Soil type specialization and functional traits in Inga (Fabaceae): Comparison between sapling and adult stages

Background/Question/Methods Several recent community-wide studies have shown that leaf functional traits exhibit convergence in relation to habitat specialization across a broad taxonomic range of tropical trees. Comparison among closely related taxa, however, should allow more direct assessment of the evolutionary patterns of leaf functional traits. Here, we focused on a speciose genus, Inga (Fabaceae) in the wet tropical forest at La Selva Biological Station, Costa Rica, where rich alluvial soil and infertile residual soil of volcanic origin occur in close proximity. Our objective was to determine how functional traits associate with preference for soil types at juvenile and adult stages. The soil type preference was assessed by surveying along 5.2 km of trail and recording all Inga individuals above 50 cm tall for a total of 15 Inga species. We quantified six leaf traits (leaf mass per area (LMA), lamina thickness, lamina density, blade nitrogen (N), blade phosphorus (P) and N:P ratio) for saplings (8 spp.) and adults (9 spp.), one stem trait (wood specific gravity) and four seed traits (mass, N%, P% and N:P ratio), sampling saplings (1-3 m tall) and adults (sub-canopy and canopy) growing in their preferred soil type.

Results/Conclusions For both adults and saplings, LMA was higher for the five species that specialize in the residual soil than the three alluvial specialists, with the single generalist species being intermediate. These differences in LMA largely reflect species difference in tissue density rather than thickness of leaf lamina. Ontogenetic changes in LMA within species, however, reflect increases in both lamina density and thickness. Leaf %N and %P were higher for alluvial soil specialists than residual soil specialists. Lamina density, leaf N:P, wood specific gravity, seed mass and seed %N showed the opposite trend, they were higher for residual soil specialists. These differences are likely the results of evolutionary responses rather than phenotypic plasticity, because the generalist species that occurred on both soil types did not show the effect of soil type in any traits except for %N (p = 0.04). Our results indicate that residual specialists exhibit traits typically associated with infertile soils and slow growth rates, compared to alluvial soil specialists. The results also indicate there is more of a trait signal in relation to soil preference in adults for LMA and in saplings for lamina density.