Quercus virginiana root attributes and lateral stability after planting at different depths

Abstract

Planting depth and irrigation can impact root and trunk growth following landscape installation in various soil types; however, impact on lateral tree stability is unknown. Quercus virginiana Mill. trees were installed at four landscape planting depths into a well drained sandy soil and grown for six years under two irrigation regimes. There was no impact of planting depth on trunk diameter or height in the first five growing seasons after planting; however, trees irrigated regularly had 10 mm larger trunk diameter than trees not irrigated. There was no impact of planting depth or irrigation on bending stress required to tilt trunks to 1°, 2° and 5° from vertical non-deformed start position six growing seasons after planting. Planting depth and irrigation also had no effect on diameter of the ten largest roots to a soil depth of 122 cm, which might explain why bending stress required to pull trees was similar for all planting depth and irrigation treatments. However, trees planted deeper had deeper roots measured 115 cm horizontally from trunk. Root cross-sectional area (CSA) 20–30 and 40–50 cm deep was positively correlated with bending stress six growing seasons after planting. Trees planted deep had some roots that ascended toward soil surface at a steeper angle than trees planted shallow, and had a deeper root flare and more roots growing over the flare that could potentially form stem girdling roots. Diameter of roots over the flare was not impacted by planting depth; however, trees irrigated for the duration of the study had more roots over main flare roots than trees not irrigated. Irrigation increased root number (>5 mm diameter) in the top 30 cm soil profile. Irrigation had no impact on any other measured root parameter. Trees planted deeper settled down below soil surface more than shallow planted trees.