Whether a tree grew into its lean or has started to lean shows in the trunk and at the base.

A lean a tree grew into is often a reach for light. NC State Extension's gardener handbook calls the growth response to light phototropism, in which "auxins accumulate on the dark side of a stem, elongating those cells so that the shoot tissues grow towards a light source," and says shoots also grow up in response to gravity. Shaded on one side by a house or bigger trees, a tree can grow toward the open side all its life, its trunk curving and its top turning upward. The USDA Forest Service's guide is careful both ways about leaning trees: "A leaning tree may be a hazard," and not all leaning trees are dangerous.

A lean the tree never had is a different event. The NC Forest Service's storm-damage checklist puts it in one line: "The tree/trunk is straight and appears to be leaning now but was upright before the storm." A trunk that stood upright and now tilts along its whole straight length has not grown into anything; it has moved, from the bottom, where the roots hold it.

What the ground at the base shows, and the root damage that can come before any storm, is on lifted or cracked soil at the base of a tree. For a lean that arrived with a storm, begin with what not to touch after a storm, and the emergency or Monday job triage holds the storm tiers.

Telling the two apart begins with what the tree used to line up with. An older photo of the house or yard, or the view from a window you look through every day, shows the trunk against things that do not move: a corner of the house, a fence post, the roof edge, a chimney. A trunk that now crosses an edge it once stood beside may have moved; a crown can grow into a roofline on its own, so the trunk is the better guide.

Then read the trunk from a few steps back, from two sides. A bend with an upright top suggests a lean the tree grew into; a straight trunk tilted from the soil up suggests a newer one. A tree can carry both, so neither clue settles it alone.

A lean also points. A leaning tree's weight already sits on the side it leans toward, so the direction names what it would most likely strike: the house, the driveway, a neighbor's roof, the wire running to the house from the pole. In the USDA guide, a tree with defects is not a hazard unless part of the tree is within striking distance of a target, a pairing that runs through the hazard tree field guide. A lean toward a wire brings in the utility, as with a tree on a power line.

The fence-line check on the home page reads a new lean as Call now, the same label it gives lifted or cracked soil. A new lean points to a change at the base, under a tree still standing over whatever it leans toward. The USDA guide's action list under poor architecture includes "A tree leans excessively," and it says "any leaning tree of concern should be examined by a professional arborist."

At the tree, a walk-through reads the trunk from more than one side, the root flare where the trunk widens into its roots, the ground opposite the lean, and what stands in its path. For a leaning tree that has to come out, the service is tree removal.