Broadleaved hedgerows as complementary habitats for small mammals in pine plantation landscapes

While broadleaved hedgerows in pine plantation landscapes support higher abundance of the generalist *Apodemus sylvaticus* compared to plantation edges, they fail to restore forest-specialist small mammal communities or increase overall species richness, indicating that such edge-based interventions alone are insufficient to counteract the biodiversity limitations of intensively managed plantations.

The Gist

Imagine the French landscape as a giant, green ocean. But instead of water, it's filled with a single type of tree: the Maritime Pine. This is the Landes Forest, a massive area of pine plantations managed like a factory farm for wood. While these trees are great for making furniture and paper, they are a bit of a "food desert" and "shelter desert" for many of the small creatures that usually call forests home.

This study asks a simple question: If we plant little strips of different trees (like oaks) along the edges of these pine forests, can we bring back the missing wildlife?

Here is the story of what they found, told through simple analogies.

1. The Setting: A Monotone World vs. A Colorful One

Think of the pine plantation as a giant, uniform room painted entirely beige. It's clean, organized, and easy to manage, but it's boring for the animals living there. The trees are all the same age, the ground is often cleared, and there aren't many different kinds of seeds or bugs to eat.

In contrast, the natural broadleaf forest is like a vibrant, chaotic garden party. There are oak trees, bushes, tall grass, and a mix of everything. It's messy, but it's a buffet for wildlife.

The researchers looked at the hedgerows—the narrow strips of oak trees and bushes running along the edges of the pine plantations. They are like green ribbons or corridors trying to connect the beige room to the colorful garden.

2. The Characters: The Generalists vs. The Specialists

The study focused on small mammals: mice, voles, and shrews. They found two main types of characters in this drama:

• The "Survivors" (Generalists): The Wood Mouse (Apodemus sylvaticus) and the White-toothed Shrew (Crocidura russula). These are the "jacks-of-all-trades." They can eat almost anything, sleep anywhere, and handle change well. They are the ones who show up to the beige pine factory and say, "I can work here."
• The "Picky Eaters" (Specialists): The Bank Vole (Clethrionomys glareolus). This animal is like a gourmet chef who only eats specific ingredients found in a complex, leafy forest. It needs dense undergrowth and specific seeds. It is very sensitive to change.

3. The Findings: What Happened in the Green Ribbons?

The Bad News: The Picky Eaters Didn't Show Up
The researchers hoped the green ribbons (hedgerows) would act as a bridge, allowing the "gourmet" Bank Vole to move from the big forests into the pine plantations.

• Result: The Bank Vole was almost completely absent from both the pine plantations and the hedgerows.
• Why? The hedgerows were too narrow and isolated. Imagine trying to build a house for a gourmet chef in a tiny, isolated shed in the middle of a desert. Even if the shed has a nice kitchen, the chef won't stay because the neighborhood is too empty and the "food supply" (the big forest) is too far away. The pine forest acts like a wall that keeps these sensitive animals out.

The Good News: The Survivors Thrived
While the picky eaters stayed away, the "Survivors" (Wood Mice) loved the hedgerows.

• Result: There were twice as many Wood Mice in the hedgerows as in the pine plantations. In fact, the hedgerows were almost as good as the big natural forests for them.
• Why? The hedgerows provided a mix of seeds (like acorns) and shelter that the pine trees couldn't. It was like turning a plain cafeteria into a bistro. The Wood Mice, being flexible, moved in and ate well.

The Shrew Surprise
Interestingly, the White-toothed Shrew actually preferred the pine plantations over the hedgerows.

• Why? Shrews like warmth and open ground. The pine trees let in more sun and kept the ground warmer, while the hedgerows were too shady and cool for them. It's a reminder that "nature-based solutions" aren't one-size-fits-all; what helps a mouse might hurt a shrew.

4. The Seasonal Twist: Timing is Everything

The study also showed that the story changes depending on when you look.

