Volcanic ash deposition shapes spatial patterns of insular woodiness

Insular woodiness is common on oceanic islands, yet its drivers remain poorly understood. A recent hypothesis proposes that volcanic ash deposition acts as a selective mechanism promoting woodiness in flowering plants. We test this hypothesis on the island of La Palma (Canary archipelago) by comparing plant communities in the volcanically active south and the dormant north. We surveyed 132 plots in two main ecosystems, Canary pine forest and succulent scrub, recording species richness, cover, and woodiness type (ancestral/derived vs. insular) of the vegetation. When controlling for climatic differences, we found that woody species, particular insular endemics, were significantly more common in the southern succulent scrub, whereas herbaceous species dominated the north. In contrast, the Canary pine forest showed no significant regional differences. This discrepancy may be attributed to the low inherent species richness of the Canary pine forest understory, which limits the potential for detectable shifts in woodiness compared to the more diverse succulent scrub. These patterns provide evidence that long-term volcanism influences local plant communities, but that this effect is also likely mediated by ecosystem gamma diversity.