Relationship of Structural Root Depth on The Formation of Stem Encircling Roots and Stem Girdling Roots: Implications on Tree Condition

Abstract The relationships of structural root depth, stem girdling roots, stem diameter, and boulevard width were studied on the condition of four tree species (Acer saccharum L., Celtis occidentalis L., Fraxinus pennsylvanica Marsh, and Tilia cordata Mill.) grown as street trees. The relationship between depth from the soil surface to the structural roots and development of stem encircling roots and stem girdling roots was also determined. Stem girdling roots, boulevard width, and root depth were significant predictors of tree condition. Tree condition was greater as boulevard width increased, but stem girdling roots and structural root depth had a negative relationship on tree condition. Depth to structural roots was positively related to the percentage of the tree stem circumference with stem encircling roots and also for stem girdling roots. For every cm the structural roots were below the soil surface, 3.3% of the stem was encircled. Thus, a 10 cm root depth translates to approximately 1/3 of the stem with encircling roots. With stem girdling roots, an approximate 1% of the stem was girdled for each cm that structural roots were below the surface. Results from the measurement of 398 trees that were approximately 10 to 20 years post planting provide additional justification for maintaining structural roots at the soil surface. Results also demonstrate the importance for planning tree planting locations with adequate boulevard widths to foster tree health. Findings have implications with nursery production, tree planting, and arboricultural treatments to remove soil away from tree stems and expose structural roots at planting and subsequently with established trees.