| European Tropical Forest Research Network |
Disucssion summary: Degradation, causes and remedy / Root suckering versus invasive species
| By Jörn Laxen, for the moderators team |
This a continuation of the discussion during the previous week. Many invasive species produce many seeds (without suckers). It is not clear whether all the root suckering species are invasive, but some of them are. Further some species have shown definite potential to become weeds due to their capacity for abundant (seed) regeneration. A sucker is "a stem which grows from a pre-existing root, irrespective of whether the root is inside or outside the soil (connected or not connected to its origin ?)" (Bellefontaine et al. 2002).
The suckering phenomenon generally speaking has not really been investigated except for rare cases of poplars, Ailanthus, cherries, Apple, or certain fruiters, etc, and mainly in temperate countries. In the tropical and Mediterranean African subregion, it has not been analysed or studied. Not all tree species have the ability to sucker; some do so prolifically while others do so sparingly. From the existing literature it would appear that in the Sudano-Sahel region, if more than 100 woody plants species sprout from the stump, 56 % of them may have the ability to produce suckers (Bellefontaine 1997).
As opposed to the suckers, there are the "layers" (marcottes in French) which arise from the newly formed roots that come up from a flexible branch which is in contact with soil, or from branches which are still attached to the mother tree. In West Africa (up to mean annual rainfall > 1000 mm) very few appear to produce layering naturally. The regeneration mode of Guiera senegalensis and Combretum micranthum, species which are economically important and widespread in West African stripped bushes, has been studied. Often the reproduction of these species through natural seeding is compromised by climatic factors. The layering rate changes according to the species and the topography: C. micranthum layers much on continental shelves and G. senegalensis layers better on shallows. This layering system is increased by the plagiotropic branches. Various traumatisms can stimulate it (broken branches, cambers). Such reforestation in semi-arid areas with fodder or multipurpose species is cheaper than other techniques (artificial seeding, planting) (Karim et al. 2002).
On the Indo-Asian continent, Populus euphratica is widely used because of its productivity and its ability to survive under harsh environmental conditions, particularly when submerged. Within three years, seven hectares, which had been submerged, were replanted with root cuttings, and have subsequently produced a lot of suckers (Sharma et al. 1999).
Root cuttings are perhaps another alternative to seeding and planting in drylands. From the existing literature, it appears that the distinction between 'suckers' and 'root segments (cuttings)' is not well defined. The fundamental difference between a root segment and a sucker is that in a root segment the separation from the mother tree occurs before the neo-formation of the buds, which develop and give rise to the shoot. The confusion is justified by the fact that natural root cuttings can occur, for example during retreat of water in clay soil, the soil cracks which causes roots to break into individual pieces (Bellefontaine and Monteuuis 2002). Depommier (1996) observes that in Faidherbia albida parklands, roots remaining in the soil after the windfall of old trees, produce a lot of suckers within a few weeks of the decayed stem falling.
Compared with planting seedlings, the socio-economic advantages of vegetative propagation in drylands are important (they do not need nurseries, planting-holes, watering after planting, proximity of nurseries). Some small-scale local reforestation using root suckering ability (like in french "drageonnage", with human induction) or layering and maybe root cutting could be proposed on special sites, where i) fuelwood is vital for cooking or for heating, ii) in heavily degraded areas or submerged lands, iii) where food is sometimes scarce (non wood forest products like honey etc.), and iv) in marginal lands to cover the soil surface.
In conclusion, the two terms "root suckering" and "invasive species" should be separated with the exception for a few tree species. More knowledge about suckering habits, especially in Africa is needed. Currently e.g. the CABI's Forestry Compendium or the Prosea books (and monographs) only provide simple information (for example: abundant root suckering; root suckers are produced; the tree often suckers freely from the roots).
Rehabilitation of Degraded Lands in Sub-Saharan Africa
Up to now, the discussion has mainly focused on the causes of forest/land degradation. As a logical continuation, the below contribution is on rehabilitation of degraded lands in Sub-Saharan Africa.
According to FAO (2001), Africa contains about 650 million hectares of forest, corresponding to 20% of the total land area. The net change of forest area is the highest among the world regions, with annual net loss, estimated at 5.3 million hectares annually (ca. 0.8%). The global average deforestation is estimated to be about 0.2%. As a result, the size of pristine natural forests are dwindling and are being replaced by extensive areas of secondary forests, grasslands, and degraded lands. That land degradation has been particularly severe in the montane zone of Africa and Ethiopia is a case in point. It is estimated that 28% of the Ethiopian highlands are seriously degraded, and a further 23% are moderately degraded (EFAP 1993).
