Changes in nutrient and fibre tissue contents in Nothofagus pumilio trees growing at site quality and crown class gradients

Estimation of biomass and nutrient contents in tree components are essential for evaluating the impact of harvesting on carbon fixation capacity, bio-element recycling, and long-term effect on the balance that influence over net primary productivity. In many reports, fixed values were roughly considered at tree and stand level; however, the nutrient contents can vary across natural gradients (e.g. site quality and crown class) and according tree components (e.g. leaves, branches, bark, wood); and also, can be related to their fibre contents. The objective was to determine these changes in one tree species (Nothofagus pumilio) growing in Southern Patagonia (Argentina), comparing carbon (C), nitrogen (N) and fibre tissue contents of above-ground biomass across site quality and crown class gradients. We found that C, N and fibre (cellulose, hemicellulose, lignin) at different tree compartments varied with the site quality and crown class gradients (dominant, codominant, intermediate, suppressed). High tree growth rate (better site qualities and dominant trees) affected tissue density and fibre content by increasing C content (e.g. lignin generated higher carbon content). Also, we found the same trend for N content, which was more evidently related to some tissues (e.g. leaves). The use of fixed values in the modelling for tree or stand approaches in C or N estimations can generate significant biases in N. pumilio trees, being necessary consider specific values for the different natural gradients that influence over this tree growth species. Calculating accurately the stored and sequestered C or N contents can improve management strategies and modelling of the natural forest stands. The information provided in the present study suggests the need of incorporate the natural gradients into the tree models for C sequestration and N storage estimations.