A practical application of tree biomechanics in ecology

Research on plant biomechanics over the last decade has increased enormously, resulting in a large database of available knowledge. However, although certain fundamental aspects are understood in great detail, biologists, foresters, and mechanical engineers are not yet sure of how this knowledge can be applied in a practical way with regards to ecological sustainability. Nevertheless, several domains exist that benefit hugely from the recent collation of data typical of that presented in this chapter. Two examples of subject areas where biomechanics and ecology can be combined to produce highly effective ecological solutions to long-term sustainability issues are ground bio-engineering and eco-engineering [88].

Both eco-engineering and ground bio-engineering fall within the framework of "ecological engineering." Ecological engineering has been described as "the management of nature" [161] or as the proactive design of sustainable ecosystems that integrate human society with its natural environment for the benefit of both [162,163]. Ecological engineering has largely been devoted to the sustainability of wetlands, wastewater, and aquaculture [162], but can be applied to a larger range of environments. Focusing more on the restoration or protection of sites, the term "eco-engineering" has recently been defined as the long-term strategy to manage a site with regards to natural or man-made hazards [88]. Eco-engineering is not to be confused with ground bio-engineering. Ground or soil bioengineering methods integrate civil engineering techniques with natural materials to obtain fast, effective, and economic methods of protecting, restoring, and maintaining the environment [164,165]. The use of, for example, geotextiles or brush mattressing to arrest soil run off and the planting of fast-growing herbaceous species to fix soil are typical ground bio-engineering techniques. Although ground bio-engineering and eco-engineering can be used on all types of land from arid land prone to desertification to riverbanks subjected to flooding, these practices have largely been carried out on hill slopes [165,166]. Hill slopes are highly prone to soil loss once vegetation has been removed through deforestation, cropping, or natural hazards. For natural slopes, such hazards can involve the mass movement of soil, such as landslides, avalanches, and rockfall, or erosion, including sheet and gully erosion or river bank erosion. By combining ground bio-engineering techniques with long-term solutions, slopes can be managed effectively to minimize the risk of failure. The correct choice of plant material is difficult because bio-engineers need to know the ability of the plant to grow on a particular site as well as the efficiency of the root system in fixing and reinforcing soil on an unstable soil. This efficiency depends largely on the mechanical properties of individual roots as well as the overall anchorage and architecture of the root system [71,167]. Although such information may be available for a particular species, its performance in the long run also needs to be known: fo example, grasses often die back in summer and should be combined with shrubs so as to avoid slippage or erosion problems. Shade intolerant species will also decline as shrubs and trees grow taller over time. Long-term solutions therefore need to include the use of appropriate management strategies, the employment of decision support systems [168,169], and the integration of such tools into geographic information systems to predict future risks. Such management techniques are particularly effective in large-scale areas of Europe, e.g., ski resorts, mountain slopes, and forest stands [170].

Not only do a bio- or eco-engineers need to consider the suitability of a species to a particular site with regards to its ability to grow and to reinforce the soil through the fixing efficiency of the root system, but they also need to take into account the long-term effect of planting vegetation on the site, especially with regards to tree species. One example where the careful long-term planning of tree growth is crucial is in mountain forests. Mountain forests are more and more frequently used as protection forests, i.e., as natural barriers against the effect of snow avalanches, landslides, and rockfall [171-174]. Recent studies have shown that the spatial distribution of trees within a forest stand can have serious consequences on the ability of a mountain forest to withstand soil mass movement such as rockfall [10,175]. The choice of tree species, as well as the aerial and root architecture, is also of utmost importance because mechanical resistance to overturning and stem rupture will influence enormously a tree's ability to withstand abiotic loading [10,88,175]. Studies in the French Alps have shown that beech (Fagus sylvatica L.) is extremely resistant to rockfall, compared with fir (Abies alba Mill.) and Norway spruce (Picea abies L.), particularly if planted in a certain spatial pattern [10,88,175]. Nevertheless, even if a tree species is useful as a barrier against one particular type of hazard, the same species may not be suitable in protecting against a different type of hazard, for example, Norway spruce is not especially wind firm [68] nor resistant to rockfall [10,174]. However, in preventing snow movement, because of its aerial architecture, Norway spruce is highly effective in holding the snow mantle in place [174]. Therefore, it is always necessary to determine which species is best suited to a particular function. Similarly, in conifer forests subjected to frequent windstorms, appropriate long-term management of the upwind border of the stand could decrease the probability of damage during a storm; such borders could be planted with windfirm broadleaf species such as oak and pruned to create a "ramp," or shelter belt type structure. This type of structure would cost little to maintain and would allow the prevailing wind to pass over the plantation, rather than penetrate into the stand [176].

One further example of how effective planting and management can improve the stability of a slope was shown in a study by Roering et al. [79]. These authors calculated the additional cohesion in soil by carrying out tensile testing of tree roots in natural and planted forest stands found within a short distance of landslide scarps. It was shown that a mixture of species, even at a young age, proved more effective than a mature monospecific plantation in preventing against soil loss. If a forest is clear felled, roots decay within a period of 3 years, resulting in a decrease in tensile strength and the ability to fix soil, hence resulting in lost soil cohesion and compromised slope stability [77].

Examples of where eco-engineering techniques would be most useful are in situations whereby human safety is not an immediate issue or where protecting structures are already in place, such as safety nets or avalanche barriers. When deciding to carry out eco-engineering techniques on an unstable slope, the engineer must first determine the nature of the slope, type of soil, type of native or desired vegetation, and the likelihood of any catastrophic event occurring that would decrease slope stability during the restoration time. If the risk of danger to human life and infrastructures is low, the user must consider the size of the site and costs to be incurred throughout the life of the project. If the site is on a small scale and the cost of construction (e.g., fascines, gabions, and geotextiles), planting, and upkeep is equal to the economic, aesthetic, and safety gain at the end of the project, ground bio-engineering techniques can be considered. If the site is large scale, such as a mountain slope, the expenses incurred in carrying out certain bio-engineering techniques may be too high for the gain produced, and eco-engineering techniques may be used. However, it must be remembered that any gain as a result of an eco-engineering project will only be incurred in the long-term.

Still a relatively young subject, eco-engineering is beginning to emerge as a future research area in Europe, an area that engineers and ecologists should consider both in terms of education and application. Human activity over the last 100 years has been concerned with increasing productivity through technological progress at the cost of environmental degradation [162]. It is now necessary to repair this damage, although with limited resources, certain countries are unable to invest heavily in environmental restoration of degraded lands. Bio- and eco-engineering techniques can therefore provide a low-cost, long-term solution in many cases.

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