I live in zone 3, central Alberta. Temperatures are well below freezing for 4-6 months of the year, with cold snaps lasting weeks between -20 and -35 C.
I have noticed that some conifers go quite yellow in winter. One cause of this: Once water transport has shut down, the tree cannot repair the damage when chlorophyll is split by UV.
If this were the only cause, then I would expect it to be most prominent in thin needled trees such as eastern white pine, bristlecone pine, and swiss stone pine, and less prominent in the much thicker needles of the two needle pines.
Overall the worst yellowing occurs in Scots, Lodgepole, and jack pines. The least in mountain, ponderosa and korean pine.
Balsam fir and concolor fir seem unaffected.
Among the spruce, colorado (both green and blue) are not affected much, while white is the worst. Meyer's and Black Hills and black spruce are intermediate.
Is there a way to reduce or eliminate the amount of yellowing of conifers in cold climates. I am looking at either a fertilizer/watering regime and/or a spray to reduce UV damage.