The transition from summer to autumn brings a remarkable transformation to numerous trees, as their vibrant green hues are replaced by striking shades of yellow, orange, and red. Subsequently, the leaves fall off the trees in preparation for the cold weather, leaving behind exposed trunks and skeletal branches.
Changing colors
Different chemicals in trees create different colors of leaves. The main chemical, chlorophyll, produces a green color and is essential for photosynthesis, which is how trees convert sunlight into energy. This is why most growing plants are green. During the summer, trees fill their leaves with chlorophyll to maximize energy production.
As autumn arrives, trees have already stored energy and sugars from sunlight for the upcoming winter. As temperatures drop, chlorophyll production stops, and the levels of this chemical decrease in the leaves. This allows other chemicals in the leaves to become more prominent.
With less chlorophyll, the yellow carotenes that have been present in the leaves all year finally become visible in autumn. If the temperature remains above freezing, anthocyanins are produced, resulting in reds and pinks. The drier and sunnier the autumn, the more anthocyanins are produced, leading to redder leaves. This process occurs gradually throughout autumn and varies within the tree, resulting in a mix of greens, yellows, and reds that constantly changes.
Shedding leaves
During spring, a layer of cells known as the abscission layer forms where a leaf grows from a branch. Throughout the growing season, the tree’s hormones and chemicals remain stable, and this layer enables the transfer of energy from the leaves to the branch and trunk for storage. However, in autumn, as temperatures drop and days become shorter, the production of a hormone called auxin decreases. As a result, the abscission layer weakens, causing the bond between the leaf and branch to loosen. Eventually, the join becomes weak enough for the wind to blow the leaf away.
Only a deciduous tree loses its leaves at the end of the growing season. Deciduous trees shed their leaves for several reasons:
It reduces the tree’s energy expenditure during the harsh winter.
It helps retain moisture within the trunk, preventing it from drying out.
It allows the wind to pass through the branches, reducing strain on the tree, which is particularly important during winter storms and gales.
On the other hand, evergreen trees keep their foliage all year round. Conifer trees are unique because they have needles instead of leaves. These needles are coated with a resin that helps them withstand cold and wet weather. The shape of conifers, tall and pointed, allows them to handle strong winds and heavy rain or snow by letting moisture slide off easily. Some conifers can even remove moisture from their cells to prevent freezing and continue growing. These trees are well-suited for cold and harsh conditions that deciduous trees cannot tolerate, which is why they are common in northern areas. Although most coniferous trees are evergreens, there are some deciduous conifers such as golden larch (Pseudolarix amabilis) and dawn redwood (Metasequoia glyptostroboides).
Sources:
https://www.thespruce.com/why-do-trees-lose-their-leaves-6826069
https://forestryandland.gov.scot/blog/trees-in-autumn#:~:text=In%20autumn%2C%20with%20the%20cooler,will%20blow%20the%20leaf%20away.