Plant growth responses to resources may be an important mechanism that influences species coexistence and community structure. While irradiance is regarded as the most important resource for seedling growth in wet tropical forests, the spatial variation of multiple soil resources have yet to be evaluated across a range of species and at a fine seedling-appropriate scale. The objectives of this research were to compare soil resource availability among sites and characterize their spatial structure, determine the appropriate spatial scale to measure soil resources to understand seedling responses, and identify the resources that best predicted Neotropical seedling growth. We measured irradiance and an array of soil resources over a fine spatial scale (1 m2) in 5 sites of an old-growth, wet tropical forest at La Selva Biological Station, Costa Rica. Soil resource availability and heterogeneity appeared to correlate with soil type and their importance in plant-nutrient cycles, thus to understand seedling responses soil resources, essential plant nutrients, such as inorganic nitrogen, phosphorus, and base cations, should be measured at a seedling-level spatial scale, while other resources, such as total nitrogen and carbon may be measured at coarse spatial scales (>10 m). Light limitation in the forest understory also did not preclude seedling growth correlations with soil nutrients, which were equally prevalent and as strong as with irradiance. These results challenge the idea that soil resources are not important to seedling performance in the light-limited understory.
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