Why Do Trees Lose Their Leaves in Autumn?

As days shorten in autumn, deciduous trees detect the declining daylight through phytochrome pigments and begin a controlled process called abscission. Trees form a layer of cells (the abscission zone) at the base of each leaf stalk that gradually seals off the connection. Before dropping, the tree reabsorbs valuable nutrients (nitrogen, phosphorus) from the leaves. Chlorophyll breaks down, revealing yellow and orange carotenoid pigments that were masked by green; red and purple anthocyanins are produced freshly in some species. The leaves then fall, reducing water loss through transpiration and protecting the tree from the weight of snow and ice on broad leaves.