INFLUENCE OF LEAF-CUTTING ANT NESTS ON SECONDARY FOREST GROWTH AND SOIL PROPERTIES IN AMAZONIA

Abstract

Leaf-cutting ants (Atta spp.) often increase in abundance following deforestation and may have an important effect on forest succession on abandoned land. In this study, we evaluated the effects of leaf-cutting ant (Atta sexdens) activity on physical and chemical properties of soil, root distribution, and tree growth in a 17-yr-old secondary forest in eastern Amazonia, Brazil. We compared the soil properties and root distribution in shafts excavated into five mature nests and at 15 m distanced from each nest mound. We evaluated the nest effects on vegetation measuring the stem diameter growth of nine tree species and predawn leaf water potential of the tree species, Banara guianensis, along a gradient of increasing distance from the nest mounds. The growth of seedlings (Cecropia sp.) in pots containing different proportions of mineral soil and organic matter removed from nest refuse chambers was also compared. The deep soil beneath A. sexdens nests at different depths (100, 200, and 300 cm) presented a low (fivefold, P < 0.01) resistance to penetration and was rich in Ca (three- to fourfold, P = 0.06–0.02), K (7–14-fold, P < 0.05), and Mg (two- to threefold, P = 0.09 for 200 cm depth) when compared to non-nest soil. These changes in nest soil properties were accompanied by increases in coarse root biomass (>2 mm diameter, three- to fourfold) and fine root biomass (<2 mm, 10–50 fold). However, stem diameter growth was generally not affected by distance from the nest. The growth of Cecropia sp. seedlings in a pot experiment was favored by the addition of organic matter from the refuse chambers. Banara guianensis trees experienced greater water stress (low predawn water potential) close to the nest mound, perhaps indicating increased competition for soil water that may have accompanied the proliferation of roots of several species in the nest. The competing Atta effects of defoliation vs. deep soil tillage and nutrient enrichment upon secondary forest growth are difficult to compare. Atta's modification of soil physical and chemical properties point to its potential role in facilitating the rapid recovery of deep root systems and soil water uptake previously observed in this secondary forest. Corresponding Editor: P. T. Bohlen.