Combining mechanical control and tree planting to restore montane Atlantic forests dominated by the Neotropical bracken (Pteridium arachnoideum)

Abstract

Climate and land-use changes have driven the dominance of native herbaceous plants in degraded tropical forests, leading to losses in biodiversity and ecosystem services. However, controlling these super-dominant species to promote forest regeneration is often ineffective and may favour undesirable species. Native clonal ferns from the Pteridium genus often dominate degraded tropical forests subjected to repeated fires. Although many ways of controlling these species have been proposed, the efficiency of different combinations of strategies and their effect on forest recovery remain unknown. Here, we assessed to what extent removing the above-ground biomass of the shade-intolerant Pteridium arachnoideum and planting trees contributed to the control of this fern and the recovery of montane Atlantic forests in south-east Brazil. In four sites dominated by P. arachnoideum we applied five treatments in adjacent plots: (1) control without interference, (2) cutting twice per year, (3) cutting twice per year plus tree planting, (4) cutting three times per year, and (5) cutting three times per year plus tree planting. Over two consecutive years we repeated the treatments and monitored P. arachnoideum height, rhizome and above-ground biomass, as well as the composition of the established vegetation and soil seed bank. We also assessed the survival and growth of the planted trees. Tree planting accounted for over half of the treatment costs, whereas increasing the cutting frequency only increased costs by 15 %. After two years, the three times per year cutting treatment caused the largest decrease in above-ground biomass (1 to 0.1 kg/m2) and abundance (80 to 15 %) of P. arachnoideum. Planted trees exhibited low survival (47 %) rates and growth (30 cm/year), and hence had no effect on the dominance of P. arachnoideum. Treatments did not affect rhizome biomass, suggesting that P. arachnoideum could regain its dominance if cutting treatments were discontinued. Although two annual cuts slightly favoured tree regeneration, trees were rare in both the established vegetation (less than 10 %) and seed bank (less than 0.3 seeds/dm3). Conversely, under three annual cuts the African grass M. minutiflora became more abundant in both the established vegetation (5 to 25 %) and seed bank (1 to 4 seeds/dm3). Our findings show that intensive mechanical removal decreases the dominance of P. arachnoideum in Atlantic forests. However, its benefits to forest recovery seem to be limited by lack of tree regeneration, impoverished seed banks and invasive grasses. Therefore, planting fast-growing and drought-tolerant tree species may be essential to the recovery of these degraded Neotropical forests.