An experimental study of regeneration on landslides in montane rain forest in Jamaica

Tree seedlings of Alchornea latifolia (Euphorbiaceae), Clethra occidentalis (Clethraceae), and Vaccinium meridionale (Ericaceae), were transplanted into five recent 'debris flow' landslides, and five adjacent understorey areas, in montane forest between 1450 m and 1770 m in the Blue Mountains, Jamaica. Organic fertilizer and protective barriers were applied in a 2 X 2 factorial design. Mortality (all species) in the 15 months following transplanting was 30% on landslides, and 46% in the understorey. Mortality was significantly higher in the understorey than on landslides for Alchornea and Vaccinium, but was not different for Clethra. In the understorey there was a peak of mortality for Alchornea and Vaccinium in the dry season, but no associated peak on landslides. There was no effect of fertilizer or barrier treatment on seedling survival. Height growth of unfertilized Clethra and Vaccinium (but not Alchornea) seedlings was greater on landslides than in the understorey. On landslides fertilization increased biomass (but had no effect on height) for all species. There were no significant effects of fertilizer in the understorey. Soil nutrient concentrations were much higher in understorey soils than landslide soils, pH values were similar. Foliar nitrogen concentrations for transplanted seedlings were lower on landslides than in the understorey, while foliar phosphorus concentrations were not different. Fertilization did not change foliar nitrogen concentrations on either landslides or the understoreys, whereas there was a significant increase in phosphorus concentration in fertilized seedlings on landslides. Soil water content varied less on landslides than in understoreys; soils from three of the landslides were judged to be near field capacity throughout the year. In the understoreys there were significant shortages of water in the dry season which correlated with increased mortality in the transplanted seedlings. Thus we have shown that growth of seedlings on landslides is limited by nutrient supply; water supply and soil movement do not limit growth of established seedlings. In the understoreys seedling growth is limited by photosynthetically active radiation, nutrient supplies may limit seedling growth due to root competition, and seasonal drought is correlated with increased mortality.