Tree Supports—A Method for Managing the Protection of Habitat Trees, Increasing Biodiversity and the Resilience of Urban Ecosystems

Abstract

Protecting valuable trees with habitat significance in urban environments is not a new problem; however, it has only been addressed to date based on practitioner experience, rather than a proper scientific foundation. The lack of a developed methodology and the failure to combine tree measurements and assessments with structural calculations often led to failures, destruction, and damage. The proposed method, based on static studies, calculations, and an action algorithm, enables effective responses and the creation of tree-specific solutions. This article presents an analysis of solutions based on 10 years of research on over 1500 trees. The method supports the long-term preservation, protection, and conservation of valuable habitat trees, which are crucial to the urban ecosystem. This paper presents a new algorithm for managing conservation issues, along with methods and examples of its implementation. The methodology has been tested on real-world examples and is recommended for use in new cases of valuable trees identified for protection. Building ecosystem resilience and increasing biodiversity, especially in urban areas, requires implementing solutions grounded in research rather than relying solely on implementers’ experience.

1. Introduction

Protecting trees from external factors and attempts to secure them mechanically is a human achievement that predates arboriculture in the broad sense, and is now an applied science that is a speciality, combining the achievements of the natural, technical, and applied sciences in horticulture, forestry, and landscape architecture. Many earlier practitioners can be appreciated and admired for their intuition and for combining the laws of mechanics and engineering with biology. The materials and solutions used were not always adequate for the needs of trees, especially ancient and venerable ones. Many trees have survived to the present day, despite imperfect solutions, allowing us to analyse both the behaviour of trees in their senile phase and the behaviour of these structures under years of variable loads.

Contemporary arboriculture has progressed significantly, influenced by evolving perceptions of trees and their biology following Alex Shigo’s introduction of the CODIT model [1,2,3]. The CODIT model, rooted in Hepting’s research [4], was developed and expanded by Shigo to explain that decay can develop within the trunk due to fungal infection, desiccation, and damage. This decay is countered by the formation of walls—barriers that isolate the affected area. The tree produces four types of walls, which can be visualised as parts of a cylinder forming a triangular shape:

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Two walls, No. 1—the weakest, compartmentalising (fencing off) changes/decomposition from above and below

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Wall No. 2—not very resistant, with compartmentalising changes or decomposition from the middle of the trunk or horizon.

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Two walls, No. 3—moderately resistant, compartmentalising changes/distribution from the sides

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Wall No. 4—named by Shigo “barrier zone”—the strongest, and compartmentalising changes/decomposition from outside the trunk/horizon.

His groundbreaking articles and books [5,6,7] sparked an explosion of activity and research, leading to the development of methods for tree care and management aligned with scientific advancements. The development of these methods was based on many branches of knowledge, especially plant anatomy and physiology [8,9]. Ideas related to tree protection were developed through an understanding of the mechanisms that trees use to defend themselves against external factors. Dujesiefken, Liese [10,11], and Morris et al. [12] significantly advanced the CODIT model, enabling the development of methods for tree care and protection. Simultaneously, methods and techniques for testing tree stability have been developed, beginning with the work of Sinn [13,14] and Wessolly [15]. In the 21st century, research has also started to highlight the benefits of maintaining trees in urban environments in good condition for as long as possible [16].

Models have been developed to describe trees not only from planting to maturity but also during their remaining years in the city, including in old age or the senior phase [17]. The concepts of veteran trees and veteranisation were developed as methods, inspired by natural solutions, to extend the lifespan of trees in human environments and to enhance their role within them [18]. Veteranisation is fundamentally based on two synergistic processes, including reducing size by shedding branches. The natural phenomenon of spontaneous branch shedding is called cladoptosis. Reasons for this process include energy deficits, water-transport issues, and excessive bending moments that the wood cannot withstand. A damaged, broken, or weakened tree can produce new, lower-level secondary branches, which cause higher branches to shed, enabling the tree to rebuild itself and improve its structural vitality. A naturally occurring process driven by water deficit and decreasing cell turgor is known as the summer drop branch. Trees develop defence mechanisms against infection, such as resin, wetwood, and CODIT, which help control decay and gradually shed “unnecessary internal mass.” The actions of tree care specialists can mimic branch shedding and, in many cases, anticipate it to assist the tree and limit damage to the surrounding area from falling branches or other parts of the tree. The second parallel process involves the formation of ecological niches at points of cuts and breaks, which encourage biodiversity, water retention in the decaying trunk, and soil enriched with humus around the tree, thereby enhancing ecosystem services. Both processes ultimately enable the tree to decrease in size, extending its lifespan in the environment [19]. These ideas initially gained significant momentum, primarily in the United Kingdom. The conscious development of methods and techniques in this area first occurred in the UK, where data collection began with the Ancient Tree Inventory [20]. This involved a realistic assessment and a desire to preserve ecological niches [21], as well as research concluding that such trees strengthen habitat protection and biodiversity by creating suitable habitats [22]. This methodology translates into tangible actions at valuable natural and cultural heritage sites, such as Waszyngtona Avenue in Krakow (Poland), which will be described in detail in the next section.

