---
title: Reaction Wood: How Leaning Trees Build Themselves Up
url: https://battleborntreeservice.com/glossary/reaction-wood/
business: Battle Born Tree Service
address: 6301 US Highway 50 East, Carson City, NV 89701
phone: 775.232.8243
hours: Office Monday-Friday 8:30 am-5:00 pm; phones answered seven days a week; 24-hour emergency service
credentials: ISA Certified Arborist WE-11728A on staff; Nevada Business License # 20131580674; fully insured
type: Tree service (removal, crane work, pruning, stump grinding, defensible space, snow removal, firewood)
---

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- [Arborist glossary](https://battleborntreeservice.com/glossary/)
- Reaction wood

# What is reaction wood?

Reaction wood is the specialised wood a tree builds on a leaning trunk or a heavy, slanted limb to push it back upright or hold it in place. It forms in different spots in different kinds of trees.

## How it forms

In conifers such as pines and firs, reaction wood grows on the underside of the lean and pushes the stem upward; it is called compression wood and is often darker and denser. In broadleaf trees such as cottonwoods, elms and maples, it grows on the upper side and pulls the stem; it is called tension wood. Either way, the tree is making a structural repair.

This gives a clue to how long a lean has existed. A curved trunk with a smooth, thickened base on the lean side is usually a tree that has been leaning, and coping, for years.

## Why it matters in Northern Nevada

The Washoe Zephyr and other persistent winds shape exposed trees, and trees on exposed ridges and valley edges grow with a permanent lean and a one-sided crown. A long-standing lean with reaction wood is different from a new one with lifting soil. Our guide on a [tree leaning toward the house](https://battleborntreeservice.com/guides/tree-leaning-toward-house/) lists the signs; new soil movement or cracks near the base mean call right away, and that can be [windthrow](https://battleborntreeservice.com/glossary/windthrow/) in progress.

Reaction wood also matters for firewood and milling, since it splits and warps differently.

## What an arborist does about it

The arborist looks at the lean's history: curvature, the shape of the base, the [root plate](https://battleborntreeservice.com/glossary/root-plate/) on both sides and the weight of the crown over the lean. A tree that has stood at an angle for decades can be stable, but nobody can promise that from a description. A change in lean calls for an in-person look. Never tell yourself a leaning tree is safe because it has not fallen yet. Battle Born's certified arborists assess the lean, the roots and the crown together, and the estimate is free.

### Sources

- [International Society of Arboriculture, Trees Are Good: Recognizing Tree Risk (brochure)](https://www.treesaregood.org/Portals/0/TreesAreGood_Recognizing%20Tree%20Risk_0721.pdf) (read October 17, 2026)

### Talk to the office

[775.232.8243](tel:775.232.8243)

| Monday to Friday | 8:30 am - 5:00 pm |
|---|---|
| Saturday and Sunday | Office closed, phones answered |

Phones answered seven days a week. 24-hour emergency service.

[Free estimate](#quote)

### Keep reading

- [Arborist glossary](https://battleborntreeservice.com/glossary/)
- [Tree leaning toward the house: what to check](https://battleborntreeservice.com/guides/tree-leaning-toward-house/)
- [Warning signs before a tree fails](https://battleborntreeservice.com/guides/warning-signs-tree-failure/)
- [Washoe Zephyr and downslope winds: preparing your trees](https://battleborntreeservice.com/guides/washoe-zephyr-wind-tree-prep/)

## See also

[Windthrow](https://battleborntreeservice.com/glossary/windthrow/) [Root plate](https://battleborntreeservice.com/glossary/root-plate/) [Apical dominance](https://battleborntreeservice.com/glossary/apical-dominance/) [Woundwood](https://battleborntreeservice.com/glossary/woundwood/)