14.2.1.2. Dry Forests
Seasonally dry tropical forests have wide global distribution and coverage.
Nearly 42% of tropical forests around the world are seasonally dry plant communities
(Murphy and Lugo, 1986). Ancient Mesoamerican cultures developed in these regions.
Domestication of animals and plants (e.g., maize, beans, sweet potato) has occurred
mainly in dry forests (Challenger, 1998). Degradation of these seasonal forests
is similar to or even greater than that of tropical rain forests, and only a
small fraction remains intact (Janzen, 1988; Gentry, 1995; Murphy and Lugo,
1995). Janzen (1988) argues that because only a small proportion of the original
distribution of dry forest remains intact in Mesoamerica, neotropical seasonally
dry forests should be considered severely threatened. The estimated deforestation
rate for Mexico, for the 1973-1989 period, is 1.4% yr-1, which
is equivalent to loss of 17.9 km2 yr-1 (Trejo and Dirzo,
2000).
Costa Rican and Nicaraguan forests will be more severely affected by changes
in precipitation than by changes in the annual mean temperature (see Section
14.1.5). In Venezuela, 40-50 Mha of moist forest will shift to dry or very
dry forest under climate change scenarios (Mata, 1996). Between 44 and 51% of
the total covered area of the Mexican deciduous tropical forest will be affected
(Villers-Ruiz and Trejo-Vázquez, 1997).
Burning in the Cerrado shrubland that borders Amazonian forest to the south
has increased in frequency in recent decades. This appears to create an unfavorable
nutrient balance for the entire Amazonian ecosystem (Coutinho, 1990).
14.2.1.3. Savannas, Grasslands, and Deserts
Latin American dryland ecosystems are seriously threatened by desertification
processes that have negative social, economic, ecological, cultural, and political
consequences (Benedetti, 1997; Anaya, 1998). Desertification is defined as land
degradation in arid, semi-arid, and dry subhumid areas resulting from various
factors, including climactic variations and human activities (conclusion from
Earth Summit of Rio de Janeiro in 1992—UNCED, 1992). Evaluation of desertification
around the world is complex because there is no unique measure of aridity. For
example, using Thornthwaite's aridity index, 75% of Mexico is considered
arid land (Thornthwaite, 1948). However, Garcia (1988) states that the arid
region constitutes just more than 50% of Mexico. Large variability in the temporal
and spatial distribution of precipitation complicates determination of arid
and semi-arid region extension and consequently analysis of land degradation
(Balling, 1994; Williams and Balling, 1996; Hernández and García,
1997).
At a global scale, the main desertification processes are degradation of vegetation,
water and wind erosion, and salinization and waterlogging (Dregne and Chou,
1992). Major land-use activities in arid regions, such as irrigation and rainfed
agriculture and livestock on rangelands, also are common factors in land degradation
in various Latin American countries (see Table 14-4).
In irrigated lands, salinization and waterlogging mainly cause the desertification.
In rainfed cropland, the dominant processes for desertification are water and
wind erosion; in this case, the percentage of the affected areas ranges from
10% for Argentina and El Salvador to 78% for Peru. Rangeland desertification
is caused by overgrazing that results in vegetation degradation, as well as
deforestation of woody species for fodder, fuel, charcoal production, and construction
materials. Rangelands correspond to the major surface of drylands, where the
percentage of desertified areas reach the highest levels (70-90%) compared
to other land uses. On the other hand, according to the Global Assessment of
Human-Induced Soil Degradation (GLASOD) survey (Middleton and Thomas, 1997),
deforestation and removal of natural vegetation cover is the primary cause of
soil degradation in South America, affecting 41.7% of the 79.1 Mha of drylands.
The most affected regions are in northeast Brazil, along the Caribbean coasts
of Venezuela and Colombia, and in northern Argentina (semi-arid Chacoan). Secondary
causes of soil degradation are overgrazing (26.2%) and agricultural activities
(11.6%) (<100 mm annual precipitation).
According Greco et al. (1994), precipitation changes projected under
various climate change scenarios are unlikely to produce major ecosystem changes
in this region. However, normal variations in rainfall patterns that are characteristic
of this region may induce cyclic changes in vegetation physiognomy. These variations
probably are more important than the total amount of precipitation. Very humid
years may affect the vegetation of the region. For example, in San Luis Potosi,
Mexico, in 1955, heavy rainfall from a large number of hurricanes caused the
"mezquital" (Prosopis) shrub to disappear as a consequence
of extremely wet soils, and the region became a grassland (Medellín-Leal
and Gómez-González, 1979).
In the same way, the presence of El Niño in 1997-1998 in coastal
arid zones of northern Peru generated drastic temporal changes in dry forest
ecosystems (Torres Guevara, 1992). That area, where the historical average of
annual precipitation is only 20-150 mm, received 1,000-3,000 mm of
rainfall between December 1997 and May 1998. This precipitation had positive
and negative effects in the region: NPP increased in all vegetation communities
(Torres Guevara, 1992), reactivating rainfed agriculture activities. However,
there was an outbreak of insect pests that reduced NPP.
| Table 14-4: Estimated land use, drylands,
desertification (modified from Dregne and Chou, 1992).
|
 |
| Country |
Irrigated
Area
(103 ha)
|
%
Desertified
|
Rainfed Cropland Area
(103 ha)
|
%
Desertified
|
Rangeland Area
(103 ha)
|
%
Desertified
|
Hyperland
Area
(103 ha)
|
Total Drylands
(103 ha)
|
|
| Argentina |
1,680
|
31
|
12,068
|
10
|
178,878
|
70
|
0
|
192,626
|
| Bolivia |
160
|
19
|
1,458
|
31
|
31,069
|
85
|
0
|
32,687
|
| Brazil |
2,300
|
11
|
3,904
|
69
|
74,558
|
90
|
0
|
80,762
|
| Chile |
1,257
|
8
|
1,281
|
47
|
20,976
|
80
|
11,740
|
35,254
|
| Colombia |
324
|
3
|
322
|
40
|
9,376
|
85
|
0
|
10,022
|
| Cuba |
390
|
1
|
35
|
14
|
10
|
90
|
0
|
435
|
| Ecuador |
540
|
7
|
400
|
62
|
7,986
|
90
|
0
|
8,926
|
| El Salvador |
110
|
5
|
10
|
10
|
15
|
93
|
0
|
135
|
| Guatemala |
75
|
8
|
88
|
11
|
719
|
89
|
0
|
882
|
| Mexico |
4,890
|
36
|
10,005
|
54
|
113,142
|
90
|
1,738
|
149,775
|
| Paraguay |
65
|
8
|
42
|
5
|
16,32
|
31
|
0
|
16,433
|
| Peru |
1,210
|
34
|
1,027
|
78
|
40,121
|
85
|
8,097
|
50,455
|
| Venezuela |
324
|
12
|
345
|
29
|
9,728
|
70
|
0
|
10,397
|
|
|