• Spring: The forest specialists (Bank Voles) were more visible in the big forests.
• Autumn: The Wood Mice population exploded in the hedgerows.
• The Lesson: If you only took a snapshot of the forest in one season, you might miss the whole picture. The animals are like actors in a play; their roles and numbers change with the seasons and the years.

The Big Takeaway

Hedgerows are helpful, but they aren't a magic wand.

• They are a "Refuge": They are excellent for flexible animals like the Wood Mouse, boosting their numbers and providing a safe haven.
• They are not a "Home": They are too small and isolated to bring back the sensitive, forest-specialist animals that need a large, connected forest to survive.

The Metaphor:
Think of the pine plantation as a desert. The hedgerows are oases.

• The "Survivors" (Wood Mice) can travel between oases and thrive.
• The "Specialists" (Bank Voles) need a continent of green to survive. A few oases in a desert aren't enough to save them; they need the whole desert to turn green again.

Conclusion for the Future:
To truly save biodiversity in these pine forests, we can't just plant a few ribbons of trees. We need to think bigger. We need to connect these ribbons into a network (like a "bocage" landscape) and perhaps restore larger patches of forest. Until then, these green ribbons are a great start for some animals, but they can't save everyone.

Technical Summary

1. Problem Statement

The global expansion of commercial tree plantations, particularly monospecific and intensively managed conifer stands, has led to a decline in biodiversity compared to natural forests. These plantations often act as "environmental filters," supporting lower species richness and excluding forest-specialist species due to structural and compositional homogeneity.

• Specific Context: The study focuses on the Landes Forest in southwestern France, Europe's largest intensively managed maritime pine (Pinus pinaster) plantation.
• The Gap: While broadleaved hedgerows are proposed as a "nature-based solution" to enhance connectivity and biodiversity in such landscapes, their specific efficacy for small mammals (rodents and shrews) remains largely unquantified. Small mammals are critical for ecosystem functioning (seed dispersal, soil processes) and public health (zoonotic disease vectors), yet their responses to edge habitats in conifer plantations are poorly understood.
• Research Questions:
  1. Do native pine plantations differ from natural broadleaved forests in small mammal diversity?
  2. Can broadleaved hedgerows embedded within plantations mitigate these differences?
  3. Do environmental descriptors of hedgerows influence dominant species?
  4. How do seasonal and interannual demographic dynamics affect these patterns?

2. Methodology

Study Design & Sites:

• Location: Landes de Gascogne forest, France (1.16 million hectares).
• Sampling: 53 sites across a 1,050 km² area, categorized into three habitat types:
  • Pine plantation edges (12 sites).
  • Broadleaved hedgerows embedded in plantations (24 sites).
  • Natural broadleaved forests (17 sites, mostly riparian/remnants).
• Hedgerow Characteristics: Linear structures (>100m long, ~8m high) composed primarily of native oaks (Quercus robur, Q. pyrenaica).

Data Collection:

• Temporal Scope: Multi-year, multi-season survey (Spring/Autumn 2023, 2024, and Spring 2025).
• Trapping: 20 live traps per site (100m line, 5m spacing) set for 3–4 consecutive nights.
• Species Identification: Morphological identification in the field, supplemented by molecular methods (DNA fingerprinting, microsatellites, COI barcoding) for ambiguous Apodemus species or shrews.
• Ethical Handling: Rodents were euthanized for tissue sampling (except marked individuals released per permit); shrews were released.

Statistical Analysis:

• Models: Generalized Linear Mixed-Effects Models (GLMMs).
  • Species Richness: Conway-Maxwell-Poisson distribution.
  • Relative Abundance: Negative binomial distribution.
• Variables: Habitat type, season, year, and interactions.
• Hedgerow-Specific Drivers: Multimodel inference (AICc) to assess the impact of climatic variables (precipitation, temperature), landscape context (broadleaved cover), and local structural descriptors (geometry, plant species richness, canopy/shrub cover via LiDAR and field data) on the abundance of the dominant species, Apodemus sylvaticus.

