Tree pruning and felling in the city
Tree pruning and felling in urban contexts are necessary practices to fix planting errors and diseases also due to stress and climate change. The Agroener project analyzes work times, costs, and technologies of 33 work sites in urban green areas
Over the last few years, society has become increasingly aware of the importance of urban greenery as a critical factor in increasing well-being and quality of life in cities. In terms of quality and quantity, the most crucial structural elements of urban green areas are trees, which provide many benefits and ecosystem services. Therefore, the proper management of the city's tree heritage is a fundamental aspect of the maintenance of this common good. Two are the most frequent and costly operations of tree management and care in the city: pruning and felling. Theoretically, the pruning of ornamental trees is never necessary, except in rare cases such as, for example, during the growing phase. However, it accounts for some of the most common practices for many other reasons. In fact, pruning is performed to improve the growth form, reduce structural problems, remove died or dangerous branches, manage pests and diseases, and reduce conflicts with infrastructures (buildings, electric cables, road signs, streetlamps, etc.). Therefore, many tree prunings are due to mistakes made during the tree's planting. The best pruning should be done by minimizing the amount of foliage removed to preserve the plant's photosynthetic apparatus, reducing operating costs and without making any inter-nodal cuts. Moreover, pruning is carried out on trees that are grown in compulsory forms (Ars topiaria and pollarding).
As for felling, in recent years, this operation has increased in frequency for several reasons. Urban trees, in fact, are subject to increasing biotic and abiotic stresses due to climate change (with the occurrence of extreme weather events such as violent storms and dry and hot summers). They are also subject to the increased frequency of the emergence of new pests and pathogens and mismanagement (especially at the level of road maintenance and underground structures). Moreover, felling is required when eradication is the necessary measure to enforce compulsory control of some diseases. In urban green areas, felling is usually carried out in a controlled manner. In other words, the tree is " dismantled," and its heavier parts are tied up so that they can reach the ground safely.
In order to reach the foliage, pruning and felling can be performed in two ways: using an aerial lift (AL) or accessing the crown with ropes, a method known as "tree-climbing", a set of techniques coming from mountaineering and speleology and that today have acquired their specific characterization. The choice of the technique to be used primarily depends on the site accessibility of the tree. Tree-climbing is preferable when targeted, and selective work on tree branches is required and where trees are located in places where machinery cannot enter. In urban environments, tree-climbing is often the only way to work on trees. This is partly because there are many sites where machinery can damage different elements laid on the ground (lawns, archaeological areas, sidewalks, etc.), or the pavement is not load-bearing and does not support the weight of the AL.
Study of pruning and felling sites
Within the Agroener1, project, thirty-three urban green sites have been studied so far (with a total of 152 hours of observation), analyzing the working time, the associated costs and equipment and materials used.
To this end, the pruning and felling operations, carried out both with AWP and in tree-climbing, were divided into the following distinct and recognizable phases that were recurrent at each worksite: 1) "Preparation and stacking" (a work phase consisting of the sum of the operations carried out at the beginning of the work - e.g., preparation of operators, initial positioning of the AL, launching of the climbing line in the tree-climbing, etc. - and those at the end - transport and stacking of residual woody biomass, cleaning of the site, etc.). 2) "shift" (work phase consisting of the movement of the operator inside or around the tree); 3) "cutting" (phase during which the branches or trunk are cut); 4) "delay times" (divided into unavoidable - e.g., refueling of vehicles, rest breaks, etc.) and avoidable (e.g., interruptions for phone calls, smoke, delays in the arrival of vehicles, etc.).
Many valuable pieces of information for analyzing site organization, optimizing the use of resources, and occupational safety come out of the results of the study of work times and associated cost estimates.
First, it was noticed that work time is highly variable depending on the many (and sometimes unforeseen) factors in determining work time. For example, sites differ in 1) the distance from the tree to the point where the residual wood can be stacked; 2) accessibility; 3) the presence of facilities near the tree (e.g., traffic lights, streetlights, fences, etc.); 4) the presence of busy roads, etc.; and 5) the location of the tree. In some cases, downtime is due to cars that must be removed for safety reasons or is due to interference with people not involved in the work (residents, passers-by, tree owners, etc.). Then, there are some site conditions that, as usual in urban green areas, influence the working time, such as cables, dimensions, infrastructures (roads, buildings, tram lines), and the static conditions of the tree itself. In addition, the working time can be influenced by the operators' skills and physical condition. It should be remembered that many aspects related to the timing and organization of work result directly from the regulations on safety in the workplace and the need to work with workers adequately trained following the law.
Comparing the sites carried out in tree-climbing with those using AL, it was found that they differed in the "preparation and stacking" time, which was shorter with AL. This is because often, because of their location, the sites where tree-climbing is chosen are far from the point of stacking and loading of residual biomass that must be transported manually since it is not possible to access the tree with machines. Meanwhile, tree-climbing has less avoidable delay times, which is often conditioned by the operator's difficulty of positioning the basket with the AL, who experiences problems getting a full view of the foliage and the inability to reach inside the crown to make specific cuts. Another aspect that emerged from our observations concerns is cutting equipment. In recent years, in fact, there has been a considerable increase in the use of battery-powered electric chainsaws. This equipment is particularly appreciated for use in cities, as it is quieter. Moreover, when working in a tree-climbing mode, they allow better ergonomics, particularly concerning the start-up, which can be operated with a button instead of the tiring pull system of chainsaws with an endothermic engine.
Felling or pruning trees growing in our cities is not a simple operation. In the urban environment, many varied factors influence the technical choices that operators must make before carrying out any intervention. All decisions are affected by: type of intervention, botanical species, accessibility of the site, type of ownership (public or private), phytostatical stability of the plant, its value and perception by the community, technical means used. Our goal is to characterize and model the factors involved in these works in order to provide operators in the sector with valuable elements for the technical choices to be made, with a view to economic, environmental and social sustainability.