A Tree is not a Carrot!
A few years ago, we referred to the description of a tree’s root system as the Carrot Theory because it is quite illustrative and playful. Here, we will explain the development of the Carrot Theory.
We are approached by designers who have received comments to their construction projects by local municipalities
The need for formulating the Carrot Theory arose from the requests we received from our clients. Specifically, designers received comments from the local municipality (hereafter referred to as LM) during the building permit application process regarding their design project, the content of which they did not understand and for which they turned to us for help. The main message of the LM comments was as follows: Provide section drawings for trees numbered 1, 2, and 3, which demonstrate that the design (installation of water and sewage pipelines, removal of an outbuilding foundation, construction of the designed building foundation) do not have an adverse effect on the growth conditions of the existing trees. The section should indicate the crown dripline and trunk diameter of the tree, the expected extent of the root system (based on the root protection zone formula), the existing ground elevation, and the proposed solution (including the distance and depth of excavations from the trunk and the proposed elevation level).
Translated into “designer language,” the content of the comment is as follows: LM sees the proposed solution in the drawing and is interested in how the existing trees (meant to be preserved) should actually survive when the proposed solution (installation of water and sewage pipelines, removal of an outbuilding foundation, construction of the designed building foundation, etc.) is very close to those trees. The interest arises from the concern of how the growth conditions of the trees will not be degraded during construction and how the trees will not sustain physical damage, i.e., how the design creates conditions for these trees to genuinely survive. Therefore, the requirement for drawing sections is very illustrative, as it shows the minimal placement of the root system necessary for preserving the tree and the positioning, extent, and impact of the proposed solution (installation of utility networks, excavation for the removal of the existing foundation, excavation required for constructing the designed building foundation, etc.) on the tree in question. Once this is drawn out, the harsh truth is revealed – whether the proposed design allows for the preservation of the tree or not. This is precisely what LM seeks: for the designer to determine how the proposed design affects the trees and whether the situation could be resolved by altering construction techniques to achieve the same outcome or through tree removal or any other means. Standard EVS 843:2016 Urban Streets allows for exceptional cases to design and construct civil engineering works within the root protection zone of trees, but not closer than 2.0 meters from the center of the trunk. It must be remembered that this is an exceptional case and such a decision must be made based on the species of the tree and its health conditions, as different species of trees have varying resilience to changes in growth conditions.
Why do we need to “translate” the local municipality’s comments?
Every designer has their strengths based on their profession, and they do not need to know everything from this field. This is the case here when our clients turn to us for help interpreting and explaining what the comments mean and how they can be resolved. The misunderstandings primarily arise from a lack of understanding of the relationships between the living and built environments.
Since Pihamaa Stuudio has various specialists dealing with the living environment (landscape architect, dendrologist, arborist, botanist), we gladly explain how the built environment impacts the landscaping designated for preservation in the design. We offer evidence-based solutions; for example, we can calculate the minimum extent of the root protection zone that must remain untouched by construction activities for the tree to survive. Trees, depending on the species, have different resistance levels to changes in the growing environment, and this must be taken into account. Furthermore, several other aspects affect the possibility of preservation, such as the tree’s age, its growing location, health condition, and the extent of hard surfaces above the root area, among others.
The Carrot Theory
The fact that the relationships between the living and built environments are not understood is expressed in the mistaken belief that a tree’s root system consists of a single main root going straight down into the ground and a few short lateral roots branching out. In other words, this understanding is the Carrot Theory – the tree is a carrot! We explain that, in reality, a tree’s root system is found at a depth of 10-80 cm (up to 1 m), with the majority of it as close to the surface as possible (about 10-60 cm) to access more air, and only a few of the roots that the tree uses to anchor itself extend deeper. See the illustration.
Minimum Root Protection Zone of a Tree
Depending on the age of the tree and its growing conditions (including proximity to hard surfaces), the tree’s root system extends further than the dripline of the tree’s crown. The roots that are very important for the tree (fine roots are with special water-absorption capabilities) are located at the perimeter of the root system. When we calculate the minimum root protection zone for a tree, it can be seen that is smaller than the actual extent of the tree’s root system, which is where the term “minimum” comes from. The minimum root protection zone is the smallest area that must remain untouched by construction activities so that the tree’s growing conditions are preserved sufficiently for it to continue growing. In this area, the mass of tap roots and lateral roots (which ensure the tree’s stability) and fine roots (which, together with other roots, ensure the supply of water and nutrients) is located, and therefore, it is particularly important that no soil is excavated, no soil is stripped, no significant elevation changes are made, no hard surfaces are constructed, and the soil is not compacted (by machinery moving over it), etc.
In calculating the minimum root protection zone, we typically use the standard EVS 939-3:2020 “Woody Plants in Greenery. Part 3: Protection of Trees During Construction Works.” According to this methodology, various factors are taken into account, including the species of the tree, as different tree species have varying resilience to changes in their growing environment. This means that species with poor resilience do not tolerate environmental disturbances well, and a significantly larger portion of the root area needs to remain completely untouched to preserve those trees. However, there are also tree species that are resilient to environmental changes, and for those trees, it is possible to design buildings and structures closer to them.
A Designed Tree is Not a Carrot!
According to the standard EVS 843:2016 “Urban Streets,” the project must provide a certain volume and depth of soil for the trees. It should also be considered whether the selected tree’s soil can fit within the design area or if the area of the soil needs to be expanded under hard surfaces.
Due to the fact that the tree’s root system is actually located at depths of 10-80 cm (up to 1 m), the EVS 843:2016 “Urban Streets” states that for small trees, the project must ensure a soil depth of 80 cm, while for medium-sized and big trees, a depth of 1 m must be designed to provide sufficient space for the anchoring roots to grow deeper. Most fine roots (and the transporter roots) tend to grow more horizontally, which is why it is necessary to design a specific volume of soil for different trees where the roots can spread laterally. The required volume of soil, according to EVS 843:2016 “Urban Streets,” is 6 m³ for small trees, 14 m³ for medium-sized trees, and 26 m³ for big trees. If it is noted that the green strip is too narrow (less than 3 m wide), the need to consider designing a structural soil or a modular structural soil cell system should also be evaluated, which can provide adequate soil volume for the trees without reducing the area of hard surfaces.