Tropic Forest Restoration

Biodiversity Consequences of Long-Term Active Forest Restoration in Selectively-Logged Tropical Rainforests

Authors and Affiliations

  • Nadine Keller a,*

  • Pascal A. Niklaus b

  • Jaboury Ghazoul a

  • Tobias Marfil a

  • Elia Godoong c

  • Christopher D. Philipson a,d
    a Ecosystem Management, Institute of Terrestrial Ecosystems, Department of Environmental Systems Science, ETH Zurich, Switzerland
    b Department of Evolutionary Biology and Environmental Studies, University of Zurich, Zürich, Switzerland
    c Faculty of Tropical Forestry, Universiti Malaysia Sabah, 88400 Sabah, Kota Kinabalu, Sabah, Malaysia
    d Permian Global Research Limited, 3 Cavendish Square, London, W1G 0LB

Article Information

Keywords

  • Restoration ecology

  • Natural regeneration

  • Tree species diversity

  • Tree species composition

  • Silvicultural interventions

  • Malaysian Borneo

Abstract

  • Active forest restoration is a focal point in climate action plans worldwide, promoting forest recovery and carbon storage. However, it may alter forest biodiversity and functioning in unforeseen ways. This study investigates the long-term effects of active restoration on adult trees and seedlings in a forest recovering from selective logging.

  • A site in Sabah, Malaysian Borneo, was evaluated for recovery after selective logging (1981-1991) with some areas naturally regenerating, while others were actively restored between 1994 and 2004 (through climber cutting and planting solely indigenous species).

  • Hypotheses posited that active restoration would negatively impact diversity and composition due to increased density of planted species and lower climber presence. Results indicate that active restoration increased adult tree diversity (via the Shannon Diversity Index) and promoted rare species, reduced liana seedling density, and showed effective, long-lasting impacts on forest structure. However, no differences were observed in seedling numbers or species diversity between restored and natural regeneration plots. All sites were dominated by late-successional species.

  • Findings suggest that active forest restoration can enhance species diversity and that restoration focused on biomass production does not necessarily compromise biodiversity, emphasizing its potential in mitigating climate change and ecological crises.

1. Introduction

Background of Tropical Forest Degradation

  • Tropical forests face significant degradation, primarily from anthropogenic activities like selective logging.

  • Since the 1990s, the highest decline in undisturbed forests has been noted in Asia-Oceania (Vancutsem et al., 2021).

  • This degradation raises concerns for irreversible ecological impacts and biodiversity loss, leading to increased interest in active restoration of degraded rainforests.

Concept of Active vs. Passive Restoration

  • Active Restoration refers to restoration efforts that involve interventions aimed at promoting species and ecosystem recovery, such as tree planting or invasive species management (Atkinson and Bonser, 2020; Chazdon et al., 2021).

  • Passive Restoration (or natural regeneration) occurs without interventions besides stopping logging, allowing nature to take its course (Atkinson and Bonser, 2020; Chazdon et al., 2021).

  • Natural regeneration could effectively restore large landscapes, but active restoration may often be necessary due to factors such as limited seed dispersal of dominant tree families in Southeast Asia, particularly Dipterocarpaceae, which only produce seeds every 2-10 years (Ashton et al., 1988; Williamson and Ickes, 2002).

  • Seed dispersal of dipterocarps is generally limited within 10 meters from the parent tree (Smith et al., 2015).

Need for Active Restoration

  • Degraded forests often present challenging microclimates that hinder growth due to light and temperature stresses (Holl, 1999; Philipson et al., 2012; Brown et al., 1999). Active restoration can create favorable conditions (e.g., improved light through canopy adjustments) and promote tree growth by eliminating climbers, which compete for vital resources (Finlayson et al., 2022).

Impacts and Results of Previous Research

  • Evidence suggests active tropical forest restoration can significantly boost carbon recovery, showing biomass accumulation up to 50% more than naturally regenerating forests (Philipson et al., 2020).

