The comments came at the end of a two-day meeting at Uppsala, Sweden, convened this month by researchers in Future Forests to discuss the use of estimates of mineral weathering rates as a tool to guide forestry policy in that country.
Mineral weathering is the way in which forest soils and rocks slowly break down to release nutrients such as calcium and magnesium needed for soil and plant growth. In Sweden, decision-makers have used estimates of mineral weathering rates to guide decisions on what harvesting methods to allow forestry industry to use, on the assumption that such estimates translate as the extent of self recovery a forest can undergo after harvest. Harvesting depletes soils of nutrients which, in turn, may cause acidification of nearby soils and waters.
In a country good for 15 per cent of the world´s timber exports and generating more energy from forest-sourced materials than from either nuclear or hydro power—the other ‘big two´ in terms of non-fossil-oil based energy in Sweden—the pressure is on for increased biomass production from forests. Other richly forested nations in the northern hemisphere, of which Canada, the U.S.A., Finland and France were represented at the workshop, are faced with similar issues. Indeed, Marie-Claire Pierret of the Strasbourg Laboratory for Hydrology and Geochemistry in France said, acidification resulting from forestry operations was again on the increase, along with calls for increased forest production.
A combination of approaches
While there was support at the workshop for the role of mineral weathering rates in some management decisions, the scientists seemed to agree that a combination of approaches should be used to support sustainable forest management, tailored to local conditions. This latter also held good for the use of different sets of estimates of mineral weathering rates which could not be used indiscriminately.
Summarising a group discussion Mike Starr of Helsinki University said, “We have got to use weathering estimates on some sites but be careful of which", adding that “a combination of different approaches is needed".
Moreover scientific models to predict the consequences for soils of tree removals should take further account of time and spatial scales, Starr went on, as well as critical loads of nutrients, deposition of chemicals in the form of air pollution or rainfall, nutrient leaching to water and, also, uptake of essential elements by plants, trees and fungi.
Others said that forest soils also had to be considered in terms of their depth. Shallow soils could be left untouched, several speakers held, hinting that the prospect of poor yields might have forestry operators turn away from these soils for economic reasons. Moreover, Richard Lucas of the Swedish University of Agricultural Sciences (SLU) proposed that ways to extend the rooting depth of trees might be looked into as wind throw by storms could become more frequent in a changing climate.
Kevin Bishop, the SLU head of a freshly launched research venture to deliver mineral weathering estimates and predictions on how these will be impacted by climate change, suggested that scientists turn their attention to the health of small streams in forests that were up for harvesting in the next 30 years. This he felt could have a real chance of better informing the debate on mineral weathering rates and forest harvesting, Bishop said.
Moreover, studying stable isotopes, whether as a way to gauge nutrient distribution in trees or chemical deposition by rainfall, was a promising line of research that needed to be continued, he said with a nod to the workshop´s expert on boron isotopes, Damien Lemarchand of the Strasbourg Laboratory for Hydrology and Geochemistry.
A model for understanding
To agree on ways forward and allow for the workshop findings to be effectively communicated, the scientists agreed on a conceptual model—a graphic depiction of factors that are part of or affect the system and the many feedbacks between them. Missing links to a fuller understanding of the forest system were identified, such as the biology of the system and effects of secondary mineral fallout from weathering processes. Again, the character and effects of substances brought to soils by deposition, including by rainfall or other weather events, were “black holes" to be explored. A further anonymous creature in the literature, so-called biofilms or the thin layer of slime clinging to rocks and soils in forests should be studied as they might well impact on nutrient cycling, of which the rate of mineral weathering, in still unknown ways.
To finish, some workshop participants pointed out that, apart from tree removal itself, the fact that forests were being managed, such as by using machines and providing space for them, meant that soils were being disturbed. This had to be accounted for in attempts to gauge human impacts on forests and, by extension, give advice on sustainable forest management.
“We are disturbing the soils", said Stefan Löfgren of SLU; “Stem outtake is a tiny part of the impact on soils during a rotation period. If we don´t take that into account we won´t arrive at the right conclusions".
The use of methods to turn up forests soils in a way similar to ploughing a field or “disc trenching" brought disturbance to the soil, he explained later, as well as compacting or tearing up soils by driving heavy-duty machinery over them.
For this and other reasons it was paramount, Löfgren and other speakers told the workshop, that long-term forest monitoring be let continue.
The Future Forests´ soils and water scientists behind the workshop will go on to summarise the workshop discussions in a scientific article, after which a stakeholder meeting will be held in November, in Stockholm, to inform policy makers and forestry stakeholders of the findings. The SLU scientists Martyn Futter and Hjalmar Laudon will be leading this process.
“We have seen that trying to take a simple approach is not the way forward", said Laudon who heads Future Forests´ Soils and Water team in conclusion to the workshop.
“That is something we need to put forward to the management" in policy circles and the forestry sector, Laudon said;
“We are going to give them the best tools that we´ve got... built on the best available science."