Currently, arboriculture seeks solutions that align not only with the needs of trees and their biology and physiology but also with broader goals, such as protecting biodiversity—especially in cities—and supporting valuable habitats and ecological niches, which merge tree care with wider conservation biology [23,24,25,26]. An additional concern, particularly from the perspective of urban trees and habitats, is the protection of cultural heritage, closely linked to the preservation of gardens, parks, avenues, and entire urban layouts. The challenges of safeguarding old trees within valuable heritage sites or those integral to entire urban frameworks have been and continue to be topics of discussion among many authors [27,28,29]. The author’s method innovatively and synergistically integrates the results of specific activities, combining distinct fields—dendrology, ecology, arboriculture, mechanics, construction, and materials engineering—into a unified, comprehensive approach. This method outlines a process for generating effective solutions to protect individual trees. Old, mature, and venerable trees are the most valuable elements of our environment; however, they vary greatly and require an individualised approach. The primary advantage of this method is its algorithmic scheme, which is model-based and applicable to nearly all trees.

Contemporary methods of assessing tree statics and biomechanics [30,31,32,33] are developed and interpreted in similar ways, and, importantly, in the context of ancient trees, they offer an opportunity for their active protection. Methods for evaluating and measuring tree statistics are currently among the fastest-growing branches of dendrology, biomechanics, and physiology; therefore, the proposal to distinguish dendromechanics from biomechanics arises, as research on trees increasingly diverges from the fundamental studies conducted in biomechanics. The author does not insist that the concept of dendromechanics is essential, but highlights that its introduction into science would organise a specific group of activities and address a research gap in the nomenclature and classification of activities undertaken worldwide in this field.

The use of supports, considering current knowledge, is an interdisciplinary activity that combines calculations related to tree statics [34], structural calculation methods [35], and material technologies with an understanding of the physiology and mechanics of the tree itself [36], along with processes that lead to wood decay in different parts of the tree [37]. Well-known examples of supporting large and historic trees can be found worldwide. Such supports have been created for Robin Hood’s Oak—Major Oak [38,39] and Bartek Oak, recognised both in Poland and internationally [40,41,42]. For these trees and many others, their support systems can be modified, adjusted, and improved [43] based on more precise data, such as load testing [44] or laser scanning [45].

The primary aim of the research and its main achievement was to develop a coherent model of tree support. This algorithm enables systematic resolution of issues related to the stability of ancient trees, which are valuable to the urban ecosystem and enhance biodiversity due to the numerous niches they provide [46]. It integrates the results of physical measurements and calculations in dendromechanics as a foundation with structural design and material selection. The concept of dendromechanics is the author’s proposal to distinguish the specific mechanical properties of trees from plant biomechanics. While biomechanics or plant physics [47] is a broad field, not all ideas, theories, or research are applicable to trees, a particular group of plants for which mechanical understanding is crucial for human safety.

2. Materials and Methods

The research methodology relies on multiple studies and calculation techniques to accurately assess a tree’s dendromechanical condition. There is extensive research on trees, their behaviour in the environment, and their interactions with external factors, justifying a dedicated section. Existing calculation methods were based on moderate-precision wind-pressure analyses of the crown [48]. Now, thanks to significant advancements in LIDAR laser-scanning techniques [49], it is possible to gather data on actual dimensions [50], which greatly enhances accuracy when determining crown area [51]. Precise calculations of moments acting on a tree, especially the bending moment, improve the accuracy of structural assessments and help select appropriate material parameters for support structures. The main aim of supporting trees—preventing breakage or tipping—is to avoid removal for safety reasons and to prolong their role as vital ecological niches and biodiversity hotspots. Currently, aside from the author’s own research, pulling tests and wind load analyses typically do not utilise the widely available LIDAR laser-scanning method, which allows for precise mapping of a tree’s shape and exact measurement of its dendrometric parameters. An innovative additional application of LIDAR scanning, introduced by the author, involves modelling trees to design detailed, precise protective measures—such as supports, pylons, bracing, and cabling.

