Included Bark Formation: Explained by Arborists

You heard the crack during last night’s storm, or maybe your neighbor mentioned that the big oak looks “different” lately. Perhaps you’re buying a house with mature trees and want to know what you’re dealing with. Whatever brought you here, you’re smart to investigate tree failure risk before gravity makes the decision for you. Between 1995 and 2007, wind-related tree failures caused 407 deaths in the United States, with thunderstorms and high winds accounting for 76% of these fatalities. As Michael Hartman, ISA Board Certified Master Arborist, I’ve spent three decades investigating tree failures after they happen. This guide will teach you what to look for before the fall.

Tree risk assessment isn’t about fear—it’s about understanding the biological and mechanical limits of these living structures. By the end of this article, you’ll know how to spot the warning signs that separate a healthy tree from a potential hazard, understand when to call a professional, and learn what factors turn an otherwise stable tree into tomorrow’s insurance claim.

The Science Behind Tree Structural Failure

Trees fail when the forces acting upon them exceed their structural capacity. This happens through three primary mechanisms: uprooting, trunk breakage, or branch failure. Each failure type leaves distinct signatures that knowledgeable observers can recognize before catastrophic collapse.

Root system failure accounts for approximately 60% of tree falls in urban settings. When soil becomes saturated or compromised, the anchoring system loses its grip. The critical root zone extends 2-3 times the drip line, but 85% of the structural roots occupy the top 18 inches of soil. This shallow placement makes trees vulnerable to windthrow when construction, trenching, or soil compaction damages this vital zone.

Trunk failure occurs when decay, cracks, or included bark create weak points that can’t handle bending stress. A mature tree’s trunk experiences enormous forces—wind loading on a 24-inch diameter oak generates over 15,000 pounds of force at the base during 60 mph winds. Sound wood can handle this load, but decay columns, vertical cracks, or poor branch unions create stress concentrations that initiate failure.

Visual Indicators of Potential Failure

Professional risk assessment follows a systematic evaluation of three zones: the root collar and trunk base, the main trunk and branch unions, and the crown structure. Each zone reveals specific warning signs that compound when present together.

Start at ground level. Look for fungal fruiting bodies like honey mushrooms or artist’s conk on the trunk or root flare. These indicate internal decay that can reduce wood strength by 70% before external symptoms appear. Exposed roots showing decay, cavities, or missing sections signal compromised anchorage. Soil mounding or cracking on the windward side suggests root movement under load.

Moving up the trunk, examine for vertical cracks, especially those extending from old branch scars or included bark unions. A crack longer than one-third the trunk diameter indicates severe structural compromise. Cavities larger than 2 inches deep or occupying more than 30% of the trunk diameter require immediate professional evaluation.

Site Conditions That Amplify Risk

The same tree presents different hazard levels depending on its environment. Trees in forests develop differently than those in urban settings, and recent construction changes everything. Understanding these site-specific factors helps prioritize which trees need immediate attention.

Recent construction within the root zone dramatically increases failure risk. Driveway installation, utility trenching, or foundation work severs structural roots that may take decades to regrow. Compaction from heavy equipment reduces soil porosity, effectively suffocating roots over 2-3 years. Trees showing decline within five years of construction have a 60% higher failure rate during storms.

Drainage changes create another hazard category. Redirecting downspouts, installing irrigation systems, or regrading can saturate soils that previously drained well. Saturated soils lose 50% of their holding capacity, making windthrow more likely. Conversely, removing mature vegetation can dry clay soils, causing shrinkage that exposes anchor roots.

The Professional Assessment Process

Certified arborists use a standardized three-level assessment process that progresses from visual inspection to advanced diagnostic tools. Understanding this hierarchy helps property owners know what to expect when hiring professional help.

Level 1 assessment involves ground-based visual inspection using binoculars for crown evaluation. The arborist systematically examines all defect categories: lean, cracks, decay, cankers, root issues, branch attachment problems, and deadwood. This limited assessment identifies obvious hazards but misses hidden defects.