  • However, contrasting conclusions exist regarding tree diversity impacts, with a metanalysis suggesting risks of reduced diversity due to the often singular focus on late-successional species for planting (Crouzeilles et al., 2017).

  • Recent studies emphasize the need to separate ecological restoration from monocultures and highlight the value of naturally regenerating forests, which typically harbor higher species diversity (Gilman et al., 2016; Trujillo-Miranda et al., 2018; Staples et al., 2020).

2. Methods

2.1 Study Site

  • The study was conducted in the Ulu Segama Forest Reserve, near the Danum Valley Conservation Area in Sabah, Malaysian Borneo.

  • The region has an annual mean temperature of 26.7 °C and receives approximately 2700 mm of rainfall (Walsh and Newbery, 1999).

  • The soil type is primarily Ultisol, formed from weathered geological formations (Clennell, 1996; Chappell et al., 1999).

  • Sampling involved lowland mixed dipterocarp forest that underwent selective logging from 1981 to 1991 (Saner et al., 2012; Reynolds et al., 2011).

2.2 Field Plots

  • The primary aim was to compare forest community structure and diversity across different restoration strategies, using a network of 15 actively restored forest plots and 15 naturally regenerating plots.

  • The plots were selected to represent the area's average biomass and avoid edge effects by situating them a minimum of 100m from roads and keeping a maximum inter-plot distance of 20km.

2.3 Measured Forest Data

2.3.1 Adult Tree Data

  • Data on tree height, diameter at breast height (DBH), and species identity was collected for all stems with a DBH greater than 10 cm. Height was measured using an ultrasound hypsometer, while DBH was measured at 1.3 m above ground.

  • Above-ground biomass was estimated using allometric models and species-specific wood density values, with total biomass converted to carbon density per hectare based on established methodologies (Martin et al., 2011).

2.3.2 Seedling Data

  • Seedling data were recorded for all seedlings below 2 cm DBH and less than 1 m in height, measured using calipers and heights using measuring tapes. Seedlings were classified as either "new" (germinated post-mast-fruiting) or "old".

2.4 Data Analysis

2.4.1 Statistical Analysis

  • Forest parameters including the Shannon Diversity Index, number of individuals of planted species, and liana density were analyzed through ANOVA. Non-parametric tests were utilized for non-normal data distributions.

2.4.2 Species Community Composition

  • NMDS analyses with the Bray-Curtis distance method were conducted to assess species community composition, evaluated with rank-abundance and PERMANOVA to determine variations between restoration strategies.

3. Results

3.1 Spatial Autocorrelation

  • No significant spatial autocorrelation was detected for any forest parameters evaluated.

3.2 Influence of Restoration on Forest Structure and Tree Diversity

  • Statistically significant differences were found in the Shannon Diversity Index for adult trees between restored (3.61) and naturally regenerating (3.21) forests (p-value: 0.025).

  • Actively restored plots showed a significantly higher number of adult tree individuals of planted species (122.6) compared to naturally regenerating plots (86.3) (p-value: 0.031).

3.3 Influence of Restoration on Species Composition

  • No significant shifts in species composition were noted for adult trees or old seedlings; however, for new seedlings, significant shifts were observed (p-value: 0.016). Composition predominantly consisted of late-succession species.

4. Discussion

4.1 Influence on Tree Species Diversity

  • The results yielded a positive influence of active forest restoration on tree diversity and forest structure in the sampled areas. Active restoration did not impair natural forest dynamics and enhanced rare species visibility, contradicting initial hypotheses.

4.2 Comparison to Other Studies

  • The results align with studies suggesting better outcomes for biodiversity through careful active restoration techniques as opposed to those leading to monocultures.

  • Notably, although Hayward et al. (2021) did not find meaningful differences in tree diversity, our larger sampling area yielded results indicating increased diversity through active interventions.

4.3 Conclusion

  • Active forest restoration can significantly enhance biodiversity without compromising ecological integrity. Such methods offer critical potential for restoring ecosystems impacted by human activities. Future studies should focus on broader ecological processes impacted by restoration practices.