The entire tree examination process before implementing the support project involves several phases, starting with species identification, dendrometric measurements, and a description of the tree’s condition [52]. Describing trees in their natural environment is vital for subsequent measurements and static values used in calculations, especially those from the Stuttgart Strength Catalogue [53]. Identifying the species enables the selection of properties such as the wood’s compressive strength (Rs), wind drag coefficient (cw) [54], or elasticity limit, which are species-specific. Measuring basic dendrometric parameters—such as tree height, crown spread (diameter), trunk circumference at 130 cm above ground, and trunk diameters at 100 cm above ground—should be carried out with specialised equipment, allowing subsequent calibration of photographs and setting other parameters for calculations [55]. Currently, laser scanning is increasingly employed for these tasks [56]. In the study, laser scanning was utilised to accurately design the supports. Additionally, it is necessary to assess the phytosanitary condition and surrounding vulnerability, visually inspect for any damage, and evaluate the trunk’s condition using an acoustic hammer or tomography [57,58,59].

For the visual assessment of trees, in addition to collecting information on the dendrometric parameters of individual specimens, it is also necessary to assess and analyse other elements of the tree’s structure, including visible/audible signs of weakness. In this case, the tree’s reaction symptoms should be used to interpret its structural symptoms/defects/deformations and assess whether they have a positive or negative impact on its static and sanitary condition [60,61].

Since wind is the primary load on a tree and the main factor in tree destruction, wind pressure analysis is an indispensable element of static calculations. The method consists of determining the wind pressure on the tree crown from a specific direction [62]. The choice of direction is essential and should be aligned with the prevailing winds or another direction that is locally relevant, e.g., due to possible damage, changes in buildings, spatial restrictions, or inclination. For the analysis itself, a directional photograph is taken in a plane normal (perpendicular) to the wind direction or another previously determined relevant load direction. Alternatively or additionally, laser scanning is used. It allows for checking the degree of distortion of the photograph and dendrometric parameters.

Once the outline of the crown has been determined, its surface area can be calculated. Based on this data, we then determine the centre of wind load (h_z) on the crown (sail) surface (A). By comparing the wind load moment (M_w) on the tree with the strength parameters—the compressive strength of “green” wood (Rs) and the moment of resistance for the trunk cross-section (Mor), we can calculate the basic safety factor of the tree (SFb).

The equation shows the value of the basic safety factor (SFb):

SF_b = (M_or × R_s)/M_w, where

R_s—compressive strength of wood;

M_w—wind load moment in the centre of the wind load on the surface.

The basic safety factor is the numerical or percentage expression of the relationship between the bending moments acting on the tree, caused by the wind load and the tree’s own weight, if the tree is tilted, compared to the response the tree gives in terms of material properties (R_s) and the moment of resistance (M_or) related to the shape of the trunk. The moment of resistance under bending is defined as follows:

M_or = d³ × π/32, where

M_or—moment of resistance;

d—diameter of the trunk.

The bending or tipping moment caused by wind load on a tree is defined as follows:

M_w = M_t = M_b = f × cw × ρ/2 × Σ(u(z)2 × h(z) × A(h(z))), where

M_w = wind load moment;

M_t = tipping/uprooting moment related to the stability of the tree;

M_b = bending moment according to the breaking safety of the trunk;

f = natural frequency factor;

ρ = air density;

u_z = wind velocity;

h_z = height of specific area unit in the crown surface;

A = crown surface in m² at respective height;

c_w = aerodynamic drag factor.

The pulling test method, also known as SIM (Static Integrated Measurement), combines two methods: testing the elasticity of representative wood fibres using an elastometer and measuring the deviation of the tree from the vertical using an inclinometer. The basis for the measurements is a methodology developed in the 1980s by Sinn and Wessolly, later revised and described in scientific and popular science articles and publications, primarily by Wessolly [63].

