Disentangling the roles of bracken fronds and litter on natural seedling recruitment in fire-disturbed tropical montane habitats

Abstract

Tropical montane forests are diverse ecosystems often affected by uncontrolled human-induced fires causing tree mortality and creating large deforested areas. After fires, Pteridium spp. ferns (bracken) often dominate, and forest regeneration in these areas is slow. In this study, we evaluated the effects of bracken fronds and litter, as well as the micro-environmental conditions created by the fern, on the density and species diversity of naturally recruiting seedlings. At eight sites, 120 experimental plots were established among forest and bracken-dominated areas with the following treatments: (a) fronds and litter intact (F+L+); (b) fronds intact and litter removed (F+L-); (c) fronds removed and litter intact (F-L+); and (d) fronds and litter removed (F-L-). After one year, all seedlings were registered, identified and classified according to their life-form (tree, shrub, herb, vine), dispersal vector (wind- or animal-dispersed) and successional status (early-, mid-, and late-successional). For all treatments we assessed 12 micro-environmental variables. We identified 3649 naturally-recruiting seedlings corresponding to 278 species from 70 families. We found positive effects of bracken fronds particularly on tree seedling recruitment: treatments with fronds had greater densities of both animal- and wind-dispersed tree seedlings, 1.8 and 1.4 fold higher, respectively, compared to treatments without fronds. Similarly, the density of early-, mid- and late-successional tree species was 1.3, 1.7 and 1.9 times higher in treatments with than without fronds. Furthermore, species diversity of early-, mid- and late-successional tree species was higher in the treatments with fronds. The environmental conditions generated by bracken presence, such as photosynthetically active radiation, soil temperature, live bracken biomass and litter depth, had positive effects on seedling density of all, animal-dispersed and early-successional tree species, and negative effects on seedling recruitment of other life-forms, notably from early successional stages. To promote forest regeneration in bracken-dominated areas, active restoration measures such as direct seed addition and transplants of nursery-raised seedlings of mid- and late-successional species should be considered. Since bracken can hinder the establishment of some life forms and species but favor others, it is recommended to include a trait-based approach to understand species responses to environmental factors and select species that are optimally adapted to bracken-dominated habitat conditions to promote forest regeneration.