The morphophysiological mechanisms of light-induced net primary production of Amazonian tree species

We investigated the morphophysiological mechanisms indicative of resource uses behind net primary production in seedlings of three valuable forest species of socio-economic value in the Amazon:mahogany (Swietenia macrophylla), Brazil nut (Bertholletia excelsa) and Laurel (Cordia alliodora). The hypotheses tested were: (i) the three species will grow better under mild to high light but with different magnitudes of response; and (ii) the three species will show different mechanisms to reach a better growth under mild to high light We measured biomass production and a set of morphophysiological leaf traits and calculated the relative growth rates (diameter and height) of seedlings subjected to low(0.9), moderate (11.3) and high (45.0 mol m(-2) day(-1)) irradiance for 80 days. For all species, biomass and growth increase with irradiance and the magnitude of the effect were species-dependent. The changes in total biomass by irradiance varied 1.5-fold (B. excelsa) and twofold (S. macrophylla). Light-induced changes in biomass partitioning and morphophysiological traits varied among species. S. macrophylla and C. alliodora significantly increased root/aboveground mass ratio (R/A) under high irradiance. Gas exchange increased with irradiance for C. alliodora and B. excelsa. The non-structural carbohydrates of C. alliodora and S. macrophylla were greater in high (total soluble sugars) and moderate (starch) light environments. The traits associated with total biomass were starch, stomatal conductance and transpiration for B. excelsa; leaf area ratio, leaf area root mass ratio, total soluble sugars and starch for C. alliodora; and RIA, leaf area ratio, and leaf area root mass ratio for S. macrophylla. The species obtained a high total biomass under mild to high irradiance but following different morphophysiological mechanisms. We discussed the potential to use these mechanisms in silvicultural systems designed to supply the plants with intermediate to high levels of light, for example enrichment planting in secondary forests or reforestation on degraded areas.