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 lists the signs; new soil movement or cracks near the base mean call right away, and that can be 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 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) (read October 17, 2026)