Restoration of tropical montane cloud forest in bracken dominated pastures: The role of nurse shrubs

Bracken (Pteridum arachnoideum) is an invasive fern species that slows or inhibits forest regeneration over vast areas worldwide, affecting biodiversity, ecosystem functions and productivity. To overcome arrested succession, effective restoration interventions are necessary. We examined the effects of planting pioneer shrubs on establishment success in mid- to late-successional tree species transplants, and their combined effects on bracken dominance in abandoned pastures of former cloud forest in Mexico. We hypothesised that the rapid development of nurse shrubs will reduce bracken growth and facilitate mid- to late-successional tree species establishment. We analysed the effects of planting cuttings of the shrubs Tithonia diversifolia and Sambucus nigra on bracken and on the seedling performance of Juglans pyriformis, Quercus insignis, Meliosma alba and Tapirira mexicana, in three sites under four treatments: (1) trees without shrubs, (2) trees with T. diversifolia, (3) trees with S. nigra, and (4) trees with both T. diversifolia and S. nigra. Increased transplant performance was expected under mixed shrub species, compared to single shrub species plantings, due to complementarity in the use of resources (niche separation). After two years, plantings with shrubs and trees reduced bracken cover by >60% and bracken live rhizome biomass by >50%. Shrubs effects were species-specific, reflecting a complex combination of facilitation and competition mechanisms. In comparison to the trees without shrubs, survival of M. alba and Q. insignis significantly increased with T. diversifolia, T. mexicana survival decreased with S. nigra and J. piriformis survival decreased with T. diversifolia and the mixture. The results support the use of cuttings of T. diversifolia or S. nigra separately, rather than in combination, and the inclusion of mid- to late-successional trees, to assist recovery of degraded cloud forest areas where succession has been stalled by bracken. An integrated management approach that considers synergistic factors causing plant mortality in bracken sites needs to be addressed.