This study investigated the effects of light and soil fertility, on arbuscular mycorrhizal fungi (AMF) colonization, and the growth responses (height and dry mass) of Syzygium seedlings. Seedlings of four Syzygium spp. were grown for 2 y in six different light treatments at the research station of the Sinharaja Forest, Sri Lanka. The light treatments exposed seedlings to: (1) 3%; (2) 16%; (3) 50%; (4) 100% of full sun (control); (5) short periods (2 h d−1) of direct sunlight; and (6) long periods (6 h d−1) of direct sunlight. In the 16% of full sun treatment five sets of fertilizer applications supplied: (1) magnesium; (2) potassium; (3) phosphorus; (4) all three nutrients combined; and (5) no fertilizer (control). The Syzygium species had the greatest mycorrhizal colonization in brighter treatments that provided direct light. Comparison across species revealed S. firmum to have moderate mycorrhizal colonization but high total dry mass. Syzygium operculatum had high percentages of mycorrhizal colonization while S. rubicundum had low percentages of mycorrhizal colonization especially in deep shade. Syzygium makul showed moderate levels of mycorrhizal colonization and dry mass, but low height growth. Among fertilizer applications, phosphorus enhanced seedling growth and mycorrhizal colonization for all species. However, species showed decreased growth with high amounts of potassium and combined fertilizer applications. Results suggest that AMF colonization will be highest, and Syzygium spp. growth greatest, beneath canopy openings large enough to receive direct sun in phosphorus-rich soils.