Monitoring recovery of tree diversity during tropical forest restoration: lessons from long-term trajectories of natural regeneration

Abstract

Given the importance of species diversity as a tool for assessing recovery during forest regeneration and active restoration, robust approaches for assessing changes in tree species diversity over time are urgently needed. We assessed changes in tree species diversity during natural regeneration over 12–20 years in eight 1-ha monitoring plots in NE Costa Rica, six second-growth forests and two old-growth reference forests. We used diversity profiles to show successional trajectories in measures of observed, asymptotic and standardized tree diversity and evenness as well as sample completeness. We randomly subsampled 1-ha plot data to evaluate how well smaller spatial subsamples would have captured temporal trajectories. Annual surveys in eight 1-ha plots were missing substantial numbers of rare or infrequent species. Older second-growth sites showed consistent declines in tree diversity, whereas younger sites showed fluctuating patterns or increases. Subsample areas of 0.5 ha or greater were sufficient to infer the diversity of abundant species, but smaller subsamples failed to capture temporal trajectories of species richness and yielded positively biased estimates of evenness. In tropical forest regions with high levels of diversity, species diversity from small sample plots should be assessed using methods that incorporate abundance information and that standardize for sample coverage. This article is part of the theme issue ‘Understanding forest landscape restoration: reinforcing scientific foundations for the UN Decade on Ecosystem Restoration’.

A practice-led assessment of landscape restoration potential in a biodiversity hotspot

“…This underscores the importance of separate treatment of savannas and forests in landscape restoration plans [7] to conserve and promote landscape heterogeneity and diversity [89]. For improved input to spatial restoration modelling, planning and monitoring, permanent vegetation plots should thus aim to span all the vegetation types and chronosequences in a given region [90], including both old growth and secondary savanna [81].…”

Rarefaction and extrapolation with beta diversity under a framework of Hill numbers: The iNEXT.beta3D standardization

“…The six forests, with their abbreviated names and ages in 2005 (the selected reference time; see below) are: Finca el Bejuco (FEB, 10 years), Juan Enriquez (JE, 10 years), Lindero Sur (LSUR, 20 years), Tirimbina (TIR, 23 years), Lindero el Peje (LEP, 28 years), and Cuatro Rios (CR, 33 years). The species abundance data for all stems ≥5 cm in DBH were recorded annually from 1997 to 2017 for four older second‐growth forests (CR, LEP, TIR, and LSUR), and from 2005 to 2017 for the two youngest ones (FEB and JE); see Chazdon et al (2022) for sampling details and summary statistics. All data are available through Dryad (Chazdon, 2021).…”

“…Similar conclusion is also valid for abundance‐sensitive ( q > 0) Hill numbers (Chao et al, 2014). For this reason, Chao et al (2020) and Chazdon et al (2022) recommend the use of coverage‐based rarefaction and extrapolation to meaningfully compare diversity across multiple assemblages, and the use of size‐based curves to determine whether the asymptotic estimates are unbiased or subject to negative bias.…”

Fifteen essential science advances needed for effective restoration of the world's forest landscapes

“…Monitoring data from Ecuador and Southeast Asia, respectively, show that ecological monitoring has potential for maximizing restoration outcomes for plant survival and growth [ 69 ] and for seed dispersal [ 118 ]]. We also see that indicators of forest structure or biomass are less sensitive to sampling area than are indicators of tree species diversity and composition, but small sample plots require standardizing for sample coverage when comparing species diversity [ 166 ]. Various articles in the theme issue also emphasize the importance of socio-economic measurement and monitoring, for pre-assessment of community restoration potential [ 47 ], evaluating human–wildlife conflict [ 31 ] and ensuring human wellbeing benefits [ 45 ].…”