Why do leaves fall/die only on autumn?

Because the leaves are where the tree loses valuable moisture and minerals etc. In order to survive the winter the tree loses its leaves so as to avoid unnesesarily losing nutrients.

http://www.na.fs.fed.us/spfo/pubs/misc/leaves/leaves.htm It's a two step process: first increasing night hours causes the chlorophyll production to slow down and stop. The green fades revealing the under colors which were always there but hidden by the green. These colors are caused by carotenoids and anthycynins....amount of each determines color of autumnal leaf. Then the veins that conduct water and minerals into the leaves and food back out to the rest of the plant begins to grow a corkly layer shutting off the vein. When this layer is complete, the leaf falls. You can see the scar left behind on some tree species. Sumac and tree of heaven are two excellent examples of deciduous leaf scars being large enough to see in winter. There is no mass migration of sap to the roots. If it happened, tree roots would swell up and crack the ground, streets and sidewalks. The liquids within the tree becomes thicker......concentrating sugars, much like antifreeze. But they stay in the limbs and trunk........and the roots.

Because during the autumn, the day will be shorter than usual (the day is less than 12 hours per day). therefore, it's hard for plants to produce enough food through photosynthesis. So, plants "kill" their own leaves in order to have less cells to be fed.

Because the days are shorter, there is less available sunlight for plants to carry out photosynthesis. Photosynthesis is the process by which green leaves and plants make food for themselves, and it gives them their green color (from chlorophyll). With less sunlight, the plants make less food and lose the green color. You get red, orange, and yellow, depending on the type of tree it is.

They are going into a dormant stage. Like you need sleep to rejuvenate, they need this time to replenish. It has a lot to do with the sun being at a further distance. The sun helps produce the agent they need for leaves to be green and fruit to grow and ripen. There is beauty in this stage of the season also.

They drop their leaves to prepare to be dormant. That way the energy only has to focus on the root system.

google.com/autumn its cold they drop there leaves so they can meeet in the bark talk over a nice cup of hot chcolate :-) while we freeze to death:'-( they look out through holes of the tree and make fun of us they open the doors to welcome hibernating animals and they all laugh at us not sleep during winter!!!

I would agree that since light levels are much lower, and temperatures are colder, there's little reason for many trees to keep their leaves in the fall and winter. Their metabolism would just be too slowed down; they may as well just rest. There's at least one other consideration, though, for plants that live in cold areas. Ice or snow accumulating on a tree can obviously lead to broken branches--if you've ever been through a bad ice storm you know what I mean. Now if the tree has leaves on it, there's a lot more surface area that the ice and snow can accumulate on. The tree can suffer a lot of damage--without leaves much of the snow or freezing rain will just slide off. An additional thought: a test of all these ideas. If losing leaves in the fall makes sense with cold and and short days, what about the subtropics and tropics, where the days never get too cold, and light is more constant? Are there fewer deciduous trees? And of those that are deciduous, are they not tied to winter and fall (maybe dry season instead). Personally, where I live (Southern California) there seem to be a few things that are unusual. A couple of our most striking subtropical flowering trees--Jacaranda and Chorisia (silk-floss tree) loser their leaves at odd times. The Jacaranda does't lose them until late winter, then regrows them around early June. The Chorisia looses its leaves--but only briefly--in fall. I also have a Plumeria that loses its leaves like clockwork in late March. Except for the Plumeria, these are normal times. However, there are a lot of plants from colder climates that get really screwed up around here. .