why do pine trees have needles instead of leaves

Needle leaves are very narrow, so they don't have a great deal of surface area to expose to the sun. Since plants generate food through photosynthesis, a process that converts light into energy for the plant, narrow leaves might not seem like a great tool for accomplishing this. However, because needles tend to grow in masses, the actual surface area exposed to the sun is greater than it seems. Also, evergreen needles can photosynthesize in winter, if necessary, and have a greater amount of time over the year in which to gather energy than trees that lose their leaves.
Coniferous forests do indeed contain narrow pointed leaves.

By reducing the surface area of the leaf(as compared to large broad leaves as found in a tropical rainforest), there is little water loss due to transpiration through the stomates found in the needles. Because the ground is frozen in the cold, northern forests in winter, the ability to absorb water by the treeВ roots is diminished.

Being able to retain whatever water is inside the tree is important during the months when moisture isn't readily available. Grasslands are fairly dry most of the year. Again, reducing leaves to the size of a blade of grass cuts down on water loss when droughts or dry periods occur. In temperate deciduous forests, rather than have needles like in coniferous forests, the leaves are rather broad, as in oaks or maples.

However, these fall off during autumn thus preventing water loss by transpiration. This keeps the tree alive until the spring time, which brings rain again. In a tropical rainforest, because it may rain several times a day, every day, water is not a limiting factor. Therefore, most tropical plants have very large, broad leaves. These contain many stomates and while a lot of transpiration occurs, the water that is lost, is quickly replaced whenever it rains again.

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