Trees lose their leaves in the winter season in cold areas. The reason for this is to save energy in the unfavorable climate which prevents the tree from damaging. It starts breaking the organic compounds and the valuable nutrients present in it which are again absorbed by the trees from the leaves so that it can be used when the next growing season will come. Phosphorous and nitrogen are the nutrients that are the absorbed from the leaves.
Leaves are the very important and vital part of the tree but during the unfavorable climate it becomes difficult to manage the water level because less amount of free water in the soil is available during these conditions moreover leaves have large surface area and the dry winds take more moisture from the leaves which will make difficult for a tree to survive. So the trees cannot keep their leaves turgid and it starts losing leaves to save the moisture. Near the petiole has a layer of cells is formed. It releases some particular enzymes which can break cellulose of primary cell and pectin of middle lamella easily. Finally the cells in the leaves get parted causing the leaf to fall from the tree. Tree goes into dormant period. In this period very less energy is required by the tree to survive.
During summer days which are long and the leaves get plenty of sunlight for photosynthesis but in winter nights get longer and less of sunlight is available for photosynthesis so leaves will take energy from the tree which will make it hard for the tree to survive.