Determining the retention value of trees on development sites

Andrew Morton

ABSTRACT/SYNOPSIS

The planning for any new development requires consideration of many different constraints, but by and large the preservation of trees on development sites is rarely considered from the initial planning stages. Trees are often viewed as potential problems to the development, rather than opportunities or assets. An arborist’s report is commonly completed as a requirement of the Planning Authority toward the end of the design phase, rather than guiding the site layout from the beginning of the project.

It is generally recognized by the community and developers alike that good quality, well-sited trees can make a significant contribution to the amenity of an individual property and the local area and make any new development more attractive and saleable. By understanding which trees are worth preserving and how much space is needed for their long-term survival during the initial design phase, it may be feasible to achieve a harmonious relationship between trees and buildings and thereby enhance any new development and sustain amenity within the local area.

To date, the methodologies used to establish the retention value of trees on development sites have concentrated on their sustainability in the landscape (that is their health, condition and longevity) as the main determining factor. However, the environmental, amenity and heritage values of trees are also important factors and need to be considered in balance with the trees sustainability using a systematic, logical and defensible approach.

INTRODUCTION

There is always a compromise between retaining trees on a development site and the economic imperatives of land development. It is clear that the community values trees given their contribution to the amenity and visual character of the local area. Retaining trees on development sites is a fine balance between sustaining that amenity and the economic development of the land. Establishing priorities for the retention of trees is an important part of the planning process if amenity is to be sustained in the long term. Unfortunately the assessment of the values that trees contribute to our urban areas is highly subjective and difficult to quantify.

Methodologies for establishing the retention values of trees on development sites is not a new concept. There has been an evolution of methodologies for establishing retention values since the introduction of BS 5837, but these have progressively moved away from the more subjective parts of the assessment.

BS 5837:1991

The British Standard (BS 5837:1991 – Guide for Trees in Relation to Construction) first published in 1980, provides guidance on preparing Pre-development Tree Surveys and establishing retention values for trees within development sites. BS 5837:1991 provides some basic categories and sub-categories that can be assigned to trees (refer Appendix 2). These categories were an attempt to sort-out trees in terms of their desirability or suitability for retention within the context of a development site in order to provide some guidance for planning purposes.

The short-coming of this method is that it is fairly conceptual and offers no criteria for assessment of these values or any detailed method to assign a tree to a particular category. In essence though, it promotes the idea that retention values should be based on an assessment of the amenity values of a tree as well as their overall health, condition and longevity (sustainability).

SULE

In 1993, the British Standard approach was superseded by a methodology known as Safe Useful Life Expectancy (SULE) developed by Jeremy Barrell, a British arboriculturist. SULE was based loosely on the British Standard, but is more systematic and rigorous in its approach. SULE made an attempt to assimilate the health, condition and value of a tree, using remaining life expectancy (in consideration of safety issues) as a measure of its sustainability in the landscape. This system was promoted as best practice in pre-development tree surveys and has been widely used throughout Britain, the United States and Australia. The concept behind SULE is that of sustained amenity, the longer a tree can contribute to amenity in a sustainable way (in consideration of safety and the proposed development), the higher the retention value. The methodology is summarized in Appendix 3.

In spite of the objective of sustaining amenity though, there is little in this methodology that assists in determining the relative amenity value (or other value) of a tree or trees. The method focuses primarily on determining a tree’s sustainability in the landscape (i.e. how long a tree can be retained in the landscape in consideration of its health & structural integrity (safety). However, other factors such as the size and appropriateness of retaining the tree in light of the proposed development (usefulness) are also considered.

Tree A-Z

In 2000, Jeremy Barrell developed a new methodology known as Tree AZ. The emphasis of Tree AZ is negative selection. That is, all trees should be assumed to be worth preserving until such time as it can be demonstrated otherwise. Once the trees that aren’t worth preserving are identified (based on a predominantly arboricultural assessment), the trees that are worth preserving become the primary focus of the planning exercise.

In this method, safety is considered to have absolute priority in decisions regarding tree retention. As such, the contribution to amenity that a tree provides is overruled by safety considerations, that is, amenity value is a secondary consideration. In simple terms it translates that a tree with a high amenity value, if structurally defective and hazardous or diseased with a short remaining life expectancy should not be assigned a high retention value. In theory this is a logical assumption.

Barrell acknowledges that nuisance and other negative values (of trees) are also of importance and must be taken into consideration when assigning retention values. He also acknowledges that a tree’s environmental values (such as a tree’s wildlife habitat value) also need to be considered, but the methodology does not attempt to establish these relative values. Barrell also recognizes that under the British planning framework, preserving amenity is the main focus of local planning instruments in relation to tree preservation.

