The memory of trees in a modern climate: epigenetics

Several recent articles on MyMinnesotaWoods have discussed the need to recognize seed zones in reforestation efforts to avoid the consequences of moving seed too far. There are several good reasons outlined in these articles. You might be surprised to learn of another reason: trees maintain a memory of their origin that helps them adapt to their local conditions. In this article I will discuss epigenetics: a novel area of research that pertains to both modern medicine and forestry.

So what’s in a tree seed? Tree seed contains DNA, the genetic blueprint of the tree, along with carbohydrates for the developing embryo and a seed coat for protection. But DNA alone does not determine what the tree will look like. Scientists are learning that chemicals bound to the DNA influence how the tree looks and functions. These chemicals are referred to as the “epigenome,” and they function to turn genes ‘on’ or ‘off,’ much like a light-switch. This means you can have genes for a trait, but those genes might not be expressed. In fact, there is a field of science devoted to studies of the epigenome called epigenetics, Latin for “outside the genome.” This article in the Science Daily describes an example in fruit fly research.

Epigenetics diagram (click for original and explanation)

Your DNA contains genes that you inherited from your parents, and the epigenome maintains a “record” of life experiences that you inherited from them. Sounds like a science fiction novel? Here’s the rub: the epigenome shuts genes on or off based on life experiences. For example, a child’s brain is in a heightened state of development and wiring. Life experiences can switch genes on or off through the epigenome, essentially leaving a record on your DNA. The really crazy part about epigenetics is that the “position” of the DNA switches, whether “on” or “off,” can be passed on to their offspring. In this way, your grandparents’ life experiences may influence the way your genes are expressed. This story highlights such an epigenetic link between obesity and diabetes. In medicine, scientists are just beginning to understand these trans-generational links between health and inheritance that complicate studies of disease and susceptibility to disease. The epigenome provides an important mechanism by which experiences are imprinted onto our DNA to help us adapt to modern life.

Back to trees. Trees, like people, experience a huge range of environments during their long lifespan. Unlike people, they cannot run from bad environments, and spend a great deal of energy reproducing to disperse their offspring to better novel environments. In this way, trees are masters at adaptation. Like humans, experiences can be imprinted on seeds. In this case there is an evolutionary advantage at stake: trees imprint clues about the local photoperiod and possibly local temperatures onto developing seeds.

Scientists recently, and unexpectedly, observed this mechanism in Norway spruce trees.

A white spruce seed orchard in Minnesota. Photo by C. Pike)

Scientists in Norway conducted a simple experiment. They selected Norway spruce trees with established pedigrees that reliably produced tree seed adapted for reforestation in the northern part of the country. These parent trees were copied through grafting, and the new grafts were planted into a location farther south. After the trees matured, seed was collected from them and planted back north. Much to their shock, the seed from this southern orchard more closely resembled trees growing in the southern environment than their kin in the northern part of the country. The growth rhythms of the seed from this new southern orchard were more in tune with the day lengths and temperatures of the southern environment. In fact, the seed from this southern orchard was not suitable to plant in the northern part of the country. Genes, assumed to be the blue-print for tree growth patterns, had been trumped by the effects attributable to the epigenome.

The scientists later learned that they had just witnessed adaptation due to epigenetics. This was one of the first reports of this phenomenon in trees. The effect was pronounced within a single generation. I had the good fortune to meet one of the scientists at a meeting in Thunder Bay, Canada last summer. I asked Dr. Johnsen how his colleagues accepted the news that he had essentially made a discovery that contradicted Darwin’s basic theories of evolution. He paused, and quietly stated “they threw tomatoes at me.” Tomato-throwing, in the hallowed halls of a University? Yes, novel discoveries in science often face uphill battles, even in today’s modern world.

Epigenetics works alongside natural selection to provide an additional mechanism for trees, and other organisms, to adapt to their environment. As the climate changes, developing seeds receive environmental cues that allows them to make adjustments to improve their ability to grow in a novel climate. At some point, our climate may change too drastically for even epigenetics to overcome. That’s where gene conservation and assisted migration may need to be implemented. The memory of a tree is strong, but the forces of climate change may be stronger yet.

MN DNR seed zones. Click for details.

How does this knowledge of epigenetics affect decisions about what seed source to plant? It enforces what we already know and are already doing. Keep seed local. I recommend using the MN DNR seed zones as a guide to determine locality since it accounts for gradients in precipitation, temperature, and day-length. Move seed too far, and the DNA and epigenetics may be poorly matched to the site. Tree planting is an important tool to reduce the impacts of climate change on our forests, but the trees you plant are only as good as the genetics and epigenetics hidden under their seed coats.

Carrie Pike directs the Minnesota Tree Improvement Cooperative, based at the University of Minnesota's Cloquet Forestry Center.

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