El Niño Phenomenon Could Help Reforest Semi-arid Regions

Arid and semi-arid systems around the world have lost a large part of their woody vegetation and biodiversity due to the overuse of wood cutting, cattle grazing, and subsistence agriculture. These systems are used for cattle production, but their productivity are usually very low and erosion of the unprotected soil is often a major problem. This situation is extremely worrying for the international community, not only because of the poor productivity, but also because it is the start of the path towards desertification. Countries such as Peru and Chile have been suffering these effects for many years.

The El Niño phenomenon is the main cause of climate variability on our planet. The frequency of El Niño varies between 3 and 6 years. Its intensity also varies, and therefore so do its effects. On the Latin American Pacific Coast, the effects can be seen in increased rainfall, with varying intensities according to the location. Rainfall in Peru, for example, can increase by up to 25 times, as happened in 1983 and 1998; in Chile, however, the increase in rainfall is much lower, and sometimes non-existent.

The main mission of the international "ELNIÑO" cooperation project was to study how the decrease in herbivories and the increase in water availability due to El Niño could be used to stimulate the regeneration of trees and shrubs in semi-arid ecosystems in Chile and Peru. This is why those participating in the project needed to know the relationship between the climate and the establishment and growth of vegetation. They chose two species that can be found in great number in Latin America and are used frequently by rural communities-the Prosopis pallida in Peru and the Prosopis chilensis in Chile-and took samples using a latitudinal gradient from the north of Peru to the centre of Chile.

The results showed that, despite the distance between the distributions of the two species, both gave similar responses. The growth of the two species is positively correlated with precipitation, but not with temperature. In northern Peru, precipitation and growth of the species occur in three-year cycles and coincide with El Niño. Furthermore, in southern Peru and central Chile, precipitation, growth of the species and El Niño also have the same cycle, but here the cycle lasts more than three years.

On a local level, it has been demonstrated that these species respond positively and significantly to precipitation. This means that when the intensity of El Niño is only moderate, zones protected from herbivores can be recovered through reforestation programmes, and the intensity of the phenomenon can be forecast months earlier.

Global warming could cause an increase in precipitation in many semi-arid zones around the world. El Niño could see the intensity of precipitation and frequency increase, according to these models. This means that although semi-arid zones currently play an almost insignificant role in global carbon balance, the expected increase in vegetation in these zones, due to climatic change, could turn these systems into important consumers of carbon.