Interactions in tropical reforestation - how plant defence and polycultures can reduce growth-limiting herbivory

Questions Can the growth of saplings be improved by limiting herbivory during reforestation? Can chemical ecology and diverse planting designs be applied to decrease herbivory in tropical reforestation? Location Reforestation plantings in Heredia, Costa Rica. Methods This study directly evaluates the effects of herbivory on seedling growth and the role of two putative plant defences, saponins and leaf toughness, in limiting herbivory in reforestation. Four planting treatments were studied in a replicated block design in cattle pastures in Costa Rica: (1) a monoculture of a fast-growing species low in saponins (Dipteryx panamensis); (2) a monoculture of a slower-growing, saponin-rich species (Cojoba arborea); (3) a polyculture consisting of half D. panamensis and half three other defended species; and (4) a polyculture of half C. arborea and half the same other defended species. Growth and herbivory were measured every 6 mo during the first 2 yr of plot development and again after 5 yr of growth. Results Dipteryx panamensis was the fastest-growing species, and individuals planted in polycultures grew faster in terms of height than individuals in monoculture. Herbivory was negatively related to sapling growth, and damage during the first 6 mo of plot establishment decreased growth even after 5 yr. Patterns of herbivory varied through time, resulting in changes in the importance of plant defences. For example, leaf toughness, which is an effective defence against many herbivores, was negatively related to herbivory at multiple time periods. In contrast, saponins were not a deterrent to all herbivores, so they were not consistently effective as a defence; however saponins were negatively related to Atta cephalotes (leaf-cutter ant) damage. Saponins are therefore a promising defence against leaf-cutter ants but not against all herbivores. Conclusions Plantinsect interactions influence reforestation through growth-limiting herbivore pressure on seedlings, and this herbivory is likely facilitated by reforestation methods that favour monocultures of fast-growing species that lack strong antiherbivore defences. This study demonstrates the potential for reducing herbivory and improving sapling growth by reforesting with polycultures of fast-growing and well-defended species.