Study Notes on Forest Restoration in the Southern Amazon

Native Species Seedlings in Forest Restoration in the Southern Amazon

Overview

  • Authors: Alexandre Ferreira do Nascimento, Ingo Isernhagen, Jorge Lulu, Antonio Okada, Jussane Antunes Fogaça dos Antunes, Austeclínio Lopes de Farias Neto

  • Institutional Affiliations: Embrapa Wheat, Embrapa Forests, Embrapa Territorial, and others.

  • Keywords: Natural regeneration, Carbon sequestration, Active restoration, Tropical ecosystems

Abstract

  • Forest degradation in the Amazon has caused severe environmental impacts.

  • Active restoration using mixed plantings of native and/or exotic seedlings is critical for ecosystem recovery.

  • The study evaluates soil organic carbon (SOC) and total nitrogen (TN) stocks from legal reserve restoration treatments in the southern Amazon after 10 years (2012-2022).

  • The experiment utilized a randomized block design with five treatments and four replicates.

    • Treatments include:

    1. Three active restoration techniques with seedling planting

    2. One passive regeneration treatment (area isolation)

    3. A secondary forest used as a reference

  • Soil samples were collected down to 1 m depth to determine SOC, TN, and bulk density.

  • Monitoring of photosynthetically active radiation (PAR) occurred from 2015 to 2019 using automatic data loggers.

  • Findings:

    • Active restoration significantly increased SOC stocks compared to passive regeneration (p < 0.05).

    • SOC stocks in seedling-based treatments reached up to 120 Mg ha⁻¹ versus ~100 Mg ha⁻¹ under passive regeneration.

    • Annual SOC accumulation rates ranged from 1.4–2.5 Mg ha⁻¹ year⁻¹.

    • Total nitrogen was significantly higher in the surface layer of the secondary forest.

    • PCA revealed a negative correlation between SOC and understory PAR, indicating increased canopy development enhances organic matter input and SOC.

    • Inclusion of exotic species may delay increases in soil C and N; therefore, planting native species is deemed the most effective restoration strategy for increasing SOC stocks rapidly.

Introduction

  • Forest degradation generates significant environmental debt, particularly in southern and eastern Amazon regions.

  • The Brazilian Forest Code designates legal reserves (LRs) as crucial for sustainable resource use, ecological processes, and biodiversity conservation.

  • Ecological restoration links directly to the provision of essential ecosystem services that support human well-being.

  • Strategies encouraged by the Brazilian Forest Code include passive/natural regeneration and active planting of native species, with temporary use of exotic pioneer species intercropped with natives.

  • Restoration can mitigate climate change but often requires decades for measurable ecological responses, although soil attributes may show faster results within the first decade.

  • Increases in SOC and TN depend on various factors, including region, strategy, land-use history, and soil class.

Objectives

  1. Evaluate SOC and TN stocks down to 1 m depth across active and passive restoration treatments and a secondary forest reference.

  2. Assess associations between structural variables (tree density, diameter at breast height (DBH), canopy cover, height) and SOC/TN accumulation.

  3. Determine whether exotic species inclusion affects early restoration trajectories and soil recovery.

Location

  • The experiment was located at Embrapa Agrossilvipastoril in Sinop, Mato Grosso, southern Amazon (latitude 11°51'S, longitude 55°35'W).

  • The soil, classified as Plinthic Dystrophic Red Yellow Latosol (Santos et al., 2018), is very clayey with flat relief and is an ecotone between the Amazon and Cerrado biomes.

  • Prior land use was mechanized agriculture with a history of invasive exotic grasses.

Experimental Design

  • Four LR restoration treatments were evaluated:

    1. Nat+Euca: native seedlings intercropped with eucalyptus (Eucalyptus urograndis)

    2. Nat+Rubber: native seedlings with rubber tree seedlings

    3. Passive: natural regeneration through area abandonment

    4. Native (Nat): only regional native seedlings

  • Total of 17 native species selected based on regional occurrence and ecological roles.

  • The design followed a randomized block layout, with each treatment plot measuring 0.48 hectares (60 × 80 m).

Maintenance and Treatment Specifics

  • Non-selective herbicides were used to control weeds in planted areas.

  • Eucalyptus and rubber trees received fertilizer, observing maintenance actions for weed control and pest management.

  • Passive regeneration was left unmanaged regarding weed control.

  • Control measures for pests, especially leaf-cutting ants, and measures to maintain firebreaks were implemented.

Photosynthetically Active Radiation (PAR) Assessment

  • PAR measured from 2015 to 2019, with a station established in each treatment to continuously monitor conditions.

  • PAR values compared between treatments indicate that all treatments initially showed similar values (~900 µmol m⁻² s⁻¹) that diverged over time.

  • Canary cover data highlighted the relationship between canopy density and PAR readings, indicating the structural impact on light transmission.

Soil Collection and Analysis

  • Soil was sampled down to 1.0 m depth using manual augerings and trenches to collect composite samples representing stratified soil layers.

  • Samples analyzed for carbon and nitrogen content following established protocols to determine SOC and TN stocks.

Statistical Analysis

  • ANOVA used to assess significant differences among treatments regarding C and N content, SOC and TN stocks.

  • Statistical methods also evaluated rates of SOC and TN accumulation compared across all treatments applied.

  • PCA conducted to examine relationships between tested parameters and treatments.

Results

  • PAR Dynamics: Decreasing trend in PAR values was noted in active treatments, contrasting with stable levels in the secondary forest.

  • Carbon Stocks:

    • SOC content varied across treatments, with the lowest in passive treatments.

    • Highest SOC stocks occurred in secondary forest and Nat treatments, confirming the effectiveness of restoring with native species.

    • Annual SOC accumulation rates under tree-based treatments were significantly higher than passive regeneration.

  • Nitrogen Content:

    • Decreased with depth, with notable differences only in the surface layer across various treatments.

  • C:N Ratio: Indicated variations amongst treatments, correlating with underlying soil health and organic matter quality.

  • PCA Insights: Strong separation between treatments denotes sensitivity of species composition to canopy structure and soil recovery.

Discussion

  • Increased soil C and N contents observed in active restoration reinforce the importance of native species and ecological interventions for soil health.

  • Differences in active versus passive restoration strategies yield different outcomes, particularly regarding C sequestration efficiency.

  • Findings underscore the significance of local conditions, historical land use, and species selection in achieving successful restoration outcomes.

Conclusion

  • Active restoration practices using native species significantly improve SOC stocks within a decade, underscoring their role in effective soil recovery processes in degraded areas.

  • The role of understory PAR and structural variables profoundly influence soil C dynamics, establishing connections between canopy growth, organic matter deposition, and SOC storage strategies.

  • Passive regeneration alone proved inadequate for achieving substantial SOC accrual during the study period, highlighting a strong advocacy for active restoration methods.

References

  • Allek, A., et al. (2023). How does forest restoration affect the recovery of soil quality? A global meta-analysis for tropical and temperate regions. Restor. Ecol.

  • IPCC. (2006). 2006 IPCC Guidelines for National Greenhouse Gas Inventories. Volume 4: Agriculture, Forestry and Other Land Use.

  • Nascimento, A. F., et al. (2025). Increased Shading in Integrated Agricultural Systems in Southern Amazon Reduces Potential to Store Carbon and Nitrogen in Soil. Geoderma Reg.

  • Other cited references as per the transcript content.

Acknowledgments

  • Recognition of the contributions by laboratory technicians and students involved in laboratory analyses.

  • Financial support from Sustainable Rural Project.