A computer simulation tool that predicts the influence of forest clearings on soil fertility

The process of clearing consists of cutting down trees in such a way that those remaining have more resources and can grow more. The question was if too many had been cut down, with the concomitant removal of nutrients, and the manner, therefore, in which this process might affect long-term soil fertility.

A PhD thesis presented in the Public University of Navarre specifically analysed the internal cycle of nutrients in two wooded areas in Pinus sylvestris forests, this species, together with the beech, being the most common tree in the Navarre mountains, especially in the Pyrenees. The areas chosen were in Aspurz and Garde, locations situated at different altitudes.

In the first stage, the above-ground biomass of the wood was measured before and after the clearing is carried out, i.e. we analysed the amount of wood, leaves and bark and each one of the nutrients (such as nitrogen and phosphorous) that existed in the wood. Then the biomass of the dry leaves, branches, etc. of the trees in question fallen onto the floor is measured in order to assess what nutrients returned naturally from the trees to the soil and which, therefore, would be natural fertiliser. The results concluded that the clearings reduced both the above-ground biomass as well as the mineral mass, although no differences in this drop were registered between different intensities of clearings.

In the second study, we start calculating the removal of nutrients from green leaves before they dry out, given that trees have a mechanism to reduce this nutrient loss and increase efficiency in their use. These parameters were not observed to have been influenced by the clearings.

The third experiment consisted of analysing the loss of weight and the chemical composition of the dead leaves. We analysed how this layer of fallen leaves deteriorated, how it breaks up and reconstitutes itself on the ground. In this way we could observe that both woods underwent a slowing down of this process after the clearings took place.

The fourth trial studied the respiration of the soil and its chemical composition and, in this way, the cycle is closed in which the tree first gathers the nutrients to be used to generate biomass, part of this biomass falls, degrades and, once more, reconstitutes itself in the soil to be reused by the tree. We found that respiration was greater in the location at higher temperatures and only in this wood was there detected a passing increase in respiration after the clearings had taken place. Neither did the clearings affect the chemical composition of the soil.

Subsequent to these studies, in the last stage of the thesis, we evaluated the changes in the reserves of nutrients. To this end, we created a computer tool that is a simulation model of the circulation of nutrients based on balances of masses. This has enabled us to predict what will happen to these trees in 60 years time.

A prediction that provides a number of possibilities in this process. It not only tells us that we should carry out clearings, but how often they should be carried out, how many trees should be cut down and whether the whole tree should be cut down or just the trunk or whether the branches and leaves should be left.

Acceptable limit

In any case, it can be affirmed that the management of forest clearings currently comes close to the acceptable limit, although we have to have more data to assess if the mountain is losing fertility. But, in order not to reduce the reserves of nutrients of a forest, the same form of clearings should not be applied to different zones without a prior evaluation of the consequences in each particular case, given that the effect of the clearings on internal cycle of nutrients largely depends on the features of each wooded area.