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Freshwater Management Series No. 5

Guidelines for the Integrated Management of the Watershed
- Phytotechnology and Ecohydrology -

G. The threat of increasing agricultural productivity

It is highly probable that, by the year 2050, 10 billion people will live on Earth (Clarke 2000). According to Smil (2000), the production of a sufficient amount of food for such a large number of people will have serious environmental consequences, such as the loss of topsoil and biological diversity. Therefore, special attention must be given to the outcome of the global "fertilisation experiment." The global fertilisation experiment is examining the effect of increasing the "pool" of reactive nitrogen (i.e., through conversion of N2 to the more reactive N) by releasing synthetic nitrogen fertilisers into the atmosphere. This process changes not only atmospheric chemistry but also causes acidification of freshwater ecosystems and can lead to marine eutrophication (Lepisto 2000).

In this context, Smil (2000) examined the question of whether or not there is enough arable land to feed an expanding global population. He concluded that, while the nitrogen problem might be manageable at the global level, constraints on water and food at the local level are likely to be increasingly significant. Two problems emerge: first, the patchy distribution and the severity of environmental problems must be resolved (i.e., localised environmental problems in Africa, Asia, and South America), and, second, the uncertainties within the biological and abiotic (climatic) systems must be addressed, because of their non-linear response to megascale environmental disturbances. These threats are connected with human population growth, especially where such growth exceeds the carrying capacity of the host environment. The decline in carrying capacity is further aggravated by poor management of natural resources. A key component of the solution should be to restore and enhance the local, regional, and global carrying capacity by restoring biogeochemical and water mesocycles, of which, plants are a crucial component. Thus far, as the restoration of natural phytocenosis has been unrealistic in most of situations, the opportunities for restoration and sustainable use of land-based bioproductivity for food, energy, employment, and the elimination of pollution rest on the application of "phytotechnology" - the creation of plant cover under given climatic and edaphic conditions, can be effective as a technology to address environmental considerations within the socio-economic hierarchy of problems to be solved in a region.

Fig. 4.3. Changes in the rates of precipitation (P), runoff (R), and evaporation (E) along a European - African transect: 18,000 years BP, 9,000 years BP, and at present. Emax = present day potential evaporation (changed from Starkel 1988)

H. Enhancement of plant biomass as a potential remedy for global climate change

According to Starkel (1988), the evolution of climate determines the distribution and form of plant cover, while plant cover, to a great extent, modifies the heat balance and water budget (Figure 4.3).

Concerning the role of biota in shaping biogeochemical processes, Schlessinger (1991) stated that ".. living systems exert major control on the composition of the oceans and atmosphere and on the rate of weathering of the Earth’s crust." Forests and shrubs cover more than 40% of the Earth’s surface. In the face of the steady increase of CO2 in atmosphere, progressive deforestation, and inefficient emission reductions, it is necessary to stress that forests can store between 20 to 100 times more carbon per unit area than crop lands. As a consequence, the world’s forests are estimated to contain almost 80% of global, above ground terrestrial carbon (Rykowski 1999). Forests constitute, however, only a temporary carbon pool within a dynamic, cyclical system: half of this pool is contained in the less dynamic (more stable) boreal forest and one third is in the more dynamic tropical areas of the globe. The significance of these pools can be expressed by the organic matter decomposition rate, which in a tropical forest can be more than 30 times faster than in a boreal forest. This difference highlights the important role of tropical forests in regulating nutrient dynamics, as well as their role in carbon and nitrogen sequestration.

Recent progress in the understanding of the role of the land surface on climatic processes (Gash et al. 1996, Silva and Dias 2000) exemplifies the effect of deforestation within the Amazon region, where, especially during the dry season, significant differences in surface fluxes of heat occur between forested and deforested areas. The consequences of this phenomenon are seen in the atmosphere, and include convection and boundary layer effects which seriously influence the local concentrations and long range transport of atmospheric constituents (i.e., aerosols, trace gases, greenhouse gases, etc.). According to the BAHC Project of the IGBP (2000), these consequences have led to a new understanding of the role of plant cover in the climatic and hydrological cycles. This new understanding has led to various hypotheses about the relationship between recent El Nino events and Amazonian deforestation (Figure 4.4). It has further been suggested that atmospheric and oceanic circulation patterns over the Eastern Pacific and North America could be changed dramatically.

