Wednesday, January 20, 2021

50 Shades of Brown

Yes, yes, that title is a reference to the 2011 erotic romance novel “Fifty Shades of Grey,” which went on to spawn four more books, three movies, two song albums, a parody movie (“Fifty Shades of Black”), and a guest appearance as a book club selection in the movie “Book Club.” But enough for a belabored frame of reference – this column will have no sex scenes, no sadism, no masochism, no bondage, no dominance, no submission. Instead, it has leaves. Lot of leaves. In winter.

As to why leaves turn brown, that is more of colors departing than brown being added. Spring to summer to fall, each leaf is in a dynamic state. Leaves are an energy factory, continually synthesizing more chlorophyll so as to convert sunlight to sugar. Toward the end of our temperate zone growing season, as days shorten, chlorophyll synthesis stops. Green fades. In school, we might have been taught that the yellows and oranges and reds and purples of fall leaf color were always present, only becoming visible with the loss of green. Current thinking is more complex. Plants need time to withdraw sugars and carbohydrates from leaves, to be stored in roots over winter. More of the color compounds – anthocyanins – are made in leaves to protect against ultraviolet damage, i.e., sunburn, while nutrients are being salvaged.      

Another part of the leaf life cycle is “abscission,” the means by which leaves fall from trees. At the time of each leaf’s creation there is a specialized layer of cells where the leaf stalk joins the stem – the abscission layer. As days shorten, a tree hormone, auxin, decreases. This triggers a structural weakening on the abscission layer. Leaves are blown off by wind, or even on a quiet morning, can detach by their own weight. A branch broken in summer will have leaves that go straight to brown without intervening color, and the leaves will stay attached into winter.

Oak leaves on a fallen tree that turned brown 
but did not fall off. Click to enlarge photo.
There are a few species of trees that hang onto their brown leaves all winter: American beech and some of the oaks. Preternaturally retaining leaves in winter is called marcescence. As to why it happens, one theory is that beech trees do this, especially on young trees and lower branches of older trees, to discourage deer from feeding on the tree’s twigs and buds, which are a high-protein food source. Another theory is that not dropping winter leaves until spring serves to self-mulch the trees against competing plants trying to get started close to the tree trunk.

Back to brown. The major constituents of winter’s brown leaves are cellulose, hemicellulose, lignin, pectin, tannins and mineral content. The percentages of each dictate leaf shade from the pale brown of beech to the rich, dark brown of oak. Falling in between are ash, poplar, maple, birch, pine, hemlock and the various understory plants. Looking downward when out for a woods walk will reward you with myriad shades of brown.   

Winterberry berries, on ice
There are also accents of winter red. When berries go from green to ripe colors, the term is “veraison.” Same for grapes and many types of fruit. Two competing theories here – either these fruits are synthesizing polyphenolic compounds, which just happen to be colorful, to combat fungus growth, or the plants are signaling to animals that the fruits are ripe, and therefore OK to eat. Thinking for the latter concept is that by enclosing seeds in edible fruits, the plants are borrowing the animals’ digestive tract and wandering lifestyle to disperse seeds far farther than just falling to the ground. There is a name for this: “endozoochory.” Mammals and birds are the majority of seed-spreading participants, although plant-eating reptiles such as turtles can be involved.

The red we spy on winter walks included rosehips on Multiflora rose and berries on Burning bush, Japanese barberry, Oriental bittersweet and Winterberry. That is four invasive species and a native, respectively. Several bird species and small mammals will eat the rosehips, burning bush, and barberry, but the bittersweet and winterberry stay around until found by robins, either those who decided to not migrate or the north-migrating birds of spring. With the exception of barberry, none are considered people-edible, but rather mildly poisonous.        

European barberry is used to make jam. In Persian cuisine, barberry is “zereshk”, and “zereshk polow” is a traditional barberry and rice dish. While Japanese barberries are considered edible, strong reasons to why not bother to collect any include a need to separate fruit from seeds, nasty thorns, and the plants being a preferred habitat for deer ticks.  


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