Image above: Branches of white spruce (Picea glauca) in a treeline study plot south of the Alaska...
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Image above: Branches of white spruce (Picea glauca) in a treeline study plot south of the Alaska Range on June 30, 2013. The newly expanded branch tips have a bluish color compared to older needles.
It's still spring at treeline in Interior Alaska (fall starts in a couple of weeks), and this year's spruce needles are not yet fully formed. The current crop of needles is very bluish compared to older needles. I think this is because of their protective layer of waxy cutin, but it might also be because the photosynthetic pigments inside the leaves are not developed yet. I was hoping an infrablue photo might shed some light.
Image above: Infrablue photo of the same white spruce branch. Photo is from a modified Canon A810 with Rosco #74 filter which was custom white balanced on blue origami paper in the sun.
Image above: The red channel of the infrablue photo which records mostly near infrared (NIR) light. Lots of NIR light is being reflected from both new and old needles.
Image above: The blue channel of the infrablue photo which records mostly visible blue light. Blue light is used by leaves for photosynthesis, and more of it is being reflected from the new needles than the old. We can therefore infer that more blue light is being absorbed by the older needles and being used for photosynthesis.
Image above: Infrablue NDVI of the spruce branch. New needles are clearly distinct from older needles which have higher NDVI values. These NDVI values highlight the difference between the blue and red channels, so the distinction is due to the greater reflection of blue light from the young needles. This is consistent with higher photosynthesis in the older needles.
Image above: False color infrared image from infrablue photo. Infrared light is displayed as red, and the new needles are not very red. They are bright because lots of blue (and probably green) light is being reflected from them.
So we can conclude that the bright, bluish needles on newly expanding spruce branches are not photosynthesizing as much as older needles on the same branch. Photosynthetically active radiation is not being absorbed by the young needles as much as by older needles. I still don't know why that is, so maybe this didn't shed much light after all.
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