Why do leaves fall off trees
We generally tend to associate falling leaves with autumn, but in truth, it is actually a daily process that goes on throughout the year. Of course, it is just before the onset of winter that most trees shed their leaves completely; it is during this time that the phenomenon becomes more visibly obvious to us. In the hotter months, the trees shed very few leaves and even those are the ones which have lived their quota. As these old leaves fall out, they are soon replaced by fresh new leaves to continue the process of photosynthesis for the tree. The shedding of leaves during autumn months are however a different process, carried on by the plant for a separate set of reasons. The main difference in the two processes is that this time around, the fallen leaves are not replaced by new ones and the tree starts to seem more and more barren with each passing day.
It is to be noted though, that all trees do not shed their leaves during winter. Trees that keep their leaves throughout the year are known as Coniferous trees (pine, hemlock, spruce, fir, etc.). The kind of trees that we see on a regular basis, shedding their leaves during colder months is known as deciduous trees (maple, oak, birch, aspen, etc.); the ones with broad, proper leaves. In fact, it is the structure of their leaves that decides primarily whether a plant will shed its leaves or not and we will elaborate on that now.
Why do deciduous trees shed their leaves?
The primary reason behind the phenomenon is adaptation for survival. As autumn is followed closely by winter, the trees prepare for the cold air and the lack of sufficient sunlight through various internal processes and shedding the leaves is one of those processes. What the deciduous trees do is they close the pores near the node of the leaves in order to stop any kind of fluids from getting out of the tree. As a result of this, the leaves cannot remain functional and eventually fall off. The entire process of photosynthesis involves losing quite a bit of the water that the tree acquires from the ground and therefore, this is a way to conserve water for the harsh, cold and dry winter months. If these trees kept their leaves on even during the winter, they would not survive.
The change of color
The changes in color as is seen during the autumn months are a result of various chemical reactions that occur inside the leaves brought on by a lack of sunlight and water. As the green chlorophyll becomes invalid at this time, it is replaced by anthocyanin; the pigment responsible for the majority of the various colors in leaves that we are used to seeing during the autumn months. The anthocyanin also serves an actual purpose, the purpose of giving the leaves an increased ability to tolerate freezing temperatures. The increased resistance to frost gives the plant some extra time to prepare itself before all leaves are lost and it completely shuts down. Other pigments responsible for affecting the color of leaves in absence of chlorophyll are carotenoids and xanthophyll.