Seed dormancy explains plant response to mass mortality events

Abstract Mass mortality events (MMEs) are large‐scale, rapid die‐offs resulting in extreme inputs of carrion biomass. Recent work demonstrates the effects of increasing carrion biomass on plant communities modulated by vertebrate scavengers and herbivores. However, the mechanisms underlying plant response to MMEs remain unclear. We hypothesized that carrion decomposition would interact with vertebrate herbivory and scavenging to influence plants grouped by three seed dormancy classes (no dormancy, physiological dormancy and physical dormancy). We designed a replicated field experiment crossing two levels of carrion biomass (~30 and ~360 kg) with three levels of vertebrate exclusion (open/no exclusion, scavenger exclusion and herbivore exclusion) to quantify plant turnover, plant performance and the response of seeds belowground and on the soil surface. We measured carrion decomposition rate, plant tissue nutrients (N, P and K), seed survival, plant height, flower production and plant community changes across 3 years. Carrion biomass levels associated with mass mortality increased plant tissue nutrients and plant mortality and decreased belowground seed bank survival, likely promoting plant colonization from seed rain. Then, vertebrate exclusion determined resulting plant turnover in different ways, depending on seed dormancy classes. Vertebrate scavenger exclusion delayed advanced decomposition, and the resulting environment provided a relative advantage to seeds with impermeable seed coats (physical dormancy). In contrast, vertebrate herbivore exclusion promoted rapid germinators (no dormancy) that arrived as seed rain and quickly capitalized on the nutrient‐rich exposed soil. With both functional roles intact (open/no exclusion), vertebrate scavenging ameliorated the negative effects of decomposition on seeds and plants, and selective herbivory on nutrient‐rich plant tissue reduced overall plant height and flower production. The activity of both functional roles favoured plants of the dominant physiological seed dormancy class and those with physical dormancy. Our results suggest that seed response to carrion decomposition and vertebrate functional activity can inform plant community response to MMEs. Read the free Plain Language Summary for this article on the Journal blog.