La clausura como estrategia de restauración en bosques heterogéneos comunales del Chaco semiárido. Un enfoque socio-ecológico

RESUMEN: Las clausuras estacionales son utilizadas en África y Sudamérica para diferir forraje a la estación seca y restaurar vegetación en áreas degradadas. En esta tesis se evaluó el potencial de la clausura como estrategia de restauración en bosques de Santiago del Estero, Argentina, mediante dos enfoques complementarios: (a)Ecológico y (b)Socio-ecológico. (a)Se estudiaron los efectos de la clausura en tres bosques con creciente nivel de degradación sobre la vegetación del sotobosque y factores bióticos y abióticos determinantes de su potencial de regeneración, mediante un experimento de 4 años. (b)Se analizó el rol de la clausura en diez comunidades campesinas, mediante un enfoque participativo de sistemas complejos. Se encontró que: (a)La clausura mejoró algunas propiedades físicas del suelo y el banco de semillas. Generó un rápido aumento de cobertura y productividad de gramíneas en bosques, pero no en arbustales, donde la productividad aumentó recién a partir de un ciclo extremadamente húmedo (evento Niño). (b)La degradación responde a interacciones múltiples entre procesos de diferentes dimensiones y escalas. Fueron considerados clave: reducción de superficie de bosque, cambios en composición comunitaria-familiar, demanda de dinero extra-predial y pérdida de diversidad productiva (debidos a la expansión agrícola y las migraciones campo–ciudad). La creciente susceptibilidad del sistema degradado a fluctuaciones climáticas naturales (años secos) exacerba la degradación. La clausura es usada para diversificar usos de la tierra de la finca. Permite estabilizar la provisión de alimentos, bienes e ingresos, ante las fluctuaciones climáticas y de mercado, mejorando así la resiliencia del sistema socio-ecológico.

ABSTRACT: Chaco region is the largest extension of land covered by xerophytic vegetation in South America. During the last century, grazing, logging and deforestation have transformed the natural landscape of forests and savannas into a heterogeneous matrix of shrublands and forests in different stages of degradation. The symptoms of this process include less tree cover and herbaceous, bush encroachment, more nude soil, and less water and nutrients availability in the soil, due to less litter contribution, soil compaction and exposition to high temperatures. All these changes reduce the forage production and increase its seasonality, therefore worsening the effect of a marked dry season that lasts 5-7 months over the livestock production. Closures, where grazing is excluded during the rainy season, are used in communal rangelands of Africa and South America, to defer fodder to the dry season and to restore vegetation. Nevertheless, its effectiveness could be conditioned by ecological and socio-ecological factors. The objective of this thesis was to assess the potential of closures as restoring strategy in communal heterogeneous woodlands of semi-arid Chaco region. This was done through two complementary approaches: (a) ecological and (b) socio-ecological. (a) It was assessed the effect of seasonal closures (versus open areas) in three patch types of woodland, with a progressive level of tree canopy degradation (mature forests, secondary forests and shrublands) on the vegetation of the understory and biotic and abiotic factors that determine its regenerative potential. For this purpose, a four-year experiment was done in four sites (true replicates) in different communal woodlands of Santiago del Estero Province (Argentina). The central hypothesis of the experiment was that the higher the degradation status of the woodland the lower the effectiveness of enclosures as restoring strategy because differences in the dominant woody layer would impose differential resource availability in the understory, so controlling vegetation responses to grazing rest. (b) It was developed a theoretical model about factors and processes (ecological, productive, cultural, economic and institutional) associated to degradation. In this context, the role of closures was studied through methodological and theoretical frameworks for complex systems analysis. Perceptions form producers from 10 rural communities and also from technicians (n > 50), were revealed; decision makers who affect directly or indirectly to the system. This data was complemented with field data collection and bibliography. It has been found that: (a) with the rising of the degradation of the tree canopy, the availability of soil nutrients decreased and the incident radiation increased in the understory. The closure generated a quick rise (2 years) of coverage and productivity of grasses in mature forests and secondary forests, but this did not happen in the shrublands. The shrublands might have overcome a certain degradation threshold, related to scarcity of water and/or seeds, which is shown in the lack of grass. In a longer period (3-4 years), the closure improved some physical properties of the soil, which contribute to the infiltration, and increased the grass soil seed bank, in both forests and shrublands, with no grass, that would receive seeds from the former. This would reduce the degradation threshold in the shrublands and would increase the possibility of rehabilitation in extremely (cyclical) humid years, such as in “El Niño” events. These conditions occurred in the fourth year of the experiment, in which a significant increase in grass productivity in the closures of shrublands was generated. That year the coverage of low shrubs reduced significantly in the three patch types of woodland. (b) All the decisive actors interviewed, ascribe degradation to the rising of pressure over the forest, due to the reduction of forest-covered surface and the productive specialization, bound to the agriculture expansion and migration processes. In this context, the closure applies as an adapting strategy, used to re-introduce key species (native, exotic) and to diversify production (agriculture, forestry and livestock). This would offer alternative answers to weather and market fluctuation, and it would stabilize the provision of goods and income, thus avoiding negative emergency practices, such as the excessive production of charcoal for sale. It is concluded that the seasonal closure contains a high potential as restoration tool in heterogeneous woodlands, as well as an adaptive strategy in the adverse socioenvironmental conditions of the semi-arid Chaco. The importance of addressing the subject comprehensively is highlighted, in order to understand the problems of degradation in the search of solutions.

Doctorado

Doctor en Agroecología