Genetic diversity and adaptation to climate change

Trees live for a long time. Climate change can rapidly alter the environment in which trees grow. As a result, trees are unable to adapt to climate change quickly enough.

That’s a common argument for offering forests human assistance to promote their adaptation, an New leaves, Fagus sylvatica, Central Apennines, Italy. Photo: B.Vincetiargument that FORGER researchers have themselves used. But it may also be a misconception, according to two papers by Koen Kramer and his colleagues at Wageningen University in The Netherlands. The misconception arises, they say, because people forget that although the tree is long-lived, roughly every year natural selection takes place as some seedlings thrive and others die, and this selection adapts the population to the environment, even as the environment is changing.

There’s another misconception too, that differences among species are more important for maintaining the health and productivity of European forests than genetic differences within each individual species. In support of the importance of genetic diversity, Kramer and colleagues point to transplant experiments, where trees adapted to different environmental conditions are grown under standard conditions. These usually reveal that the different provenances respond differently to, for example, changes in temperature regime or water availability. Indeed, in some cases trees from one provenance may grow faster, while those from a different provenance grow slower, when both are offered more water than they get in the environment where their parents grew.
Genetic diversity matters most at the limits of the species distribution. At the leading edge, where the species is expanding into new areas, genetic diversity is likely to be lowest, because the new populations are founded by a few individuals who represent a subset of all the genetic diversity. At the trailing edge, where the species is going locally extinct, diversity is highest, because that is where the species has had the longest history since the end of the most recent ice age and the most time to diverge. And in the centre of the distribution, high genetic diversity will allow the species to recover most rapidly from extreme environmental events.

At the moment, the researchers say, models that assess the ability of long-lived perennials such as trees to respond to climate change ignore both ongoing selection and genetic diversity. To overcome this, they turned to the Forest Genetics, Ecology and Management (ForGEM) model they had previously developed. ForGEM essentially bridges eco-physiology and quantitative genetics, marrying the way an individual tree responds to such things as water availability to the way in which selection acts on trees with different genes, changing gene frequencies and adapting the population to different conditions.

In a more detailed paper, the researchers explain that the model is built from accurate data of individual trees in a stand. The key characteristic of interest is the date on which the buds burst and the leaves begin to unfurl. The date of bud burst is the result of a complicated interaction between genetic and environmental factors, including cold temperatures during the winter dormant period and warmth as spring progresses.

The date of bud burst matters because it involves a trade-off. An early bud burst allows a tree to get going sooner in the growing season and thus to benefit from greater productivity. On the downside, if there is a late frost, the tree risks losing its leaves and flowers. A late frost will also kill any seedlings with unfolded leaves. By setting back trees with an early bud burst, a late frost simultaneously favours trees with a late bud burst. In the absence of a late frost, late bud burst is a disadvantage, because it allows earlier trees to gain a head start in capturing resources.
In the model, each tree is assigned a genome of 10 different genes that affect the date of bud burst in different ways. In addition to simulating the effect of temperature on the productivity of trees with differing dates for bud bursts, the model also simulates sexual reproduction, which shuffles the genome, and the flow of pollen from neighbouring trees. Each season, the model calculates the productivity of the trees and the likely survival of seedlings with different genomes, with survivors joining the population as they grow and mature.

In this way, ForGEM brings together the genetic changes that underlie adaptation to environmental conditions with the physiological processes that govern the growth of the trees.
Using a base model that accurately reflects the structure of an actual stand of beech trees in The Netherlands, the researchers watched to see how the model trees performed when they were exposed to climate data for seven sites that span the distribution of beech in Europe, using predictions of likely weather patterns at each site and intensive forestry management. After 400 years of simulated growth, timber harvesting and seedling selection, the model trees had clearly adapted to their “new” environment. Bud break was earlier in warmer sites, and at each site bud burst was predicted to be about 10 days later 300 years in the future.

The model offers two conclusions. Firstly, forest trees can indeed adapt to important aspects of climate change, such as water availability and temperature regimes, within two or three management rotations. Secondly, the rate of adaptive change is strongly affected by forest management. These conclusions point to the need for future assessments of how forests might adapt to climate change to pay more attention to both genetic diversity and local adaptation.

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