Functional trait divergence of juveniles and adults of nine Inga species with contrasting soil preference in a tropical rain forest


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1. Community-wide studies have shown that functional traits of tropical trees vary with soil-type preference, but few have examined trait diversification among closely related taxa. In this study, we asked how functional traits of adults and saplings within a speciose genus, Inga, differ in relation to their soil-type preferences.

2. We quantified soil-type preference and functional traits of nine Inga species (Fabaceae) in the wet tropical forest at La Selva Biological Station, Costa Rica, where rich alluvial soil and less fertile residual soil of volcanic origin occur in close proximity.

3. From the extensive trail network, we randomly selected 104 trail segments, each 50 m long, along which all Inga individuals >50 cm tall were counted and identified to species. Five and three common species showed significant bias to residual or alluvial soil, respectively, while the remaining one showed no significant bias. For these nine species, we quantified five leaf traits for saplings and adults, as well as wood-specific gravity (WSG) and three seed traits.

4. For both adults and saplings, leaf mass per area (LMA) and lamina density were higher for residual-soil specialists than fertile alluvial-soil specialists. The opposite trend was found for nitrogen (N) and phosphorus (P) concentrations in leaves. Lamina thickness did not differ between soil-type preference groups. All leaf trait values except N concentration increased from saplings to adults, with adults showing larger interspecific variations in trait values than saplings.

5. Saplings and adults of the single species found on both soils had trait values intermediate between residual and alluvial-soil specialists.

6. Seed mass and seed N, as well as WSG of adults, were higher in residual-soil specialists than alluvial-soil specialists. Higher seed N indicates higher maternal investment of species growing on less fertile soil.

7. These differences in traits among closely related species across continuous soils suggest that habitat filtering has contributed to the evolution of seed, juvenile and adult traits within a speciose tropical tree genus.