Aim: There has been a debate about the effectiveness of passive restoration for recovering tropical forests, but few studies quantify the success of passive restoration. The aim of this study was to better understand tropical forest succession under a passive restoration scenario. We compared floristic and functional attributes of seedlings in a passively restored and an old-growth forest, and assessed the effect of restoration age and distance from the old-growth forest on seedling attributes. Location: Kibale National Park, western Uganda. Methods: We measured seedlings in a passively restored and an old-growth forest in 2011, 2014 and 2017. We determined species diversity, structure and composition and searched the literature for functional traits. We used generalized linear mixedeffects models to compare seedling attributes between the restored and old-growth forest and determine the influence of restoration age and distance from the oldgrowth forest. Results: Seedling species abundance, evenness, basal area and height were similar between the restored and old-growth forest. Wood density and abundance of seedlings of different dispersal modes, habitat types, fruit size categories, and regeneration strategies were also similar between the restored and old-growth forest. However, richness, diversity and composition of seedlings were different. We found a positive effect of restoration age on species abundance and abundance of non-zoochorous, medium-fruited, forest-dependent, non-pioneer light demander and shade-tolerant species, and a negative effect on evenness, wood density, abundance of pioneers and compositional dissimilarity. Basal area of seedlings and the abundance of zoochorous and forest-dependent species declined while compositional dissimilarity increased with distance from the old-growth forest. Conclusions: Our results provide empirical evidence on the potential of passive restoration to recover the structure and functionality of tropical forests in a relatively short period of time. We demonstrate that the effect of restoration age and distance from the old-growth forest is not straightforward and depends on the attributes measured.
The growing trend of agricultural abandonment necessitates understanding the development of regrowth forests on old fields in the context of forest restoration. However, the successional patterns of ecosystem functioning and functional diversity of afrotropical regrowth forests are rarely examined. We assessed whether aboveground biomass (AGB) and functional diversity (FD) vary with restoration age and proximity to old-growth forests, compared AGB and FD between regrowth and old-growth forests to measure restoration success and investigated the FD – AGB relationship. We sampled trees in 63 plots (2000 m2 each) in a regrowth forest and 5 plots in an old‐growth forest in 2011, 2014 and 2017. We calculated AGB using diameter, height and wood density. We collated species functional traits (dispersal modes, habitat types, fruit sizes and regeneration guilds) and computed FD measures (richness, evenness, dispersion, divergence and RaoQ’s entropy). AGB and FD measures (richness, dispersion and RaoQ) increased with restoration age. Functional divergence declined with increasing distance to the old-growth forest. Within 22 years, regrowth forests regained 22% of the AGB and recovered all FD measures of the old-growth forest. We found positive, negative and quadratic relationships between AGB and FD depending on the FD measure and forest type. We demonstrate that regrowth forests increase ecosystem production and functional diversity in abandoned areas, however they cannot substitute old-growth forests. Considering multiple measures of functional diversity in different habitats provides a better understanding of the influence of functional diversity on ecosystem functioning.
The growing trend of agricultural abandonment requires an understanding of the development of secondary forests on old fields in the context of restoration. However, few studies examine the regeneration trajectories of functional composition and functional diversity in afrotropical secondary forests. We tested how functional composition, diversity, and aboveground biomass (AGB) change with age and determined restoration success for a secondary forest restored through assisted natural regeneration in Uganda. We assessed the influence of distance to forests on regeneration.We sampled trees in 63 plots (2000 m 2 each) in the secondary forest (16-22-year old) and five plots in an old-growth forest in 2011, 2014 and 2017. We computed functional composition (community-weighted means-CWM) and diversity using categorical (habitat type, dispersal mode, fruit size, and successional group) and continuous traits (wood density and maximum height) of the species and calculated AGB. The secondary forest showed dissimilar trajectories of functional composition, diversity, and AGB. After 16-22 years, the secondary forest had not yet reached equivalent values of most attributes of functional composition, diversity and AGB in the old-growth forest. The distance to forests had a negative effect on CWM of forest-dependent species, nonpioneer light demanders, and functional divergence and a positive effect on CWM of pioneer species. We show that assisted natural regeneration can enhance the functional composition, functional diversity, and AGB of degraded forests and that continued monitoring is needed to attain full recovery. In planning passive restoration, sites closer to existing forests should be prioritized in order to achieve faster recovery.
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.
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