I don't have any trees that need wintering yet, but since I only have access to a balcony I've been thinking about how it might be done before I get any .
For one thing I thought creating a tent out of plastic against the balcony door.
But today I saw heat mats for seed propagation and thought that might provide the extra few degrees that keep the soil & roots from freezing.
For trees that don't need light, maybe a cooler with a small light bulb for heat might work too.
The the average low temperature here is around -10C, with the lowest extreme being -32C (in 1981).
Any thoughts, input?