The measurement is performed by applying a load to the tree using a rope tension, within safe limits, typically not exceeding 4–5% of the hurricane force. As the load is increased, readings are recorded from an elastometer, which measures the stretching or compression of representative tree fibres, and an inclinometer, which measures the angle of the tree’s deviation from the vertical. By comparing the load capacity of the trunk at its weakest point with the forces caused by a hurricane, we calculate the current safety of the trunk against breaking. By comparing the results of the tree’s deflection to the generalised windthrow curve developed by Wessolly, it is possible to calculate the current safety of stability in the ground. This provides an answer as to whether the tree has sufficient stability in the ground. The pulling test method requires accurate interpretation of the data obtained, based on the necessary knowledge and experience in statics, dynamics, and dendrology.

A safety factor of k = 1.5 has been adopted as the limit value for the current breaking safety of the trunk and for the stability of the tree in the ground under dynamic loads, meaning that the safety factor cannot/should not be lower than 150%. The safety factor, k, is an unmeasured number used in engineering that expresses the ratio of the dangerous value to the allowable value, i.e., the ratio of the tipping/braking moment for the tree to the maximum moment related to the maximum allowed wind speed equal to 33 m/s. The safety factor varies with material and construction, and for trees, a value of 1.5 has been adopted, which is the lower limit for elastic materials.

The safety values calculated and measured as a result of the test are either greater or less than the threshold value of 150% [64]. If the values are greater, the tree can remain. If the values are lower, there are usually four options: removal of the tree, reduction in the tree to reduce the crown sail area, securing the trunk with a support, or a combination of the latter two.

The methods discussed for assessing the safety of trees for people and the environment have been repeatedly tested for effectiveness [65,66] and applied in forests and safety work [67]. This is important in the context of the latest, increasingly precise research on trees, especially their forks, branches and twig attachments [68].

The aforementioned methods of tree inspection and assessment influence the type and selection of structures suitable for building supports. The choice of solutions is of great importance for the tree. The advantages of simple metal structures ideal for this purpose are highlighted by Heyman [69], and the principles of their design are outlined by others [70]. Gerard Passola, director of Doctor Árbol, has been involved in tree protection in this way for decades and has presented numerous solutions at industry lectures and webinars [71].

3. Results

3.1. Beginning of the Implementation of the Method

A detailed description of the history, condition, and the entire process of the pilot multi-stage veteranization of selected trees on the avenue, along with the methodology, is presented in the author’s article [72]. The author conducted the first study of the Avenue, published in 2007 [73]; hence, the author’s method is firmly grounded in many years of research and observation. The avenue has a long history, dating back nearly 200 years. It is home to a collection of valuable trees of historical, monumental, and ecological significance, particularly important for biodiversity. Since 2017, a slow but comprehensive process has been implemented on the Avenue, focusing on thorough dendrological and arboricultural identification, measurements and static analysis, as well as necessary protective and conservation measures. The observations, measurements, and assessments carried out allow for the selection of trees that need to be removed, those that are not of significant habitat importance, and those that need to be veteranised. On the one hand, these measures reduce the size of individual trees. On the other hand, they preserve them in various forms, gradually reducing valuable trees from a naturalistic perspective—especially in terms of biodiversity protection—to a safe state. In subsequent years, the author participated in analyses and work related to the safety and care of trees as required. Last year, he carried out a detailed verification of the condition of all trees growing in the Avenue, supplemented this year by sound tomography and load testing of the indicated group of trees. On this basis, further guidelines for the management of the avenue are being developed to preserve as much of the avenue’s biodiversity potential as possible. These measures are being consulted on an ongoing basis and implemented by the Municipal Greenery Authority in Krakow.

3.2. Method Scheme

The primary aim of the research and the main research achievement were to develop a consistent tree-support model. Based on many years of research, an algorithmic decision-making scheme was developed (Figure 1). The algorithm’s operation has been tested to varying degrees across at least a dozen cases, and the text presents the most illustrative examples from various locations that have had a significant impact on the method’s development. The algorithm (Figure 1) provides a methodical way to address the static nature of old-growth trees, which are valuable to the urban ecosystem and enhance biodiversity through the multitude of niches they provide. Consistency of action is essential in the management and protection of usable trees, and therefore in the planning, design, decision-making, supervision of the process and control of the effectiveness of the solutions adopted.

