Trees play a critical role in landscape design, offering numerous environmental and aesthetic benefits. As Professor William 'Chip' Winslow from Texas A&M University states, "The biggest things that trees provide for us are ecosystem services, which helps the entire environment."
However, incorporating trees into boardwalk, trail, or bridge designs can present challenges, particularly when building around their delicate root systems.
This article aims to provide landscape architects and engineers with key information on navigating tree root crossings when designing boardwalks and pedestrian bridges. By understanding critical root zones, exploring design strategies, and collaborating with arborists, designers can create functional and visually appealing structures while preserving the health and beauty of the surrounding trees.
Table of Contents:
- Understanding the Critical Root Zone (CRZ)
- Tree Ordinances and Heritage Trees
- Design Strategies for Navigating Tree Root Crossings
- Durability Considerations
- Tree Root Crossing Case Studies
- Collaborating with Arborists
- Designing Your Next Boardwalk Over Tree Root Crossings
Understanding the Critical Root Zone (CRZ)
The Critical Root Zone (CRZ) is an important concept in tree protection and preservation during construction projects. The CRZ is defined as the circular area around the base of a tree, with a radius equal to the tree's canopy spread. This zone contains the essential roots responsible for anchoring the tree, absorbing water and nutrients, and maintaining its overall health.
Protecting the CRZ is vital for the tree's survival, as any damage to the roots within this area can lead to decline, instability, or even death. When designing boardwalks or trails near trees, it is essential to minimize disturbance to the CRZ.
As Professor Winslow explains, "When preserving a tree, the usual rule of thumb is to stay out of the tree's drip line. You look at the extent of its canopy and draw a line straight down, and anything in that area you leave alone, and the tree has a much better chance of survival during the construction."
To determine the CRZ of a tree, designers should follow these steps:
- Measure the tree's diameter at breast height (DBH), which is typically 4.5 feet above the ground.
- For every inch of DBH, allow for 1 to 1.5 feet of critical root zone radius.
- Draw a circle around the base of the tree using the calculated radius to establish the CRZ.
For example, a tree with a 20-inch DBH would have a CRZ radius of 20 to 30 feet. By identifying and marking the CRZ, designers can plan boardwalks and trails to minimize impact on the tree's root system, ensuring its long-term health and stability.
Tree Ordinances and Heritage Trees
Wake Forest University has an extensive campus tree care plan to protect heritage trees.
When planning boardwalk or trail projects, designers must be aware of local tree ordinances and regulations. These ordinances are put in place by cities or counties to protect and preserve trees, particularly those of significant size, age, or historical importance.
Tree ordinances may dictate which trees can be removed, pruned, or disturbed during construction, as well as the required mitigation measures, such as replanting or maintaining a certain number of trees.
Heritage trees, also known as landmark or protected trees, are those that hold special significance to a community due to their size, age, species, or historical value. These trees are often protected by specific ordinances or regulations, which may prohibit their removal or require special permits for any work within their CRZ.
Complying with tree ordinances and regulations is crucial for the success of any boardwalk or trail project. Failure to adhere to these rules can result in fines, project delays, and damage to the designer's reputation. Moreover, preserving heritage trees and other significant specimens helps maintain the cultural and ecological value of the landscape.
To ensure compliance with tree ordinances, designers should:
- Research and understand the specific tree protection laws and regulations in the project area.
- Identify and document any heritage or protected trees on the site.
- Collaborate with local authorities and arborists to develop a tree protection plan that meets the requirements of the ordinances.
- Incorporate tree protection measures, such as fencing and root bridging, into the project design and construction process.
Design Strategies for Navigating Tree Root Crossings
When designing boardwalks or trails in areas with significant trees, it is essential to employ strategies that minimize impact on the critical root zones (CRZs) while still achieving the project's goals.
By carefully mapping out trails, using elevated structures, and creating observation platforms, designers can navigate tree root crossings effectively.
1. Mapping out trails to avoid CRZs
- Conduct a thorough site analysis to identify the location and size of trees and their CRZs.
- Align trails and boardwalks to avoid CRZs whenever possible, minimizing the need for root disturbance.
- Where CRZs cannot be entirely avoided, consider alternative routes or construction methods that reduce impact.
2. Using elevated boardwalks and bridges
- Elevated structures, such as boardwalks and bridges, allow trails to pass over tree roots without causing damage.
- Benefits of elevated structures include:
- Minimizing soil compaction and root disturbance
- Allowing for air and water circulation around roots
- Reducing the need for grading and excavation
- PermaTrak Concrete Boardwalks offer a durable, low-maintenance solution for tree root crossings.
- Concrete boardwalks provide a stable, slip-resistant surface suitable for wet, shaded conditions.
- The modular design allows for flexibility in layout and easy installation with minimal site disturbance.
- Concrete's longevity and resilience make it an ideal choice for areas with falling branches and heavy foot traffic.
Main Street Tree Root Crossing in Broussard, LA
3. Creating observation platforms near significant trees
- Observation platforms offer opportunities to showcase and appreciate heritage trees or other significant specimens.
- Benefits of observation platforms include:
- Enhancing the user experience by providing a unique perspective and educational opportunity
- Protecting tree roots and the surrounding landscape by concentrating foot traffic on a designated structure
- Allowing for ADA accessibility and inclusive design
- Platforms can be designed to cantilever over the CRZ, minimizing root disturbance while bringing users closer to the tree.
By employing these design strategies, landscape architects and engineers can create boardwalks and trails that navigate tree root crossings effectively while promoting tree health and visitor engagement.
