Tree Trimming & Arboriculture Basics

Tree care is one of the most physically demanding and hazardous occupations in the green industry. It combines knowledge of tree biology and physiology with the skilled use of chainsaws, climbing equipment, and aerial lifts at significant heights. The fatality rate in tree care is among the highest of any occupation. This guide covers comprehensive tree biology, pruning science, equipment operation, and safety practices to prepare you for day-1 work on a tree care crew.

Tree Biology for Arborists

Understanding how trees grow, heal, and respond to pruning is what separates an arborist from someone who just cuts branches. Every pruning decision should be informed by biology.

Tree Structure

• Roots - Anchor the tree and absorb water and nutrients. Most absorbing roots are in the top 12-18 inches of soil, extending well beyond the drip line (the outer edge of the canopy). The root zone of a mature tree often extends 2-3 times the canopy radius. Protect roots during construction by avoiding grade changes, compaction, and root cutting within the drip line.
• Trunk - The main structural support. Made up of concentric layers:
  • Bark - The outer protective layer. Protects against insects, disease, weather, and physical damage. Once bark is damaged, the underlying tissues are exposed to pathogens.
  • Cambium - A microscopically thin layer of actively dividing cells just beneath the bark. This is where all new wood (inward) and new bark (outward) growth occurs. Damage to the cambium (from weed whacker hits, vehicle impacts, or flush cuts) directly reduces the tree's ability to grow and compartmentalize wounds.
  • Sapwood (xylem) - The outer wood that actively transports water and dissolved minerals from the roots to the leaves. Living tissue.
  • Heartwood - The inner wood that no longer transports water. Dead tissue that provides structural support. Not all species form distinct heartwood.
• Crown - The branch structure and leaves. Where photosynthesis occurs. The crown is the tree's food factory.
• Branch collar - The swollen area at the base of a branch where it meets the trunk or a larger parent branch. The branch collar contains specialized cells (the branch defense zone) that wall off decay after a pruning cut. Cutting into the branch collar destroys the tree's primary defense mechanism.
• Branch bark ridge - A raised line of bark on the upper side of a branch union, where trunk tissue and branch tissue meet. Together with the branch collar, the branch bark ridge marks the boundary of a proper pruning cut.

How Trees Respond to Wounds

Trees do not heal the way animals do. They cannot regenerate lost tissue. Instead, they compartmentalize wounds - they wall off the damaged area with chemical and physical barriers and grow new wood over and around it.

This process is called CODIT (Compartmentalization of Decay in Trees), developed by Dr. Alex Shigo:

• Wall 1 - Above and below the wound, the tree plugs the water-conducting vessels with gums and tyloses, limiting vertical spread of decay.
• Wall 2 - Inward, the annual ring present at the time of wounding limits inward spread of decay. This is the weakest wall.
• Wall 3 - Laterally, the ray cells limit horizontal spread.
• Wall 4 - The barrier zone. New wood formed after the wound creates a continuous boundary between old (potentially decayed) wood and new, healthy wood. This is the strongest wall.

What this means for pruning:

• Make proper cuts that the tree can compartmentalize efficiently
• Never cut into the branch collar - this removes the branch defense zone
• Never leave stubs - they cannot be compartmentalized and become decay entry points
• Never apply wound paint, tar, or sealant. Research by Dr. Shigo and subsequent studies have shown these products do not prevent decay and can actually slow compartmentalization by sealing in moisture and pathogens.

Tree Growth Patterns

• Trees grow in length only at the tips (apical meristems). A branch that is 6 feet high today will always be 6 feet high. The tree does not push it upward.
• Trees grow in girth (diameter) by adding new rings of wood each year from the cambium layer.
• Apical dominance means the leader (central trunk) grows fastest, suppressing lower branches. Removing the leader disrupts this pattern and stimulates multiple competing leaders, creating weak structure. Topping a tree (cutting the leader and major branches to stubs) is the most destructive pruning practice and should never be done.

Pruning Science

Pruning is the selective removal of plant parts to achieve specific objectives. Professional pruning follows ANSI A300 standards, the industry standard for tree care operations.