The detailed operation of the method, based on the algorithmic scheme presented (Figure 1), is based on several essential steps:

• The problem identification. The basic approach is to focus on screening and baseline surveys to identify problem trees in urban or park stands quickly. In the near future, processes can be automated with the support of artificial intelligence. Greehill is implementing such solutions [74,75].
• Basic tree assessment based on standardised sensory assessment methods, which are being implemented throughout Europe [76] and guide most tree professionals. This allows advanced diagnosis of tree needs, allowing the next steps of the algorithm to be carried out efficiently.
• Wind load analysis is a calculation to determine the basic safety factor. It should be performed based on a photo or a cross-section of the tree obtained from a LIDAR scan. Such analysis can and should be combined with a sonic tomography of the trunk or a pulling test. This allows the mechanical parameters of the tree to be determined, which will serve as the basis for further structural calculations.
• Analyse the results of the tree condition in the context of the vulnerability of the surroundings, to assess which solution is the most optimal, not only for the tree itself, but also for the protection of the surroundings. Assessing the vulnerability of the surrounding environment is a fundamental element of comprehensive tree risk assessment methods [77,78].
• Modelling of the tree based on LIDAR scans, allowing effective selection of tree support solutions—supports or pylons. A well-prepared model enables greater design precision. In fact, it reduces the time required to produce and install a protective element. That makes a real difference in actively protecting valuable trees.
• The selection of the final protection solution allows for structural calculations and the choice of material solutions. It is essential to design a foundation with a limited impact on the root system. Optimal solutions should have the least possible impact on the tree’s current physiology. They should not block the reaction mechanisms of the wood, which are responsible for the construction of load-bearing structures.
• Model and visualise the solution for a specific tree, making it easier for decision makers to familiarise themselves with the chosen solution. This also facilitates the subsequent installation of the solution on the tree. This allows us to prepare the tree protection documentation.
• Carry out work on the tree to install the chosen protection solution. A contractor usually carries this out under the supervision of inspectors for the best tree protection.
• Monitoring the effects of the protection system and making adjustments if these prove necessary or if the tree develops significantly or deteriorates due to decay.

3.3. Selection of Trees for Analysing the Effectiveness of the Method

These selected trees are a representative group of valuable yet problematic specimens from a structural perspective, particularly in terms of stability and resistance to breakage, for which the site and green space managers decided to implement the proposed method as a pilot programme to protect these valuable trees. The managers above chose to implement an algorithmic decision-making process to secure trees, perform the necessary measurements, and identify dedicated solutions for individual cases. This ensures that the method is not just a theoretical consideration, but an action proven in practice. These selected trees represent a representative group of valuable and, at the same time, problematic specimens from a static perspective, mainly in terms of stability and fracture resistance. Based on this, the site and landscape managers decided to implement the proposed method as a pilot programme to protect these trees. These managers decided to implement an algorithmic process for determining whether to preserve a tree, taking the necessary measurements and finding dedicated solutions on a case-by-case basis. This ensures that the method is not just a theoretical consideration but an action proven in practice. All conceptual and implementation work took place in Poland in selected cities (Kraków, Warsaw, Polkowice) and parks (Nieborów). The first decisions on the author’s part to work on the method and improve the algorithm took place in 2015, where, as a member of a team led by Marek Siewniak, a specialist in dendrology and arboriculture [79], he supported the process, the successful rescue of a London plane (Platanus × hispanica) damaged and partially destroyed by fire. The solution was based on a pulling test conducted in the historic Nieborów Park near Łódź. The main scope of activities was to prepare a concept and visualisation of the pylon supporting the tree. A further step in 2016 was the assessment, based on measurements with a sonic tomograph, of a valuable, aged small-leaved lime tree (Tilia cordata) in Polkowice, for which a care plan and a concept for modifying the support protection were developed. In the following years, 2017–2020, the development of the method was based on consultations and the creation of guidelines and supervision of nursing works for valuable trees (Waszyngtona Avenue—Kraków 2017, Osobowicki Cemetery—Wrocław 2018, Royal Castle Wawel—Kraków 2019, Aleja Fryderyka Chopina Avenue—Duszniki-Zdrój 2020). In 2018, the first trial solution was proposed to support a Norway maple (Acer platanoides) in Planty Park in Krakow. In 2021, a pilot implementation of the whole method—from identification through assessment and measurement to the concept and design of a support structure for tree protection—was conducted. This was a support for a Norway maple (Acer platanoides—tree no. 2 in

Table 1. Results of basic safety tests on eight trees selected for the design of supports—supporting structures.

