Regression analysis was used to determine the relationship between root depth and tree performance, with depth as the independent variable and crown vigor or trunk diameter (only on sites where trees of uniform size from the same source were all planted at the same time) as the dependent variable. Logistic regression was used for crown vigor and trunk condition ratings. Linear regression was used for trunk diameter measurements. Where insufficient trees were available for some species, data is reported by genus.
| Genus/species | Crown vigor | Trunk diameter | Trunk condition | |||
|---|---|---|---|---|---|---|
| Sommers’D | P (χ2) | r2 | P | Sommers’D | P (χ2) | |
| Greensboro, NC – July 2008 | ||||||
| Acer rubrum | 0.306 | 0.050 | 0.042 | 0.153 | 0.306 | 0.017 |
| Snoqualmie, WA – November 2007 | ||||||
| Quercus rubra | 0.005 | 0.712 | 0.020 | 0.326 | -- | -- |
| Tilia cordata | −0.048 | 0.936 | 0.081 | 0.045 | -- | -- |
| Fraxinus oxycarpa | 0.107 | 0.559 | 0.130 | 0.010 | -- | -- |
| Glen Ellyn, IL – June/July 2007 | ||||||
| Fraxinus sp.z | 0.229 | 0.231 | -- | -- | -- | -- |
| Malus | 0.283 | 0.088 | -- | -- | -- | -- |
| Quercus bicolor | 0.481 | 0.002 | -- | -- | -- | -- |
| Quercus sp. | 0.202 | 0.166 | -- | -- | -- | -- |
| Ulmus hybridy | 0.081 | 0.763 | -- | -- | -- | -- |
| Tilia cordata | 0.571 | 0.118 | -- | -- | -- | -- |
| Gleditsia triacanthos | 0.153 | 0.386 | -- | -- | -- | -- |
| Acer × freemanii | 0.229 | 0.428 | -- | -- | -- | -- |
| Taxodium distichum | 0.429 | 0.478 | -- | -- | -- | -- |
| Gymnocladus dioica | 0.556 | 0.096 | -- | -- | -- | -- |