Cherry blossom time

Our Weeping Cherry tree started to bloom on March 28, an old lady now yet still graceful. She has a voluminous floral dress spread wide from her ‘hips’ by branches like the hoops and side panniers of a woman in the court of George III. She cheekily displays through the cascade the one silvery leg she stands on. We hope she dances in the spring breeze for more years.

The same day, the National Park Service announced the famous lines of cherry trees lining the National Mall reached peak bloom. Fewer people stroll there in a pandemic year but can view them at #BloomCam. This year the blossom that celebrates beauty and grace is a brilliant contrast to the chaos and violence viewed from the Mall of the Capitol steps on January 6. But it also symbolizes the impermanence of life.

The trees were gifted to Washington DC in 1912 by the Japanese, who celebrate bloom time with spring festivals (hanami). This year the peak occurred in Kyoto on March 26, earlier than usual, as in the Mall. Bloom times have been recorded in Japan for 1,200 years. The date varied depending on when winter lost its grip, but on average stayed constant over centuries or rose slightly until the 19th century since when it has steadily advanced.

The ancient recorders of first blooms and shoots could not imagine why they should interest us today. But there are no more blazing signs of a  warming planet than trees exploding in color. On March 28, Red Maple buds burst at Mechanicsburg, PA and Pawpaw at Gibsonville, NC, although Redbud is still dormant at Spring Hill, TN (already rose pink here in Williamsburg, VA). If you doubt our climate is changing, ask the trees.

Shelterbelt Trees in Snow and Fog

Have you noticed how trees hollow out melted sleeves from snow around their boles as our arms would if we could hold them long enough in a snow blanket? And have you wondered why winter fog sinking over open fields is denser than in adjacent woodland? I never gave them much thought until a recent winter walk, but isn’t it often so that the familiar and banal is suddenly thrust to attention to look fresh and strange?

The brain suppresses absorption with too much detail until the detail becomes important or something or someone points it out. I can’t put my finger on what drew my attention to melting snow and thinning fog one day, and it wasn’t more important than the other stimuli bombarding my sense organs. Most noise is filtered out in the conscious brain to avoid being swamped and making us go crazy. But looking intently at something commonplace for the first time can stir uncommon curiosity.

Almost everyone prefers a simple explanation to an obscure one, which is the wisdom of Occam’s razor. When I had students in class they loathed to hear me say: “I have several hypotheses to offer for this observation, and there is evidence for all of them …” That was a way of admitting ignorance without sounding ignorant. Nature is more complicated than we are sometimes wont to admit or accept.

Foggy morning at Jamestown Island

We are drawn to the single explanation, but when we look more closely and are better informed we often find two, and on even closer examination we realize there are four, and then we begin to wonder if we’ll ever get to the bottom of the mystery. We are tempted to turn back to simplicity and turn our backs on provisional knowledge to cling to intuition. I don’t believe in fairy stories, but I know the temptation to be willfully ignorant and it must be resisted.

There is a bottom to the pit of curiosity, a place where facts are finally robust against doubt, but plumbing the depths can take a long time, even generations of science and philosophy. If we boast about our knowledge we fool ourselves because science is still young. Besides, isn’t mystery something to celebrate except when it holds down human welfare or harms our environment (basically the same things)? Mystery is a call to act and understand and is often more intriguing than the mere appearance of knowledge, and perhaps that’s why it is so popular in fiction.

I started musing about snow crystals melting around trees and foggy water droplets vanishing in shelterbelts. Why do they?

Elementary physics offers an easy first answer to my sudden curiosity. Tree bark absorbs some radiant energy, especially the darker shades, even on a cold day when we hardly feel heat of the sun on our faces. It may seem too trivial to change the physical state of water in snow and fog, but the heat stored by day is slowly released to help a slow thaw continue at night. The scientist wants to test a bare theory by experiment. Next time it snows I will check a natural experiment by comparing the melt around maples and oaks with the lighter boles of silver birches, which should in theory remain snow-bound longer.  And here is another question for an inquisitive walker. Does snow melt as quickly on rocks as tree trunks of the same color, and if not why the difference?

A second explanation is that trees offer shelterbelts against chill winds. More heat is lost by radiation from open ground on cold days and freezing nights than among trees, which also reduce wind velocity to stabilize a warmer zone. If you ever get stranded on a bitter night in the countryside it’s wise to find shelter in woods. But it’s hard to see how shelter accounts for melted ‘sleeves’ of snow around trees.

Casting around for other explanations, I wonder how much heat is generated by the thin cylinder of living cells under the bark when trees are looking dead in winter. To be alive is to be engaged in combustion because heat is the by-product of metabolism, generating 500 kJ per mole of oxygen when complete, to be precise.

