Planting set-up and local environmental conditions affect outcomes in tropical agricultural restoration

Abstract

1. Fine-scale environmental metrics, including spatial configuration and identity of surrounding plants, can strongly influence plant survival and ecosystem functioning. However, tropical restoration projects rarely test their role. Within the Riparian Ecosystem Restoration in Tropical Agriculture (RERTA) Project, located within oil palm plantations in Indonesia, we investigated how the spatial configuration and position of enrichment-planted trees influenced restoration outcomes within actively restored riparian margins. 2. We compared the enrichment plantings' response to fine-scale spatial conditions, either planted with conspecific (same-species) or heterospecific (mixed-species) neighbours for six planted tree species (Peronema canescens, Shorea leprosula, Alstonia scholaris, Artocarpus integer, Calophyllum inophyllum, and Albizia chinensis), in restoration areas with mature oil palms either left in place (shaded) or removed (unshaded). We also tested whether restoration outcomes varied with position within the margin (river, middle, or edge). We measured plantings response as seedling survival and height (monitored over eight years) for all six species of trees. 3. We also carried out more detailed observation on multiple ecosystem functions associated with focal trees of three of the species (Peronema canescens, Alstonia scholaris, and Albizia chinensis), measuring differences by planting configuration and distance to river edge. Functions measured included: herbivore predation, leaf herbivory, litterfall, soil chemical properties (organic carbon, cation exchange capacity, total nitrogen, and phosphorus), and soil physical properties (bulk density, aggregate stability, friable soil content, and A-horizon depth). 4. Survival and height did not differ between conspecific and heterospecific neighbours, but did with position within the riparian margin, but only when analysed by treatment and time. Planting configuration and position influenced ecosystem functions; heterospecific plantings had greater soil aggregate stability and higher ant densities, while increasing distance from the river was associated with lower arthropod abundance and herbivory but greater predation. 5. Our findings demonstrate that planting outcomes are strongly context-dependent, with local seedling configurations, position within the riparian margin, and restoration treatment interacting to influence restoration success and local ecological conditions. Our findings provide experimental results to inform planting strategies and restoration protocols in agricultural landscapes.