Volcanic eruption and wildfires as compounding drivers of first-year seedling establishment in Canary pine

On the island of La Palma, located in the Canary Islands, Spain, the Canary pine forest is largely unmanaged and depends on natural regeneration for sustainable population dynamics. Canary pine ( Pinus canariensis C.Sm. ex DC.) has been continuously exposed to volcanic eruptions over evolutionary time scales. The species exhibits many adaptations to wildfires, but the current fire regime is likely not natural. While both volcanic eruptions and wildfires can devastate existing vegetation and unbalance ecosystems, they can also facilitate plant growth through mechanisms like nutrient release and increased light availability. Occasional successful regeneration events, driven by high first-year seedling establishment following a disturbance, could be essential to maintaining population structures. We investigated the interactions of volcanic eruption and past wildfires on first-year seedling establishment in the Canary pine forest after the 2021 Cumbre Vieja volcanic eruption. We combined in-situ seedling abundance data from 117 plots (5 m radius) with remote sensing to test the hypotheses that (1) the favorable conditions created by the eruption triggered a localized pulse in first-year seedling establishment of Canary pine and (2) seedling establishment was diminished in areas affected previously by wildfires (2012 and 2016). Using a two-part approach, consisting of univariate analysis of individual factors and multivariate analysis with generalized additive models, we find evidence consistent with our hypotheses. Plots located closer to the volcano and more heavily impacted by the eruption were significantly associated with higher seedling abundance (> 50 per plot). Furthermore, plots that had experienced prior burning in addition to volcanic impact showed lower seedling densities compared to plots impacted solely by the volcano. This suggests that fire history negatively influenced first-year seedling establishment following the eruption. We discuss the role of both wildfires and volcanic eruptions in the evolutionary history of Canary pine and highlight the task of disentangling the legacies of these two disturbances. Serotiny, traditionally considered to be a fire-specific adaptation, appears to also function after and increase the resilience of Canary pine to volcanic eruptions at the stand-level. Lastly, we raise the question of whether recurrent disturbances exceeding natural system dynamics could endanger the future demography of Canary pine by limiting infrequent but necessary forest regeneration events. • First-year seedling establishment was increased in the forest near the 2021 volcanic eruption. • Stands affected by the 2016 wildfire exhibited comparatively lower seedling establishment after the eruption. • Serotiny, normally a fire-specific adaptation, seems to function alongside epicormic resprouting after volcanic eruption. • Both volcanic eruptions and wildfires probably shaped the evolutionary history of Canary pine. • Compounding disturbances with shortened return intervals due to human triggers may threaten long-term population dynamics.