Why do trees loose their leaves in autumn?
Trees shed their leaves in autumn as a survival strategy in a process known as abscission. During this season, daylight hours decrease, which limits the amount of photosynthesis and, therefore, the energy the tree can produce.
By dropping leaves, the tree conserves energy and water that would otherwise be used to maintain these structures. Additionally, without leaves, trees are better able to withstand strong winds, which would increase transpiration (the process where water evaporates from the leaf surface). Since water is harder to absorb in colder weather, losing leaves helps prevent excessive water loss, helping the tree survive through winter.
What causes tree leaf colour to change?
In autumn, trees change colour as part of a resource-conservation strategy. Building leaves requires significant energy, so to avoid waste, the tree “recycles” nutrients from its leaves before letting them fall.
The tree breaks down chlorophyll, the pigment responsible for green colour, and retrieves valuable nutrients like nitrogen and phosphorus, storing them in its woody parts until spring.
As chlorophyll fades, other pigments become visible:
- Carotenoids, responsible for orange and yellow hues.
- Anthocyanins, which create red and purple colours. These may help protect the leaves from sunlight and assist in the breakdown process.
Through these adaptations, trees recover up to 50% of their nitrogen and phosphorus, resources they’ll need to grow new leaves in spring.
Why are leaves more colourful some years than others?
The intensity of autumn colors varies significantly from year to year, largely depending on weather conditions. The most vibrant displays typically occur when specific weather patterns align:
- Dry, sunny days encourage sugar production in leaves
- Cool nights (above freezing) help trap these sugars in the leaves
- These trapped sugars promote the production of anthocyanins, intensifying red coloration
- Calm weather helps leaves stay on trees longer, extending the colorful display
