Pastures abandonment

The recovery of deep soil water uptake in the Paragominas secondary forest was possible because of the re-establishment of deep root systems following pasture abandonment (Fig. 9.2). The root systems of secondary forest trees, vines, and palms rapidly penetrate to at least 8 m depth during the first 15 years of regrowth. We identified one third as many morphos-pecies of roots to 8 m depth in the secondary forest as in the neighboring mature forest, with a prevalence of vine and palm roots in the secondary forest. The vine Davilla kunthii, for example, penetrates to at least 8 m depth by the time its stem has attained 1 m height (Restom 1998). Vines in... 

Nepstad, D. C., P. Jipp, P. Moutinho, G. Negreiros, and S. Vieira. 1995. Forest recovery following pasture abandonment in Amazonia Canopy seasonality, fire resistance and ants. In Evaluating and Monitoring the Health of Large-Scale Ecosystems, ed. D. Rapport (NATO AS Series, New York Springer-Verlag), pp. 333-349. 

Nepstad, D. C. 1989. Forest recovery following pasture abandonment in eastern Amazonia. Ph.D. Dissertation, Yale University. 

Silver, W. L., Ostertag, R., and Lugo, A. E. (2000). The potential for carbon sequestration through reforestation of abandoned tropical agricultural and pasture lands. Restoration Ecol. 8(4), 394 107. 

Mobilization of water from the soil is closely related to root depth and root density in each layer of soil. Fine roots of active B. brizantha pastures, established in deeply weathered clayey soils in eastern Amazonia, reach depths of 8 m or more (Nepstad et al. 1994). In abandoned pastures (50% B. humidicola and P. maximum cover and 50% invading shrubs and small trees), fine roots ( < 1 mm in diameter) were found at depths of 12 m (Nepstad et al. 1994). Fine-root biomass in the superficial soil layers of an active pasture in Paragominas, eastern Amazonia, was 3 times higher than that found in an adjacent primary forest area. Fine root biomass in the active pasture decreased by a factor of 100 between the surface and 6 m depth. In an abandoned pasture area, the distribution pattern of fine-root biomass was similar to that observed in the deeper soil layers of the forest ecosystem. This pattern is associated with the fine roots of the existing dicotyledonous invading species. 

Under normal conditions of water availability, values for LAI in pastures of B. brizantha have been measured above 4.0. However with the establishment of a water deficit in the soil, these values decrease to below two or even lower in pastures of P. maximum (Roberts et al. 1996). A similar situation is found in abandoned pastures in eastern Amazonia, where a reduction of approximately 68% of green tissue has been observed in the dry season, while in an adjacent area of primary forest this reduction was only 16% (Nepstad et al. 1994). Primary forests, which have deep root systems and little seasonal variation in LAI, maintain stable subcanopy microclimatic conditions and transpirational flux, even during the dry season. Because of an evergreen forest canopy, the return of the rainy season has less impact on the microclimate near the soil in the forest than in the pastures, and the deep soil water stores are also more efficiently recharged in the forest. 

As abandoned pasture areas become increasingly invaded by shrubs and small trees, which in time lead to secondary forest formation, the biogeochemical cycles of plant nutrients and the hydrological cycle are expected, eventually, to resemble the cycles originally found in the primary forest. The rate of recuperation of biogeochemical cycles in secondary forests and the factors that influence those rates, however, deserve further attention. 

Buschbacher, R., C. Uhl, and E. A. S. Serrao. 1988. "Abandoned pastures in eastern Amazonia K. Nutrient stocks in the soil and vegetation." Journal of Ecology 76 682-699. 

Fig. 93 Arbuscular mycorrhizal associations in fine roots of adjacent mature forest, secondary forest on abandoned cattle pasture (l6 years old), and active cattle pasture planted with Brachiaria brizantha forage grass. Each data point is the average value for six root samples, with 75 1-cm root segments stained and analyzed per sample following Phillips and Hayman (1970). Percent infection refers to the percent of each root segment s length in which fungal hyphae or arbuscles were visible.

Nepstad, D. C., C. Uhl, C. A. Pereira, and J. M. C. da Silva. 1996a. A comparative study of tree establishment in abandoned pasture and mature forest of eastern Amazonia. Oikos 76 25-39. 

Silva, J. M. C. da C. Uhl. and G. Murray. 1996. Plant succession, landscape management, and the ecology of fru-givorous birds in abandoned Amazonian pastures. Conservation Biology 10(2) 491-503. 

Vieira, 1. C. G., C. Uhl, and D. C. Nepstad. 1994. The role of the shrub Cordia multispicata Cham, as a succession facilitator in an abandoned pasture, Paragominas, Amaz6nia. Vegetatio 115 91-99. 

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