The mountains are getting warmer, and willow colonies are shrinking

Salix herbacea, as other arctic-alpine species, likely found a refuge from the Ice Age in the Apennines. As the climate changed around them, the trees survived in a fragmenting population. This fragmentation has genetic consequences.

The Apennines are the mountains that form the spine of Italy. In some ways their forests can act as guardians of the past. Among the Apennine trees wolves and bears still roam. But some of the trees themselves are endangered species. Michele Carbognani and colleagues have been investigating populations of Salix herbacea L. In a paper in Annals of Botany, the authors say this is a plant in urgent need of study: “The current and unprecedented warming trend calls into question whether these S. herbacea populations can keep up with fast and dramatic environmental changes, including prolonged snow-free periods, a higher frequency of summer heatwaves and increasing competition with taller sub-alpine shrubs.”

Female Salix herbacea
Salix herbacea. Image: El Grafo / Wikipedia

S. herbacea should be a plant that’s well-equipped to survive. Not only does it sexually reproduce but, when that’s not possible, it can clone itself too. These clones can transfer water and nutrients through the group, so that when challenges come, there are always trees that can help the genet, the colony of clones, survive. It’s this ability to survive, and living in conditions where it has to be a survivor, that attracted the botanists to study the plant, as co-author Andrea Piotti explained. “Our research group has a long tradition of studying the ecological consequences of climate change on snowbed species. Not only is Salix herbacea, the dwarf willow, is fascinating on its own – Linnaeus defined it as “minima infra omnia arbore“, the World’s smallest tree – but is also a species with a widespread distribution in the Northern Hemisphere which is highly threatened by increasing temperatures. For these reasons we, therefore, believe S. herbacea has the right credentials to become a model species with which to study adaptation to a changing environment.”

The S. herbacea colonies in the Apennines are now under increasing pressure in modern times, Carbognani said. “The populations of S. herbacea are retreating from sites where the growing season is becoming longer and warmer. Since the snow cover period is decreasing in snowbeds, other species such as Vaccinium spp. are becoming more and more competitive, strongly reducing the S. herbacea ecological niche at Mediterranean latitudes. It is, however, interesting to note that Mediterranean mountains represent the most likely places where genetic variation useful for facing climatic changes are most likely to have arisen. This history means these remnant populations could become a reservoir of potentially pre-adapted genotypes of invaluable importance for the species persistence in a warmer world.”

While S. herbacea can survive through clonal reproduction, there is a cost to this lifestyle. The more the plants depend on cloning, the more the population loses genetic diversity. Co-author Alessandro Petraglia said this led to surprising differences between the Apennine willows and their alpine relatives. “We expected a difference between alpine and Apennine populations, but we were astonished by how different the two Apennine populations were in terms of clonal and genetic structure. Apennine populations are both small and extremely isolated, but the fact of being a few dozen meters smaller has nearly driven the smallest population to the edge of extinction. Besides this, it is impossible not to be impressed by the size of the largest clones found. This is an extremely slow-growing species – it is estimated that 4-m wide individuals are approx. 500-yrs old – but here we found diameters of individuals of up to 70 m. We are currently collaborating with dendro-anatomists from the University of Cambridge to obtain a reliable estimate of their age based on growth rates…expect surprises!”

However, for the smallest population, at Mt Cimone, time is running out. The genetic diversity of the colony is extremely low. With this lack of diversity, it doesn’t have much chance of mixing and matching genes to hit upon better-adapted combinations as temperatures climb. While the problem is for S. herbacea at Mt Cimone today, it the study has lessons for plants elsewhere in cold, but warming, locations, say the authors. ” These results will be relevant for several other Salix species that are widely distributed across the Arctic and alpine areas of the world. Dwarf shrubs constitute a very prominent functional group across the Arctic and alpine tundra landscape, therefore several ecological processes rely on the presence and growth of these species. We would like to test the relationship between isolation and genetic diversity in other S. herbacea populations across the southern European mountain ranges, and then expand our investigation to several other tundra species that, unfortunately, are nowadays reduced to few, extremely isolated populations at their southern edge.”

Extinction is final, for the species concerned, but for ecologists it is more of a moving target. As species become extinct, so others get closer to the edge. The team say that understanding how these plants disappear will help ecologists prepare for future projects. “It is likely that some southern populations of several arctic-alpine species will disappear in the future. There is general agreement about the likelihood of such an outcome among plant ecologists but, surprisingly, the mechanisms and causes of this phenomenon have been scarcely studied.”

“Describing in unprecedented detail what the ongoing demographic and genetic dynamics at the extremes of species’ distribution are will raise our understanding of the impact of climate change and, hopefully, provide information leading to possible conservation strategies for threatened plant communities and plant genetic resources likely on the verge of extinction.”