A green lung which needs revitalising

sample-plot‘It is the very first time in fifty years that a multi-resource inventory has been carried out at the Sart Tilman site,’ states Jacques Rondeux. ‘And what is more, this has been carried out on proven scientific bases. Up until now we had completely no idea of the yearly growth volume of the forest stands . This gap could soon be filled in, as long as new measurements are taken at a periodicity of several years and, of course, at the same sites.’ With what results, at this stage? The distribution of tree species is broadly speaking two thirds of deciduous (hardwood) trees (primarily oak, beech, birch, etc.) and one third softwood (mainly pines). The volume of the growing stock is around 390 cubic metres per hectare, in other words a very high value in comparison to that obtained in Liègois Condroz (but a logical value, given the absence of  silvicultural  rules). The average tree, over these 250 hectares, has a circumference of  around 79cm at a height of 1.5 metres above the ground and a volume of half a cubic metre . The inventory has also enabled an evaluation of the role the Sart Tilman estate plays as a carbon sink. The 205 hectares studies ‘stock’ about 32,000 tons in the woody biomass. If the carbon stored by the deadwood and the litter bedding is added we reach a value ranging from 52,000 to 66,500 tons depending on whether the depth of the soil is 20cm or 100cm. ‘This data is also 1.5 times to  twice as high as that found in the literature as regards Liègois Condroz. That is explained by the large amont of standing trees and the large abundance of deadwood. If we extrapolate them to the whole of the Sart Tilman forest site, we arrive at a value situated between 80,000 and 90,000 tons of carbon stored in the living trees and a value ranging between 128,000 and 164,000 tons, by including the litter, the deadwood and the soil.’

No clear cutting!

The volume of 390 cubic metres of living wood per hectare could obviously be subject to reductions in the form of thinning or regeneration. But not just any old how? ‘It is out of the question to carry out any clear cutting, nor of using heavy machinery in such a site, nor of plotting out hauling pathways,’ points out the former director of the Forest and Nature Management Unit. The only form of silvicultural activity admissible on the Sart Tilman site is a soft silviculture, the so called ‘close to nature.’ It is moreover difficult to imagine a university like the ULg, which has just enriched itself with an agricultural faculty and which has the oldest forestry school in the country, would not be a benchmark as regards the subject. The felling, precise and targeted, could only be done manually (NB: with a chainsaw ), and only certain resinous zones could possibly be treated with mechanical equipment.

How can this biomass be valorised? The scenario imagined is that  harvesting is fixed according to a rotation applied to a zone of around a maximum of 20 hectares per year. That would mean that each zone would only be ‘visited’ a maximum of once every twelve years in such a way to allow the forest to regenerate itself and thus also to become younger. The quantity of living wood harvested would range from 535 to 1850 steres annually. Why these two figures? Quite simply because they represent – the first being deliberately conservative – the natural growth of the forest, in other words the volume of supplementary wood produced by tree growth. Ground down on the edge of the parcels or close to a storage site, the wood which has been cut down could be transformed into chips (pieces of wood several centimetres long) which, once dried out, could feed a biomass boiler: a particularly interesting solution for a building or a facility situated on the periphery of a district urban heating system.

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