prone to wind damage, hence rounder leaves tend to be smaller than other shapes of broad leaves. Larger leaves tend to be lobed or cut, such as oak or maple, because their shape is better able to withstand the wind – a balance.
Leaves at risk of being regularly frozen tend to be needle shaped. Needles have a reduced surface area hence there is a reduced risk of freeze damage. Leaves at no risk of freeze damage tend to large, such as the tropical palm, and can have surface area up to 1.0m² – a balance.

Summary

There is no one size fits all solution. The shape of a tree’s leaves is determined by its function and by its environment, it is determined by evolution and by perfection. A tree cannot afford to be less than 100% effective; if it is, it will simply lose out to the competition and become extinct. It is different strokes for different folks!

References: http://www.psu.edu, https://prezi.com, https://www.woodlandtrust.org.uk