The use of elevated structures, such as PermaTrak Concrete Boardwalks, and the incorporation of observation platforms demonstrate a commitment to both environmental stewardship and user experience.
Durability Considerations
Image from Audubon Beidler Forest Jan 22, 2022, Credit: Richard Covey
When designing boardwalks or trails in areas with significant tree canopy, it is crucial to consider the durability and maintenance requirements of the chosen materials.
The presence of trees can impact the longevity and performance of the boardwalk surface, and selecting the appropriate materials can help ensure the structure's long-term success.
1. Impact of tree canopy on boardwalk surface
- Tree canopy can create a wet, shaded environment that promotes the growth of moss, algae, and fungi on the boardwalk surface.
- Falling leaves, branches, and debris can accumulate on the surface, leading to increased moisture retention and potential slip hazards.
- The acidic nature of some tree species' leaves and debris can accelerate the deterioration of certain materials.
2. Comparison of timber and concrete boardwalks in wet, shaded conditions
- Timber boardwalks, while initially cost-effective, may not perform well in wet, shaded conditions under a tree canopy.
- Wood is susceptible to moisture absorption, leading to warping, splintering, and rot over time.
- The growth of moss and algae on timber surfaces can create slip hazards, particularly when wet.
- Timber may require frequent cleaning and application of anti-slip treatments to maintain safety.
PermaTrak Concrete boardwalk at Julian B Lane on a wet day in Florida
- Concrete boardwalks, such as PermaTrak, offer superior durability and performance in wet, shaded environments.
- Concrete is moisture-resistant and does not absorb water, minimizing the risk of deterioration due to dampness.
- The textured surface of concrete boardwalks provides inherent slip resistance, even in wet conditions.
- Concrete's alkaline nature helps prevent the growth of moss and algae, reducing the need for frequent cleaning.
3. Maintenance requirements for different materials
- Timber boardwalks require regular maintenance to ensure safety and longevity.
- Annual inspections to identify and replace damaged, rotting, or out of ADA compliance boards
- Periodic cleaning and application of anti-slip treatments
- Potential need for complete replacement after 10-15 years, depending on environmental conditions
- Concrete boardwalks, like PermaTrak, have lower maintenance requirements.
- Occasional cleaning to remove debris and maintain appearance
- Infrequent repairs or replacements due to concrete's durability and resistance to weathering
- Potential lifespan of 50+ years with proper installation and minimal maintenance
When selecting materials for boardwalks or trails in areas with significant tree canopy, it is essential to consider the long-term durability and maintenance implications.
While timber may be an initially attractive option, concrete boardwalks, such as PermaTrak, offer superior performance, safety, and longevity in wet, shaded conditions.
By investing in durable materials and minimizing maintenance requirements, designers can create boardwalks and trails that withstand the challenges posed by tree canopies and provide a lasting, enjoyable experience for users.
Tree Root Crossing Case Studies
Julian B Lane Park
The Julian B. Lane Riverfront Park renovation in Tampa aimed to create a centerpiece for the city while preserving the heritage live oak trees lining the existing trail.
PermaTrak designed a 538-foot elevated concrete walkway that minimized disturbance to the tree roots, providing a safe and maintenance-free solution for park visitors.
The project demonstrates the city's commitment to making the riverfront accessible to all while protecting the valuable heritage trees that contribute to the park's character.
Flat Branch Nature Preserve
The Flat Branch Nature Preserve project required a boardwalk solution that would minimize disturbance to the delicate wetland ecosystem and preserve tree root systems.
PermaTrak's modular concrete boardwalk system was chosen for its ability to sit above the ground without penetrating it, allowing for flexibility with seasonal ground movement.
The installation process, which involved setting pier footings by hand and using a top-down construction method, successfully minimized the impact on the trees and wetland surface.
Wake Forest University Davis Field Road
The Davis Field Project at Wake Forest University faced the challenge of re-establishing a pedestrian walkway through an area where tree roots had grown uninterrupted for over 25 years.
To minimize disturbance to the tree roots, PermaTrak collaborated with Stimmel Associates to design an elevated boardwalk that would allow pedestrians to cross over the roots safely.
The project successfully preserved the natural beauty of Davis Field while providing a pedestrian pathway for the campus community.
Collaborating with Arborists
When designing boardwalks or trails in areas with significant trees, project owners often require collaboration with qualified arborists to ensure the health and protection of the trees throughout the project.
Arborists can help by assessing tree health, identifying critical root zones (CRZs), and recommending appropriate protection measures.
By involving arborists from the early stages of the project and regularly communicating with them throughout the design and construction process, landscape architects and engineers can create boardwalk and trail designs that prioritize tree health and longevity while meeting the project owner's requirements.
Collaboration between designers and arborists is key to achieving a balance between human use and the preservation of valuable tree resources.
Designing Your Next Boardwalk Over Tree Root Crossings
Navigating tree root crossings when designing boardwalks and trails requires careful consideration and collaboration between landscape architects, engineers, and arborists.
By understanding the critical root zone, complying with tree ordinances, and employing design strategies such as elevated structures and observation platforms, designers can create functional and aesthetically pleasing spaces that preserve the health and beauty of the surrounding trees.
PermaTrak Concrete Boardwalks offer a durable, low-maintenance solution for tree root crossings, providing a superior alternative to traditional timber boardwalks in wet, shaded conditions.
As the importance of preserving trees in landscape design continues to grow, designers are encouraged to explore innovative solutions like PermaTrak to create sustainable, tree-friendly spaces that enhance the user experience and contribute to the overall well-being of the environment.