3. Key Results

Community Composition:

• The small mammal community was dominated by two generalist species: the Wood Mouse (Apodemus sylvaticus, 72% of captures) and the Greater White-toothed Shrew (Crocidura russula, 20%).
• The forest specialist Bank Vole (Clethrionomys glareolus) was nearly absent in pine edges and hedgerows (only 3 individuals found), appearing almost exclusively in natural broadleaved forests (36 of 39 individuals).
• Conclusion: Pine plantations and hedgerows act as filters, excluding forest specialists.

Species Richness:

• Habitat Effect: Natural broadleaved forests supported significantly higher species richness than pine edges and hedgerows, but only in Spring.
• Seasonality: In Autumn, richness did not differ significantly among habitats. In Spring, broadleaved forests had >2x the richness of pine edges. Hedgerows did not significantly increase richness relative to pine edges.

Abundance of Dominant Species:

• Apodemus sylvaticus: Abundance was twice as high in hedgerows compared to pine edges and comparable to broadleaved forests. Abundance increased significantly over the years (2023–2025) and was higher in Autumn than Spring.
• Crocidura russula: Abundance was lower in hedgerows compared to pine edges and tended to be lower than in broadleaved forests.
• Temporal Dynamics: The interannual increase in A. sylvaticus was observed in hedgerows and forests but not in pine edges, suggesting pine plantations lack the resource pulses (e.g., mast seeding) driving population booms.

Environmental Drivers in Hedgerows:

• Autumn: A. sylvaticus abundance increased with cumulative precipitation and tree species richness (bottom-up resource effects).
• Spring: Abundance decreased with increasing canopy cover but tended to increase with shrub cover. This suggests that structural complexity (shrub layer) is critical for spring survival when seed resources are scarce.

4. Key Contributions

1. Quantification of Hedgerow Efficacy: The study provides empirical evidence that while hedgerows do not restore species richness to the level of natural forests in pine-dominated landscapes, they significantly alter community structure by supporting higher densities of generalist species (A. sylvaticus).
2. Differentiation of Generalist vs. Specialist Responses: It highlights a divergence in response: A. sylvaticus benefits from the structural and resource diversity of hedgerows, whereas the forest specialist C. glareolus remains excluded, likely due to landscape-scale constraints (lack of large source habitats) and microclimatic limitations of narrow linear habitats.
3. Temporal Dynamics: The research underscores that habitat effects are not static. The benefits of hedgerows for A. sylvaticus were only detectable over multiple years and varied by season. Snapshot surveys (e.g., single-year studies) would have failed to detect these habitat-dependent population trends.
4. Mechanistic Insights: The study links A. sylvaticus abundance to specific hedgerow attributes: tree diversity drives autumn abundance (seed resources), while shrub structure drives spring abundance (shelter).

5. Significance and Implications

• Conservation Strategy: Broadleaved hedgerows should be viewed as complementary habitats rather than full substitutes for natural forests. They can enhance prey biomass for predators and support generalist populations but are insufficient alone to restore specialist communities in highly fragmented, intensively managed landscapes.
• Management Recommendations:
  • Landscape Scale: To support forest specialists like C. glareolus, interventions must go beyond isolated hedgerows. A "bocage" landscape approach (increasing the density of hedgerows and restoring larger broadleaved patches) is necessary to overcome landscape isolation.
  • Hedgerow Design: Management should focus on optimizing structural complexity (promoting dense shrub layers) and tree species diversity to maximize benefits for generalist species and potentially improve conditions for specialists.
• Monitoring: Biodiversity assessments in plantation landscapes must account for interannual demographic fluctuations and seasonal dynamics. Relying solely on species richness metrics may underestimate the ecological value of habitat interventions; abundance and population dynamics are critical early indicators of success.
• Public Health: By altering small mammal community composition and abundance, hedgerows may influence the dynamics of zoonotic pathogens (e.g., Lyme disease, leptospirosis), necessitating further epidemiological study.

In summary, the paper argues that while hedgerows are a valuable nature-based solution for increasing habitat heterogeneity and supporting generalist small mammals in pine plantations, their capacity to restore full forest-specialist biodiversity is limited without broader, landscape-scale restoration efforts.