Lean The trunk is a physiological and structural connection between the crown and root system. Serious trunk lean can cause structural problems and may cause the tree to fail. This is especially apparent if some soil cracking and/or uprooting is also observed. Some species that are on the border between being either small trees or tall shrubs have a tendency to exhibit lean. This can be the case with Russian Olive, Serviceberry, French Lilac, Hawthorns, Japanese Maple and Staghorn Sumac. However, the leaning trunk of such species is usually not serious.    Three classes of leaning can be recognized using the following rating: 0 The tree is virtually vertically positioned over the base of the stem (Figure 1). 1 Slight or minor lean (< 15ƒ from vertical) but no apparent danger (Figure 2). 2 Slight or minor lean (< 15ƒ from vertical) with some evidence of root mounding or soil cracking on the side of the tree away from the lean (Figure 3). 3 Serious lean (>15ƒ from vertical) with some evidence of root mounding or soil cracking on the side of the tree away from the lean.   Tree Problems Pruning Scars