You state that "only the bottom of the trunk survived". Are you confident that its not all rootstock?

During the first years, there is no need to trim at all. Research has shown that any cut to a young tree reduces root growth. This is due to the equilibrium between the foliage and the root system. A citrus tree is a biological unit. Cuts on a young tree discourages canopy growth and delays the tree from coming into bearing. After a few years, depending on the speed of tree growth, climate and cultural practices, a selective canopy thinning may be carried out. A harmonious citrus tree grows to an almost round shape (globe). Unpruned trees come into bearing quicker, and yield crops for many years.

@molewacker I wasn't confident it wasn't root growth but the experts on this forum suggested it was still the original tree. When I say the very bottom, I meant below the original branches. They start around 6 to 8 inches up.

@Silica The tree is probably about 4 years old now. I've had no fruit this year (I wouldn't have expected any after the freeze) but I had 7 or 8 lemons on each tree the year before. I had already trimmed one tree but I'll leave the other ones as they are. Thank you.