Effect of initial fertilisation on biomass and nutrient content of Norway spruce and Douglas-fir plantations at the same site

Norway spruce (NS) and Douglas-fir (DF) are among the main species used for production forestry in France. In low-elevation mountains and under-acidic conditions, they often occupy the same ecological situations. It is therefore of paramount interest to have a good understanding of how the two species behave under similar conditions and how they react to site improvement by fertilisation. The study stands are part of an experimental stand located in the estate forest of Breuil-Chenue in the Morvan (east central part of France). Its aim is to compare the impact of change in species on ecosystem functions. Destructive sampling of 10 trees per stand, distributed over the whole spectrum of inventoried classes of circumference at breast height (c 1.30), was carried out within four stands, e.g., fertilised and control (non-fertilised) NS; fertilised and control (non-fertilised) DF. Allometric relationships between c 1.30 and biomass or nutrient content per tree compartment were calculated. These equations were applied to the stand inventory for quantifying stand biomass and nutrient content on a hectare basis. The standard deviations of results were estimated using Monte-Carlo simulations. Specific emphasis was given to explain the origin of differences observed between species and treatments, i.e., changes in carbon allocation leading to specific allometric relationships, changes in stand structure (tree size distributions) and changes in stand density due to mortality. DF was more productive than NS (+28% for total tree biomass, +50% for ligneous biomass and +53% for stem wood). Both NS and DF were affected by fertilisation but in the case of NS, effects on the crown_c 1.30 relationship and on average tree growth were predominant while in the case of DF, the stem_c 1.30 relationship and stand density were affected by changes in soil fertility. The general fertilisation effect was an increment of 40% of ligneous dry matter for DF and only 22% for NS. In both cases, the amount of wood biomass produced per unit of leaf biomass (on a tree basis and, to a lesser extent, on a per hectare basis) was greater in fertilised plots. However, in the case of NS, the same amount of wood biomass was produced from a smaller quantity of leaves while in the case of DF, the same amount of leaves produced more wood biomass. The amount of nutrients in total ligneous biomass was higher for N, P and K, but lower for Ca and Mg, in DF than in NS. A high variability was observed between nutrient content of the different compartments, e.g., DF < NS for needles (except Mg), DF < NS for K, Ca and Mg for stem wood and DF > NS for N and P of stem wood. Fertilisation did not considerably change the hierarchy. On the basis of this study, all the indexes concerning stand production, wood density, nutrient use efficiency and response to fertilisation gave a net advantage to DF. This information is highly relevant for both ecological and practical purposes.