14.2.2. Agriculture and Plantation Forestry
22.214.171.124. Arable Farming and Tree Crops
Agricultural lands (excluding pastures) represent approximately 19% of the
land area of Latin America. Over the past 40 years, the contribution of agriculture
to the GDP of Latin American countries has been on the order of 10%. Agriculture
remains a key sector in the regional economy because it employs an important
segment (30-40%) of the economically active population. It also is very
important for the food security of the poorest sectors of the population.
Arable farming is based on annual crops of cereals (wheat, maize, barley, rice,
oats), oil seeds (soybean, peanuts, sunflower), vegetables/tubercles (potatoes,
cassava), and a variety of perennial grasses, including specialty crops such
as cotton, tobacco, tea, coffee, cacao, sugarcane, and sugar beet. Major tree/shrub
crops include a large variety of fruits, oil palm, and others. This farm production
has given rise to associated activitiessuch as beekeeping and bee productsas
well as important agro-industries that produce valuable incomes in countries
that already have developed their own markets and exporting lines.
Although the more important commercial agriculture and agro-industry businesses
are well developed in a few countries, many Latin American economies rely on
small farming system production. In smaller and poorer countries, such as rural
communities in Central America and the Andean valleys and plateaus, agriculture
is the basis of subsistence lifestyles and the largest user of human capital.
For these countries, agriculture is the main producing sector; it undoubtedly
is severely affected by climate variations and would be seriously influenced
by climate change (Rosenzweig and Hillel, 1998).
Extremes in climate variability (e.g., the Southern Oscillation) already severely
affects agriculture in Latin America. In southeastern South America, maize and
soybean yields tend to be higher than normal during the warm Southern Oscillation
and lower during the cold phase (Berlato and Fontana, 1997; Grondona et al.,
1997; Magrin et al., 1998; Baethgen and Romero, 2000). Contributions
to variability as a result of global warming and/or reduction in evapotranspiration
from forest loss would be added to this background variability, thereby aggravating
losses caused by extreme events.
Land-use choices will be affected by climate change. For example, increasing
precipitation in marginal areas could contribute to an increase in cropped lands
(Viglizzo et al., 1995). On the other hand, more favorable prices for
grain crops relative to those for cattle are causing an increase in cultivated
lands (Basualdo, 1995). The continued global trend to replace subsistence with
market crops also creates an increasing threat to soil sustainability and enhances
vulnerability to climate change.
Global warming and CO2 fertilization effects on agricultural yields
vary by region and by crop. Under certain conditions, the positive physiological
effects of CO2 enrichment could be countered by temperature increasesleading
to shortening of the growth season and changes in precipitation, with consequent
reductions in crop yields. Reduced availability of water is expected to have
negative effects on agriculture in Mexico (Mundo and Martínez-Austria,
1993; Conde et al., 1997b). However, increases in temperature would benefit
maize yields at high altitudes and lower the risk of frost damage (Morales and
Magaña, 1999). Several studies were carried out in the region to assess
the impact of climate change on annual crop yields. Most of these studies use
crop simulation models with GCMs and incremental (temperature and precipitation)
scenarios as climatic inputs. Baethgen and Magrin (1995) have shown that winter
crop yields in Uruguay and Argentina are more sensitive to expected variations
in temperature than precipitation. Under nonlimiting water and nutrient conditions
and doubled-CO2, the results for Argentina have shown that maize,
wheat, and sunflower yield variations are inversely related to temperature increments,
whereas soybean would not be affected for temperature increments up to 3°C
(Magrin et al., 1997b, 1999a,b,c). Results obtained under rainfed conditions
for different crops and management approaches in the region are summarized in
Table 14-5; most of these results predict negative
impacts, particularly for maize.
Adaptive measures to alleviate negative impacts have been assessed in the region.
In Mexico, Conde et al. (1997a) found that increasing nitrogen fertilization
would be the best option to increase maize yields, although it would not be
economically feasible at all levels. In Argentina, the best option to improve
wheat, maize, and sunflower yields would be to adjust planting dates to take
advantage of the more favorable thermal conditions resulting from fewer late
frosts (Travasso et al., 1999). However, this adaptive measure would
be insufficient for maintaining actual wheat and maize yield levels. Genetic
improvement will be necessary to obtain cultivars that are better adapted to
the new growing conditions. For wheat and barley crops in Uruguay and Argentina,
a longer growth season could be achieved by increasing photoperiodical sensitivity
(Hofstadter et al., 1997; Travasso et al., 1999).
Subsistence farming could be severely threatened in some parts of Latin America.
The global agricultural model of Rosenzweig et al. (1993) identifies
northeastern Brazil as suffering yield impacts that are among the most severe
in the world (see Reilly et al., 1996; Canziani et al., 1998;
Rosenzweig and Hillel, 1998). Because northeastern Brazil is home to more than
45 million people and is prone to periodic droughts and famines even in the
absence of expected climate changes, any changes in this region would have major
Climate changes can be expected to lead to changes in soil stocks of carbon
and nitrogen. In the Argentinean pampas, chemical degradation of soils, based
on climate changes predicted by the GISS GCM (Hansen et al., 1988) at
an atmospheric CO2 concentration of 550 ppm, would reduce organic
nitrogen by 6-10% and organic carbon by 7-20% in the topsoil as a
result of lower dry-matter production and an increased mineralization rate (Díaz
et al., 1997).
Tree crops in locations where frost risk presents a limitationsuch as
coffee in Paraná, Brazilbenefit from higher minimum temperatures
resulting from global warming (Marengo and Rogers, 2000).