The Open Door Web Site
Part XVIII: Energy and Activity : How Plants
A forest in winter
Winter poses serious problems for plants.
A deciduous wood in winter
ENERGY AND ACTIVITY
Trees in Winter
A large part of a tree's body is found above ground (the branches and the trunk). These parts need to withstand the cold winter air. In winter water freezes to ice. The trees cannot absorb water, therefore, transpiration becomes impossible. Trees can be divided into two groups: deciduous trees, which lose all their leaves in autumn and evergreens, which maintain leaves on their branches throughout the year.
A holly branch with berries
Evergreen trees, such as holly, usually have leaves which are covered in a thick layer of wax to stop them losing too much water. Conifers, such as pine trees, are evergreens which have long narrow leaves. This shape of leaf also helps to reduce water loss.
Pine trees showing their needles
The sap of trees which live close to the Arctic is also special. In winter their sap becomes rich in sugars. The sugar stops the sap from freezing, in other words, it acts as an antifreeze. In the sugar maple the sap is particularly rich in sugar. In North America the sap of the sugar maple is collected for food.
Deciduous trees lose their leaves in autumn but the branches already have buds which contain next year's leaves. It is very important that the buds do not break (open) before winter is over or else the young leaves will be damaged. The leaves are needed for photosynthesis, so if the leaves are damaged the tree will starve.
Just like seeds and the root-storage organs, the buds of trees can "tell" when winter has passed and spring has arrived. Some trees, such as birch, will only open their buds when there is a certain number of hours of daylight. Other trees need to pass through a cold period before the buds open. For example, apple trees need to be kept below 7°C for 6 to 8 weeks before their buds will open.
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