Responses of mature roadside trees to root severance treatments

Abstract

Conflicts between trees and infrastructure installation or repair occur frequently in the urban environment and may result in damage to or complete removal of tree roots. We monitored the responses of trees to linear root cutting and examined the effects of the presence of a road within tree root zones. Mature Quercus virginiana Mill trees (n = 31) were exposed to three root pruning treatments (plus a control) consisting of a trench on one side of the tree, offset by a distance as a multiple of trunk diameter (3x, 6x and 12x). Roots were severed manually, and their diameters were measured at the cut point to estimate a total cross-sectional area of severed roots. The total severed root cross-sectional area was subsequently expressed as a ratio of whole trunk cross-sectional areas (termed Ar(BH)), as well as trunk conductive sapwood areas (termed As(BH)) at breast height (1.4 m). The shortest horizontal distance between the tree bases and the edge of a nearby road was also recorded and included in statistical analyses as a continuous variable. Linear root cutting reduced shoot extension and leaf area by as much 146.13 mm (p < 0.01) and 2.92 cm2 (p < 0.001) respectively, one growing season after root severance. Trees nearer the road had significantly reduced trunk diameter growth (p < 0.05), independent of root severance effects, i.e. 12x, 6x and 3x treatments. Pre-dawn leaf water potential was negatively affected for all treatments (p < 0.05) and the symptoms of water stress persisted in the 3x treatment for 440 days. Ar(BH) and As(BH) ratios were regressed against physiological (predawn leaf water potential (Ψ), stomatal conductance (gs), leaf temperature (T) and two chlorophyll fluorescence variables (Fv/Fm and Fv/Fo)) and growth (trunk diameter, shoot extension and leaf area) responses. Ar(BH) was a significant predictor of Ψ, Fv/Fm, Fv/Fo, shoot extension and leaf area responses after one growing season, although model strength varied (R2 = 0.75, 0.14, 0.16, 0.18, 0.24 respectively). To avoid sustained water stress symptoms, linear root cutting on Q. virginiana should not be undertaken closer to trees than six times DBH, equating to ≈ 25% root system loss.