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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.

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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.

  • Good answer. Thanks. In passing: The 'blue' of blue spruce, and concolor fir isn't actually pigment but nanoscale beads of resin on the surface of the needles. Smaller beads = bluer. More beads = whiter. As the needle ages the beads wash off, so last year's needles are lighter coloured. – Sherwood Botsford Jan 20 '16 at 22:51
  • This doesn't explain the differential between species and genuses. Fir, with similar thickness needles and hemlock both maintain winter colour. Pines have the largest variation within the genus. Spruce show a very pronouced difference between P. glauca and P. glauca var densitata. By this model the north side of the tree should be substantially greener than the south side. I will check on this. Certain cultivars also seem to more resistant. E.g. Lake Superior Blue scots pine, compared to seed run from the Ukraine. Another source claimed that the greener cultivars weren't as cold hardy. – Sherwood Botsford Jan 20 '16 at 22:57
  • I have had or observed cultivars that are desired because of their yellowing traits: p. thunbergii 'Ogon', p. contorta 'Chief Joseph', and p. strobus 'Louie'. They exhibit seasonal variation in their degrees of 'yellowness', but all are greener where shaded that is especially noticeable in winter. I agree that my superficial understanding has lots of gaps - most notably p. ponderosa to me. There must indeed be more to it that I don't (yet) know. Very interesting question you've posed! – Jim Young Jan 21 '16 at 1:07
  • BTW, @Sherwood Botsford, thanks for the nano-bead info RE blue spruce and concolor fir. – Jim Young Jan 21 '16 at 3:51

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