Photosynthetic traits of tree species in response to leaf nutrient content in the central Amazon

Rainforest tree species are often found on poor soils where essential nutrients may be in low concentrations. Here, we determine the effect of nutrients (N, P, Ca, Mg and K) in photosynthetic traits in five rainforest tree species from central Amazonia. Gas exchange parameters were measured with an infrared gas analyzer in four saplings (1-3 m tall) per species using two leaves per plant. Data on gas exchange and leaf nutrient content (N, P, K, Ca and Mg) were collected between August and November, 2010. Specific leaf area (SLA) and leaf thickness (L (T)) were also determined. Potential photosynthesis per unit mass (A (pot-mass)), maximum carboxylation velocity of Rubisco (V (c-max)) and electron transport rate (J (max)) were responsive to variation in leaf nutrient content per unit mass. On a mass basis, P content was positively correlated with N, Mg, and K content; Mg content was positively correlated with K content. However, no correlation was found between the content of P, Mg, K and Ca, or between Ca, Mg, K and N. SLA and L (T) were strongly related to A (pot-area), V (c-max) and J (max) (per unit area). Our study shows in situ evidence on the effect of leaf nutrients on A (pot-mass), V (c-max) and J (max) (per unit mass) of tree saplings in the central Amazon. The magnitude of changes in photosynthetic capacity of juvenile trees in the forest understory depends not only on N and P use efficiency, but also on the availability of K, Mg and Ca, in decreasing order.