Each autumn, trees across the Northern Hemisphere turn various shades of brown, orange, yellow – and even a regal purple – while in the Northern Tropics trees shed their leaves at the start of the dry season (October-March) – but why do trees lose their leaves – and why do trees have leaves in the first place?

All Shapes And Sizes

Why do trees have leaves? For a number of reasons including: gathering sunlight for photosynthesis and taking in oxygen & expelling carbon monoxide, transpiring water which helps bring up water from the roots which in turn helps draw nutrients from the soil to the required parts of the tree.

Why Trees Shed

Why do trees lose their leaves? Again – this is a multipurpose function the tree has in order to protect itself in colder weather:

  • Unlike evergreens  that live in warm, wet climates, many trees have leaves that are vulnerable to damage in dry or cold weather (with a couple of notable exceptions). These are known as deciduous trees, a word from the Latin meaning “that which falls down.”
  • Leaf loss is also thought to help the tree conserve moisture in the trunk to keep it from drying out.
  • It also helps it to conserve energy throughout winter that would otherwise be spent on leaf production.
  • Shedding is also thought to help trees to pollinate in springtime – as shedding creates extra space to allow pollen to travel longer distances and reach other trees.
  • For a similar reason, it also allows the wind to flow through the trees, easing the strain on branches caused by gales and winter storms.

Why Leaves Change Colour

To begin with, the tree takes in valuable nutrients from the leaves and stores them in it’s roots, woody stems and significant branches – one of the first being chlorophyll, which gives leaves their green colour, creating the autumn colours we see around us every year. Once the leaves are shed, a protective layer of cells known as the abiscission layer develops over the exposed area, protecting the tree.

As the colder weather draws in, a hormone in the tree (auxin) starts a process known as abscission (another Latin word meaning “to cut”),  where the leaves are essentially cut off from the tree using specialised cells. As the weather becomes colder the pressure, auxin reduces and the strain on the abiscission layer weakens the bond between the branch and leaf, causing it to drift to the ground.

To learn more about tree care, contact your local tree surgeon Manchester, who can also advise on carrying out an arboricultural impact assessment to determine the impact of proposed building work on surrounding trees.