Coniferous are a kind of evergreen trees which distinctive feature is existence of a "prickly" cover instead of leaves. Actually, these needles (or needles) are modified leaves which in the best way are suitable for continental and sharp and continental climate. But why green coniferous always?
Deciduous plants of times a year change the green cover, most often it occurs to the middle or towards the end of autumn months. It is connected with the fact that the root system of deciduous trees have no place to take nutrients to keep the green cover. Therefore adaptive processes come into force and trees dump foliage from themselves. But even in winter time the deciduous breeds of trees continue gradually, to collect on particles from the soil nutrients that in the spring again to blossom and start photosynthesis process.
Coniferous trees, such as fir-tree or pine, keep the green cover all the year round. But actually these trees change it throughout all the life, gradually getting rid of unnecessary needles when it is necessary for a tree. It is connected with the features of course of a metabolism inherent only in this kind of trees.
One more feature of coniferous trees is receiving all necessary nutrients for existence mainly thanks to photosynthesis, than deciduous plants cannot brag. The special appearance of needles allows it to consume smaller amount of nutrients, than to a usual leaf. Therefore at a coniferous tree the need for frequent change of a cover disappears, for its maintenance it is not obligatory for it to have feed from the soil at all. The special form of needles interferes with it to evaporate water that allows it to grow and to exist on a tree throughout long time. If a part of needles dies off, then on its place will grow new as the tree always has an opportunity without special expenses of nutrients to increase to itself new. Many deciduous breeds of plants remain green throughout all the life. But it is inherent only in plants which grow in tropical and subequatorial climate. The abundance of nutrients for all year is characteristic of the soil of those places therefore plants simply have no need to lose the foliage.