Juvenile tree growth rates and species sorting on fine-scale soil fertility gradients in a Chilean temperate rain forest

Aim: We sought to clarify the role of juvenile tree growth rates in species sorting along fine-scale fertility gradients in the coast range forests of southern Chile. Location: In montane forests in the coast range, the conifer Fitzroya cupressoides occupies the poorest soils, whereas dominance shifts to the evergreen angiosperms Nothofagus nitida and/or Weinmannia trichosperma on moderately infertile sites. On the most fertile sites, N. nitida is usually the sole overstorey dominant. Methods: We examined the effects of soil depth and nitrogen availability on height growth and leaf nitrogen levels of the three common overstorey dominants. Sapling height growth rates and foliage nitrogen concentration were measured in 20 large gaps (>0.1 ha), on sites of varied soil depth and potential N-mineralization rates. Results: Height growth rates of Weinmannia and Nothofagus were strongly positively correlated with both soil depth and potential N-mineralization. Height growth of Fitzroya was strongly correlated with soil depth, but not with N-mineralization. Nothofagus and Weinmannia enjoyed large growth-rate advantages over Fitzroya on the more fertile soils of the study area, and this advantage was reduced or nullified (but not reversed) on poor soils. This pattern was associated with species differences in nitrogen use: on fertile soils, Nothofagus and especially Weinmannia, achieved more height growth per unit of leaf N than Fitzroya, whereas differences were minimal on poor soils. Main conclusion: Results suggest that comparative height growth rates of juvenile trees, associated with nitrogen productivity differences, play a key role in determining eventual dominance on productive sites, but that traits other than growth (possibly related to nutrient retention) are more important on low fertility sites. Disturbance regimes appear to vary in relation to site fertility, and differential species responses to this superimposed disturbance gradient are also probably influential in determining overstorey composition.