Testing the importance of ectomycorrhizas and nutrients for the growth of dipterocarp seedlings in Borneo

PREMISE: A number of ecologically important tropical trees form symbiotic ectomycorrhizal (EcM) fungal associations including the Dipterocarpaceae, that dominate lowland forests of South-east Asia. Whilst numerous pot-based studies have focused on the importance of EcMs for dipterocarp seedling growth and performance, few field studies have been undertaken. METHODS: In a 20-month field experiment in Malaysian Borneo, two species with contrasting light requirements-shade-tolerant Hopea nervosa and the more light-demanding Parashorea tomentella-were subjected to the factorial addition of fungicide (to reduce EcM colonization) and nutrients. RESULTS: Fungicide addition reduced EcM colonization by a small but significant percentage. Reductions in foliar nitrogen, phosphorus, and chlorophyll concentrations in both species and in calcium and magnesium concentrations in H. nervosa did not translate into reduced biomass in either species. When given additional nutrients, H. nervosa had no increase in foliar nutrient concentrations or biomass, but P. tomentella had an increase in foliar nitrogen, phosphorus, and magnesium concentrations and more than doubled its biomass. When nutrients were added but EcM reduced, P. tomentella did not increase in biomass. CONCLUSIONS: EcM fungi can play an important role in influencing dipterocarp seedling mineral nutrition, and P. tomentella may require EcMs to effectively utilize additional mineral nutrient sources. The importance of nutrient uptake for biomass production is less clear and may be confounded by the use of fungicide to control EcM colonization.