Rehabilitating abandoned lands in the Latin American humid tropics Montagnini 2001 2002

A research program to develop alternatives for the rehabilitation and use of abandoned lands took place from 1987 to 1998 in three humid forest regions of Latin America: the Atlantic lowlands of Costa Rica, the Atlantic rainforest of Bahia in northeastern Brazil, and the subtropical forest of Misiones, northeastern Argentina. In these regions, common situations of rapid deforestation, loss of biodiversity, resource misuse, and land degradation persist. Similar methods were used in the three locations: soil chemistry and nutrient cycling parameters were measured in pure stands of selected indigenous species using adjacent areas free of trees (abandoned agricultural field or pasture, secondary/primary forest) for comparison. The soils under the species, grassy areas free of trees, and adjacent young secondary forest were sampled for soil fertility and nitrogen availability measurements at the three research sites.

In Costa Rica, in just 3 years, soil conditions improved in the tree plantations compared to abandoned pasture. In the top 15 cm, soil nitrogen and organic matter were higher under the trees than in nearby pasture, with values close to those found in adjacent 20-year-old forests. The highest values for soil organic matter, total N, Ca, and P were found under Vo-chysia ferruginea, a species common in mature and secondary forests in the region (Montagnini and Sancho 1990). Subsequent measurements revealed similar trends in the soil parameters in the following years. Based on the standards determined by the Ministry of Agriculture of Costa Rica for soil fertility assessments, the cation levels (Ca, Mg, and K) under most of the tree species were at or above the critical values for agriculture (Montagnini and Sancho 1990). In contrast, the cation levels in the adjacent abandoned pasture soils were too low for the subsistence crops preferred in the region (rice, beans). Soil organic matter also had positive influences on soil physical properties: soil bulk density was lower (i.e., lower compaction) while soil moisture was higher under the trees than in abandoned pasture (Montagnini and Mendelsohn 1996).

In Bahia, Brazil, values of at least five soil parameters under 15 out of the 20 species of the plantations were similar or higher to those found under primary or secondary forest. Several species contributed to increased C and N, including: Inga affinis, Parapiptadenia pterosperma, Plathymenia foliolosa (leguminous, N-fixing species), Caesalpinia echinata, Copaifera luscens (leguminous, non-N-fixing), Eschweilera ovata, and Pradosia lactes-cens (of other families). Others increased soil pH and/or some cations, such as Copaifera luscens, Eschweilera ovata, Lecythis pisonis, and Licania hypoleuca (Montagnini et al. 1994, 1995b).

In Misiones, northeastern Argentina, the greatest differences in soil C and N levels under tree species and grass were found under Bastardiopsis densiflora, where they were twice those in areas beyond the canopy influence (Fernández et al. 1997). The pH was higher under Bastardiopsis densiflora and Cordia trichotoma, while the sum of bases (Ca + Mg+K) was highest under Cordia trichotoma, Bastardiopsis densiflora, and Enterolo-bium contortisiliquum.

Most of the species identified in this research for their positive influence on soil properties are currently being used as components in productive land-use systems such as commercial plantations and agroforestry in each region (Figs. 6.15 and 6.16).

Fig. 6.15. The native tree Schizolobium amazoni-cum, 17 years old, in an arboretum at the Pau Brazil Experiment Station in Porto Seguro, Bahia, Brazil. As mentioned earlier in this chapter, this species can be grown as a nurse tree in reforestation of degraded areas. (Photo: F. Montagnini)

Fig. 6.15. The native tree Schizolobium amazoni-cum, 17 years old, in an arboretum at the Pau Brazil Experiment Station in Porto Seguro, Bahia, Brazil. As mentioned earlier in this chapter, this species can be grown as a nurse tree in reforestation of degraded areas. (Photo: F. Montagnini)

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