SHORT-ROTATION EUCALYPT PLANTATIONS IN BRAZIL: ENVIRONMENTAL ISSUES

2. ENVIRONMENTAL ISSUES

In the 1960s, little if anything was known about the silvicultural and ecological needs of the introduced eucalypt species in Brazil. Eucalypts were established across the country in varying soil and climatic conditions. Availability of land and low land prices stimulated most of the reforestation projects at that time. Ecological zoning or use of certified seeds of the correct provenance to ensure the success of the plantations was not considered. Because Eucalyptus grandis was the most popular and fastest-growing species at that time, it was used indiscriminately by most of the companies. Consequently, survival was very low for some plantations, some of which were located in regions having very low annual rainfall, a key factor for growth.

In the 1970s, criticism concerning the alleged harmful effects of the eucalypts on the environment grew. By that time, large plantations (encouraged by the fiscal incentives for reforestation) were wide-spread in São Paulo, Minas Gerais, Espírito Santo, Bahia, Mato Grosso do Sul, and Rio Grande do Sul as well as in other Brazilian states. Eucalypts were said to adversely affect the soil, the water cycle, wildlife, biodiversity, and local vegetation. These concerns were being expressed in India, Portugal, Spain, and the United States where Eucalyptus also had been introduced.

No provenance tests had been established in Brazil, thus a species would grow well in one place and very poorly elsewhere. Productivity in some places ranged from 6 to 12 m3 of wood per hectare per year and other regions, from 25 to 30 m3 of wood per hectare per year. Coppicing was not responding as expected, and even when successful, second-rotation yields were lower than first-rotation yields. Often, yields would decrease 20 to 40% from one rotation to another. In the early years, very intensive insect and fungal attacks also occurred, mainly on the Eucalyptus grandis plantations. No adequate site-preparation technologies were used. In most areas, fire was used to help eliminate the existing vegetation, and soil preparation was similar to the system used for agricultural crops that sometimes led to erosion and silting of streams. No attention was given to any physical or chemical impediment in the soil layers that could jeopardize the growth of the newly established eucalypt plantations. No adequate fertilization formulas or methods of application were available, and correct spacings and rotation lengths were unknown at that time.

In the late 1970s, this situation changed drastically as a result of ecological zoning for reforestation with eucalypts and pines done by Lamberto Golfari, an expert from the Food and Agriculture Organization of the United Nations (FAO). He established a network of species and provenance trials throughout the country with the help of the Brazilian Enterprise for Agricultural Research (EMBRAPA). He used Thorntwaite's water balance to compare eucalypt species and provenances in the various regions in Brazil. Currently, Golfari's ecological zoning for reforestation with eucalypts is being improved and refined by EMBRAPA in the South and by the Department of Forestry of the Federal University of Viçosa in the Southeast. This new technology is incorporating climatic (Martins et al. 1992), ecological and edaphic (Tristão 1992) variables into the ecological zoning by means of multivariate analysis technique (Reis and Reis 1993).

With ecological zoning and the introduction of new species and provenances tested by EMBRAPA and the forest companies (combined to better silvicultural and management practices), productivity levels of the eucalypt plantations during the 1980s improved substantially. The average growth of the plantations increased 18 to 22 m3 per hectare per year, with some hybrids and clones producing almost twice that. Today, the average productivity is around 35 m3 per hectare per year, but in some places plantations can grow at the incredible rate of 90 to 100 m3 per hectare per year (Reis and Reis 1993).

Current knowledge allows much more environmentally responsible establishment of eucalypt plantations, such as those recently established in the south of Bahia and northern Espírito Santo. At the same time, environmental and forestry legislation is limiting the options that companies have for plantation establishment, forcing the companies to associate with universities and research institutions in searching for new techniques by which to establish eucalypt plantations.

2.1 CLIMATE

One criticism of eucalypt plantations is that they can promote a change in the local climate. This is because of the very high evapotranspiration rate of eucalypts, which drains water from the soil leading to a lower water table. This high evapotranspiration rate is claimed to adversely affect local rainfall levels, resulting in possible desertification of the area. Others point out that the contribution of the continental water evaporation to the hydrologic cycle is known to be very small compared with that of the oceans (Lee 1980). Further, the mere presence of a forest in a certain area does not necessarily affect the occurrence of rainfall in that area (Penman 1963). The subject merits more study.

In some regions, such as the Amazon basin, the forest can influence local precipitation. The canopy can affect the air-circulation pattern that flows from the Atlantic Ocean into the basin (Shuttleworth 1988). There are also situations in which the forests are located in hilly regions along the coast and are subjected to a constant fog, which condenses on the canopy and falls to the forest soil adding to the rainfall level (Lima 1993). This effect also has been observed in some native eucalypt forests of Australia (Costin and Winbush 1961). This could indicate that Brazil's eucalypt plantations may have the same effects on the climate as a native forest located in the same region. Thus, the effect of planting a large area with eucalypts is likely to be the same as if other vegetation of similar structure and albedo were planted. In summary, certain research studies have shown that differences in the microclimate within eucalypt plantations may exist compared with those of other species and native forests, but the data are not conclusive (Poore and Fries 1985).

