Growth & Development

Aspen, they grow up so… weirdly.

Aspen possesses enormous levels of genotype-based intraspecific variation.

The Pando Clone made up of a single quaking aspen (Populus tremuloides) clone in southern Utah has been capturing people’s attention. It is the heaviest living organism in the world, spanning over 100 acres and weighing 6,615 tons. However, a research team found in 2018 that the Pando Clone has not been able to keep up with self-replacement and is in danger. 

Whilst trees look somewhat alike, there is great variability within one species in their leaves, branches and trunk characteristics. This intraspecific variation is due to different gene expression and environmental and developmental factors. In order to understand how individual plants with the same genetic makeup can grow in a different way, scientists previously used quick-growing plants to experiment on and leaving out many tree species as they grow slowly.

Cole and colleagues at the University of Wisconsin-Madison, the University of Maine at Ft. Kent and at the Max Planck Institute for Chemical Ecology followed the growth of thousands of clonally reproducing quaking aspen trees for a decade. The scientists found that intraspecific variation mostly varied between different genotypes and there were trade-offs among tree growth, defence and reproduction.

Quaking aspen is a foundation species, it has a strong role in shaping the local habitat. It grows from Alaska to Central Mexico and its white bark can photosynthesise. The lead scientist, Dr Christopher Cole, has previously investigated the molecular evolution of condensed tannin production for defence in Quaking aspen and also found the aspen have grown quicker as atmospheric CO2 levels rose in the past. 

Cole and colleagues set up field trials at Wisconsin Aspen (WisAsp) common garden in 2010 with 1,788 aspen trees from 510 genets. They measured the trees’ growth, defence chemistry, leaf morphology, flower production, the leaf area lost to herbivory and disease from the juvenile stage until the reproductive stage. Microsatellite markers identified different genotype (i.e. genets) and the heritability of intraspecific variability was investigated.

Caption: Scientists in action at WisAsp in 2014 and 2017, measuring tree functional traits. Source: Cole et al. 2020

All traits measured greatly varied between 2014 to 2018. Trait heritability changed over time and found that ramets (i.e. individual trees) became more similar to one another. The number of flowering twigs on reproducing trees varied 1,300-fold among genets whilst leaf area lost to herbivory and disease was 13- and 43-fold respectively among genets. The scientists found a shift from resistance to tolerance in terms of phenolic glycoside and condensed tannin levels. There were no direct trade-offs between defence and reproduction – contradictory to their initial hypothesis.

Cole and colleagues explain, “a dominant message emerging from our work is that aspen possesses enormous levels of genotype-based ITV [intraspecific variation], even after accounting for the roles of ontogeny and trade-offs among growth, defence, and reproduction”. The researchers add, “despite this wealth of phenotypic variation, the high heritabilities of traits closely associated with other species – notably defence traits – provide the foundation for connecting these traits to other organisms in the aspen community, as well as to the genes that shape them”.

This study shows that long term research can reveal unique processes in trees. By understanding how trees balance growth, defence and reproduction, conservation efforts and forest management practices can be informed. Hopefully, the Pando Clone will also continue to live on and human disturbances will not lead to the death of a tree that is over 80,000 years old.

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