Plant species were previously believed to associate with only a single mycorrhizal type, but there is increasing evidence of dual mycorrhizal colonization in a single root system. The mechanisms that regulate dual mycorrhizal colonization are still poorly understood. I propose a field experiment that will isolate the environmental regulators of dual mycorrhizal symbiosis and assess patterns of dual colonization across a natural gradient of varying abiotic and biotic factors at Harvard Forest. I will be addressing how the availability of different nutrient pools, carbon supply, and mycorrhizal propagules influence a plant’s symbiont preferences.