Persistent declines in forest-dependent birds following active restoration of logged tropical forest in Borneo

Despite accelerating carbon recovery, active restoration techniques like liana cutting and enrichment planting in Borneo's selectively logged forests have led to persistent long-term declines in forest-dependent bird populations compared to naturally regenerating areas, challenging the assumption that carbon and biodiversity outcomes in nature-based solutions always align.

The Gist

Imagine a tropical rainforest in Borneo as a massive, bustling city. For decades, loggers came in and took the biggest, most valuable buildings (trees) away, leaving the city damaged but still standing. Now, scientists and conservationists are trying to figure out the best way to help this city recover.

There are two main strategies being tested:

1. The "Let Nature Heal" Approach (Passive): You stop the damage, leave the city alone, and let the remaining residents and new growth rebuild the city naturally over time.
2. The "Active Renovation" Approach: You hire a construction crew to speed things up. They cut away the wild vines choking the remaining trees (to help them grow faster) and plant new, valuable trees in neat rows to replace what was lost. This is supposed to make the city recover its value (carbon storage) much faster.

The Big Question: Does this "Active Renovation" actually help the city's original residents—the birds—come back faster and stronger, or does it accidentally make things worse?

The Study: A 20-Year Time-Lapse

The researchers spent over 20 years watching this "city" in Borneo. They counted birds in three types of neighborhoods:

• The Old-Growth City: The pristine, untouched forest (the gold standard).
• The "Let Nature Heal" Neighborhood: Logged areas left to recover on their own.
• The "Active Renovation" Neighborhood: Logged areas where humans cut vines and planted new trees.

They looked at 176 different bird species, paying special attention to the rare and threatened ones that rely heavily on the forest.

The Surprising Discovery

The study found a twist that challenges a popular belief in conservation: Just because a forest is growing back faster (in terms of wood and carbon), doesn't mean the wildlife is happy.

Here is what happened, using some simple analogies:

1. The "Nature-Healed" Neighborhood is Winning the Bird Race
In the areas left alone, the bird communities slowly but surely recovered. After about 50 years, the number of forest-dependent birds (especially the rare ones) was almost as high as in the pristine, untouched forest. It was like a garden that, left alone, slowly filled back up with the exact wildflowers and butterflies it had before.

2. The "Active Renovation" Neighborhood is Stalling
In the areas where humans cut vines and planted trees, the bird numbers didn't just stay low; they actually declined over time. Even 50 years after the work was done, these areas still had significantly fewer birds than the natural recovery areas.

• The Analogy: Imagine a construction crew comes in to "fix" a park. They cut down the tangled, messy bushes (vines) because they think it will help the trees grow taller. But those bushes were actually the nurseries where baby birds built nests and the places where insects lived. By "cleaning up" the park too much, the crew accidentally removed the bird's apartments and grocery stores. The trees grew tall, but the birds had nowhere to live.

3. Who Got Hurt the Most?
The birds that suffered the most were the "specialists":

• The Fruit Eaters (Frugivores): They need specific trees and tangled vines to find food.
• The Insect Hunters (Insectivores): They hunt in the complex, messy layers of the canopy.
• The Big Predators: Large birds like hornbills that need big, old trees and complex structures.
• The Result: In the "Active Renovation" zones, these birds were 20% to 50% less common than in the natural recovery zones.

4. The "Carbon vs. Birds" Trade-off
This is the most important lesson. The "Active Renovation" approach is great for carbon (it pulls more CO2 out of the air faster because the trees grow bigger). But it seems to be bad for biodiversity (the variety of life).

• The Metaphor: It's like building a high-rise apartment complex. It's very efficient at housing people (carbon storage), but it has no parks, no gardens, and no space for squirrels or birds. The "Natural Recovery" is like a slow-growing, messy neighborhood that eventually becomes a perfect home for all kinds of wildlife, even if it takes longer to fill up.

The Bottom Line

The paper suggests that while "Active Renovation" (cutting vines and planting trees) is a powerful tool for fighting climate change by storing carbon, it might be a mistake if our main goal is to save endangered birds and restore a healthy ecosystem.

The Takeaway for Everyone:
Sometimes, the best way to heal a forest is to trust nature. If we want the forest to be full of life again, we might need to stop trying to "fix" it with construction crews and instead let the messy, wild, natural process do its work. However, the study also notes that if active restoration prevents the forest from being turned into a palm oil plantation (which would be a total loss for everyone), then it's still better than nothing. But if the choice is between "Active Renovation" and "Letting Nature Heal," the birds prefer the latter.

Technical Summary

1. Problem Statement

Tropical forest restoration is a critical strategy for mitigating climate change and biodiversity loss. While passive natural regeneration (allowing forests to recover without intervention) is common, active restoration (e.g., liana cutting and enrichment tree planting) is increasingly promoted in selectively logged forests to accelerate carbon recovery and timber stock regeneration.

However, a critical knowledge gap exists regarding the long-term biodiversity impacts of active restoration in logged forests. There is a prevailing assumption in nature-based climate solutions that carbon sequestration and biodiversity recovery will align. This study challenges that assumption by asking:

1. How do long-term restoration outcomes vary based on species ecology and conservation status?
2. Does active restoration enhance biodiversity recovery in logged forests over time compared to natural regeneration?

