Instead of covering the ground, they grow up through woody branches and twigs: Charles Darwin was already amazed at the strange variety of woody plants on volcanic islands. Observations after eruptions on La Palma now suggest that it is an adaptation to the frequent ash rain: In contrast to herbaceous plants, the trees and shrubs on the Canary Island can survive even after an ash covering of up to half a meter thick.
Actually, volcanoes are better known for their destructive power - but they can also develop creative potential: Especially on volcanic islands, nature is often particularly lush and a surprising number of living beings have developed into new species. This life-giving and diversity-promoting effect has to do with various factors: The fire mountains in the sea can provide for different ecosystems with different conditions in terms of temperature, humidity, light radiation and nutrient supply in a relatively small area. This results in a relatively large number of niches – life unfolds.
Strange trees in sight
One of the special features of the nature of many oceanic volcanic islands is a special woody plant community. The comparison with biologically related species on the mainland is particularly surprising. For many of the woody island species belong to families that are represented on the mainland by herbaceous plant species. In concrete terms, this means that groups whose representatives only grow on the ground there, such as thistles, have produced forms on volcanic islands whose leaves are attached to tall, wooden plant parts. So far, this development has been explained by competition for sunlight or by adaptation to drought. Nevertheless, this special species development on the volcanic islands remained a mystery.
As reported by the team around Carl Beierkuhnlein from the University of Bayreuth, the newly discovered drivers of the evolutionary phenomenon emerged as a result of the violent volcanic eruptions that occurred on La Palma in 2021. “The eruptions of the Tajogaite volcano presented a unique opportunity for biogeographical studies. For many years, geological and climatic changes on the Canary Islands have been scientifically precisely recorded, so that new observations regarding the insular vegetation can be easily integrated into larger explanatory contexts," says Beierkuhnlein.
Survival after the ash rain
Four months after the end of the eruptions, he and his colleagues studied the geological and biological consequences of the ash rain on the island of La Palma. As the team reports, the volcanic eruption had covered the entire island surface with the pyroclastic material, which reached considerable thicknesses in some areas. Surprisingly, however, many woody plants that only occur on La Palma survived the up to half a meter deep burial and were in full bloom again after just a few weeks. Most of the herbs and grasses, on the other hand, had died under the layers of ash. Apparently, therefore, the elevated and resilient structures allowed the island trees to survive the volcanic cataclysm.
As the team explains, it is therefore obvious that the selection pressure has led to the development of special woody plants on La Palma and also on other oceanic volcanic islands due to frequently recurring ash rain events. According to this, populations of herbaceous plant species developed over many generations to become endemic woody plants due to the increase in the proportion of wood.
"Other factors, such as climatic conditions, could have further favored and increased the growth of woody plants on oceanic islands. But our observations and measurements on La Palma support the assumption that volcanism is a previously underestimated driver of evolution on oceanic islands. The biogeographical and ecological consequences of volcanic eruptions should therefore be examined more closely than has been the case in the future,” concludes Beierkuhnlein.
Source: University of Bayreuth, specialist article: Nature, npj biodiversity, doi: 10.1038/s44185-023-00018-2