Tree Protection During Construction and Conflict with Building Foundations

Tree Protection & Construction

In urban areas, there is often competition between development activities and trees due to a lack of available growing space. Urban trees are frequently impacted by infrastructural maintenance, underground utility expansion, or building construction. When construction is close to or within tree root zones, soil removal, trenching, heavy machinery, and repeated foot traffic cause soil compaction and root damage (Despot & Gerhold, 2003). Sometimes, unintended movements by heavy machinery can also result in mechanical damage to the above-ground tree parts. For example, common construction activities such as paving, sidewalk (re)installations, excavation, trenching, and roadway widening involve various machinery that can severely affect the existing trees (Despot & Gerhold, 2003).

Without adequate protection during construction, trees can be damaged, leading to a decline in tree health, which can be deadly. In worst cases, these injuries can lead to functional and structural damages that appear as weak foliage, canopy decline, rot and decay, or even tree death (Hauer et al., 2020; North et al., 2017). Damage to roots caused by compaction can impact tree access to water and nutrients, ultimately compromising its health, longevity, and ability to recover (Fini et al., 2020). In many cases, tree decline and death can take years to become apparent (Fini et al., 2020). It has been documented that trees in construction zones experience a higher annual mortality rate and have worse tree health than trees not impacted by construction (Hauer et al., 2020; Hilbert et al., 2019). Trees previously exposed to construction damage are also more vulnerable to other environmental and biological stressors. For instance, a tree stressed during construction may not exhibit obvious signs of decline until a period of drought occurs, causing crown defoliation and eventually other health problems like dieback, limb loss, and increased susceptibility to insects and disease (Fini et al., 2020). This might appear as a sudden decline, but due to root reduction from construction impacts, the already-stressed tree has limited access to water, oxygen, and nutrients and can no longer handle additional environmental stress. However, the rate of post-construction tree decline depends on many factors such as the age of trees, tree species, the extent and nature of damage, the health of the tree prior to construction, and care given after construction is complete (North et al., 2017; Fini et al., 2020). 

Evaluation of green space and trees, as well as implementing strategies to save and protect urban trees, should be a critical part of urban development. An assessment should be completed before starting construction to ensure the conservation and preservation of existing trees and, thus, maintain the urban forest canopy and its integrity. Studies have shown that investing in tree protection for mature trees positively impacts the overall tree canopy within urban environments (Benson, Koeser, & Morgenroth, 2019a). Urban forest studies continuously refine tree protection recommendations based on emerging root damage and tree health studies. For example, Benson et al. (2019a) recommend providing a protection zone 15 times the diameter of the tree in question to ensure tree health. Not only do mature trees add to the aesthetic value of public spaces, but they also provide ecosystem and infrastructural services that cannot be easily replaced (Hotte et al., 2015). However, questions always remain about the appropriate extent of the tree protection zone, and this type of research continues to advance relevant knowledge (Benson et al., 2019a; City of Toronto, 2016; Matheny & Clark, 1998). 

Best management practices to protect trees during construction include construction-specific tree protection bylaws, site plans that ensure adequate space for tree roots, and tree, soil, and root protection measures. Many large Canadian municipalities mandate these measures, which are reflected in protection bylaws, guidelines, and urban forest management plans (Yung, 2018). Tree-protection plans often include physical barriers at a certain distance around trees that typically restrict access to their root zone and stem. These barriers protect the soil around the tree from compaction and can also prevent damage from machinery. Construction documents often detail what can and cannot be done within set distances from each tree (Despot & Gerhold, 2003). Tree protection techniques and guidelines are backed up by research that tracked tree health for years and decades after construction (Hauer et al., 2020; Fini et al., 2020). In special situations where additional expertise is needed, a professional arborist or forester may provide recommendations related to protecting and preserving trees near construction projects. 

Trees and Building Foundations

Many urban trees are planted too close to buildings or other gray infrastructure. This could be due to lack of space, lack of knowledge of how trees will develop over time, planning designs that disregard trees as living and growing organisms, or an inappropriate species or cultivar selection for the given space. As a result, trees often grow in conflict with structures and have the potential to cause direct or indirect damage to urban structures. An example of direct conflict between a tree and structure is when a tree trunk or stem grows into a building or a tree root grows into the pavement (Overkeke, 2008; Day, 1991). When tree roots search for water, air, and nutrients, they can grow into undesirable places; intruding root growth is often prompted by existing cracks in the structures or pavement, which allows moisture to seep through. This can be avoided by considering the mature size of a tree prior to planting, including the extent of the root zone, and by selecting the right species for the space (Overkeke, 2008). 

Tree roots can contribute to the settling of substrates under and around building foundations. Studies show that a combination of clay soil, proximity of trees to structures, and quality of construction can lead to indirect damage to buildings over time (Navarro et al., 2009; Overkeke, 2008; Day, 1991; Vorwerk, Cameron, & Keppel, 2015). Clay soil is especially prone to shrinking and expanding, which can lead to more movement around buildings as they settle and create a space in which tree roots can develop (Overkeke, 2008; Vorwerk, Cameron, & Keppel, 2015). When trees are planted too close to the foundation, they can add to the amount of water extracted from the soil (clay soil in particular) and lead to more root movement over time. Since the water demands of trees are species-specific, the soil type and species should be considered when creating a planting plan. Planting far away from buildings or structures is a good preventative measure in areas with clay soil; the notable exception to this recommendation is for rail tracks and sloped embankments built on clay soils, where vegetation provides necessary stability (Vorwerk, Cameron, & Keppel, 2015). Lastly, as structures with shallow foundations are especially prone to damage, infrastructure solutions such as deeper perimeter foundations are also a helpful preventative measure (Day, 1991).