2. Methodology

Study Site and Design:

• Location: Malaysian Borneo (Yayasan Sabah concession), specifically the Ulu Segama Forest Reserve (logged) and Danum Valley Conservation Area (primary forest reference).
• Treatments: The study compared three habitat types across a 30+ year post-logging chronosequence:
  1. Primary Forest: Unlogged reference sites.
  2. Naturally Regenerating (Once-Logged): Logged forests left to recover passively (19–62 years post-harvest).
  3. Actively Restored (Once-Logged): Logged forests subjected to the INFRAPRO project (early 1990s), involving liana cutting and enrichment planting of >50 native tree species (21–41 years post-harvest).
• Data Collection: Over two decades of avian survey data (2008–2022).
  • Sample: 200 survey points, 10,368 detections of 176 bird species.
  • Method: Point counts conducted by expert ornithologists, stratified across forest types.

Statistical Analysis:

• Modeling: A hierarchical Bayesian multispecies detection-occupancy model was fitted to account for imperfect detection.
• Variables:
  • Response: Species occupancy probability ($\psi$).
  • Predictors: Management treatment (Primary, Once-Logged, Restored), time since logging, and species traits (Forest Dependency, IUCN Threat Status, Foraging Guild).
  • Covariates: Detection probability was modeled based on time of day, observer, forest structure (LiDAR-derived above-ground density), and habitat type.
• Inference: The study used the Probability of Direction (pd) to assess the consistency of effects (e.g., higher occupancy in natural vs. restored forests), applying thresholds of 0.75 (less conservative) and 0.9 (more conservative).

3. Key Contributions

• Long-Term Temporal Scale: This is the first broad-scale, long-term evaluation (up to 50 years post-logging) comparing active restoration against natural regeneration in selectively logged tropical forests.
• Decoupling Carbon and Biodiversity: The study provides empirical evidence that interventions successful for carbon recovery (liana cutting/planting) may actively hinder the recovery of forest-dependent biodiversity.
• Species-Level Granularity: Unlike previous studies focusing on community-level metrics, this analysis tracks specific responses across 176 species, categorized by threat status, forest dependency, and foraging guilds.

4. Key Results

A. Conservation Status and Forest Dependency:

• Threatened Species: Among 66 threatened or near-threatened species, 17–40% consistently showed lower occupancy in actively restored forests compared to naturally regenerating forests.
• Recovery Gap: Even 50 years after logging, median occupancy for globally concerned species in actively restored areas remained ~22% below primary forest levels. In contrast, naturally regenerating forests were only ~6% below primary levels.
• High-Dependency Species: 52% of high forest-dependency species retained distinct occupancy deficits in restored forests after 50 years, whereas naturally regenerating forests showed recovery to near-primary levels.

B. Foraging Guilds:

• Arboreal Specialists: The most negatively affected groups.
  • Arboreal Gleaning Insectivores: 27–49% had higher occupancy in natural regeneration.
  • Arboreal Frugivores: 13–30% favored natural regeneration.
  • Predators: One-third of predatory species (including large hornbills) showed higher occupancy in natural regeneration.
• Mechanism: Active restoration (liana cutting) likely removes critical nesting substrates, foraging structures, and fruit resources provided by vine tangles, which are essential for these guilds.
• Ground/Sallying Species: Ground-associated insectivores and sallying insectivores showed less consistent or uncertain responses, suggesting they are less dependent on the specific canopy structures altered by liana cutting.

C. Temporal Trajectories:

• Natural Regeneration: Showed a clear positive trajectory. High-dependency species increased in occupancy over time, with many reaching primary forest levels by year 50.
• Active Restoration: Showed stagnation or decline. Only 4% of high-dependency species increased in occupancy over time in restored stands.

5. Significance and Implications

Challenging the "Carbon-Biodiversity Alignment" Assumption:
The study refutes the common assumption that nature-based climate solutions (like active restoration for carbon) automatically yield biodiversity co-benefits. While active restoration accelerates carbon accumulation (as shown in previous work by the same authors), it appears to impede the recovery of forest-specialist bird communities.

Management Recommendations:

• Prioritize Passive Regeneration: In areas with high natural regeneration potential, resources may be better spent on protecting logged forests from conversion (e.g., via land purchases or community stewardship) rather than expensive active interventions (costing ~$1,500–$2,500/ha).
• Refining Active Restoration: If active restoration is necessary (e.g., in heavily degraded areas with no natural recovery), interventions should be modified. This could involve less intensive liana cutting or avoiding strip-planting that homogenizes the forest structure.
• Addressing Conversion Risk: The authors note that active restoration may still be justified if it prevents the conversion of logged forests to industrial plantations (a high-risk scenario in SE Asia). In such cases, the biodiversity value of a restored forest is superior to a plantation, even if it is inferior to a naturally regenerating forest.

Conclusion:
Active restoration via liana cutting and enrichment planting in Borneo has resulted in persistent, long-term declines in forest-dependent bird populations compared to passive natural regeneration. Conservation strategies must carefully weigh the trade-offs between rapid carbon sequestration and the preservation of complex forest structures required by specialized avian fauna.