Does restoration success vary with tree size under restoration plantings and regrowth forests?

Abstract

Several studies evaluate active (i.e., seeding/planting) and passive (i.e., protecting forest regrowth) restoration, but few studies examine successional patterns for different plant sizes. By using biodiversity and structure, we examined whether restoration communities approach old-growth forests over time, and whether restoration success varies for different tree sizes in both active and passive interventions. We examined how initial site conditions affect active restoration. Small (dbh ≥ 5 cm), medium (≥15 cm), and large trees (≥30 cm) were measured in 2003-2017 in permanent sample plots in restoration plantings (initially 3-8 years old) and in an old-growth forest in Kibale National Park, Uganda. Trees were also measured in regrowth forests (initially 16 years old) in 2011-2017. We collated information about site conditions from restoration reports. Biodiversity and structure increased over time towards the old-growth forest. Restoration plantings and regrowth forests recovered diversity and structure of small and medium trees except for large trees. Forest recovery increased with proportions of remnant banana plants and shrubs, while isolation from the old-growth forest slowed recovery. Disaggregating vegetation inventory data by tree size may be useful in achieving a holistic measure of restoration. Restorationists could prioritize sites with remnant banana plants and shrubs, and sites closer to old-growth forests in order to achieve better results.