Level 2 assessment adds detailed measurements and basic tools. Resistograph drilling tests internal wood density, sounding hammers detect hollow areas, and increment borings reveal decay extent without significant tree damage. Climbing inspections examine branch unions and crown structure from within the canopy. Most property evaluations stop at Level 2 unless litigation or high-value trees justify further investigation.

Level 3 assessment employs advanced diagnostic equipment including sonic tomography, pull-testing for root stability, and aerial drone mapping. These techniques quantify the exact percentage of sound wood remaining and calculate safety factors under various wind loads. Expect Level 3 assessment for heritage trees, municipal liability cases, or when removal costs exceed $10,000.

DIY vs. Professional Evaluation Thresholds

Property owners can safely assess obvious defects but should recognize when professional evaluation becomes necessary. Simple measurements and visual inspection work for routine monitoring, but certain conditions demand specialized training and equipment.

Evaluate trees yourself when defects are visible from ground level and consequences of failure remain low. Small trees under 20 feet tall, those falling away from targets, or specimens with minor deadwood removal needs fall within DIY capabilities. Document measurements, photograph concerning areas, and monitor changes every six months.

Call certified arborists immediately when trees lean toward targets, show multiple defect categories, or exceed your removal capabilities. Any tree within striking distance of occupied structures, power lines, or public areas requires professional documentation. Insurance companies increasingly deny claims when obvious hazards went unaddressed without professional consultation.

Seasonal Timing and Weather Triggers

Tree stability changes with seasons, weather patterns, and even time of day. Understanding these cycles helps predict when your assessment findings require immediate action versus routine monitoring.

Winter dormancy provides the clearest view of branch structure and deadwood. Without leaves obscuring vision, structural defects become obvious. However, frozen soils provide artificial stability that masks root problems. Trees leaning in frozen conditions may fail dramatically during spring thaw when soil anchorage weakens.

Summer assessments reveal crown health indicators like dieback, sparse foliage, or early color change. These stress symptoms often precede structural failure by 1-3 years as declining trees allocate fewer resources to defense compounds. However, full leaf canopies hide branch defects that become apparent only after autumn leaf drop.

Documentation and Monitoring Systems

Effective tree management requires systematic documentation that tracks changes over time. Professional arborists maintain detailed records, but property owners can implement simplified systems that capture critical information for decision-making.

Create a tree inventory listing species, diameter at breast height (4.5 feet), approximate height, and target assessment. Photograph each tree annually from the same vantage points, including overall form and close-ups of any defects. Include measuring tapes or coins in photos for scale reference. Date-stamp all images and store them digitally with backup copies.

Establish monitoring intervals based on risk categories. High-risk trees with obvious defects require quarterly inspection during growing season. Moderate-risk specimens need annual evaluation, while low-risk healthy trees can be assessed every 2-3 years. Document any changes in lean, crack extension, cavity size, or crown dieback immediately.

What to Do Next: Your Action Plan

Start today by walking your property with fresh eyes and a measuring tape. Photograph every tree over 6 inches diameter, noting obvious defects like cracks, cavities, and dead branches. Prioritize trees within striking distance of your home, driveway, or neighbor’s property. Measure trunk diameters and estimate heights to discuss with professionals.

Schedule professional evaluation for any tree showing multiple warning signs or targeting high-value areas. Expect to pay $150-300 for Level 1 assessment, $300-600 for Level 2 with basic testing, and $800-1500 for Level 3 advanced diagnostics. These costs pale compared to average tree failure claims of $4,500 for property damage, not including potential injury liability.

Remember that tree risk assessment isn’t a one-time event—it’s an ongoing stewardship responsibility. Trees that pass inspection today may develop problems as they age or when site conditions change. Annual monitoring during routine yard work takes minutes but can prevent disasters that unfold in seconds. When in doubt, the cost of professional consultation always exceeds the price of learning you were wrong too late.