It seems unlikely the low ebb of metabolism in sleeping winter trees warms the snow, and heat is more likely to be generated on tree bark than under it. To think of microbes and fungi is to remember hot compost and sweaty manure, as well as bubbles of carbon-dioxide popping in the air trap of a home brewer’s dewar. They can be sources of prodigious heat, and commercial breweries sometimes struggle to keep temperatures down.

Tidewater Virginia is marching toward spring and unlikely to see more hard weather so my questions about silver birches and rocks will dangle until I am trudging through snow again next winter.

Next Post: Snow tracks

 

Fall Colors in Appalachia

Most people in this region look forward to the fall more than any other season, despite this being the prologue to winter. Starting in September at the atlas or far end of Appalachia, color changes ripple down the spine to the coccyx at the southern extremity a month later, but the finest sights are in the north and at high elevations. Colorful sugar maples like it cool.

Fall colors in Appalachia

People have probably marveled at the spectacle ever since they first set eyes on it eons ago, and long before they wrote about it. I find it perfectly understandable if Native Americans believed fall beauty was the deliberate artistry of a Great Spirit who painted the trees, flowers and creatures for human pleasure, like the Huron story of a great bear’s blood dripping on the forest from heaven and other stories passed down in tradition and lore. More than anyone else, Charles Darwin deposed humans from the center and purpose of creation, but accommodated his feelings for natural beauty in an evolutionary perspective (‘There is grandeur in this view of life …’), which is surely true though we have paid a price by losing a naïve and supremely arresting joy out of mystery.

When the first people saw the Appalachian Mountains there were no trees because the land had been scraped by glaciers and it took centuries for forestland to replace retreating plants that were the first colonizers after the last Ice Age. We are lucky to live in an era when conditions favor trees with fall splendor, though probably one of the last generations to witness them as climate warming pushes back the season and trees struggle with summer heat. The change is not so much a loss to the tourist industry as to the human spirit.

This year’s weather has muted fall colors in Central Appalachia. The maples were almost denuded a month ago, although the oaks are still green and beeches golden-yellow.

Leaf color chemistry is molded by weather. For the most brilliant display, summers should have plenty of rain because drought triggers leaf abscission. Then, late summer should have warm, dry days and cool nights with little wind. Those conditions prevailed this year, save one—the nights stayed mild. We had our first frost on October 17, but it was slight and night temperatures were mostly above average. An Indian summer spoiled a painted fall.

The closer you look at leaf chemistry the more complicated it seems. This is surely a principle in science as, for example, whether physicists study elementary particles or the cosmos the closer their examination the deeper and further they are borne. Science mines nature, but we never get to the end of the seam. We celebrate this richness, but it must be frustrating for politicians who commission research because they hope complexity will turn into neat answers, yet at the end of a study the conclusions are often provisional and there are plenty of new questions.

Once upon a time, fall colors were regarded as consequences of healthy biochemistry yielding to organic decay. The fact of the matter is half opposite because they involve a rather active process. Some genes switch off while others turn on. Each leaf is more in command of its fate than left to the consideration of entropy. Of course, photosynthesis does shut down in the absence of enough sunshine or moisture, and, to add complexity, deciduous leaves are more responsive than evergreens. It’s a familiar story because a sun-loving pot plant left in a shady place when we go on vacation will be a pile of dry leaves and bare stems when we return. It is a protective mechanism for plants and deciduous trees in autumn to withdraw vital nutrients and minerals from leaves into their ‘body’ for storage until needed at the start of the new growing season when the sun breaks out.

When the green pigment disappears, the yellows and oranges that were present all along are revealed. Beech leaves go a step further when the pigments turn into brown tannins that we notice dangling on stems all winter. But the red and purple pigments of maples and gums are synthesized de novo shortly before their leaves fall.

These are anthocyanins, which are molecules that are abundant in ripe berries and grapes and lend red wine its virtuous reputation. They are beneficial for leaves too where they serve as sunscreens and antioxidants to protect valuable molecules synthesized in the summer from solar rays shortly before the fall. There is another theory that bright colors warn away pests, as if rouge leaves can tell insects they ought to buzz off to find a less vigorous tree. It doesn’t square with the widespread lack of receptors needed for seeing red.

Despite its brisk pace, I believe science will never end and its ambition will never find a final goal. That’s worth celebrating. Life would be boring, almost pointless, if everything was predictable and nothing was mysterious. Mystery is sacred.

Fortunately, there are still countless enigmas in nature to stimulate our curiosity, and keep scientists employed. In this post I can mention only one, though it is relevant here. I wonder why evolution hasn’t given all deciduous trees the same glorious reds and purples in the fall if those pigments are so beneficial. Isn’t natural selection supposed to steer genetics to an optimum fit for the environment? Europeans must be satisfied with their yellow fall leaves and no native reds at this season compared with the hot colors we enjoy most years in eastern North America and Asia. Are there any bright theories out there?