The main questions are: Is it possible to restore the composition, structure and function of the ecosystem prior to perturbation on degraded lands and fragmented secondary forests and by doing so, can these areas be converted into more productive ecosystems? The answer to these questions is 'yes'. However, the re-constructed ecosystem will not be exactly the same as the original one, but it will be similar. The exact recreation of the original forest is not even theoretically possible.
Passive methods like area closures can have amazing results even in severely degraded areas. The other approach is by using foster plant assemblages (e.g. forest plantations) that can accelerate the colonization of degraded lands by native species. In a study (Yirdaw 2002) conducted in the Ethiopian highlands, about 80% of the native woody species found in the nearby natural forest were able to colonize and establish themselves in forest plantations, which were established on former agricultural land. The density of the spontaneously established woody species seedlings was also high (up to 8000 seedlings/ha). This was achieved in 15 years. With this method it is possible to convert degraded lands to a diverse natural forest at a comparatively low cost. In other words, biological diversity, hydrological and ecosystem stability can be restored on degraded lands. Restoration can also be viewed as an active form of conservation. The lack of seed sources and severely eroded soils may slow down the recovery process though. Lack of land, which can be set aside for rehabilitation purposes (especially in densely populated areas) may be a problem. In Ethiopia, areas with steep slopes, which are unsuitable for crop production purposes can be designated for restoration. About one-third of the land area in the Ethiopian highlands have slopes exceeding 30°.
Rehabilitation of degraded lands is a long-term process and may cause resentment in local people. In order for rehabilitation endeavours to succeed, first and foremost the participation and awareness of the local people, and the commitment of the relevant decision makers at the different levels of government administrative structure is crucial. Moreover, ways and means should be devised (cut and carry method, collection of dead branches, etc.) to benefit the local people without endangering the regenerative capacity of the areas to be restored.
There was also a passionate plea for plantations as a solution to deforestation which included the following arguments. Plantations can practically achieve annual growth rates of 20-40 m3/ha/a, whereas the growth of mixed tropical natural forests is 0.5-7.0 m3/ha/a (FAO 1995; Tomberline and Buongiorno 2001). Accounting for only 5 percent of global forest cover, forest plantations were estimated in the year 2000 to supply about 35 percent of global roundwood. This figure is anticipated to increase to 44 percent by 2020 (FAO 2001).
Plantations can provide timber with lower production costs, a higher concentration of merchantable species, and better accessibility than naturally regenerated forests (Evans 1992). Furthermore, plantations have the potential to produce much more in the future due to tree breeding, the use of clones and better silvicultural practices. This may further increase their competitive advantage over natural forests (Tomberline and Buongiorno 2001). In the future a significant shift in timber production from natural forests to plantations is anticipated particularly in the tropical and sub-tropical countries.
This growing reliance on plantations for woody biomass has a high potential to decrease the pressure on the remaining natural forests. The high productivity achieved from lands allocated to high-yielding forests will allow large areas of forest land to be dedicated to other purposes, including biodiversity and ecosystem preservation and watershed protection.
At present, Africa has the smallest plantation area, corresponding to 4% of the total global plantation area (FAO 2001) and has a low rate of plantation establishment. At the same time, Africa has among the highest rates of deforestation and population growth. Forest plantations are one of the promising options for catering to the needs of its growing population and for the conservation of the remaining natural forests. Plantations can reduce deforestation and the subsequent land degradation.
Obviously, the establishment of plantations cannot totally eliminate deforestation in Africa. However, it can reduce it substantially. In some cases, plantation establishment has not resulted in substantial retarding of deforestation. This is mainly due to lack of appropriate policy and/or regulation enforcement failure rather than the failure of plantations to reduce deforestation per se.
There are legitimate concerns about the negative impact of plantations on biodiversity, soil and hydrology. However, with proper management the negative effects can be mitigated. To this end, an effort to define criteria and indicators for the sustainable development is underway, focusing on soil and water quality, non-declining productivity, and social benefits (cf. Tomberline and Buongiorno 2001). Generally, the positive impacts of plantations outweigh the negative ones, especially if plantations are established on a marginal area or on a degraded site.
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