Description
Afforestation is the establishment of a forest or stand of trees in an area where there was no forest. Indeed the forest helps in dissipating energy of rocks falling downslope. This type of measure was started already in the late 19 century in the Alpine regions where the mountain forests were considered to play a protection function by stabilizing the slope itself, and by protecting objects and infrastructure located further down a slope from natural hazards such as avalanches and rockfall. In New Zealand an interesting overview of landslide control measures adopted, among them afforestation from the 70s to now is reported in Brown (1991). The forest having as a primary role the protection of people from natural hazards is also called "protection forest". Protection forests may or may not be managed. The protective ability of a protection forest is mainly provided by the presence of trees that accomplish three main functions: 1) the tree stems help in stopping the falling stonesn (Figure 1), 2) the tree crowns and canopy prevent, by snow interception and by snow release, the build-up of a homogeneous snow layer that may glide as a compact blanket, 3) the tree roots are well known to reduce shallow landslide hazards, to reduces surface erosion and to increase the water-holding capacity of the soil by the build-up of organic layer (Brang et al., 2001). However, the effectiveness of the forest to the slope protection depends on many factors such as the forest management (Rammer et al., 2008), the age of the trees, the stem density and spatial distribution, as well as aspects related to the type of natural hazards (e.g. volume of the avalanche).
Figure 1. A rock stopped by a tree (Brang et al., 2006)
In the Europe Forests Alps most rockfall are low magnitude/high frequency events with single rocks with volume lower than 5 m3, thus in this case forests can act as effective barrier providing a protection for the elements at risk (Stokes, 2006). In other cases, very often afforestation is combined with other engineered measures. An example of afforestation projects in Switzerland included the planting of 150'000 trees, as well as the construction of avalanche fences and masonry respectively in the upper reaches and along the middle reaches (Mueller, 2009). The Stillberg case site is an open experimental site in Switzerland monitored by the WSL Institute for Snow and Avalanche Research SLFto study the growth of trees and treelines and their interaction with snowpack and avalanches (Figure 2).
Figure 2. Stillberg site: on the left tree lines along the slope; on the right the upper part of the slope with avalanche fences (https://www.slf.ch/en.html).
Advantages
- Cheaper than rockfall nets, which instead are difficult to install, expensive and deteriorate with time;
- afforestation projects are likely to contribute to climate change mitigation, restoration of forest services, storing carbon and strengthening communities;
Disadvantages
- When trees are subjected to rockfall, they may uproot, suffer stem breakage, or kinetic
energy may be transferred to the crown, causing breakage;
- Not all trees can play a protection function depending on where they are placed. It is therefore advisable to remove unsuitable trees from highrisk source areas, such as cliff tops.
Design methods
Afforestation is usually a large-scale measure. Before any action is taken, the exact nature of the risk on the slope in question should be examined carefully (Stokes, 2006). The selection of the tree species depends on the type of natural hazard to be addressed. For example to provide protection against rockfall broadleaved species may be considered because they can easily regenerate after damage and heal more quickly if wounded by a falling rock. To protect against snow avalanche, broadleaved species are not useful since they do not prevent the formation of homogeneous snow layers because of their reduced canopy area not able to intercept snow. When both rockfall and snow avalanches may occure, a mixed forest would be the most effective protection measure.
Sometimes an additional external support is needed when young trees are planted. Wooden tripods can be placed around the tree stem to prevent snow gliding and enable planted trees to pass the threatened juvenile phase (Figure 3).
Figure 3. Wooden tripods in Vaujany, Savoie, France (Brang et al., 2006).
Simulations can be carried out in order to quantify the retention capacity of a forest of selected hazards. Within the project “Effectiveness of biological protection measures”, Protect Bio was developed to determine the effect of the forest and other biological protection measures and to take them into account accurately in hazard protection projects. The site gradient, stem density and other factors are incorporated into the simulation for the determination of the forest’s retention capacity (FOEN, 2015).
The "ecorisQ" association proposed a software for the calculation of the probability of rockfall along a slope by taking into account the role of vegetation in dissipating energy an stopping a certain percentage of stones downslope. The input data include stone physical characteristics, slope characteristics, trees density and land cover.
Protection forests may or may not be managed and their effectiveness can change depending on the type of management as found by Rammer et al. (2008) who used a simulation approach to evaluate alternative silvicultural strategies for a case study rockfall protection forest project in Austria using a coupled rockfall and forest ecosystem modelling.
Functional suitability criteria
Type of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 5 | Applicable for topple or fall landslides, it can only slightly reduce the velocity of possible flows. Use of adequate vegetal species is recommended. |
| Topple | 3 | |
| Slide | 7 | |
| Spread | 6 | |
| Flow | 6 | |
Material type |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 6 | It is suitable for rock or debris materials with dimension of the mass volumes big enough to be entrapped or stopped by the tree stems. |
| Debris | 7 | |
| Rock | 4 | |
Depth of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 9 | Applicable irrespective on the depth of movement, is more effective when surficial detachments occur |
| Shallow (0.5 to 3 m) | 8 | |
| Medium (3 to 8 m) | 3 | |
| Deep (8 to 15 m) | 1 | |
| Very deep (> 15 m) | 0 | |
Rate of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 3 | Applicable irrespective on the rate of movement, however the effectiveness of the measure is reduced as the velocity of the movement increases. |
| Slow | 4 | |
| Very slow | 6 | |
| Extremely slow | 6 | |
Ground water conditions |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 3 | Applicable irrespective on the ground water conditions |
| High | 7 | |
| Low | 6 | |
| Absent | 5 | |
Surface water |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 7 | No particular constraints for the surface water conditions |
| Snowmelt | 6 | |
| Localized | 5 | |
| Stream | 3 | |
| Torrent | 3 | |
| River | 3 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 7 | A well designed afforestation can be reliable over the years and decades. Sometimes it requires engineered measures to address the risk mitigation. |
| Feasibility and Manageability | 10 | Simple technique, the management can be simply the removal of possible fallen trees after an event or reforestation along the years. |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 6 | As all vegetative measures, the vegetation establishment is fundamental for a well functionality of the intervention. It may take years, sometimes decades or even centuries to reach the optimum for protection forests to be effective against natural hazards. |
| Environmental suitability | 10 | Afforestation projects are highly recommended because they have several positive impacts on the environment: contribute to climate change mitigation, restoration of forest services, storing carbon and strengthening communities. |
| Economic suitability (cost) | 8 | The financial costs for silvicultural interventions and further maintenance for effective protection are relatively low. |
References
- Federal Office for the Environment, FOEN (2015). Natural resource in Switzerland - Living with Natural HazardsLiving with Natural Hazards
- Brang, P., Schönenberger, W., Ott, E., & Gardner, B. A. R. R. Y. (2001). Forests as protection from natural hazards. The forests handbook, 2, 53-81.
- Brang, P., Schönenberger, W., Frehner, M., Schwitter, R., & Wasser, B. (2006). Management of protection forests in the European Alps: an overview. In For. Snow Landsc. Res.
- Brown, W. J. (1991). Landslide control on North Island, New Zealand. Geographical Review, 457-472.
- Rammer, W., Brauner, M., & Lexer, M. J. (2008, June). Evaluating the long-term rockfall protection effect of silvicultural strategies at project level. In Proc. Interdisciplinary Workshop on Rockfall Protection (pp. 90-92).
- Stokes, A. (2006). Selecting tree species for use in rockfall-protection forests. For. Snow Landsc. Res, 80(1), 77-86.
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