La foresterie analogue en tant que Réalisation conjointe

Dans le contexte de la foresterie analogue, une forêt se définit comme étant l’apogée écologique qu’une région donnée peut atteindre sans intervention humaine. Ainsi, le mot forêt s’applique autant aux forêts de l’Amazonie qu’aux forêts de broussailles australiennes. La foresterie est la science de gérer les forêts. Pour alléger l’impact des activités humaines sur les écosystèmes naturelles, deux méthodes sont employées : d’une part, la conservation des ressources forestières naturelles; d’autre part, la création de forêts faites par l’homme.

Dans l’ensemble, les projets de reboisement ont été lacunaires à l’échelle planétaire puisqu’ils ne tenaient compte que de la production du bois. Les forêts créées par intervention humaine sont dans la plupart des cas des monocultures d’espèces ayant un taux de croissance élevé. Les impacts négatifs de ce type de culture ont bien été documentés.

La diversité des plantes ouvre la voie à l’évolution d’une variété d’herbivores et d’une variété correspondante de carnivores. Il est nécessaire de compenser les pertes de forêts par des forêts créées selon une structure architecturale appropriée et incluant une diversité appropriée d’espèces de plantes.

Analog Forestry as a Tool in Joint Implementation

Ranil Senanayake - Sri Lanka
Member of The Analog Forestry Network,
sponsored by The Falls Brook Center
August 1998

i.gif (173 bytes)n the context of Analog Forestry, a forest is defined as an ecological climax community that any geographical area can attain without the intervention of humans. Thus, forests will range from Brazilian Rainforest to Australian Bush. The most important feature is that such forests are natural formations. Forestry, on the other hand, has all to do with human intervention. It is the art and science of managing forests. As human impact on natural ecosystems increases, the the burden increases for the practitioners of this science to ensure that the stability conferred on the biosphere by natural forests continues uninterrupted. This is accomplished by two principal methods. The first is by the conservation of natural forest resources, the other by creating human made forests.

However, the nature of such human created forests has to be carefully designed.

The response of foresters has been sadly lacking in any consideration other than that of producing timber. Consequently, monoculture planting of fast-growing species is the major response to deforestation worldwide. The negative impacts of these plantings on both social and ecological levels have been well documented. It is now becoming apparent that the loss of natural forest cannot be substituted for by the present trends in forestry.

The replacement of natural forests with exotic monocultures has not provided any solutions to these trends. In most cases, these monocultures have amplified environmental problems and created social dislocation. The problems created by Eucalyptus plantations in Portugal and India, or by Pinus plantations in Sri Lanka are examples. In all these cases, the local population and the native biota have been negatively affected.

It is also evident that the loss of natural climax forests has reached critical proportions. The extinction of species, particularly in the tropical rain forest, is increasing exponentially due to two related factors. One is the destruction of habitat and the introduction of biocides; the other is the reduction in size of remnant forest patches so that they become too small to retain the full complement of species found in the original forest ecosystem.

Forests possess an architectural form that creates a modified environment. This form, consisting of a closed diverse canopy held far above the surface layer, mitigates erosive effects of rainfall by breaking the force and frequency of raindrops; reduces the desiccating effects of direct sunlight and wind; and regulates the ambient temperature of the environment under the canopy by reducing the amplitude of diurnal/nocturnal temperature fluctuations. It is this modified or organism-controlled environment that allows most forest species to maintain their populations.

The diversity of plants allows the evolution of a diverse array of herbivores and, thereby, a corresponding diversity of carnivores and detritivores. Any response to compensate for the loss of forest will have to include the requirements of architectural structure and diversity of plant species.

 

ANALOG FORESTRY

Analog forestry is a system of silviculture that seeks to establish a tree-dominated ecosystem analogous in architectural structure and ecological function to the original climax or sub-climax vegetation community. The tree and plant species act in an analogous fashion to the original natural structure and function of the forest while also being useful to humans as crop plants. In this manner, analog forestry seeks to empower rural communities, both socially and economically. It encompasses the diverse forms of tree farming commonly termed Village Forests, Forest Gardens and Mixed Tree Farming.

