How much energy wood does the forest provide?

In connection with climate change, there are calls for more intensive management of forests and greater use of forest biomass. On the one hand, wood should be increasingly used instead of other construction and materials, and on the other hand, wood should replace fossil fuels as an energy source.

But how much forest energy wood is actually available for use?

A interactive map of the Swiss Federal Institute for Forest, Snow and Landscape Research WSL provides an answer. It shows the ecologically sustainable forest energy wood potential ESP (from the English Ecological Sustainable Potential). The ESP represents the upper limit of the available forest energy wood quantities. It denotes the amount of wood that is available for energy use while respecting ecological aspects. This means that protected areas such as forest reserves, but also harvesting losses and dead wood as well as wood used for material purposes were previously deducted. Our web map now shows the temporal and spatial resolution of the provision costs at which the ESP can be used.

The interactive web map allows queries and comparisons on ESP according to three different intensive management scenarios:

1. Continue as before (increase in inventories)
2. Increased utilization (moderate stock reduction)
3. much greater utilization (strong stock depletion)

In addition, the following variables can be adjusted:

1. Wood species: hardwood or softwood
2. Wood market situation: energy wood friendly or less energy wood friendly
3. Four time periods: 2017-2026, 2027-2036, 2037-2046, or 2047-2056.

This results in the potential energy wood volumes in the cantons, broken down into seven cost classes. The costs include the processes of wood harvesting including chipping and transport to the energy wood plant. In addition, the potentials can be retrieved by dimensions of the wood assortments (thickness classes, R1 to R6) and by further tree compartments (brushwood, bark and residual deadwood). The results appear in the form of histograms and as exportable files (.csv).

Methodical

The calculations are based on data from the National Forest Inventory LFI. They follow a hierarchical approach in which the above-mentioned restrictions (e.g. protected areas, harvesting losses, deadwood) are successively subtracted from the theoretically usable forest energy wood potential. This allows quantifying the impact of ecological and economic restrictions on the spatial and temporal availability of forest energy wood.

The starting point of the calculations is the theoretical potential (TP), which describes the maximum amount of annually usable wood in a region. It includes the annual increment, mortality and - depending on the management scenario - the amounts of a stock depletion. In reality, ecological and economic restrictions limit the use of wood, so these must be taken into account when determining its availability. Accordingly, the ESP is derived from the theoretical potential by subtracting the following restrictions:

1. Mortality, i.e. dead wood (depending on management 10-15% of increment).
2. Wood from Natural forest reserves(0.3-25% of forest area per region).
3. Harvest losses, due to incomplete utilization during felling and processing as well as transport in the forest (8% of coniferous and 13% of hardwood, 50% and 58% of wood with a diameter smaller than 7 cm, respectively).
4. Share of forest wood, which is used for more valuable material uses (construction timber, furniture, paper, etc.) can be used. For this purpose, two wood market situations are distinguished:  energy wood friendly or less energy wood friendly.

The ESP represents a more realistic upper limit of the availability of forest energy wood than the TP (for detailed information on the calculation cf. Thees et al. 2020).

Three forest management scenarios

The treatment of the forest stands significantly determines the resulting energy wood potentials. The results of the potential analyses are based on three management scenarios, which represent different intensities of intervention in the forest stands:

1. Business as usual: Corresponds to a continuous Increase in inventories and reflects current timber harvesting and management practices in Switzerland, which is why it can be considered a reference scenario.
2. Stronger use, which has a moderate reduction in inventories represents: Management reduces stocks in stands to ~300 m³/ha by 2046 and is thus expected to accelerate the growth of the stock.
3. Much greater use, which has a strong reduction in inventories on ~250 m³/ha by 2046 with more frequent thinning and 40% shorter rotations.

The three scenarios were simulated with the simulation model MASSIMO (Stadelmann et al. 2019) is calculated. This model simulates forest growth and management and is based on data from the Swiss National Forest Inventory. The linkage with the timber harvest productivity models HeProMo (Holm et al. 2020) allowed to estimate the expected harvesting costs. Calculations were made for 5086 LFI sample plots with productive forests based on surveys from LFI2 (1983-1985) and LFI3 (2004-2006).

Two timber market situations 

The wood market situation is a decisive factor in determining the proportion and quantity of forest wood that can be used for energy. Therefore, all potentials were calculated for two wood market situations, one energy wood friendly and one less energy wood friendly.

The energy wood-friendly market situation corresponds to an increased use of wood for energy directly after harvesting. In the less energy wood-friendly wood market situation, a larger proportion of the wood is geared towards a material pre-use and is only available for energy use at a later point in time. Both wood market situations were defined in consultation with the Swiss forestry and timber industry associations.

Results and conclusions

In the Alps and Pre-Alps, as well as on the southern side of the Alps, large stocks of wood have been able to build up in the forests in recent decades because considerably less wood has been used than has grown over the same period. These high stocks reflect the problems of exploiting forests economically under difficult terrain and development conditions: It is often not worth harvesting the wood in these locations.

As a result, the economically available potential of forest energy wood in the mountain regions drops to less than half of the ecologically sustainable potential in the stock reduction scenarios and to slightly more than half in the stock increase scenario. Thus, the ESP is far from being utilized here in all three forest management scenarios due to economic restrictions. In the Jura and the Central Plateau, where the forests are more accessible and are often hardwood stands, the reductions in forest energy wood potential due to economic restrictions were significantly lower.

The results show that ecological and economic restrictions strongly limit the potential of forest energy wood and only 20-30% of the annual wood increment is available for energy purposes in the long term. In this context, the intensity of forest management has a significant influence on the amounts of potential. While the reference scenario "continue as before" ensures the most constant forest energy wood supply over several decades, both scenarios of stock reduction result in larger cumulative total potentials for Switzerland. The variant "moderate stock reduction" results in an increase of the ESP by 32%, the scenario "strong stock reduction" even increases it by 52%.

A less energy wood market supports the circular economy and cascading use of wood, provided that the industries necessary for wood processing are in place and the concepts of cascading and circular recovery are implemented (Erni et al. 2020).

Original scientific article: Thees, O., Erni, M., Lemm, R., Stadelmann, G., & Zenner, E. K. (2020). Future potentials of sustainable wood fuel from forests in Switzerland. Biomass and Bioenergy, 141, 105647 (11 pp.).