The aim of both systems (SULE & Tree AZ) is to convert what amounts to a fairly complex arboricultural assessment into a few broad categories that are easily understood by planners and can be considered together with other constraints to determine the best layout and design of the site. Ultimately this is the main aim of establishing tree retention values.

Essentially these categories can be defined as follows:

• Trees that are important to preserve (most suitable),
• Trees that are desirable to preserve (suitable),
• Trees that are undesirable for preservation (not particularly suitable); and
• Trees that should be removed (unsuitable).

SRIV

The Institute of Australian Consulting Arboriculturists (IACA)^© has also developed a methodology called the Sustainable Retention Value Index (SRIV)^©. This method mainly concentrates on the viability of the tree within the development site (i.e. it’s sustainability in the landscape) without consideration of its landscape or amenity value. SRIV^© acknowledges that arboricultural elements can be assessed fairly objectively by trained personnel, but assessing amenity values is highly subjective and fraught with difficulty, therefore it is ignored. In this method, trees that are normally considered of low retention value (such as dead trees and noxious weeds) are excluded from the assessment since the requirement to remove these trees would generally be a foregone conclusion.

DISCUSSION

All of these methodologies focus primarily on the sustainability of the tree in the landscape as a way of determining its value for retention, thus a tree that is in good health and condition with a long remaining life expectancy is considered the best candidate for retention on a development site. Decisions about sustainability rely predominantly on a detailed arboricultural assessment. These assessments can be relatively objectively assessed by a trained arboriculturist with good knowledge and experience in evaluating biological and biomechanical indicators (Refer to Appendix 2).

The difficult part to assess consistently and objectively is a tree’s amenity, heritage and environmental values. There is no doubt that the criteria developed in the past to assess these factors is highly subjective and provides inconsistent results, even when trained professionals undertake the assessment. However, this doesn’t mean that these factors are unimportant and should be ignored. Despite the inherent difficulties, some measure of the relative value of a tree or trees in consideration of these factors is necessary. No method currently provides a systematic way of measuring and balancing both sides of the equation to arrive at an informed decision on tree retention value.

A trees amenity, environmental and heritage values are the main reason that the community supports tree preservation initiatives. That support is reflected in relevant legislation to protect those values at a local, state and federal level. Like the British planning system, Tree Preservation Orders in NSW are primarily designed to preserve amenity, not the inherent values that trees contribute to environmental quality. Most TPO’s in NSW are derived from the Model Provisions of the Environmental Planning and Assessment Act 1979 (EP&AAct), which permit a local planning authority to make a Tree Preservation Order for the purpose of “preserving or enhancing amenity”. It is the trees contribution to the amenity, landscape quality and visual character of an area that is important from a planning perspective.

Since methodologies for establishing retention values are undertaken primarily within a planning context, amenity value must be given due consideration. Likewise heritage conservation and environmental protection are important planning issues and constraints to any new development.

Section 79C of the EP& A Act lists the general matters for consideration in evaluating a development application. Aside from the local environmental planning instruments (generally embodied in a Local Environment Plan), Development Control Plans and other regulations, the Planning Authority must consider “the likely impacts of that development, including environmental impacts on both the natural and built environments, and social and economic impacts in the locality”. Therefore, in order to assess the potential impact of a development proposal, the relative contribution that individual trees and groups of trees make to amenity and the natural and built environment must be evaluated and considered.

To ensure a balanced decision on tree retention, these values also need to be weighed up and considered together with a tree’s sustainability in the landscape. A measure of sustainability is important, since it is an estimate of the relative length of time that that amenity and other values can be provided. The heritage value of an artifact or building diminishes or increases according to its rarity and condition. So too, a tree’s environmental value diminishes with reduced longevity. The amenity value of a tree diminishes if it will only be maintained over a short time frame. Therefore the sustainability of a tree in the landscape is an important part of the equation and has a clear relationship with those values.

The amenity, heritage and environmental values of a tree should not be confused with the inherent values that most or all trees provide, such as carbon sequestration and improvement in air quality. Rather, these values relate to individual trees and groups of trees in particular situations. These values can be positive and negative. They vary according to the species and size of the tree and its position in the landscape. Whilst it may be difficult to assign categories and values to categories, the opposite ends of the spectrum can be readily understood and in some instances are currently defined by relevant state or federal legislation or local planning instruments.

