Keep conifers green in winter

Question

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.

Answer 1

My superficial understanding of the machinery of photosynthesis is that the role of carotenoids is to protect against the destructive effects of light and reactive oxygen. The yellow coloring xanthophyll, in particular, combats reactive oxygen.

Photo system 2 (PSII) captures photons to hydrolyze water, creating reactive oxygen in the thylakoid lumens. The rate that this happens is controlled by the photon flux (light intensity) and does not depend on temperature. The rate at which the biochemical processes controlling the reactive oxygen levels, on the other hand, decrease with decreasing temperature. There are only two possible ways to head-off destruction of the thylakoid membrane:

1. increase the ratio of xanthophyll/chlorophyll (i.e., turn the foliage color toward yellow)- photobleaching that you mention is one such way to do this
2. shade the PSII light-harvesting complex (i.e., reduce the incident light level)

So, shading your trees is the best way I can think of to keep them green through the winter. I speculate that blue to almost white coloration of spruce and ‘candy cans’ white fir, for example, is because of a layer of cells that reflect sun-light, shading PSII.

As best I know, the thylakoid are contained in cells that are a few cell below the needle/leaf surface. I think this would explain why your conjecture about needle ‘thickness’ fails. Your observations may, in part, be related to needle lifetime. For instance, I know that white pines have a short needle life of only 2 seasons (last year’s new needles will be dropped in the fall next year - I think this occurs because the xylem connections are lost). As such, it seems to me that the investment in maintaining the chlorophyll would not return much benefit; hence these species would be prone to photobleaching. It would be interesting to contrast the propensity for long lifetime needle species such as mugo pine to yellow in your climate.