Weak allelopathic effects of bracken fern (Pteridium esculentum subsp. arachnoideum) on early life stages of native tree species in a Neotropical montane forest

The allelopathic effect of Pteridium ferns (hereafter bracken) is considered an important factor hindering forest recovery. We examined the effects of bracken allelopathy on seed germination, seedling emergence, survival, and relative growth rate (RGR) of two native tree species, Morella pubescens and Vismia plicatifolia, in a tropical montane forest. Two potential allelopathic sources, fronds and litter, were considered, along with site age (using soils from young and old bracken-dominated sites, burned < 5 and > 10 years ago, respectively). In a greenhouse experiment, resembling bracken conditions of shade, we conducted a fully crossed block experiment with a Control and three treatments: (1) soil + activated carbon (AC, to adsorb allelopathic compounds), (2) soil + bracken (Bracken), (3) and soil + activated carbon + bracken (Bracken + AC), using soils from old and young sites. In a laboratory experiment, four aqueous extracts of rhizomes, litter, and fronds (from old and young sites) were used to water seeds at low and high concentrations and to compare their effects with the control treatment (water). Results showed low allelopathic effects on seedling emergence in soils from young sites for both species, with activated carbon mitigating these effects. RGR was higher in soils from young sites for M. pubescens seedlings, contrasting with soils from old sites. Aqueous extracts from bracken fronds exhibited a 20% reduction on seedling survival of V. plicatifolia, while other extracts had no significant impact on germination or seedling survival. We discuss the implications of these mild allelopathic effects on the impediment to forest succession in tropical bracken-dominated areas.