Number of a Tree	Location	Species	Height	Circumference of Trunk	Crown Area	Wind Load Centre Height	Tipping/ Bending Moment	Basic Safety Bp
1	Krakow, Axentowicz Square	Japanese pagoda tree Styphnolobium japonicum	21.5 m	235 cm	198.2 m2	12.9 m	261 kNm	243%
2	Krakow, Krakowski Park	Norway maple Acer platanoides	22 m	235 cm	243.8 m2	11.9 m	331.3 kNm	233%
3	Warsaw, Polin Museum	Indian bean tree Catalpa bignonioides	7 m	168 cm	32.9 m2	3.2 m	15.8 kNm	1121%
4	Warsaw, Piękna Street	Black locust Robinia pseudoacacia	9 m	146 cm	50.1 m2	5.3 m	27.1 kNm	397%
5	Warsaw, Agrykola Park	Boxelder maple Acer negundo	13.5 m	186 cm	135 m2	7 m	160.8 kNm	178%
6	Warsaw, Wilson Square	Boxelder maple Acer negundo	14 m	319 cm	144 m2	7 m	171.5 kNm	1088%
7	Warsaw, Arkadia Park	Eastern crack-willow Salix euxina	19 m	466 cm	284.2 m2	8.5 m	328.8 kNm	114%
8	Warsaw, Skaryszewski Park	Kobus magnolia Magnolia kobus	13.5 m	158 cm	128.5 m2	6.3 m	137.7 kNm	143%

and

Table 2. Constructions of tree supports—materials and dimensions.

Tree Number	Foundation Dimensions [cm]	How the Tree Was Supported	Dimensions Profiles [mm]
1	240 × 140 × 50	Single, made of a traction pole. The pole consists of two C180 channels with ties. Two ties from the pylon to the trunks with GEFA 8t ropes.	C180
2.1	Aggregate and wooden pegs, diameter 8	Temporary, wooden, A-shaped. No adjustment.	140 × 140
2.2	100 × 210 × 40	A-shaped. Adjustment—spacer pads.	fi 88.9/5.0
3	90 × 50 × 30	Two struts (external and internal). Adjustment—change in internal rod extension and screw lock.	external diameter 63.5/4.0 internal diameter 54/4.0
4	50 × 50 × 40	Two seat posts (external and internal). Adjustment: Change the extension of the inner rod and secure it with a screw.	external diameter 63.5/4.0 internal diameter 54/4.0
5	50 × 50 × 40	Consists of two seat posts (outer and inner). Adjustment—change in internal rod extension and screw lock.	external diameter 63.5/4.0 internal diameter 54/4.0
6	50 × 50 × 40	Two seat posts (external and internal). Adjustment: Change the extension of the inner rod and secure it with a screw.	external diameter 70.0/4.0 internal diameter 60.3/4.0
7	50 × 50 × 40	Two wooden posts fixed in square tube sockets. Internal posts are made of wood, and external posts form sockets. Adjustment involves extending the internal post and locking it with a screw.	C24, 70 × 70 R 80 × 3
8	200 × 70 × 40	It consists of two rods (outer and inner). Adjustment: Change the extension of the inner rod and secure it with a screw.	external diameter 70.0/4.0 internal diameter 60.3/4.0

) in Krakowski Park in Krakow. In 2022, several implementations of the method were developed in Warsaw (trees Nos. 3–8 in Table 1 and Table 2) and in Krakow (the Japanese pagoda tree, Styphonolobium japonicum, tree No. 1 in Table 1 and Table 2).

From the selected trees surveyed and analysed, the following stood out:

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Important cultural context—Indian bean tree—Catalpa bignonioides—city centre, neighbourhood of the Museum of Polish Jews—Polin, Warsaw;

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As an element of a modernist, nearly 100-year-old composition—Japanese pagoda tree—Styphnolobium japonicum—Axentowicz Square, Kraków; Kobus magnolia—Magnolia kobus—Skaryszewski Park, Warsaw;

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As a witness to history—a tree that survived the bombing of the city during World War II—Black Locust—Robinia pseudoacacia—Piękna Street, Warsaw;

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Important social and compositional context (Boxelder maples—Acer negundo—Agrykola Park and Wilson Square, Warsaw, Norway maple—Acer platanoides—Krakowski Park, Krakow.

