UV-B-induced forest sterility: Implications of ozone shield failure in Earth's largest extinction.

Although Siberian Trap volcanism is considered a primary driver of the largest extinction in Earth history, the end-Permian crisis, the relationship between these events remains unclear. However, malformations in fossilized gymnosperm pollen from the extinction interval suggest biological stress coinciding with pulsed forest decline. These grains are hypothesized to have been caused by enhanced ultraviolet-B (UV-B) irradiation from volcanism-induced ozone shield deterioration. We tested this proposed mechanism by observing the effects of inferred end-Permian UV-B regimes on pollen development and reproductive success in living conifers. We find that pollen malformation frequencies increase fivefold under high UV-B intensities. Surprisingly, all trees survived but were sterilized under enhanced UV-B. These results support the hypothesis that heightened UV-B stress could have contributed not only to pollen malformation production but also to deforestation during Permian-Triassic crisis intervals. By reducing the fertility of several widespread gymnosperm lineages, pulsed ozone shield weakening could have induced repeated terrestrial biosphere destabilization and food web collapse without exerting a direct "kill" mechanism on land plants or animals. These findings challenge the paradigm that mass extinctions require kill mechanisms and suggest that modern conifer forests may be considerably more vulnerable to anthropogenic ozone layer depletion than expected.