The function of a forest can be measured by many variables including the provision of microhabitat, clean water, produce and environmental stability. Analog forestry recognises the varied functions of a forest and is modelled to meet with those needs. It is a very effective tool in arresting biodiversity loss and also provides a greater range of crops, thus spreading the risks for the individual manager. It provides specialised products of high value and products amenable to community level processing, as well as providing the highest carbon sequestering value of any silviculture crop.

This value is gained by a management system that seeks to maintain a canopy cover over the land under Analog Forestry for the longest possible time horizons. This is accomplished by: maintaining the mature state of the forest, using long term maturing species like Ebony (100 years) in the architectural design, and managing the forest soils so that long age humates accumulate in the A and B horizons.

The most widely recognised product of a forest has been the wood in the trees. Different species produced timbers of different quality. Some species were rarer or more difficult to obtain than others. Thus, timber is categorised into different classes reflecting its quality and rarity. The differential in price between Pinewood and Walnut in temperate zones or between Albizzia and Ebony in the tropics serve as good examples. In addition, the age of the tree also has a bearing on the quality of the wood within a species. Tree age is seen to have importance in setting value because: a) a greater amount of low-density sapwood is generally found in young growth trees, and b) with increasing age, cambium produces longer and thicker walled cells, a feature important in the modern market (Resch 1967). In this study it was shown that as a tree matures and grows in girth, there is an increase in fibre quality as well as in the specific gravity of commercial wood.

Thus forests should be valued in respect to its carbon sequestration potential. Trees are being widely appreciated as a useful tool in sequestering carbon from the global atmospheric pool of carbon dioxide. While the distinction between the value of 'biotic' carbon and fossil carbon has still to be made, tree growing is being promoted as a method by which economic benefit through the sequestering of carbon can be gained.

However, the value of a forest ecosystem in enhancing this effect by the differential sequestering value of different tree species has not been fully appreciated. In a growing forest the processes of sequestering or incorporating into long term carbon cycles have two distinct pathways. One is by photosynthetic activity, which sequesters atmospheric carbon in living biomass. In this process, the effective rate of sequestering is confined to the life of the individual organism. The other is by respiration activity, which uses the energy fixed by photosynthetic activity, such as in the synthesis of humates. Here, the effective rate of sequestering is dependent on the nature of the respiring ecosystem.

The photosynthetic activity of plants takes carbon dioxide out of the atmosphere and fixes it in a solid state as organic matter. This act of sequestering carbon is what provides forest biomass. It's quality, in terms of sequestering value has to be measured in time. While all plants sequester carbon, trees and woody plants are most efficient as they produce resistant compounds such as lignin. Consider the fate of two photosynthetically derived objects of similar biomass - a large pile of seaweed and a log lying on a beach. Both are plant products, but one (the tree) is strengthened with lignin. The same biological, chemical and physical forces will impact both. The seaweed will disappear within a few weeks, while the log may remain more or less the same for years.

An important attribute of the wood in terms of its sequestering value is its durability. Natural durability is a reflection of the wood's ability to withstand the attacks of decay organisms. Archeological finds often demonstrate wooden construction items dating back about 1000 years. In America a durability standard has been devised by using White Oak as the standard. In this method of evaluation White Oak is given a rating of (100). Wood with higher scores, such as Red Cedar (150-200) or Black Locust (150-250), is more durable. Wood with a lower score, such as Hemlock (35-55) or Birch (35-50), is less durable.

The rate at which carbon can be sequestered by a forest is a product of its primary productivity. The rate of production is reported as net annual volume-growth in stemwood(cubic metres/ha/yr). Different tree species have timber of different densities, so that a cubic metre of softwood weighs about 0.43 t. and hardwood about 0.63 t.(Stewart et al 1979). An asymmetric sigmoidal function has been described (Richards 1969) to approximate the pattern of carbon sequestration into organic matter as a forest grows (Fig1).