Again, assessing the values of trees in this fashion is not new. There have been numerous attempts to develop methodologies that assess these types of values in order to arrive at some type of monetary valuation for a variety of purposes. In a similar way, monetary valuation methods are primarily designed to provide a relative value, and then assign a dollar value to that. But most give unreliable and inconsistent results, simply because the criteria for assessment are very subjective and open to interpretation. In addition, trees are not commodities that can be bought and sold, so the values given can never be tested in an open market situation.

One of the better known and utilized valuation methods in Australia is the Thyer Method, developed by Peter Thyer. This method considers the overall size of the tree and relates its size factor to a relative measure of the trees environmental, heritage and aesthetic values. Whilst there are some measures of sustainability (health, vigour, suitability & life expectancy) built in to the formula, they are not considered independently. The independent assessment is a significant issue, since it is the sustainability in the landscape that increases or diminishes these values to some extent, therefore it needs to be considered first. For example, tree can be significant (for heritage, environmental or amenity reasons) regardless of its health, condition and longevity (or its size!). At first glance this may appear to contradict my previous discussion. However, I am not suggesting that a tree that is significant necessarily has a high retention value. A high retention value would depend on two parts of the equation being satisfied. That is, is must be both sustainable in the landscape for a reasonable time frame and be significant in the landscape to have a high retention value. That is why sustainability and landscape significance need to be assessed independently then weighed up to provide a meaningful retention value.

ASSESSING ENVIRONMENTAL, HERITAGE & AMENITY VALUES

The subjective nature of the criteria could be sharpened somewhat if there was a closer reference between the assessment criteria and our current legislative and planning framework, since these embody at least some of the definitions. This is a useful reference for two reasons. Firstly, this framework in essence reflects community standards, expectations and values; and secondly, the reason for the evaluation (of retention values) is to assist in the planning process and determine the extent of impact (i.e. potential loss of amenity and other values). Even though there may still be some grey areas, the opposite ends of the spectrum are easily understood.

For example, consider the environmental value of a tree. How can the environmental value an individual tree be defined? Using the existing legislative framework in NSW, at one end of the spectrum we have Threatened and Vulnerable Species and at the other end we have Noxious Weeds. Both of these are defined under State and Federal Legislation, (i.e. The Threatened Species Conservation Act (TSCAct) and the Noxious Weeds Act) therefore they can be assessed objectively. Logically, a species that is listed as a Threatened Species under the TSCAct is considered of high value, and a species listed as a Noxious Weed (within the relevant Local Government Area) is considered of very low value. The attached tables (Appendix 4) provide some suggested criteria that could be used to rank the importance of a tree or trees in the context of these values.

In between these two ends of the spectrum, we have trees that may be listed or recognized as environmental weeds or nuisance species (but not necessarily Noxious); trees that are protected under local Tree Preservation Orders; and trees that are locally-indigenous to the area, but are not threatened species. These categories can be assessed fairly objectively by reference to the local planning codes and instruments and schedules and definitions in state or federal legislation.

Likewise, consider heritage values. At one end of the spectrum we have a tree that is listed as a heritage item under the relevant Local Environment Plan with a local, state or national level of significance. At the other end we have a tree with no known heritage significance or association. In between, there may be trees that are listed on Significant Tree Registers, but have no legislative protection or trees whilst not listed, have strong association with a particular heritage item or building, or are representative of a particular era or style of landscape design within that landscape.

Likewise, consider heritage values. At one end of the spectrum we have a tree that is listed as a heritage item under the relevant Local Environment Plan with a local, state or national level of significance. At the other end we have a tree with no known heritage significance or association. In between, there may be trees that are listed on Significant Tree Registers, but have no legislative protection or trees whilst not listed, have strong association with a particular heritage item or building, or are representative of a particular era or style of landscape design within that landscape.

DETERMINING A LANDSCAPE SIGNIFICANCE RATING

Most previous monetary valuation methodologies use some form of relatively complex mathematical equation or scoring system. Whilst this may be necessary for monetary valuation, in my view it is unnecessary for assigning retention values. Ultimately all that is necessary is for a considered evaluation of the trees amenity and other values and to place trees into one of four broad categories to aid in the planning process and guide the layout of the site. As such, a fairly simple system based on the rankings shown in the previous tables (Appendix 4) can be used to assign a landscape significance rating to each tree (refer to Appendix 5).

DETERMINING TREE RETENTION VALUE

Weighing up sustainability and landscape significance to arrive at a retention value is the next step in the process. We have seen that these two elements must be assessed independently, since they have a relation ship with one another. The health, condition and longevity of an item (in this instance a tree) increases or diminishes depending on its level of intactness, quality and potential longevity.

