Volcanic deposits select for woodiness on islands

Oceanic islands’ unique biodiversity often arises from endemic woody plant radiations derived from non-woody continental ancestors. Since Darwin, various theories have attempted to explain the evolution of woodiness on islands (insular woodiness), but the mechanisms remain controversial. Recently, vegetation burial by recurrent volcanic pyroclastic deposits (tephra) has been linked to the abundance of woody species on oceanic islands. 

We investigated plant community and trait composition using a chronosequence of five tephra fields on La Palma (Canary Islands), ranging in age from 54 to 6,000 years. We tested whether woody species abundance and the evolutionary origin of woodiness – insular woodiness vs. woody species with woody continental ancestors – were modulated by tephra field age and distance to the crater. 

Woody growth forms dominate tephra fields, providing first evidence that pyroclastic deposits can select for woody species. Endemic, insular woody species are dominant on young fields and areas close to the crater, whereas woody species with woody continental ancestors become dominant on older fields and more distal areas. Functional trait space strongly overlaps among tephra fields indicating trait convergence. We found no trend in functional richness and dispersion. All these observed patterns occur independently of phylogenetic relatedness.

Volcanic pyroclastic deposits impose a strong environmental filter favoring woody species and their traits on islands. Endemic, insular woody species show increased colonization and establishment ability on extreme habitats following volcanic eruption, highlighting their acquired functional advantages. This illustrates that pyroclastic deposition structures communities ecologically. Over evolutionary timescales repeated environmental filtering by pyroclastic deposits favors the emergence and persistence of woodiness in oceanic island ecosystems. This sheds light on the role of repeated environmental filtering during evolution.