Minimum depth of root collar where effect of deep structural roots was demonstrated for various species. Effects observed and possible contributing factors are summarized. Reports that showed no effects are not listed and are instead discussed in the text. Some studies did not include shallower depths than those indicated. only controlled experiments demonstrating the effect described was due to deep structural roots are listed.
| Species | Minimum depth of root collar where effect was evident (cm) | Possible contributing factors | Effects observed | Published Report |
|---|---|---|---|---|
| Acer rubrum | 15 | Adventitious root formation | Increase in girdling roots | (Wells et al. 2006) |
| Corylus colurna | 15 | None | Girdling root present | (Day and Harris 2008) |
| Corylus colurna | 30 (not observed at 15) | Flooding | Mortality of established trees associated with flooding | (Day and Harris 2008) |
| Lagerstroemia indica × faureiei; Fraxinus pensylvanica; Nerium oleander; Platanus occidentalis | 7.5 | Clay soils, trees rely on irrigation | Lower survival 3 years after transplant | (Arnold et al. 2007) |
| Koelreuteria bipinnata | 7.5 | Sandy loam underlain by clay pan, trees rely on irrigation | Reduced growth and survival | (Arnold et al. 2005) |
| Quercus virginiana | No minimum depth, effect increased with planting depth up to maximum tested of 18 cm | Only occurred when minimal irrigation was supplied after a dry period—effect therefore attributed to water not reaching rootball | Decrease in plant water potential (water stress) | (Gilman and Grabosky 2004) |
| Phoenix roebelenii | 30 (not observed at 15) | Root initiation zone extends 15 cm up the trunk on these trees | Foliar Mn deficiency, reduced survival | (Broschat 1995) |
| Prunus × yedoensis | 15 | Wet soils—effect was exacerbated on lower portion of the slope | Increased mortality during establishment | (Wells et al. 2006) |