Dendroecology of Prosopis flexuosa woodlands in the Monte desert: Implications for their management

VILLAGRA, P.E.; BONINSEGNA, J.A.; ALVAREZ, J.A.; CONY, M.A.; CESCA, E.M. Y R. VILLALBA

Mendoza

Workshop; Fourth Annual Science Meeting, IAI CRN 03. The Assessment of Past, Present and Future Climate Variability from Treeline Environments; 2003

IAI-IANIGLA

The species of the Prosopis genus are dominant elements in the woodlands of the arid and semiarid zones of Argentina. The structure of these woodlands varies among the different biogeographical regions. For instance, at the subhumid Chaco they can reach cover close to 100%, whereas in the drier zones there is a decreasing structural complexity and a decrease in the cover of the tree layer (Cabido et al., 1993). The Monte desert is bioclimatically defined  as arid or semiarid. Mean annual rainfall ranges between 30 and 350 mm, being the water deficit the main limiting factor over all the area. It shows a latitudinal gradient in rainfall seasonality (more concentrated on summer in the Northern part) and thermal (6ºC hotter in the North). A shrub steppe dominated by species of Zygophyllaceae is the typical plant formation, while open woodlands of several species of Prosopis are likely to occur in areas with accessible groundwater (Morello, 1958; Cabrera, 1976). Prosopis woodlands of the arid zones have been a source of subsistence for numerous human communities during several centuries, and even presently continue being exploited by the zone inhabitants (Abraham & Prieto, 1999).  From the economical point of view, these woodlands have been used with a mining concept, with no adjustment of the extracting rate to the rate of renewal of the wood production. In addition, these arid ecosystems have a scarce natural restoration capacity due the extreme environmental conditions and the present high degradation conditions. This has led to severe desertification and consequently to the impoverishment of the local people..             Two strategies have been used to conserve this ecosystems: restriction to wood extraction which have conduced to the transference of the wood demand, and its consequences, to other areas; and the establishment of arbitrary biological rotation or minimum basal diameter of the logging individuals without any knowledge of the biological productivity. In order to pose strategies for the management and recuperation of these woodlands we studied the structure and productivity of P. flexuosa woodlands in relation to a latitudinal gradient. We studied three P. flexuosa woodlands: Pipanaco (Catamarca, 28ºS), Telteca (Mendoza, 32.5 ºS) and Ñacuñán (Mendoza, 34ºS). The structure of the woodland was different between the three site studied. A decreased in density, mean basal diameter, height, canopy cover and the total wood biomass was observed from North to South. The total wood volume varied between 30 and 76 m3/ha in Pipanaco, between 13 and 19 m3/ha in Telteca and is about 13m3/ha in Ñacuñán. P. flexuosa growth varies between the different woodlands and among the individuals of the same population. Nevertheless it is observed that the mean growth values decrease from north to south along the Monte. For those individuals approximately 60 years old, the mean current diameter increase varies from 8 mm at the Valles Calchaquíes (26° S) (Calzon Adorno 1995), 6 mm at Pipanaco, 4 mm at Telteca to 2 mm at Ñacuñán (Fig.1).   These results suggest that the economic potential of the Prosopis woodlands in the Monte greatly vary according to the ecological conditions and that the three sites studied should received different management in order to get sustainability. The Pipanaco woodland is susceptible of forestry management, including lumber production, while in both Mendoza woodlands the structure of the P. flexuosa population, the dominant bioform of the trees and the low productivity suggest that the management should combine firewood production at local scale with other activities such as livestock. The growth ratio observed in different  algarrobales indicate that the biological turns for cutting are longer than those presently used. The higher basal growth, and consequently the highest wood contribution, occurrs between 40 and 90 years, and the cutting biological turn is of approximately 130 years. Results suggest that it is necessary to rethink most of the existent forestal practices. For example, it has traditionally been advised to cut those trees with more than 30-cm diameter. These means felling trees before they have reached the higher growth ratio. Moreover, the forestal legislation should be adjusted to the biological productivity, including the  integral used of wood and other resource from the woodlands.