Slope aspect influences plant association patterns in the Mediterranean matorral of central Chile

The most important processes shaping communities are likely to leave observable imprints on their spatial structure. The net outcome of positive and negative interactions among plants has been proposed to vary inversely with environmental harshness. In the Mediterranean-type matorral of central Chile, slope aspect strongly influences mesoclimatic conditions: equatorial-facing slope are xeric and are dominated by spiny shrubs and cacti, while evergreen sclerophyllous trees dominate polar-facing slopes due to their more mesic conditions. Here we use spatial pattern analyses to infer on the interactions that shape community structure in this matorral. We sampled the vegetation on two polar- and equatorial-facing slopes and assessed the dominant interaction at each slope through: (1) a species co-occurrence index (C-score) for the entire community; (2) the frequencies of negative and positive associations between species pairs; and (3) the species richness at each slope. Our results indicated that negative spatial associations dominated plant assemblages on all slopes, but the importance of negative associations decreased from mesic (similar to 30%) to xeric slopes (similar to 16%). Species richness was higher on xeric than on mesic slopes. Although competition seems to be the main biotic force structuring communities on both mesic and xeric slopes, we suggest that the decrease in importance of negative interactions from mesic to xeric habitats could produce an increase in diversity. (c) 2004 Elsevier Ltd. All rights reserved.