Types of Pruning (ANSI A300 Definitions)

Crown Cleaning (most common, most important)

• Removing dead, dying, diseased, broken, and weakly attached branches from the crown
• This is the first priority in any pruning operation. Dead wood is a falling hazard and a disease entry point.
• There is no limit to how much dead wood you can remove - remove all of it.

Crown Thinning

• Selectively removing live branches to increase light penetration and air movement through the crown, reduce weight on heavy limbs, and reduce wind resistance.
• Remove no more than 25 percent of the live crown in a single pruning cycle. Removing more than 25 percent triggers a stress response: epicormic sprouting (water sprouts), reduced root growth, and increased susceptibility to pests and disease.
• Remove branches from throughout the crown, not just the interior or lower portion. "Lion-tailing" (stripping interior branches, leaving only tufts at branch ends) is a harmful practice that shifts weight to the branch tips and increases the risk of branch failure.

Crown Raising

• Removing lower branches to provide clearance for pedestrians (minimum 8 feet over walkways), vehicles (minimum 14 feet over roadways), buildings, sight lines, and mowing equipment.
• Avoid removing more than 25 percent of the live crown. On young trees, maintain a ratio of at least 60 percent live crown to total tree height.

Crown Reduction

• Reducing the overall size or spread of the crown by cutting branches back to lateral branches. The lateral branch should be at least one-third the diameter of the branch being removed (the "one-third rule" for reduction cuts).
• Crown reduction is not the same as topping. Reduction cuts maintain the natural form of the tree and leave properly sized laterals that can assume the terminal role. Topping makes indiscriminate heading cuts to stubs, which is never acceptable.

Structural Pruning (young trees)

• Developing a strong branch structure in young trees by selecting a single dominant leader, removing competing leaders (codominant stems), removing branches with included bark (bark trapped in the branch union creating a weak attachment), and spacing scaffold branches.
• Structural pruning is the most valuable and most neglected pruning operation. Investing in structural pruning on young trees prevents costly removals of mature trees with structural defects decades later.

Making Proper Pruning Cuts

Small branches (under 2 inches diameter):

• Cut just outside the branch collar and branch bark ridge
• Angle the cut so it mirrors the angle of the branch bark ridge on the opposite side
• Use hand pruners (up to 3/4 inch), loppers (up to 2 inches), or a hand saw

Large branches (over 2 inches diameter) - The Three-Cut Method:

This method prevents bark tearing, which occurs when a heavy branch falls before the cut is complete, stripping bark down the trunk.

1. Undercut - 12-18 inches from the trunk, make a cut on the underside of the branch, about one-third of the way through. This is your safety cut.
2. Top cut - 2-3 inches farther out from the undercut (away from the trunk), cut through the branch from the top. The branch will break away cleanly at the undercut, preventing bark tearing.
3. Final cut - Remove the remaining stub by cutting just outside the branch collar. Identify the branch collar (the swollen ring at the base) and the branch bark ridge (the raised bark line on top). Cut from the outside of the branch bark ridge to the outside of the branch collar. Do not cut flush with the trunk (flush cuts remove the branch defense zone). Do not leave a stub (stubs cannot be compartmentalized).

What NOT to Do

• Topping - Cutting main branches or leaders to stubs. Creates hazardous regrowth, exposes the tree to massive decay, destroys the natural form, and is the single most harmful thing you can do to a tree. If a tree is too large for its space, removal is preferable to topping.
• Lion-tailing - Stripping interior branches, leaving only foliage tufts at branch ends. Shifts weight to the tips, increases wind resistance at the tips, and increases the likelihood of branch failure.
• Flush cuts - Cutting into or removing the branch collar. Removes the tree's defense zone and creates a large wound that is slow to compartmentalize.
• Heading cuts on mature trees - Cutting a branch to an arbitrary point rather than back to a lateral. Creates weak, bushy regrowth from the cut point.
• Over-pruning - Removing more than 25 percent of the live crown. Starves the tree and triggers a stress response.

Chainsaw Operation and Safety

Chainsaws are among the most dangerous tools in any trade. The Bureau of Labor Statistics consistently ranks chainsaw injuries among the most common causes of occupational injury in tree care and logging.

