Volcanic ash deposition as a selection mechanism towards woodiness

Abstract

The high proportion of woody plant species on oceanic islands has hitherto been explained mainly by gradual adaptation to climatic conditions. Here, we present a novel hypothesis that such woodiness is adaptative to volcanic ash (tephra) deposition. Oceanic islands are subject to frequent eruptions with substantial and widespread ash deposition on evolutionary time scales. We postulate that this selects for woodiness through an increased ability to avoid burial of plant organs by ash, and to re-emerge above the new land surface. We sense-checked using observations of plant occurrences and distributions on La Palma (Canary Islands) in April 2022, 4 months after the end of the eruptions of the Tajogaite volcano (Cumbre Vieja ridge). In contrast to herbs and grasses, most woody plants persisted and were already in full flower in areas with 10+ cm ash deposition. Remarkably, these persisting woody plants were almost exclusively endemics.

Fig. 1. Volcanic ashes.

Ash (tephra) deposition during the recent eruption of the Tajogaite volcano in 2021 on La Palma, including the depth of the newly formed ash layer, the location of the new craters, and the lava flow (own records for ash thickness).

Fig. 2. Impact and recovery.

a Flowering Single Island Endemic shrub Echium brevirame at a site with 30 cm fresh ash layer, 4 months after the end of the eruptions. b Resprouting canopy branches of Pinus canariensis that were exposed to toxic sulphur gases during the eruptions, sticking out of metres-deep ash deposits. (Fotos by C. Beierkuhnlein).

Fig. 3. Theoretical concept.

Hypothesised selection of woody plants through ash deposition. a Understorey plant life-form spectrum before ash deposition on developed oxidized soil. b New volcanic ash deposition covers previous vegetation structure (here ~0.5 m depth), causing photosynthetically active organs to die. Exposed leaves may become chlorotic (yellowish) from toxic gas impact and are likely to be shed. c Death of plants that cannot reach the new surface (geophytes, graminoids, forbs). These may become locally extinct (red X). Resprouting of woody plants that protrude from the ashes (trees, larger shrubs) or reach the new surface with new shoots (small shrubs). d flowering and reproduction of remaining species populations. e dispersal of propagules, germination, and establishment on the newly formed soil surface while parent plants remain alive.