It is not the purpose to present all cases used as test or implementation cases, and the successful protection of selected trees is sufficient to confirm the relevance of the developed method.

3.4. Results of Measurements

The tree studies focused on the step-by-step execution of the algorithm. Trees were selected and assessed for basic dendrometric parameters, such as height and trunk circumference at breast height, in relation to crown canopy area and wind pressure centre height. This allowed the bending moment and basic safety level to be determined in accordance with the methodology described above. The results of these studies, for eight representative examples, were compiled (Table 1).

The author has been involved in the testing of around 1500 trees over the last 10 years. The results of these studies show that the average basic safety value for the trees included in the measurements ranges from 250 to 300%, so the basic safety factor varies between 2.5 and 3. This means that most of the large, valuable trees growing in cities and parks have good or very good biomechanical properties. The survey of a large group of trees shows that only 10 to 15% of specimens (150–200 trees) have noticeable mechanical problems and require further analysis, mainly involving leaning, valuable, old, and habitat trees. Those trees indicate the need to select systems and materials for a planned support protection project. Of the 150–200 trees in need of action, many were removed due to the inability to implement protection solutions or the lack of financial resources to prepare a dedicated solution. The author managed to convince the owners or managers to implement the treatment model in whole or in part for only about 10% of this group—more than 20 trees—on which the method was tested. Eight trees in Table 1 are selected examples from over 20 trees, as the most characteristic set of specimens that required external support to maintain stability or resistance to breakage.

3.5. Case Studies of the Implementation of the Methodology

The example of tree no. 1—an approximately 100-year-old Japanese pagoda tree—illustrates the dilemma associated with protecting a valuable tree with internal decay and high habitat potential, but with problems of stability in the ground, which years ago were unsuccessfully attempted to be secured, especially in the context of the tree’s increasing dimensions and growing bending moments. The strongly leaning tree, 33 degrees to the south-west (Figure 2a), required a new solution. The assumptions made should consider the least invasive approach to the proposed solutions in relation to the tree. Based on calculations (Figure 3a), a beam–pylon protection type was added at an angle opposite the inclination (Figure 3b), and the tree itself was secured with dynamic ropes —branches to the pylon (Figure 4b). A chosen tree combines several issues. Firstly, the basic safety value accounts for the presence of an old, deformed support (Figure 2b), which could fail at any time. Secondly, a large bench was designed directly under the tree (Figure 4b), thereby significantly increasing the surrounding vulnerability and increasing the risk of damage. Thirdly, the tree is one of the four original 100+-year-old specimens in the square’s modernist layout and thus has high historical and cultural value. Fourthly, decomposition is developing in the tree, which is essential for increasing biodiversity.

A major challenge in selecting design solutions was the need to relocate the pylon foundation away from the tree roots and adapt the element to the square’s composition (Figure 4). The adopted solution works well, and there are no noticeable issues with its effectiveness as protection for a valuable tree.

The solutions adopted for this tree can be considered a practical application of the model–algorithm, although the same type of solution will not always be equally effective.

In tree no. 2, according to the method, after LIDAR scanning of the tree (Figure 5b), three-dimensional models of the trees were generated, and, with their help, support and pylon solutions were selected in detail. In this case, a quick, temporary solution was implemented by building a wooden support (Figure 5a).

For individual trees, such as the Kobus magnolia in Skaryszewski Park in Warsaw, the type of support, the material used (Figure 6a,b), and the size of the foundations required for the tree protection elements (Table 2) are considered. This selection of dedicated solutions is essential if the tree’s presence in the environment is to be maximised.

In each case discussed, implementing the methodology preserved the tree in as unchanged a form as possible, protecting not only the specimen itself but also the habitats associated with it.

4. Discussion

The protection of old and habitat trees is becoming increasingly crucial for preserving the essential features of urban ecosystems. For years, many authors have emphasised the role of old trees in shaping ecological niches and microhabitats, thereby influencing biodiversity [80]. The functions of old-growth trees in environmental systems are increasingly described [81]. The benefits of retaining old trees, even dead trees and dead wood, on properties, and their impact on property prices and value, are analysed and discussed [82]. As mentioned in the introduction, the effects of aged trees on ecosystem development and restoration are studied, measured, and quantified [83]. Many studies and articles focus on how care [84] creates wounds, cavities, niches, and habitats for different organisms, especially birds. The benefits of intensive tree grooming, which cause wounds and defects that, over time, become niches and sites for microhabitat development, are indicated [85]. With the increasing number of valuable yet sometimes dangerous trees that pose risks for people and properties, the issue of safety around them is raised [86]. For these reasons, the author advocates extending the use of tree protection systems, such as supports and pylons, to protect both the surroundings and the old trees themselves, thereby extending their lifespan in urban and parkland ecosystems.

