O Christmas Tree: It’s Not Easy Being Green

Evergreens are a remarkable mainstay in the evolution of plants. Evidence suggests that they have existed more or less in their present form for the past 300 million years. In other words, the evergreens are so resilient and exquisitely adapted to their environment that nature has not tweaked with their genetic recipe since the Permian. The evergreens can survive just about anything nature can throw at them, except humans. Nearly 40 million of these stoic conifers are chopped down each Christmas season in North America alone.

"Christmas Tree" farms cultivate a variety of evergreens that will grace one of 40 million homes each season. This makes it a lot easier than hiking into the forest to cut one down yourself.

Humans have long been fascinated by the evergreens because these trees and shrubs do not lose their leaves (needles) in autumn like the broadleaf trees. Seemingly in defiance to the harsh winter, the aptly named evergreens stay full and green all year long. Impressed with this act of endurance, early humans thought that evergreens must hold special powers. The ancient Pagans would place evergreen branches over their doors and windows to ward off evil spirits, especially during the winter solstice when the days were at their shortest and the nights at their coldest. Evergreens served as a reminder that the days would lengthen and the crops would grow once again in the spring.

A decorated evergreen is now synonymous with “Christmas Tree”, but this ritual has its “roots” in Paganism. Interestingly, it has even been argued that this passage from the Bible forbids emulating this Pagan practice.

So how do evergreens stay green year round? In winter, shorter days mean less sunlight. As sunlight is required for photosynthesis, plants face a dramatic reduction in energy during winter. To cope with this, broadleaf plants stop making chlorophyll, the molecule that drives photosynthesis and reflects green light. Consequently, the leaves change color and eventually fall off as the tree goes dormant.

By way of comparison, evergreen “leaves” do not have a lot of surface area; they are more resistant to lower temperatures and decreased moisture. Chlorophyll in these needle-like leaves is retained and photosynthesis can still generate energy from light, albeit at a much slower rate than spring or summer.

In addition to keeping chlorophyll, retaining moisture is equally important:  trees cannot extract water from frozen ground, and occasional sunlight in the winter can draw out precious moisture. Evergreen needles have a thick coating of wax and a slender shape, characteristics that help them hold water in and prevent evaporation, respectively.

A recent study has shown that the conifer’s ability to survive arid times involves the coordinated evolution of tissues regulating water supply (xylem) and water loss (stomatal pores) in the needle leaves. A plant hormone called abscisic acid helps keep the leaf’s pores sealed when water isn’t available. Another mechanism allows leaves to dehydrate and resist damage via a water transport system.

Close up image of pine needle – the small pores are stomata, which open and close to regulate gas exchange. When open, water vapor can escape.

Conifers have thousands of needle leaves, which help maximize energy production while not losing water to dehydration. Of course, evergreen needles do not last forever. They do need to be replaced, but conifers do this intermittently and a green appearance is always observed.

Ever since ancient times, the evergreens have been admired for their stamina and hardiness through the winter. They are a source of inspiration reminding us that better times are ahead. In this light, the ritual chopping down of the tree for decoration seems a most bizarre way to honor the mighty evergreen. Consider, instead, a Festivus Pole.
 

Contributed by:  Bill Sullivan

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Brodribb TJ, McAdam SA, Jordan GJ, & Martins SC (2014). Conifer species adapt to low-rainfall climates by following one of two divergent pathways. Proceedings of the National Academy of Sciences of the United States of America, 111 (40), 14489-93 PMID: 25246559