Sun — The universal source of energy to sustain life on our planet, but only autotrophic, that is green, the plants can use its energy directly. However, the intensity of sunlight falling on a territory, not a constant in the Northern Hemisphere, the maximum solar radiation in the summer months, at least — for the winter. Its strength varies depending on the altitude of the sun and the atmosphere. Peak radiation during the day falls on the afternoon. If the tree is not always enough light, it produces less and less nutrients, and finally they become so small that it has to go to the harsh regime of economy: no longer laid out new leaves or needles, old leaves, twigs and sometimes die, lower the barrel is laid bare. Light is the lever that controls the life of the forest. That is why the forest life runs in cycles, in accordance with the seasons. Heat is though important but subordinate role, because trees can exist in a fairly wide range of temperatures.