Giada Centenaro and an international team of researchers have discovered that three clonal plants of the dwarf willow, Salix herbacea, growing in the Northern Apennines mountains of Italy are at least 2000 years old, making them the oldest known clones of this species. The findings were published in the American Journal of Botany. The ancient ages give clues into how this arctic-alpine species has persisted for millennia at the southern edge of its range.
Challenges in Dating Ancient Clonal Trees
Traditional tree dating methods don’t work well for clonal trees like these ancient Salix herbacea individuals. Unlike non-clonal trees, where counting annual growth rings on the trunk gives a tree’s age, clonal genets consist of interconnected subunits called ramets. The ramets are the things that look like the above-ground trees and they can persist for decades but not nearly the centuries or millennia lifespan of the entire genet. As ramets continually die off, they are replaced by new ramets regenerating from the same parent genet.
So the genets can live for centuries or millennia, even as individual constituent ramets live much shorter lives. This modular growth pattern makes accurately ageing ancient clonal genets very difficult. Their growth spans changing environmental conditions over enormous timescales.
Studying the Ancient Clones
To study these ancient willow clones, Centenaro and colleagues first used genetic analysis to map out the spatial extent of the clonal population on Mount Prado, identifying which sampled ramets belonged to the same genet based on their genetic fingerprints.
The entire clonal population covered an area of 1.5 hectares. Within this, they found some genets with a 60-metre diameter. In these genets, the scientists then examined how fast the ramets grew.
The researchers then took detailed growth measurements along marked stem sections to calculate annual growth rates. They counted growth rings and bud scars on cross-sections at different distances along the stems. This data allowed them to model annual longitudinal growth rates.
Finally, they estimated the minimum and maximum ages of the three target genets by dividing their diameter by the annual growth rates. This indirect “linear method” accounts for variability in the growth rate measurements.
Unfortunately, as the plant is constantly renewing itself, more direct dating methods are impossible. In an email to Botany One, Giada Centenaro explained:: “
Unfortunately, as the plant is constantly renewing itself, more direct dating methods are impossible. In an email to Botany One, Giada Centenaro explained: “Radiocarbon has not been used because there is no remaining part (roots, root collar, or stems) of the first ramet (for each genet) from which the genets growing on Mt Prado originated (the ramets last on average less than 50 years before deteriorating).”
What These Ancient Clones Reveal
Centenaro and colleagues estimate these Salix herbacea clones likely established themselves somewhere between 2000 and 7000 years ago in the Northern Apennines mountains. This wide range comes from estimating a minimum and maximum age for each genet based on the annual growth rate measurements.
The estimates indicate the clones established after glaciers retreated from the mountain tops following the last glacial maximum. While the most conservative minimum estimate is around 2000 years, the clones could be even older. The maximum date is 7000 years, which would be around the start of the Neolithic.
The ancient ages and large sizes of these clones give them great value in understanding the longevity and persistence of clonal plant species. Their location near the southern edge of the Salix herbacea range also makes them valuable models for studying the dynamics of range edge populations. The researchers noted the clones’ ages represent the oldest recorded individuals of this species.
Remaining Questions and Next Steps
While these growth ring analyses add to knowledge of ancient clonal plants, some questions and uncertainties remain that could be addressed by future work.
Potential uncertainties from missing or double-counted rings could skew the age estimates. The simple assumptions of consistent radial growth also do not capture real complexities over millennia. Additionally, continued modelling and measurements could better account for environmental factors influencing nonlinear growth over centuries. This could provide further details about the origins and development of the clones.
Despite the questions that persist, this work significantly pushes back the confirmed ages for Salix herbacea clones. The findings provide an improved understanding of clonal plant biology and longevity that will inform future climate change studies. Ongoing research can build on the insights gained into these remarkable ancient willows growing in the Italian mountains.
READ THE ARTICLE
Centenaro, G., Petraglia, A., Carbognani, M., Piotti, A., Hudek, C., Büntgen, U. and Crivellaro, A. (2023) “The oldest known clones of Salix herbacea growing in the Northern Apennines, Italy are at least 2000 years old,” American Journal of Botany. Available at: https://doi.org/10.1002/ajb2.16243.
Updated 6th October 2023 to clarify details on the date range, and to clear up a mess I’d made of the radiocarbon section. Many thanks for a kind email to Giada Centenaro.