Photosynthesis in winter

I began to get very interested in the possibility of the bark of some trees photosynthesizing in winter as a result of my predawn meanderings in the Bosque. I noticed, for example, the pale skin of Mexican Privet and the young branches of Cottonwood trees. Both had a pale greenish tinge. I also recalled the Aspen and Poplars on my land in Maine that also had greenish bark.
 
When I was researching to write an article on Aspens I learned that the willow family that included Aspens, Cottonwoods, and Poplars as well as our Coyote Willows did indeed photosynthesize all winter long as long as temperatures stayed above freezing. If sunlight warms bark on the south - and southwest - facing sides of trunks and branches, it also makes it possible for bark to photosynthesize even when air temperatures are below freezing.
Energy produced by bark photosynthesis is thought to support regular cell maintenance in the trunk and branches and can help trees recover from defoliation due to insects, storms, or severe drought.

​I keep a sharp eye on the Coyote Willows because I don’t want to miss the changes that are subtle; they are already starting to turn. Anthocyanins and carotenoids are plant pigments that produce the red, brown and purple colors in willow stems while carotenoids produce yellow and orange hues. Both anthocyanins and carotenoids protect photosynthetic pathways from being damaged by New Mexico’s intense spring light before the narrow leaves appear. My observations suggest that the only time the willows approach dormancy is during December and January (at least this year).
 
Because we have spent most of the winter with above freezing conditions this may have been a particularly good year to notice subtle changes in young bark. It turns out that Birch and Beech, two northern trees, are adept at this process as well. I only recently discovered that some northern deciduous trees continue to photosynthesize even under the snow! There is filtered light, beneath the snowpack. And plants are able to harvest that light, once temperatures get above freezing, most notably in wetlands – good examples are pitcher plants and cranberries. And, not surprisingly, low growing alpine plants, also avail themselves of this strategy in the harsh alpine zone, where the growing season is so short.
 
We all know that plants and trees use their leaves to convert sunlight into sugar, or carbohydrates during the warmer months, and some folks are aware that evergreens continue to photosynthesize all winter as long as the temperatures are above freezing; one reason we continue to water our evergreens at regular intervals in New Mexico.
 
Although I don’t have adequate research available to support my hypotheses I suspect that many trees and bushes with thin bark in our areas take advantage of this phenomenon. Certainly Chamisa bushes must; their lime green bouquets are stunningly beautiful right now (mid February). I also suspect that my two pear trees may be doing the same thing. In just the past three weeks the bark has lightened to a pearly eggshell. Unfortunately, for young trees and bushes this tender sweet bark with cambium beneath provides a sugary treat for hungry rodents.  
 
Many people don’t know that extreme temperatures of 100 degrees or more will stop photosynthesis completely in trees, and around here summer temperatures hover well above the 100’s in the sun. Thus, beginning the photosynthetic process early in the year has definite survival value for trees and bushes especially as the Southwest heats up. Did you know that according to NOAA, 2019 was the hottest year in recorded history?

Another aspect worth mentioning is that early photosynthesis helps with buds that are getting ready to swell. I am fortunate to live near a cottonwood bowery so I can watch those photosynthesizing buds and twigs every single day, and they have definitely begun to become engorged. But even in the Northeast the buds are visible. Those of red maples are swelling, weeping willow twigs develop a yellow tinge, as do the pussy willows. There is a period when it is easy to spot these changes between snowmelt and when the buds burst into flowers and leaves. Scientists call that space the vernal window.

According to Rebecca Sanders-Demott, a research scientist at UNH, who studies this gap the length of time between melt and blooming can have implications for how much carbon dioxide goes into or out of a tree’s system on an annual basis. If snow melt occurs very early in mid-February for example, we know that leaf out won’t happen until early May so there is an extended vernal window.

That opening has different effects on different species, but scientists are in agreement that changes to the window impact how much photosynthesis occurs during the rest of the season.
In New Mexico the vernal window is a long one that helps the trees and bushes to maximize photosynthesizing before  summer heat strikes its lethal blow.

Photosynthesizing tissue, whether its buds, the year’s new shoots, or tasty branches and saplings are a welcome arrival for animals in any region this time of year.
 
 Speaking of animals, yesterday I had a couple of very unwelcome cows who were just about to devour my crocus, planted only inches from the house; one had already begun to feast on my favorite juniper when my dogs went berserk as did their mother. As a self-responsible animal ‘owner’ I balk when others allow their animals to trespass illegally. At the very least cow owners could feed their livestock so they stay home.
 
Just for fun I am experimenting with willow twigs, but the wily rabbits are onto me; they systematically demolished my first experiment with ease. Undeterred, I have devised a different method to foil them, but I carry grave doubts of its success because ‎Lagomorphs and other wild animals are much smarter than me!