Spatial association of Aspidosperma quebracho-blanco juveniles with shrubs and conspecific adults in the Arid Chaco, Argentina

The survival of Aspidosperma quebracho-blanco juveniles in the Arid Chaco is facilitated under the canopy of nurse plants. The possible effects of nurse plants were studied at intra- and interspecific levels by analysing the spatial distribution of juveniles and adults of A. quebracho-blanco, of the main shrubs Larrea divaricata and Mimozyganthus carinatus, and of the group of deciduous and evergreen shrub species, and their pair associations. Data were analysed using the SADIE (Spatial Analysis by Distance Indices) software. A. quebracho-blanco seedling abundance followed the distribution pattern of the main shade-providing species: an aggregated spatial distribution pattern in most of the categories studied. The seedling bank of A. quebracho-blanco also showed an aggregated pattern and was spatially associated with shrubs and adults of its own species. The intensity of the association depended on the functional types: deciduous Fabaceae, deciduous non-Fabaceae, evergreen and conspecific adults, each of which provides a different canopy structure and therefore different amounts of shade. The spatial association was significant with the evergreen group, and less significant with the deciduous Fabaceae group. There was no positive association with deciduous non-Fabaceae, or with gaps (open sky). The differences generated by canopy cover may influence the nurse effect, as observed in the intensity of association of A. quebracho-blanco with shrubs and conspecific adults.