No abstract

We aimed to determine the role of soil nutrients in the herbaceous-tree boundary in a tropical mangrove forest. We assembled plots in this boundary such that each was initially covered by~50% of each life form. We added NPK to three of the plots monthly and monitored them, along with the control plots, for 18 mo. In plots with added NPK, mangrove branches grew over and shaded the herbaceous plants, resulting in the presence of mostly bare soil. The mangrove trees eventually covered~80% of the experimental plots, but the proportion of cover in the control plots remained unaltered. We conclude that soil nutrient level modulates the mangroveherbaceous plant interaction in steps, and from a given level there is a shift in the dominant life form. This mechanism leads to a sharp transition over a smooth nutritional gradient.Abstract in Portuguese is available with online material.

Although fire occurrence plays a central role in the dominance of the super-dominant bracken fern Pteridium arachnoideum in Neotropical forests, it is unclear whether this applies to the Brazilian savanna (Cerrado), which vegetation is naturally adapted to fire. We assessed fire effects on the aboveground biomass of P. arachnoideum in the Cerrado Ecological Reserve in Central Brazil. We measured frond height, density, standing biomass, and litter biomass before and for two years following a prescribed fire, and also sampled a control site to control for other mortality events and for seasonal variation. The average P. arachnoideum frond height and aboveground biomass decreased over three times in the burnt site 18 months after the fire. The frond density decreased both in the burnt and control site, indicating that this may have been caused in another way than by fire. Conversely, litter biomass decreased shortly after the fire in the burnt site, but returned to pre-fire levels in one year. Possible explanations for the lasting negative effects of fire on the height and biomass of P. arachnoideum include the implementation of the prescribed fire early in the dry season, which likely limited immediate clonal regrowth, and the herbivory by leaf-cutting ants. Nevertheless, the effective clonal growth of P. arachnoideum apparently precluded the fire from having stronger effects and hence this species was not excluded from the site. We conclude that direct and indirect fire effects may cause the decline of P. arachnoideum in Cerrado, so that prescribed fires may be applied as a management strategy in Cerrado sites where this species is super-dominant, ideally in combination with other control measures.