The effects of root pruning on growth and physiology of two Acer species in New Zealand

Abstract

Trees in the urban environment are affected by anthropogenic activities that have the potential to negatively affect tree function, health, and growth. The provision and maintenance of new underground utilities, footpaths, and street furniture all have the potential to result in conflicts with tree root systems leading to root severance. Traditional investigations into the responses of trees to root severance usually involve severing roots indiscriminately in one or more trenches, although root loss may not be fully quantified for individual trees. In the urban environment, root zone alterations may be localised and involve more discriminate methods, and practitioners must rely on experience and often anecdotal information when making decisions about root pruning. In this study, we investigate the suitability of allometric modelling as a tool to quantify root removal treatments. Two species of Acer were exposed to four different root severance treatments (trenching on one, two, three or four sides plus control). Fragments of the severed roots of all trees were excised and their diameters measured at the point of severance and used to estimate root cross-sectional area. Total root cross-sectional areas for each tree were expressed as a ratio of trunk cross-sectional areas at ground level (Ar(GL)) and at 1.4 m (Ar(BH)) as well as conductive sapwood areas in the trunk at each height (As(GL) and As(BH)). Ar and As ratios were used to explain differences in physiology (stomatal conductance and dark-adapted chlorophyll fluorescence) and growth (trunk diameter, shoot growth and specific leaf area) responses using linear regression models. Increasing root removal intensity (trenching treatments) revealed significant differences between treatment trees and control for stomatal conductance in both species (p ≤ 0.02), as well as growth responses (p <0.05). Ar(BH) was a significant predictor of stomatal conductance (p < 0.05), trunk diameter growth (p < 0.05), specific leaf area (p < 0.001) and shoot growth (p < 0.01); although model strength varied (R2 = 0.56, 0.31, 0.16, and 0.03 respectively). This research takes the first steps towards developing a practical tool for practicing arborists who make day-to-day tree management decisions relating to roots and root care practices.