Facilitation by tussock-forming species on seedling establishment collapses in an extreme drought year in a post-mined Sphagnum peatland

Questions Does the facilitation of herbaceous species seedling establishment by tussock-forming species collapse in an extreme drought year? Do different germination phenologies, between the seedling species, influence the variation in facilitation? Location A post-mined Sphagnum peatland at Sarobetsu mire, Hokkaido, northern Japan. Methods The effects of tussock-forming species, as potential facilitators, on seedling emergence and survival of five perennial herbs were examined during a 4-yr seedling monitoring, which included an extreme drought. The general effects of tussocks on each seedling species during annual variations were simultaneously assessed using hierarchical Bayesian analysis. To examine the stress limiting seedling establishment, which was ameliorated by the tussocks, micro-environments (i.e. light, temperature, water content and erosion) altered by morphological traits of the tussocks with litter cover were compared to those in open areas. Results In general, tussocks facilitated seedling establishment of all species through a positive effect on emergence and no effect on survival; however, the facilitative effect declined for several species in an extreme drought year. Peat erosion was the limiting stress on seedling establishment in normal years, however, peat water content also limited seedling establishment in a drought year. During the drought year, the positive effect of tussocks on seedling emergence for species that germinated before or during the drought was weakened for one species, and a negative effect on seedling survival emerged in another species. In contrast, species that germinated after the drought did not exhibit annual variation in the effects of tussocks on seedling emergence and survival. Conclusions Our results illustrate that collapse of facilitation arises under extremely severe conditions through addition of infrequently occurring stress (drought) on frequently occurring stress (peat erosion). Variation in seedling emergence and survival of the five study species in response to severe drought suggests that species-specific traits must be considered when assessing how facilitation varies temporally. Further, these findings suggest that facilitation in temporally varying environments can drive plant community composition and dynamics.