Fig. 4.4. A new understanding of the role of the land surface

Some positive predictions, based upon the use of phytotechnologies for forest propagation, have been made in the face of doubling atmospheric CO2 concentrations. According to Somebroek (1991), plants reduce water vapour loss and increase water use efficiency. As a consequence, the potential growth of tropical forests could increase by 75% (Rykowski 1999). Considering the above, the creation of an appropriate forest structure for restoration of water and nutrient circulation in various landscape types should be based on a sound knowledge of:

  • climatic hydrologic and edaphic conditions,
  • the physiological performance of plants,
  • a prognosis of long term plant community succession.

The last point is particularly important from the point of view of sustainable exploitation of bioenergy and creation of employment.

According to the classic Odum (1989) model of forest succession, the phase of the intensive growth and accumulation of biomass occur at about 60 years, with maximum net primary productivity appearing at between 30 and 40 years. Under this theory, respiration remains steady after 30 years (Figure 4.5). This model has been expanded to a longer temporal scale in the Gap model (Shugart et al.1992). This model has been used to forecast succession in the Carpathian forest from 1950 to 2300 (Kozak and Mienshutkin 1999). On the basis of comparative data obtained from forest patches of different ages, the authors demonstrated that, from year 2000 until the year 2220, there will be a forecast increase in the dominance of the broad leaf Fagus sylvatica and a steady decline of Abies alba and Picea abies. A sharp decline in Fagus is forecast thereafter, with a short period of dominance by the birch. This is likely to be followed by a period of dominance by the coniferous Abies (Figure 4.6). Such models can serve as useful tools in evaluating the effects of different factors, such as pollution and climatic change, on forest structure. These models can also be useful in assessing the potential applications of phytotechnologies. Phytotechnologies can lead to the development of more resistant phases of succession in light of their potential resistance to human impacts (e.g., broad leaf forests, rather than coniferous forests, provide greater resistance to acid rain impacts). Such approaches can also help optimise timber exploitation and enhance the selection of species with specialised functional performance (e.g., legumes can increase the amount of nitrogen in the soil).

Pg - gross primary production
Pn - net primary production
R - respiration
B - Biomass

Fig. 4.5. Optimisation of phytotechnologies

(Kozak and Mienshutkin 1998 - computermodel, changed)

Fig. 4.6. The prognosis of the forest succession in the Carpathians

Thus, an understanding the physiology of the various plant species is essential for the optimal use of phytotechnologies in a given environment (i.e., defined by both water and soil descriptors). For example, some legumes (i.e., acacias) are effective tools for the reclamation of mine spoil disposal areas in the tropics because of their ability to extract nutrients from soils with very low fertility (Sprent and Sprent 1990). However, acacias can also modify native plant communities when they spread beyond the boundaries of the phytoremediation sites. Thus, it is important to recognise that a diversified biomass is needed in a given basin, based on the knowledge that various plants possess different functions and roles with the land and water system. For example, grasses are an excellent filter for suspended matter and nutrients in surfacial flows, but do not possess the same potential as forests for modifying the rate of evaporation. These rates can fluctuate by as much as 100%, depending on water deficit or flood conditions.

According to Ryszkowski and Kedziora (1999), the proper management of the temperate agricultural landscape should be focused on two fundamental rules:

  • Maximising the retention of the water in the landscape
  • Creating agrotechniques and forestry management practices which support the transfer of water to the atmosphere through plants versus direct physical processes.

The recent, increased rate of degradation of biotic structure around the globe is the result of a lack of understanding of the role of biomass diversity and spatial structure in relation to energy flow and nutrient circulation within the landscape. Even in developed countries, the management of natural resources has been carried out in accordance with a mechanistic vision of the world (Lametrie) having a view to maximising short-term economic profits.


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