Climate Change 2001:
Working Group II: Impacts, Adaptation and Vulnerability
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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

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