The Fall Rainbow: Yellow

I grew up in Girdwood, Alaska, but I was 5 when we moved to southern California. In the years since moving south, I only ever visited in the summer, and so had forgotten what a fall in the great far north is like. Well this past week, I had the chance to travel back to Alaska and timed it just right with the tail end of peak fall foliage. And wow, what a shocking reminder of the stunning, stark palette that paints the boreal forests at the dawn of winter. While hidden amongst the conifers in the summer, stands of balsam poplar, quaking aspen, and Alaska paper birch (Betula neoalaskana) pop out of the hillsides, singing a mournful, though beautiful chorus in celebration of or preparation for the long cold months ahead.

In the previous newsletter, I mentioned that the yellow and orange colors in fall foliage come from a class of photosynthetic pigments called carotenoids that (unlike anthocyanins) are present in the leaves during the entire growing season. They are one of three photosynthetic pigments found in all plants. Carotenoids play a small role in protect the leaf from UV radiation, but primarily serve to expand the range of sunlight absorbed by the chloroplast (see image below for the absorption spectra for the pigments). As the pigments absorb sunlight, the energy is used to split water and precipitate a chain of reactions (called the light or light-dependent reactions). The two forms of chlorophyll (a and b) are significantly more abundant in chloroplasts and so they mask the yellowish pigments. 

In the fall, chlorophyll production slows down and the leaf begins to deconstruct itself to retrieve nutrients and energy. All three photosynthetic pigments are deconstructed by the leaf, but it takes longer for the leaf to break down carotenoids than chlorophyll a and b. As the two greenish chlorophyll pigments break down, so too does the green coloring of the leaf drain out, unmasking the underlying yellow carotenoids. You can see this process on a smaller, less dramatic in the summer when deciduous structures like the stipules at the base of American elm leaves are shed.

The intensity of the yellow can be due to the presence of anthocyanins (which grade the yellow into an orange). Anthocyanins are produced primarily in response to sun exposure. So in the striped maples I found growing on a cliff exposure on the southern slope of Pease Mountain (image below), the leaves were turning a rich orange yellow. Another striped maple growing on the northern slope under the thick shade of sugar maples and red oaks, the shrub took on a significantly paler color (see the second photo of striped maple below). The leaves on understory species like striped maple are quite thin, with just a single layer two of photosynthetic cells in the palisade mesophyll. The greens disappear and the without any anthocyanins and only a thin layer of carotenoids, the leaves can be quite bland.