Fire enables passive directed seed dispersal by birds

Recent work proposes that passive directed dispersal (PDD) occurs when resource tracking leads animal vectors to disperse seeds at sites that favor plant establishment. For example, wildland fire can attract seed-dispersing animals via the magnet effect while simultaneously generating the environmental conditions that benefit animal-dispersed plants. To evaluate the hypothesis that frequent fire results in PDD, we first documented the effect of frequent fire on locally common bird-dispersed plants known to be promoted by fire by measuring frequency of occurence along transects and per-individual fruit production in a long-term replicated fire experiment. Two of these focal plant taxa occurred more frequently in plots burned biennially compared with plots that remained unburned for approximately six decades. All focal plant taxa produced more fruit per individual with recurring fire. Next, we used a paired crossover experiment to measure bird activity and bird-dispersed seed rain at perches after fire. Fire increased seed dispersal distance and elevated seed loss from seed traps. When adjusting for these confounding factors, the magnet effect of recent fire resulted in greater projected seed rain from focal plant taxa arriving in recent burns compared with areas burned 1-2 years prior. Collectively, these results suggest that wildland fire generates the necessary conditions for PDD. With the prevalence of fire in terrestrial ecosystems, PDD may thus be a widespread process shaping global plant communities.