Technical Aspects

Trees have a slow growth process and must be planted following suitable criteria; the species must be chosen in order to develop well in the specific conditions of the urban space (LORENZI, 1992; KULCHETSCKI et al., 2006). In cities, trees suffer numerous aggressions in their daily lives and the environmental characteristics of urban forests are very different from the conditions in rural areas (MCDONNELL et al., 1997). The negative impacts related to the presence of afforestation in cities stem mainly from the implementation and inadequate management and frequent changes in use, a direct consequence of the process of expansion and transformation of urban spaces at different scales.

Example of conflict between trees and urban infrastructure. Photo Grupo PodaLab

There are several technical aspects to be considered in the planning, implementation and maintenance of urban afforestation. Among the main aspects we can highlight:

  • Dimensions of the pavement;
  • Presence, conditions and type of electrical network and wiring;
  • Location in the road system (e.g. proximity to corners, presence of pavement recess and exposed face);
  • Typology and arrangement of furniture and urban equipment;
  • Existence of sewage network and underground wiring.

Characteristics of urban trees and impacts on their potential use

There are studies that identify characteristics common to urban trees, and that differentiate them, even from the same species grown in forests, whether planted or native. Such characteristics are equally important for planning potential uses of tree residues.

Como já mencionado, no ambiente urbano, as árvores crescem de um modo diferente, compartilhando o espaço com as pessoas e a infraestrutura urbana: prédios, postes, fiação elétrica, vias, gramados, canteiros de flores, mobiliário urbano, etc. Desse modo, as propriedades de sua madeira (troncos e galhos) também serão diferentes, em função de:

1. Rapid growth

  • These trees were planted for shade and other benefits, growing in open spaces and quickly, where they don’t compete with others for light and space.
  • Fuller crowns, with more lateral branches and smaller main trunks; the branches may still have large knots, reducing their quality.
    larger portion of sapwood (the outermost, “living” part, through which the tree’s nutrients pass), making the wood less resistant, more subject to degradation.
  • Wider growth rings, with shorter, thinner-walled cells and fewer late cells, resulting in lower density and lower resistance values.
  • Tendency towards a more spiraled grain, which results in a wood with crossed or reverse grain and with lower strength properties.
  • Greater longitudinal contraction and warping of this wood during processing.

2. Management of urban afforestation

  • While forest tree management carries out their removal when they are at the peak of their qualities, in cities trees are removed when they die, are damaged, infested or with compromised health, for safety reasons or necessary changes in the urban scenario;
  • The simultaneous removal of a significant and homogeneous set of trees in the same location rarely occurs. This does not prevent the use of urban wood, but imposes conditions to which markets and systems that work with wood from forests are not adapted.

3. tree health

  • Both forest and urban trees can be affected by similar pests and extreme weather conditions. But in the urban environment, trees often withstand difficult growing conditions and various terrain conflicts. Frequent or poor pruning, as well as the impacts suffered by the careless handling of equipment linked to urban management, create wounds on the trees, which turn into calluses and reaction tissues, which is not common in trees grown in forests. Some parts of these trees die; the wood of these limbs changes color and begins to decay. As a result, spots, color variations and stripes are more common than in forest trees, even after recovery.

4. Presence of contaminants in the trunk

  • Such as nails and fence wires, due to their frequent use as a support for signage. Metal detectors are needed, as well as special, thinner and stronger saws to process this wood.

5. Reaction wood

  • The topography of certain places, the incidence of light/shade, the frequency of winds, storms or other disturbances in the urban environment can cause trees to grow inclined or curved, forming what is called “reaction wood”. In these circumstances, the trunk’s pith becomes eccentric, giving rise to a wood with an irregular surface or with many splinters when cut (it is more difficult to work, therefore) and stains when applied some type of surface treatment (because its absorption is irregular). ; in addition, due to irregularities in the longitudinal contraction, this wood also warps more easily during drying and its mechanical properties are inferior, aggravating the inherent weaknesses of young woods, already mentioned.

6. The analysis of branches

The properties of branches are inferior to those of the trunk of trees, as a rule. In addition, when planning pruning, it is important to observe their dynamic characteristics in relation to the whole of the tree in which they are inserted, as they indicate the period in which their life cycle is found, especially the morphology of the base of the branch:

  • When the collar (lower portion of the base of the branch, at the insertion of the trunk) is barely perceptible, the branch is in clear assimilatory activity;
  • When it stands out visually, it is already in the rejection phase;
  • When the basal fossa appears (depression in the trunk below the base of the branch), it means that there is no more sap flow between the branch and the trunk, the former being about to dry up (after the branch dies it is colonized by fungi, bacteria and insects , which lead to its degradation, weakening and drying, facilitating its fall).