BayCEER Workshop 2026

Volcanic tephra deposition shapes spatial patterns of insular woodiness

Insular woodiness – the evolutionary transition from herbaceousness toward woodiness in plant species on islands – is common on oceanic islands, yet its drivers remain debated. A recent hypothesis proposes that volcanic deposits (tephra) act as a selective mechanism promoting woody growth forms in flowering plants. Here, we test predictions from this hypothesis on the island of La Palma (Canary Islands, Spain) by comparing plant communities on recent tephra depositions in the volcanically active south with the volcanically dormant north. We surveyed 132 vegetation plots in two of the island’s major ecosystems, the canary pine forest and the succulent scrub. We recorded species richness, cover, and for species with woody growth form the woodiness type: woody island species with woody continental ancestors vs. insular woody species stemming from herbaceous continental ancestors. When controlling for climatic differences, we found that woody species, particularly endemic, insular woody species, were significantly more common in the southern than the northern succulent scrub. In contrast, the canary pine forest ecosystem showed no significant north–south differences regarding woodiness. This discrepancy may be due to both the generally low understorey species richness of the canary pine forest and the frequent pine forest fires, which limit the potential for detectable shifts in woodiness. These findings are consistent with long-term volcanism being an ecological filtering mechanism that shapes community assembly, though its effect on the prevalence of insular woodiness may be mediated by ecosystem type.