2.2 WATER

2.2.1 Canopy Interception

One of the primary concerns about eucalypt plantations is that they lead to a diminished rainfall in their area of influence. The most significant hydrologic effect of a eucalypt plantation, as well as any other tree plantations or forest cover, is its interception of rainfall. A portion of this rainfall reaches the ground, while some remains in the canopy and is lost by direct evaporation (Lima 1993). When comparisons are made between forests and open areas, such as pastures and native grasslands, a higher evaporation rate can occur in the forest cover, thus diminishing the water supply of the watershed (Calder 1985).

The total loss of water by canopy interception is equal to the sum of the evaporation that occurs during the rainfall and the evaporation that occurs after the rainfall from the water retained in the canopy. Because precipitation is likely to be heavy in the tropical regions and occur over a very short time period, losses during rainfall are smaller than losses after. The capacity of a forest canopy to store or hold water can be measured by using a leaf area index. The typical leaf area indexes of some eucalypt species are smaller than those of other forest species, which suggests that the total interception by eucalypts may be comparatively low (Gash 1979).

A study in São Paulo indicated that a 6-year-old Eucalyptus saligna plantation lost 12.2% of rainfall water by canopy interception. Two 13-year-old pine plantations (Pinus caribaea and P. oocarpa) have shown losses of 12% (Lima 1976), and savanna-like vegetation showed a loss of 27% (Lima and Nicolielo 1983). Secondary Atlantic forests varied from 12.4% (Castro et al. 1983) to 18.2% (Cicco et al. 1986), and the range in Amazonian rain forest was 8.9% (Lloyd et al. 1988) to 19.8% (Franken et al. 1982). Given the number of existing eucalypt species and the diversity of species existing in the tropical forests, it is premature to draw any final conclusions about the loss of rainfall resulting from canopy interception by forests in the tropics. This subject merits more attention from researchers (Shuttleworth 1988). However, a review of the work and data available in Brazil as well as other parts of the world suggests that, on the average, the water interception loss by eucalypt plantations is less than that of other tree plantations or native forests (Lima 1993).

2.2.2 Runoff

It has been demonstrated that watersheds covered by natural forests of eucalypts in Australia produce good quality water (Hatch 1976). Charley and Richards (1983) showed that the most positive effect of forest cover on watershed water quality occurs during rapid tree growth. This correlation may suggest that eucalypt plantations in Brazil may have a beneficial effect on the water quality by virtue of their very fast growth rate and short rotation.

Although some variation exists in the influence of various eucalypt species, native forests, and other tree species on water quality, studies have shown that watershed water quality depends more on the geology, soil, and precipitation regime of the region (Lima 1993). In the valley of Paraíba river in São Paulo, a study conducted in two small watersheds covered by Eucalyptus saligna showed that the nutrient balance and quality of the water was not unlike that found in similar studies in other parts of the world (Ranzini 1990).

The substitution of the forest cover by pastures and agricultural crops will normally promote a change in the water quality and water balance. The water often becomes saltier because of the concentration of minerals. However, afforestation of the area probably will restore the initial condition over time (Borg et al. 1988). Therefore, it is possible to conclude that establishing forest plantations can be a positive factor in watershed management. However, care must be taken because inadequate site preparation, clearcutting, fertilization, and slash burning can have negative effects on water quality (Lima 1993).

2.2.3 Uptake

Perhaps the most controversial water-related issue relating to eucalypt plantations is the effect on the water content of the soil. It is claimed that eucalypt trees absorb more water from the soil than any other tree species. The uptake of soil water depends mainly on the architecture of root systems and the depth of root penetration (Lima 1993). The capacities of the more than 600 eucalypt species for water uptake vary depending on the type of root system; some have superficial root systems and others have deeper systems (Jacobs 1955). As in most natural forests and forest plantations, the roots of most eucalypt plantations are concentrated in the superficial layers of the soil (Reis et al. 1985). However, some eucalypt roots can grow to 30 m in depth (Jacobs 1955) and extract water from 6 to 15 m deep (Peck and Williamson 1987).

The growth of the eucalypt root system depends on the environmental factors, mainly soil compaction (Nambiar 1981). This has been verified for E. urophylla, E. pellita, E. camaldulensis, E. grandis, E. cloeziana and E. citriodora in the state of Bahia (Krejci et al. 1986). Of these, only E. citriodora and E. pellita were able to develop fine enough roots to penetrate a compacted layer of soil in the research area.

A study carried out in Minas Gerais compared soil moisture in a 5-year-old E. grandis plantation, a 5-year-old P. caribaea plantation, and a savanna-like native forest. A similar pattern of annual variation in soil water was found for the three forest covers. However, in terms of timber production, the eucalypts used water more efficiently than did the natural vegetation (Lima et al. 1990).

Eucalypts seem to behave as any other tree plantation or natural forest cover with respect to the soil water dynamics and with respect to the water balance of the watersheds. As with most timber harvesting, soil water increases during cuttings and decreases after afforestation is complete. Thus, eucalypt plantations (as any other managed vegetation) can have both positive and negative effects on the water quality and quantity. Whether the sum of these effects is positive depends largely on management practices.