Breeding taller trees could make them more susceptible to climate change

March 1, 2021

A first-year lodgepole pine seedline grown in the study garden. (Ian MacLachlan)

Selective breeding to promote faster tree growth may complicate future reforestation efforts for temperate pine forests by making these trees more vulnerable to changing climates because of genetic trade-offs in the flora, a new study found.

The researchers analyzed climatic and seedling data and approximately 50,000 genetic variants for several key traits — height, vulnerability to cold and timing of growth — in lodgepole pine trees in Northwestern Canada. The results, appearing in PNAS on Monday, showed extensive genetic overlap in these traits, meaning that selecting for a positive trait such as height may exacerbate a negative trait such as cold vulnerability in future generations.

“Selective breeding for one trait, in this case height, can have impacts on other, correlated secondary traits,” said Ian MacLachlan, a postdoctoral research associate in biology at Texas A&M University and first author on the study. “When those secondary traits are important for climate adaptation, you create trade-offs between growth gains you’re trying to achieve and the adaptations that the trees need to survive within the constraints of the local climate.”

The potential devastation to world forests as a result of climate change is well-documented: Climate change has negative impacts on forest health, carbon absorption and timber production. These impacts compound with existing barriers to forest expansion such as geographic obstacles and speed of seed dispersal, pointing to trouble for trees in the coming years.

However, relocating trees to cooler regions in anticipation of future warming may put the trees at greater short-term risk of frost damage, and runs contrary to the usual goals of tree breeders.

“What’s been the foresters’ mantra for hundreds of years is that ‘local is best,’” MacLachlan told The Academic Times. “But as climates shift at an increasing rate, that ‘local’ climate may now be found somewhere else where it didn’t use to be. So now what forest managers are trying to do is keep up with those changes.”

MacLachlan and his colleagues sought to address the issue by figuring out to what extent selective breeding for height can impact traits associated with climate adaptability.

The researchers compared the genomes of 105 wild seedlots with 20 seedlots that had been selectively bred, looking at genetic variations called single nucleotide polymorphisms, which behave as a genetic “currency” when compared with changes in a physical trait. These provided insight into how changes in the genome affect the expression of traits in the trees.

They found that 23 of these genetic variations were associated with height, cold vulnerability and timing of growth, and also shifted significantly in response to selective breeding, suggesting that breeding for one of these traits, such as height, could lead to tradeoffs in the others, such as cold adaptation.

“These analyses have not really been done before in temperate trees, so it was surprising to see such strong directional shifts in the markers associated with the traits we were looking at after only two generations of selective breeding,” MacLachlan said.

Next, the researchers are interested in taking these population-level findings and refining them to the family level, or a group of trees from the same parents. This will make their results more immediately useful to tree breeders, who tend to work with individual families of trees.  

While these findings point to potential complications in reforestation efforts down the road, the researchers emphasize that all is not lost. In fact, one of their previous studies found that for spruce trees, selective breeding is not yet compromising the trees' ability to adapt to local climate. So there may still be time to mitigate these effects.

However, the adaptability issue comes to light when considering the sustainability of selective breeding over the long term, especially as climate change not only continues, but accelerates.

“The selective breeding, we see now, is still based on natural variation that’s already present in the landscape, so breeding is kind of optimizing that at the current time,” MacLachan said. "But there’s so much genetic variation in these trees that it’s unclear to what extent future breeders will have to manage these tradeoffs as the climate continues to change.”

The paper “Genome-wide shifts in climate-related variation underpin responses to selective breeding in a widespread conifer” was published March 1 in PNAS. The authors of the study are Ian R. MacLachlan, Texas A&M University; Tegan K. McDonald and Sam Yeaman, University of Calgary; and Brandon M. Linda, Loren H. Rieseberg and Sally N. Aitken, University of British Columbia. The senior author is Sally N. Aitken. 

Correction: A previously published version of this article incorrectly described the genomes compared by the researchers. The error has been corrected.

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