What environmental change causes trees to become dormant in the Fall.?

Question

Many organisms use biological clocks to "tell time." The timing mechanisms are triggered by changes in the environment and changes within the organism. what environmental change causes trees to become dormant in the Fall.

Answer 1

Even though momo already answered it

I still want to stress about - Moisture and Temperature.

Answer 2

Temperate trees enter dormancy when the whole tree carbon balance shifts. For growth to continue there must be more photosynthate (carbohydrate food) made than is used to support both the tree's basic functions (including roots below ground, the trees woody parts plus reserves for spring bud break). With less light and a decrease in temperature to between 5 & 10 C the tree struggles to support foliage. So there is a break-even point between available light energy and temperature that predicts actual canopy loss trigger point but the plant continues basal respiration. This is the first level of dormancy for regions with limited cold and is called ectodormancy.

http://www.cactus-art.biz/note-book/Dict...

However there are two kinds of winter dormancy. The first related to the plants current environment so the plant will grow but only when the the conditions are right, so it will be active in warmer spells but quiescent otherwise. But to insure this basic activity the plant will super-cool with antifreeze so the cells do not rupture with ice crystal formation. This is enough for plants alive in mild winter  but they will use the second form of dormancy if in zones with severe cold. In this they shut down and stay dormant despite warm spells. The plant tracks the amount of time above 32 F and below 50 F and does not respond or grow in warm temperatures above 50 until their necessary counted cold time is met. Species adapt to latitude by having different number of chill hours that must pass before they can break dormancy. The growth-inhibiting abscisic acid (ABA) is broken down by the cold over time so allows an increase in growth promoters like gibberellins, these are the biological processes interacting.

http://www.actahort.org/members/showpdf?...

Studies have shown plants lacking ABA lack the ability for dormancy, are extremely sensitive to water deficiency and have immature seeds germinating while still on the fruit.

http://www.bio.indiana.edu/~hangarterlab...

http://cat.inist.fr/?aModele=afficheN&cp...

Answer 3

Less moisture, less daylight, cooler weather.