Dating young (<1000 yrs) lava flow eruptions of Piton de la Fournaise volcano from size distribution of long-lived pioneer trees

International audienceDating recent lava flows is a critical issue to assess the volcanic hazards for nearby human populations, but traditional methods such as radiocarbon dating are often not applicable. We propose here a simple statistical method that relates the age of lava flows of the Piton de la Fournaise volcano (PdF) to the size of Agarista salicifolia (Ericaceae), a long-lived pioneer tree that quickly establishes after eruption and able to live >600 years. We measured the diameter at base of 711 trees on 20 dated lava flows (between 1401 CE and 2007 CE). We used a log-log linear model to assess the relationship between maximum diameter at base of Agarista salicifolia and the age of lava flows, and showed a very strong correlation (R2 = 0.987, p < 2.2e-16). We then used this calibrated model to estimate the age of 11 lava flows between 1447 CE (CI: 1349–1531) and 1823 CE (CI: 1806–1839). These new ages, combined with existing radiocarbon ages and historical records, indicate three clusters of eruptions (1460–1630, 1690–1840, since 1970 CE) affecting both the caldera and the flanks of PdF. We interpret such discontinuous dynamics made of periods of intense and low activity as evidencing pulses of high magma supply since at least the 11th century. Overall, our work shows that dating lava flow with calibration based on the size distribution of long-lived pioneer trees represents an accurate alternative method to redefine the hazard map of lava flow inundation. The existence of long-lived pioneer trees in several volcanic areas provides the opportunity to use the same framework in order to better understand eruption recurrence patterns