Required PPE - No Exceptions

• Hard hat with face screen and hearing protection - Combination units are standard in tree care. The hard hat protects from falling branches, the face screen from sawdust and debris, and ear protection from noise (chainsaws produce 100-115 dB).
• Chainsaw chaps or chainsaw pants - Made of layers of Kevlar or ballistic nylon that jam the chain drive in the event of contact with the leg. They are designed to stop the chain within a fraction of a second. Chaps must be worn from waist to boot top.
• Steel-toe boots with ankle support - Logging boots with a high top and aggressive tread are ideal. Some boots include chainsaw-resistant material.
• Cut-resistant gloves - Protect hands from sharp chain teeth during handling, tensioning, and sharpening.

Pre-Use Inspection

Before every use, check:

1. Chain tension - The chain should be snug against the bar with just enough play to pull it slightly away from the bar and snap back. A loose chain can come off during operation. An over-tight chain binds and overheats.
2. Chain sharpness - A sharp chain pulls itself into the cut with minimal pressure. A dull chain produces fine sawdust instead of chips, requires the operator to force the saw, and increases kickback risk.
3. Bar condition - Check for a bent bar, worn rails, or plugged oil holes. A damaged bar causes the chain to track improperly.
4. Chain brake function - Push the chain brake forward (engage it) and pull the throttle trigger. The chain should not move. Release the brake.
5. Throttle lock - The throttle should not engage without first pressing the lockout.
6. Fuel and bar oil levels - Fill both. Running without bar oil destroys the bar and chain within minutes.
7. Air filter - Clean. A clogged filter causes poor performance and can damage the engine.

Kickback

Kickback is the most dangerous chainsaw hazard. It occurs when the upper quadrant of the bar tip contacts an object (or when the chain at the top of the bar is pinched). The saw rotates violently upward and back toward the operator's head and neck in a fraction of a second.

Preventing kickback:

• Maintain awareness of the bar tip at all times. Never let the tip contact anything.
• Use a chain with low-kickback features (guard links, reduced-kickback chain profile) for general use.
• Maintain a firm grip with both hands. Left hand wrapped around the front handle with thumb underneath, right hand on the rear handle.
• Keep the chain sharp. Dull chains increase kickback risk.
• Start cuts with the lower part of the bar (the pulling chain), not the top.
• Never cut above shoulder height. You lose control of the saw and the kickback zone is aimed directly at your face.

Starting a Chainsaw

Ground start (recommended):

1. Place the saw on level ground
2. Engage the chain brake
3. If cold starting: set the choke to full choke
4. Place your right foot through the rear handle to stabilize the saw
5. Hold the front handle firmly with your left hand
6. Pull the starter rope with your right hand in a smooth, firm pull
7. When the engine fires, move the choke to half/run position and pull again
8. Release the chain brake before cutting

Never drop-start a chainsaw (holding the saw in the air and pulling the starter). This is dangerous and prohibited by OSHA and ANSI standards.

Chain Sharpening

A sharp chain is safer and more efficient:

• Use a round file matching the chain's pitch (typically 3/16-inch, 7/32-inch, or 13/64-inch - check your chain's specifications)
• File at the angle specified by the chain manufacturer (typically 25-35 degrees from perpendicular to the bar)
• File each tooth with the same number of strokes (3-5 strokes per tooth typically) for even cutting
• Check the depth gauge height with a depth gauge tool. File the depth gauges when they are higher than the cutting teeth - this is what controls bite size.
• Sharpen in the field every 1-2 tank fills of fuel. Replace the chain when the teeth are worn to less than half their original depth.

Aerial Work Practices

Tree work frequently requires working at height, which is the primary source of fatal and serious injuries in the industry.

Climbing with Rope and Saddle

Traditional rope climbing remains essential for trees that cannot be accessed by bucket truck:

• Climbing saddle (harness) - A tree-specific saddle designed for all-day comfort and mobility. Must meet ANSI Z133 standards. Inspect before every use for wear, fraying, and damage.
• Climbing rope - A tree-specific climbing line, typically 1/2-inch (12mm) double-braid polyester or blended rope. Inspect for cuts, abrasion, glazing, and soft spots before every use. Retire rope with any visible damage.
• Tie-in point (TIP) - The anchor point in the tree where your climbing line is set. Must be a live, structurally sound branch or crotch capable of supporting the climber's weight plus dynamic loads. Set the TIP as high as practical to maximize working range.
• Climbing hitch - A friction hitch (Blake's hitch, distel hitch, or a mechanical climbing device) that allows the climber to ascend, descend, and work-position on the rope. The hitch must be tended (not left unattended while slack exists in the system).
• Second tie-in point - ANSI Z133 recommends a second point of attachment whenever possible. This provides a backup if the primary TIP fails.

