My Late Father told me a few years ago that trees continue to grow trough winter,not in lenght but in roots and thickness of trunks.
At the time i discounted it as a here say,but I notice my Grape vine did get a lot thicker recently,and I have a fig ,143x 36,that positivelly got thicker compared to last Sommer trunk diameter.
Please comment,or explain if you know more about it.
Dr Tapla is needed here???

It was always explained to me that it works like a see-saw with established plants:  The faster the leaves and branches are growing, the slower the roots will grow, and vice-versa.

This is the reason most grasses for yards are planted in the late fall (at least down south), and why fruit tree growers ship their plants in fall - this way the plant has a chance to work on a root base before top-growth begins in spring.

Indeed, when plants enter dormancy on top, there is a lot going on under the soil that most folks don't realize.

I don't know about trunk growth, but I have noticed that the two grape vines I have on my back deck seem to "fatten" during the winter.  I don't know what this swelling is caused by.

Herman2: I know this effect from my figs in the ground in the greenhouse. I think it has to do with the moderate temperatures.

That would explain then to why it is a good idea to water the stored figs from time to time. There's actually some activity going on.
This year I stored all my fig trees in my attached garage and just this morning I glanced at some of them and they showed signs that are active.

I actually saw some baby figs ( about the size of a green pea ).

Hopefully I don't lose any fig trees this year but from what I am seeing it's pretty encouraging that the loss would be at a minimum if any.

Herman2
Very good question and hope tapla also provides some input.

My other guess is that the plants may have some natural gates and switches activated by ambient (low/high of light and temperature) for making decision as to which growth can be sustained without sure death and which one can survive so it continue those operations which provides it return on its investment. Just my guess. 

JMO, but in all but the most arctic areas the bulk of the root mass of most plants should have access to soil below the frost line. It would stand to reason in with my limited lack of knowledge that there is crucial activity occurring in those roots. The plant itself still has to live and like most things in nature has a plan/method for ensuring survival. From what I have seen it is very rare for anything in nature to be truly static or non functioning even if appears to be so to us humans who are creatures of constant activity. Remember the plants that we grow often have lifespans measured in multiples of our own, they can afford to be more deliberate in their activity.
Again JMO but I believe the root system during the winter play a role in storing energy in the form of starches and other carbohydrates to include moving these stores from one area of the plant to another in preparation for re establishing vegatative growth.

Vasile, I have read many times that although frost causes the leaves of deciduous plants to drop the roots keep growing far into the winter.  Fall, after leaf drop, is the preferred planting time here for many plants.  I also read, even on this forum, that roots will grow in colder temperatures than will the upper parts of a tree.  

I read in the local gardening column that we should not let our deciduous plants get too dry in the winter.  It seems that even though leafless they use a little water.  Too little water and they get freeze damage.  (I do not know how to explain that.)

I do not know about the increase in trunk size.  I have never read anything about it, nor have I observed this phenomenon.  It is easy to check though; next year I wil put calipers on a couple of trees in the fall, after leaf-drop, and again about the middle of march, well before they leaf out. 
Ox

That is the question:Is the trunk and branches getting thicker over winter?
My father said:Yes
I knew the roots continue growing,if soil warm.
H2

I wonder if the increase in caliper is due to movement of fluids (starches/carbs) that cause vascular/cellular swelling or is it an actual addition of new cellulosic growth (woody material).

Warrior i think thats a very good assumption.
Personally i never measured my trunks girth to see if it actually grows but i think it would along with roots doing something underneath the soil that we do not see , i water my very sparingly during the winter months and think that the roots are drinking it albiet very slow but still doing something. From what i have read in past on forums the so called antifreeze moves upward into tree limbs as a safety mechanisim in cold months . This is why i personally believe the older and thicker the trunks wood the better chance of winter survival up to a point maybe just more antifreeze i call it.

Warrior:I like your input,and thanks for posting.
H2