Unlike the solutions used in the 19th and 20th centuries, research on tree protection is now conducted using a developed methodology based on detailed measurements and calculations. Previous solutions were not based on calculations related to tree statics, and supports were prepared without design analyses or structural calculations, relying on the contractors’ experience. Method outcomes provide a way to find the best solution, from problem identification through relevant surveys and measurements, including the use of state-of-the-art scanning and modelling of trees and safeguards. Many authors highlight methods for problem identification (surveys, tree assessments) [87] and measurement (by type, quality, and applicability) [88], as well as material applications (wood, steel, concrete). What is lacking is a coherent method that enables complementary actions for a specific tree. The algorithm proposed by the author bridges the gap between known but often separately operating methods. It allows determining the validity of creating tree collaterals using support and pylon systems and their subsequent iterative implementation. The technique enables exploration of new security solutions and is thus open and developmental. This approach makes it possible to systematise such measures, to evaluate them righteously after implementation and to compare results, despite the variability of the trees themselves and of individual cases.

The proposed methodology follows the expectations of modern times, related to accelerated climate change, deteriorating urban living conditions, and declining biodiversity, for which the preservation of valuable and old trees of high value, growing in large cities, facing the effects of the problems mentioned above, is a matter of increasing urgency and necessity. The methodology was first implemented in two of Poland’s most significant cities: Warsaw, the country’s capital, and Krakow, the former capital, a major centre of services and business, and Poland’s most important tourist destination, with more than 8 million tourists a year [89].

Research on the use of tree supports has been conducted for many years across Europe and in many countries worldwide. Yet, the author has not encountered a coherent methodology for implementing such hedges based on research, assessment, and measurement—especially LIDAR measurements and 3D modelling of tree supports. This is a general knowledge gap. There are no scientific papers or articles on this topic or research area. In particular, there is a lack of research combining tree supports with biomechanical measurements.

Broader discussions on the proposed solutions were held only for specific trees—most notably the Bartek oak in Poland and the Robin Hood oak in Great Britain. In many cases, this led to damage to trees or parts of them, destruction or damage to supports, and, in some cases, to entire trees. Some very spectacular tree supports were made without conducting in-depth biomechanics studies and were based more on the performers’ experience. The best examples are the Major Oak (Quercus robur) and the well-known Japanese pagoda tree (Styphnolobium japonicum) at Kew Gardens in the UK, the Pines (Pinus sp.) near the Canale Grande in Venice and the supports created by Gerard Passola in Spain.

The proposed methodology is not only applicable to new cases of valuable trees requiring protection, but also to the analysis of existing solutions in the context of changing support parameters. The author tested this method on several valuable trees with existing supports, e.g., the previously discussed Japanese pagoda tree and the Hetman oak in Warsaw. Various tree owners and managers are now using the methodology. Monitoring existing implementations has yielded positive results in protecting trees and habitats.

5. Conclusions

Every year, more and more valuable trees provide ecosystem benefits for humans, the city, and the environment. They are essential for dendrology and local history. Senile trees, despite loss and wood decay, are of great value to the depleted ecosystems of cities, habitat development, and biodiversity. Such trees very often need to be actively protected, not only by reducing the impact of external factors but also by using support or pylon systems. The model of action proposed by the author has no analogues in known solutions. The method developed, based on an algorithmic scheme that leads from identification through surveys, measurements, calculations, and analyses to design solutions, includes key steps such as combining the results of the wind pressure analysis with an assessment of basic safety, resulting in a well-chosen and calibrated solution for individual trees. As demonstrated through operational monitoring, the proposed methodology produces accurate results, supports sustainable tree protection, and is practical for protecting valuable specimens. The use of proven survey methods, combined with modern, precise laser-scanning techniques, yields more effective design solutions. With this method, it is possible to save more valuable trees in the city that would otherwise be condemned to total or partial removal, resulting in significant losses to the urban ecosystem.