W= A exp(-b exp(-kt)) (1)

Here W is the amount of carbon sequestered at any time t (years), A is the asymptotic value of W, b and k are constants (Richards op cit)

A design that incorporates carbon sequestering as a goal will also tend towards long rotation tree crops. This is due to the fact that the active sequestering or growing phase of any timber is longer and the total biomass is greater ( see Fig 50 yr. cycles and 100 yr. cycles). Such design will allow many species of trees determined to be 'marginal' to be brought into culture.

Present approaches to controlling carbon dioxide have been largely directed towards controlling release. The development of terrestrial sinks has been directed towards the growing of trees (Barton and Gifford 1990). While this is the most logical approach at present, the dynamics of the tree growing process needs to be examined more closely in order to obtain the maximum benefit.

The output from growing trees in terms of sequestering carbon can be stated as Wt. W or the carbon sequestered +TLR where;

T = Timber, trunk and branch material over y cm in diameter

L = Leaves, bark and stems under y cm in diameter

R = Roots and all other underground parts.

In addition to producing the photosynthetic products listed above, a growing tree also contributes to the creation of soil organic matter. As a forest product, soil also has great value as a carbon sink; the process of biochemical distillation of photosynthetic products can keep atmospheric carbon dioxide sequestered by the biological system for periods exceeding 4000 years (Beckermann and Hubble 1974). While about 16 percent of the long-lived fraction identified as 'old carbon' can have lifetimes from 5700 - 15,000 years (O'Brien and Stout opp cit). The role of soil in sequestering atmospheric carbon dioxide needs to be recognised. An evaluation of the sequestering potential of various forest ecosystems (Fig 2) suggests that forest soils contain a large proportion of the carbon pool. These long lived compounds are a product of the bio-chemical distillation of photosynthetic products and tie up about 20-30% of the organic matter reaching the soil from the above ground env I

Where the sum total of the plant production is its total biomass (Wt). The ratio LSc/Wt will vary according to the efficiency of a particular soil to sequester carbon into the long-lived pool and the end use of the forest. In the case of tree crops the contribution to the soil will be only from the roots, leaves and branches such that Wt = L+R, as the timber is expected to be removed from site or used for an anthropocentric purpose.

The variable (T) representing timber will have a sequestering value equal to the time of growth and biomass attained. At harvest, the value of the clear wood as a carbon sink will depend on its end use. Therefore (T) must be described with a multiplying factor dependent on the durability and end use of the wood. For instance;

End use Firewood Pulpwood Chipwood Constr Timber .

Multiplier (z) .05 1.0 1.75 2.5 .

The value Tz can then be added to LSc to give some approximation of the carbon sequestered into the long term pools so that.

Tz + LSc = p (2)

Similar calculations can be made of the short and medium term pools to obtain an idea of the value of various forestry approaches to address global greenhouse.

Thus when considering the decay function, the values of the long term pools should be reflected to provide a set of relative values for different types of forestry. The exponential decay function proposed by Barson and Gifford (1990) states that:

Wr + Wo exp(-dt) (3)

where Wr is the amount of carbon remaining after decaying for time t., Wo the carbon sequestered into the forest at the time of felling and d is a decay constant. The inclusion of the long term pools as primary values can make this function more sensitive to the type of forest grown. Thus a decay function that incorporates the long term pools will state:

Wr = Wo exp (- d/p t) (4)

For the purpose of sequestering carbon the most productive forests are those that have a long standing life as well as a high potential to develop deep organic soils. Commercial monocultures have a disadvantage in this respect as they are harvested for timber after a set period of time and develop deep organic soils very rarely. A better model is provided by a polyculture with long rotation times, such as that seen in some forms of traditional forestry. There, a high diversity of tree species with a good development of organic soil has been recorded. Further, as the trees used in this approach to forestry are crop species which produce large crops as the trees mature, there is a disincentive to fell the trees unless they are diseased or very old. The development of this type of forestry in some temperate and tropical regions can provide a very efficient method of sequestering carbon, that also provides social, ecological and economic benefits.

The following suggests that the application of the technique of Analog Forestry will provide some of the largest gains in respect of the volume and time that carbon can be sequestered away from the atmospheric pool. Thus the application of AF in both temperate and tropical systems could gain the rural sector a substantial economic yield from the emerging market in Carbon credits.