The science behind Christmas trees: How conifers brave winter's worst

As the festive season approaches, evergreen conifers like spruce and pine adorn homes worldwide. But while Christmas trees bring warmth and joy into our lives, they endure some of the harshest conditions on Earth in their natural habitats.

Most people take it for granted that they keep their needles lush and green in freezing winters and blinding sunlight in the boreal forests, but now scientists can unwrap the science behind conifers' winter survival.

The photosynthetic process of most green plants is highly conserved; it functions overall the same in green algae, tulips and redwood trees. Yet, there are differences and scientists are gradually understanding more about both the differences and commonalities.

Conifers have extraordinary winter survival strategies, some of them were not understood until recently.

Scientists from Umeå University have, together with colleagues, summarized breakthroughs in an article published in Trends in Plant Science.

Dissipates extra energy

One of the two main findings, both of which this group of researchers have contributed to, is that conifers change the structure of their thylakoid membranes—where photosynthesis takes place—making Photosystem I (PSI) and Photosystem II (PSII), which otherwise by large remain separated, come in winter closer to each other and work together in a special way named spill-over.

"This helps them to safely dissipate extra energy and avoid damage from too much sunlight in the cold," says Stefan Jansson, Professor at Umeå Plant Science Center at Umeå University.

Others have previously, without understanding the mechanism, named the process "Sustained Quenching" as it could put photosynthesis into a lock down mode for days.

The second strategy, operating in parallel to spillover, is that conifers use special routes for moving the electrons in photosynthesis.

These paths, known as alternative electron flow, involve flavodiiron proteins and help keep the photosynthesis process balanced. This also prevents the system from becoming overloaded when there's too much light and freezing temperatures.

Conifers are the dominant species

In addition, the photosynthetic apparatus of conifers differs from that of flowering plants (angiosperms) in a few other ways. They lack, for example, some so-called light-harvesting proteins found in other plants.

"All together, this can explain why conifers are the dominant species in boreal forests, thriving where few others can, perhaps at the expense of advantages during less challenging conditions. Few conifers, if any, grow where water, nutrients and temperature conditions are all favorable," says Pushan Bag, lead author who, during his doctoral studies at Umeå Plant Science Center, studied these phenomena.

Understanding these mechanisms may also aid conservation and help predict forest responses to climate change and may, in the longer perspective, inform strategies for breeding crops that are resilient to extreme weather conditions.

Co-author Alexander Ivanov adds, "This paper highlights the intricate adaptations of conifers to extreme winter conditions. By combining structural, molecular, and evolutionary insights, it advances our understanding of how these trees have come to dominate some of the harshest ecosystems on Earth."