Conflicts between landscape trees and lawn maintenance equipment - The first look at an urban epidemic

Urban forests are expected to provide numerous ecosystem benefits in challenging conditions that include environmental and anthropogenic stresses. Cities challenge the growth and survival of trees due to restricted growing space, highly modified soils, extreme soil moisture conditions, and climate that often differs from surrounding undeveloped areas. Compounding these stresses are the human factors, like vandalism - both intentional and accidental. Mechanical wounding of exposed surface roots and the lower stem by lawn maintenance equipment falls into the latter category. Anecdotally, lawn maintenance related mechanical damage is a major stressor to landscape trees, compromising their ability to thrive and thus, to provide ecosystem services. Unfortunately, no previous studies have formally quantified the incidence and extent of the problem. Here, we survey mechanical damage for 1018 trees across 308 randomly stratified plots in parks, nature reserves, cemeteries, educational institutions, and roadside grass verges in Christchurch, New Zealand. At least one wound was found on 62.9% of all surveyed trees. This was mainly driven by trees with exposed surface roots, of which 93.6% had at least one wound. This is in contrast to only 43.9% of trees without surface roots exhibiting wounds. Surveyed trees were subjected to repeat wounding with 17.8% of trees having more than 10 wounds. Maintenance activities (i.e. mulch, physical or chemical removal of grass from around the stem) reduced the incidence of mechanical wounding. In the absence of maintenance activities, 67.1% of trees were wounded, while this was reduced to 46.2%, 43.5%, and 64.2% for each of the three aforementioned maintenance activities respectively. While the reductions in mechanical wounding associated with maintenance practices are promising, alternative solutions are necessary to further reduce mechanical wounding, so that the ecosystem benefits derived from urban forests are not undermined by this blight on tree health and survival.