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Asked by to Sarah, Izzy, Clem, Amelia on 18 Mar 2014. This question was also asked by .
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Isabel Webb answered on 18 Mar 2014:
Being evergreen or not is a choice of survival strategy by different types of tree. Trees that lose their leaves in the winter do it to save energy. There is not enough water around in the winter, and it is too cold to be able to survive. The trees opt to lose their leaves , since leaves are where water is lost from the plant. It is also darker in winter, so the leaves wouldn’t photosynthesise efficiently enough to be worth keeping. They stop producing chlorophyll (which is the green pigment in leaves, so they go brown) and let the leaves die and fall off. This allows them to conserve energy over the winter.
Evergreen plants come up with a different strategy to cope with winter. They have darker leaves which have more chlorophyll, allowing them to photosynthesise more efficiently in the few hours of light they get. They also have different shaped leaves, such as needles, which limit water and heat loss due to their smaller surfaces.
These two strategies are just two different ways of survival that evolved at some point in the past. The two types both work in their environment, and so both types survived.
The only main difference in the chemicals is the amount of chlorophyll, and also evergreen plants often have thick waxy surfaces to reduce heat loss.
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Amelia Frizell-Armitage answered on 19 Mar 2014:
Many trees lose their leaves in winter because they can save resources and energy by doing this. Leaves are very expensive to maintain – they require lots of energy to be put into them to keep them healthy and working properly. In the summer time this is ok, because the leaves act as the energy factory of the tree converting sunlight to energy by photosynthesis. However, during winter when there are less hours of sunlight, the leaves are not able to photosynthesise for long enough to produce much energy and the tree has to put more energy into the leaves than the leaves can actually make. In this case it actually works out better for the tree to lose the leaves and save its resources over winter, and then re grow them ready for the following spring.
Evergreen trees that keep their leaves all year round usually live in environments where there are very low nutrient levels in the soil. This means there are simply not ever enough nutrients in the soil for the tree to lose all of its leaves in autumn and grow a whole new set in summer. Instead, the tree just replaces its leaves one at a time throughout the year whenever they fall off. This continuous process only requires a small amount of nutrients at any one time. Because of this the trees grow leaves that are much smaller, stronger, cheaper to maintain (like the needle-like leaves on a christmas tree), and live far longer than the leaves on non-evergreen.
I’m not sure exactly what chemical it is that tells a plant whether to lose its leaves or keep them. I expect the two types of trees have different genes, and also produce different hormones. There is probably a set of hormones that tell a plant when to lose its leaves that an evergreen tree doesn’t have.
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