A test of tree protection zones: Responses of Quercus virginiana Mill trees to root severance treatments

A suite of best management practice documents (BMPs) providing procedures to protect trees are available around the world. An underpinning component of these texts is the provision of a tree protection zone (TPZ), the purpose of which is to define an area of ground surrounding the tree to be excluded from development activities. TPZs 12 times the trunk diameter are common in BMPs, however, the principles of the TPZ have not been extensively tested. The objective of this study was to test the suitability of TPZs by monitoring the responses of trees to different root removal treatments. Mature Quercus virginiana Mill trees (n = 18) were exposed to five root pruning treatments consisting of a circular trench around each tree. Trenches were established with radial offsets from the tree base between 3 and 15 times the trunk diameter in increments of three, plus control. Roots were severed manually, and their diameters measured to obtain a total cross-sectional area of severed roots, which were expressed as ratios of whole trunk cross-sectional areas (Ar(1)) as well as conductive sapwood areas (As(1)). Ar and As ratios were regressed against physiological (predawn leaf water potential (Ψ), stomatal conductance (gs) and chlorophyll fluorescence) and growth (trunk diameter, shoot extension and specific leaf area) responses. Ar(1) was a significant predictor of Ψ (p < 0.0001), trunk diameter growth (p ≤ 0.01), specific leaf area (p ≤ 0.001) and shoot extension (p ≤ 0.001), although model strength varied (R2 = 0.75, 0.47, 0.57, 0.58 respectively). Significant (p ≤ 0.05) differences were observed in Ψ between control trees and all treatments except 15 × . Growth responses were affected for one or more treatments between 3x and 9 × . The results reveal that a TPZ defined by 12 times the trunk diameter was insufficient to adequately avoid short-term physiological perturbations in Quercus virginiana.