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Untangling nature and nurture in forest trees

Setting up the common garden experiment in Oulu. Credit: T. Pyhäjärvi/ University of Oulu

Have you ever looked at trees in autumn and wondered why some of them still have green leaves, some have turned red or yellow and still others are already completely bare? Or maybe, after a hard winter or hot summer, you asked why this plant survived and that one died? Well, that is what I do for living!

I am one of the many European scientists working in the GenTree project to understand the heritable differences among different forest trees as a way to improve the sustainable use forest genetic resources in Europe.

As a biologist, I know that some of the differences that we see among individuals are the result of the environment. Maybe this specific plant was subject to violent gusts of wind. Maybe it did not get enough water. Maybe it was attacked by herbivores. All these could cause visible differences among individuals and even the death of some.

But as an evolutionary geneticist, I am also aware that just as all the people on earth (except identical twins) have different DNA, so do trees. It is the genetic variation in the trees’ DNA that I am particularly interested in.

There are two broad types of evidence of genetic variation: phenotypic and genotypic. The phenotype is the combination of an organism’s observable characteristics, determined partly by the genes and partly by the environment. Further, genes and environment interact. So one tree may be more drought-resistant than another, but unless there is actually a drought, there may be no visible difference between them. Genotypic variation these days is often assessed by DNA sequencing, which reveals genetic differences among individuals. In our research, we try to assess both at the same time.

To separate the phenotypic variation caused by genetic differences from that caused by the environment, we need to minimise environmental variation. Then only the genetic differences will matter. With forest trees, we do this by growing plants in what we call common garden experiments. This simply means that we grow seedlings of potentially different genetic origins in the same location, trying to keep the conditions as similar as possible for all of them.