Climbing inspection routine before entering the tree:

1. Sound the tree - strike the base with the back of an axe and listen. A hollow, dull sound may indicate decay.
2. Visually inspect the trunk and lower branches for decay, cavities, cracks, hangers (broken branches lodged in the canopy), and deadwood overhead.
3. Check for power lines. Maintain at least 10 feet of clearance from all power lines (OSHA minimum for unqualified workers - and tree workers near power lines should have specific line-clearance training).
4. Look for bee, wasp, or hornet nests.
5. Assess the lean of the tree and the condition of the root zone (heaving roots, mushrooms at the base indicating root decay).

Aerial Lifts (Bucket Trucks)

Aerial lifts provide a stable work platform for trees accessible by truck:

• Operators must be trained and authorized. Many jurisdictions require a specific certification.
• Always wear a body harness and lanyard attached to the boom or bucket (never to the truck or an adjacent tree).
• Do not exceed the rated capacity of the bucket (typically 300 pounds for a single-person bucket, including the operator, tools, and wood).
• Maintain minimum clearance from power lines per OSHA standards.
• Set the truck on firm, level ground. Deploy outriggers and set wheel chocks. Never position the truck where it could be struck by traffic.
• Maintain communication with the ground crew. Use two-way radios or agreed-upon hand signals.

Rigging (Lowering Branches)

When branches cannot be dropped freely (near structures, vehicles, power lines, or other targets), they must be rigged and lowered:

• Rigging point - A structural anchor point above the cut where the lowering rope passes through a pulley or friction device. Must be capable of supporting the dynamic load of the falling piece.
• Lowering rope - Separate from the climbing line. Sized for the weight being lowered. Standard lowering ropes are 1/2-inch to 3/4-inch double-braid polyester.
• Ground crew - Controls the descent of the piece using a friction device (port-a-wrap) anchored at the base of the tree. The ground person manages the speed and direction of the lowered piece.
• Piece sizing - Cut pieces small enough that the rigging system can handle the weight. A green wood branch 6 inches in diameter and 10 feet long weighs approximately 150-200 pounds. Overloading the rigging system is a common cause of serious accidents.
• Communication - The climber and ground crew must communicate clearly before every cut. Standard calls: "Stand clear" (climber warning), "All clear" (ground crew confirmation), "Headache" (falling debris warning).

Emergency Procedures

Every crew should have an emergency action plan:

• Aerial rescue - If a climber is injured or incapacitated in the tree, the crew must be able to perform an aerial rescue to lower the climber to the ground. Practice rescue scenarios regularly. ANSI Z133 requires that crews have the ability to perform aerial rescue.
• Chainsaw injuries - Apply direct pressure to control bleeding. Chainsaw lacerations can be deep and severe. Call 911 immediately for any chainsaw injury. Do not remove embedded chain.
• Struck-by injuries - Falling branches are a leading cause of fatality. Ground workers should always be outside the drop zone during cutting operations. Wear hard hats at all times when in the work area.
• Electrical contact - If the tree or equipment contacts a power line, do not touch the tree, equipment, or the person. Call 911 and the utility company. If you are in the bucket and the boom contacts a line, stay in the bucket and wait for the utility company to de-energize the line.

Key Takeaways

• Always cut just outside the branch collar. Never flush, never leaving a stub. This is the fundamental skill of arboriculture.
• Use the three-cut method for any branch you cannot support with one hand. It prevents bark tearing and trunk damage.
• Wear full chainsaw PPE every time, no exceptions. Chainsaw chaps, hard hat, face screen, hearing protection, and boots.
• Maintain at least 10 feet of clearance from power lines. If the tree is within 10 feet of lines, call the utility company first.
• Never top a tree. It is the most destructive and hazardous pruning practice. Reduce, thin, or remove instead.
• Inspect your equipment (climbing gear, chainsaws, rigging) before every use. Equipment failure at height is catastrophic.