Tussock competition does not limit sapling performance of an endemic shrub in mountain forest restoration, but leaf-cutting ants may at lower elevations

Abstract

Livestock exclusion is essential for restoration of mountain forests in central Argentina but may intensify competition between saplings and tussock grasses, whereas leaf-cutting ants may limit regeneration in disturbed landscapes, particularly at warmer elevations. We tested the importance of competition and herbivory in the success of Escallonia cordobensis enrichment plantings at an elevational gradient. In a full-factorial experiment, 750 saplings were planted at three elevations (1300, 1800, and 2300 m a.s.l.), and two mowing treatments were carried out (unmowed and mowed). Sapling survival, height, winter dieback, and ant-pruning were analyzed at 30 months at the three elevations and at 90 months at the two higher elevations. Survival increased with elevation (39.3%, 75.9%, and 77.6% at 30 months; 27.2% and 60.8% at 90 months), with no effects of mowing or elevation × mowing interaction. Sapling height declined with elevation and was slightly greater in unmowed plots at 30 months but not at 90 months, whereas winter dieback showed the opposite pattern. Ant-pruning was concentrated at low and intermediate elevations, affecting 14.6% and 5.3% of the saplings in the first two years, which reduced both survival and growth of the affected saplings. Our results do not support the stress-gradient hypothesis, as slight facilitation by tussocks prevailed across the entire gradient. Instead, livestock exclusion revealed a potential top-down control by leaf-cutting ants at warmer elevations, highlighting the importance of consumer effects on the restoration of neotropical mountain forests.