I don't know why the frost on the conifers is more rare specially in the mornings like on that day I shot the picture (clear sky and absolute calm). Perhaps it has something to do with the specific heat capacity (if it differs ??) or do the coniferous trees have some icing inhibitor, I don't know. This is the way they often behave .
That freeze-fog is partly a photo-chemical reaction but it is in the air not on the branches.