The 7-8 February workshop is given in follow up to two scientific articles by Future Forests´ soils and water scientists which caution against an overreliance on existing estimates of mineral weathering rates when deciding what types of forest harvesting to allow. The studies point to a very large variability in mineral weathering rate estimates and to knowledge gaps in the scientific literature.
The scientists say that the issue carries great importance since, in Sweden, mineral weathering rate estimates have been used as a proxy by decision-makers trying to decide whether to allow forestry industry to harvest the whole tree—stem, top, branches or needles and stumps—as compared with taking out the stem only, which is the convention.
Effects of whole tree harvesting
This is because tree harvest or, more precisely, removing trees from the forest, permanently depletes soil stores of essential elements such as base cations which feed plants with magnesium, potassium and calcium and prevent soil pH to drop to cause acidification. Harvesting the whole of the tree appears to make a considerably larger dent in base cation soil concentrations than removing only the stem. Since science has been inconclusive about the extent of this loss, forestry decision-makers have referred to scientific estimates of mineral weathering to guide them when laying down policy.
“Over the long term, mineral weathering... is the only means of replacing losses (of essential elements) due to forest harvesting. More intensive forest harvesting for bioenergy production will lead to greater losses than conventional harvesting", said Martyn Futter who is a water modeler with the Swedish University of Agricultural Sciences and lead author of a study under review which showed mineral weathering rate estimates to vary by up to 600 per cent.
“We are managing (Swedish forests) on the basis of what we believe are precise numbers", said Futter. In reality, “there is a lot of variability in the weathering rates. The variability in estimates leads to uncertainty in management decisions." Moreover, “internal cycling is not included in most weathering models", he said with reference to the way in which essential elements cycle in the forest environment.
Identifying knowledge gaps
With the 20-scientist-strong workshop at Uppsala, “we want to identify what we know and... key gaps in our knowledge", Futter went on, saying that it would be an “opportunity for wide-ranging discussions", on topics such as mineral weathering, forest catchments, modeling, deposition of minerals or nutrients and their cycling in the forest environment, as well as possible impacts of climate change.
From SLU at Umeå, the head of Future Forests´ Soils and Water researchers Hjalmar Laudon remarked that even though the link between mineral weathering and acidification had been studied for 30 years, knowledge was sketchy at best when it came to knowing at what thresholds the lack of essential minerals and nutrients in forest soils, of which base cations, would cause the surface waters of nearby catchments to acidify.
“How do we avoid... causing acidification because we have taken out too many base cations from the system? That is one thing that we want to know", Laudon said. Coming back to an overarching aim of his team´s research, he said: “We want to gain a better understanding of the impacts of forestry on water quality but also to help improve the methods for avoiding negative consequences of forestry".
By Anna Strom