Wednesday, March 21, 2012

Bud-break, Leaf-out and Leaf Colors

Written March 6, 2012, published in late March, 2012

By March, there are several signs that the new growing season is reaching the Pacific Northwest coast. Salmonberries break bud, and are in flower in sheltered areas. Skunk cabbage opens its distinctive large yellow flowers. Willows flower, first the hairy outer bracts––the pussy willows, then yellow anthers, followed by white stigmas. The first Rufus Hummingbirds arrive, more aggressive and much louder than the Anna’s Hummingbirds that over-winter here. With typical night temperatures above 40 °F, male Pacific Chorus Frogs, AKA Tree Frogs, call for babes. Brant flock on Willapa Bay in larger and larger groups, restless, leaping into the air as a flock more often, settling back down to feed more slowly. The first swallows and Turkey Vultures arrive, usually in mid-March. 

Leaf-out gets underway slowly. Alders and willows flower in February and March, and after flowering, open their leaves for the season. Red alders open leaves that are light green, then darken. Willow leaves vary from silvery green to gold-green. Big-leaf maples open both leaves and flowers at the same time, with a lovely yellow-gold color. Some years, the Willapa Hills have a golden wash as the maples start leaf-out. It’s startling against a backdrop of dark green conifer foliage. Cottonwoods have a nice gold color too. Last to arrive are Garry oak and Oregon ash, both waiting well into late April or May to start; both these ‘late leafers’ have a pale gold color. 

Why are these colors important? Tree leaves are green, yes, but they don’t open up with their photosynthetic mechanisms completely in place and operational. Leaves are designed to capture light, photon by photon, and turn it into food. Simple sugars made in the leaf from basic materials––sunlight, water, carbon dioxide––become more leaves, new roots, and cellulose, the natural biopolymer that makes wood. 

As leaves unfold, the photosynthetic powerhouse inside the cells also has to assemble, and this is where light-green, gray-green, gold-green, yellow-gold, and in some cases, red, purple and near black leaf colors, come from: non-green pigments that protect the new leaf tissue from photo-destruction. Sunlight drives life, and it also can destroy plant tissues before the new chloroplasts have completely assembled.  These colors are protective pigments that keep fragile new cells alive in the presence of sunlight until their chloroplasts are green and using those photons to make sugars.

These pigments are most noticeable in spring before the chloroplasts have completely assembled. Once chloroplasts are completely operational, those leaves look green to us. The protective pigments are still there, we’ll see them again in the fall as the leaves shut down and are shed. The intense green of fully functioning leaves will hide the other colors during the summer. 

Nurseries promote plants with non-green foliage: yellow, red, purple, black. These plants were grown from abnormal plants, or in some cases, twigs on otherwise normal shrubs and trees. We like to have pleasing colors around us, including foliage that is other than simply green. And so nurseries offer conifers, hardwoods, shrubs, grasses and perennials with a range of foliage colors, all selected from naturally variable plants. The mechanism by which these colors are produced in the plants varies. Some plants produce less chlorophyll than normal, others produce higher amounts of other pigments. 

One of the most striking of the former was Kiidk'yaas (the Ancient One) also known as the Golden Spruce, a tree that lived in a forest on Haida Gwaii archipelago in northern British Columbia. This spruce had golden needles and stood like a golden spire in the forest. It lived for almost three hundred years, until a day in 1997 when it was cut down by an unemployed forest engineer making a confused political statement. His fate is unknown; he was arrested and disappeared on his way to trial. Meanwhile, cuttings of the golden spruce were grafted onto a normal green Sitka spruce by University of British Columbia researchers in the 1970s. Its progeny live today as Picea sitchensis ‘Aurea,” or “Bentham’s Sunlight.” The golden spruce lacks about eighty percent of its normal chlorophyll, and needs to grow in the shade of other trees to protect it when young. 

A very striking color change takes place in cranberries between summer and winter. Cranberries use red pigments to protect their leaves and shoots during winter, turning dark red in fall. Come spring, as plants come out of dormancy and start growing, leaf color goes back to green, though red protective colors never completely go away. 

These seasonal changes are not as striking as the fall colors and spring leaf-out of the great hardwood forests of the East Coast. In spring they herald another seasonal change: the arrival of the lawn-growing season. I’m getting my mower cleaned and its blade sharpened for another summer of tussle with my lawn. 

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