Direct and indirect relationships between logging intensity and regeneration of two timber species in the Dry Chaco of Argentina

Abstract

Dry forests with a high abundance of hardwood tree species, such as the Dry Chaco, have a long history of logging practices. Logging intensity can directly affect forest structure, both in the canopy and in the understory, by extracting larger trees, and by damaging the understory. This in turn can impact the regeneration of timber species, showing an indirect effect of logging. However, little is known about these relationships in Neotropical dry forests. In this observational study across 13 sites with different logging histories, we analyze how understory structure (shrub basal area and shrub stem density) and canopy cover are modified by logging intensity (quantified by an index that combines logging selectivity, frequency, and time since the last harvest). In addition, using structural equation models, we evaluate if the regeneration (sapling density) of the two most valuable timber species (Schinopsis lorentzii and Aspidosperma quebracho-blanco) is indirectly related with logging intensity through the modification of the structure of the canopy and the understory. We found that canopy cover and shrub basal area decreased with logging intensity (P = 0.001 and P = 0.016, respectively), while shrub stem density showed no changes (P = 0.48). The density of A. quebracho-blanco saplings was positively related with shrub stem density (P = 0.0033), and tended to be negatively related with shrub basal area (P = 0.09). Therefore, the indirect effect of logging intensity on A. quebracho-blanco regeneration, by modifying the shrub basal area, tended to be positive, although the magnitude was quite low. None of the variables considered showed a relationship with regeneration of S. lorentzii. Our results indicate that logging intensity does not negatively affect regeneration of key tree species of the Dry Chaco, nor the density of shrubs. Moreover, shrub basal area and canopy cover may recuperate after a considerable rest period since the last logging event. Together, these results suggest that woody plants in the Dry Chaco forest might be resilient to forestry practices. We propose that logged dry forests must rest decades to recover their structure in canopy cover and shrub basal area. Including other attributes (e.g. soil, fauna and interactions) is necessary to better understand the possible influence of logging intensity on the ecological integrity and sustainability of dry woodlands like Chaco forests.