I had started air-layer on a 7-ft tall single trunk fig plant at mid-height to make two out of it (wind was always toppling it). There are plenty of roots for a number of weeks now but I decided no to separate the two now but store it in a cellar as is, and separate the air-layered branch in spring. This will save me space of one more pot in the limited space in cold storage in the basement.

Any downside to this idea of postponing the separation of air-layered branch until spring such as any kind of damage to the new roots?

Will it be better in the spring to separate when still dormant or when the plant start waking up?

With some sort of protection you are doing, I am quite certain it would be OK to leave them as is. Well, 2 seasons ago I air layered a few fig trees and one of them was an in-ground tree. I never separate them till the following spring and they all grew happily.

The only issue I see is keeping the roots of the airlayer moist, even if dormant.

leon_edmond's post remind me of a question I have been having in my mind. The question may seem dumb but I will ask anyway and clear up my mind.
The question is if the moisture and nutrient transfer in roots is uni-directional i.e. moisture and nutrient always has to flow from the roots to the plant so in cases where there have been good roots developed in the air-layer ball and then the air-layer soil ball dries up then the roots will dry-up too (assuming no reverse supply from plant to the drying roots) and die even if there is no bark ring cut for initiating the roots and the pot of the mother plant is happily supplying moisture to the mother plant.
Crazy question?

I have found if the mother tree is under stress, the attached air-layer (and the limb beyond) are more likely to suffer that stress.  I had one tree with four layers go into shock.  The mother tree and all limbs survived with the exception of the four air-layered limbs.

Also, if the air-layer is not taken off in a timely manner, the roots will dry out as Leon eluded to.  If the layer is not desired, one can remove the rooting medium and let the roots dry out and fall off.  The girdle will heal, but will leave the tree susceptible to invasion. (I'm not sure if  this answers your question)

If it were me, I'd remove the layers and pot them up.  While the top is dormant, I'd keep the root zone warm and moist.  This will give the roots time to establish themselves in the container before the top breaks bud in the spring.

~james

Thanks all for the inputs.

James
Very good observations to keep in mind when dealing with air-layering. Probably the situation may be somewhat different for in-ground layering.