To do this systematically, Footprint Green (a Sydney based arboricultural and environmental consultancy firm) have developed a simple but effective table. The version following is modified from the original, but the intent is the same (refer to Figure 2).

FIGURE 2 – TREE RETENTION VALUES – ASSESSMENT METHODOLOGY

Ref:- Modified from
Couston, Mark & Howden, Melanie (2001) Tree Retention Values Table Footprint Green Pty Ltd, Sydney Australia

In effect what this table shows is that a tree that has high environmental, heritage and/or amenity value (high landscape significance) and is sustainable in the landscape in the long-term should be considered of high retention value. A tree which has low landscape significance should be considered of low retention value, regardless of its sustainability. A tree which has a high landscape significance rating, but is dead or potentially hazardous (and beyond remedial treatment), has a low retention value, since its health and condition diminishes its value.

CONCLUSIONS

Existing methodologies developed since the 1980’s to assess the retention values of trees on development sites do not adequately address the amenity, heritage and environmental values that trees contribute to a local area. Most of these systems rely purely on a measure of sustainability in the landscape to derive a retention value. Amenity, heritage and environmental values are largely ignored, since they are subjective in nature and difficult to assess consistently and objectively. Not withstanding this, it is these values that are the main focus of local tree preservation initiates, not the inherent values that most or all trees provide. It is the potential impact or loss of these values that local Planning Authorities must assess as part of any development proposal.

This is not to say that sustainability is irrelevant. It is certainly a significant part of the evaluation, since it is the sustainability of the tree that increases or diminishes these values. The health, condition and longevity of a tree or trees can be assessed in a relatively objective manner by a trained arboriculturist.

The subjective nature of most of the criteria for assessment to date could be improved by reference to Federal & State Legislation and Local Planning instruments, since it is those things that both reflect community standards and expectations and form the framework for the assessment of development proposals. Whilst there may be some grey areas, the opposite ends of the spectrum can be readily understood. An attempt to rank these is shown in the attached tables (Appendix 4).

Once there is some measure of a tree’s overall importance or significance in the landscape (i.e. its contribution to environmental, amenity and heritage values), these two factors can be weighed up using a simple table which then categorises the tree or trees according to its suitability or desirability for retention. The end result is a methodical and systematic approach, which can be scrutinized and defended where necessary, and could form a more standarised approach to the assessment of the retention values of trees on development sites.

REFERENCES:

• Draper, Danny (2006)
• SRIV – Sustainable Retention Value Index ^©
• The Australian Arbor Age, Volume 11, No.1 (June/July 2006)
• Mayne Publishing Pty Ltd, Sydney
• Mattheck, Dr. Claus & Breloer, Helge (1994) – Sixth Edition (2001)
• The Body Language of Trees – A Handbook for Failure Analysis
• The Stationery Office, London, England
• Thyer, Peter (1986) – Revised 1996
• The Thyer Tree Valuation Method
• Peter Thyer   www.intercoast.com.au
• British Standards Institute (1991)
• British Standard 5837:1991 – Guide for Trees in Relation To Construction
• British Standards Institute, London
• Matheny, N. & Clarke J. R.  (1998)
• Trees and Development – A Technical Guide to Preservation of Trees During Land Development
• International Society of Arboriculture, Illinois USA
• Barrell, Jeremy (2005)
• Detailed Guidance on Practical Tree AZ Assessment (Guidance Note 7)
• Barrell Tree Consultancy (www.barrelltreecare.co.uk)
• Barrell, Jeremy (1995)
• Pre-development Tree Assessment
• Proceedings of the International Conference on Trees and Building Sites
• International Society of Arboriculture, Illinois, USA

APPENDICES:

1. Assessment of the sustainability of a tree in the landscape
2. BS 5837:1991 – Retention Categories and Sub-categories
3. SULE Methodology
4. Landscape Value Tables
5. Landscape Significance Criteria

APPENDIX 1 – ASSESSMENT OF THE SUSTAINABILITY OF A TREE IN THE LANDSCAPE

APPENDIX – 2 BS 5837:1991 CATEGORIES AND SUB-CATEGORIES

APPENDIX – 3 PROCEDURE FOR ASSESSING SAFE USEFUL LIFE EXPECTANCY (SULE)

APPENDIX 4 – FACTORS THAT INCREASE OR DIMINISH AMENITY, HERITAGE & ENVIRONMENTAL VALUES

APPENDIX – 5 CRITERIA FOR ASSESSMENT OF LANDSCAPE SIGNIFICANCE

The level of landscape significance has been determined using the following key criteria as a guide:

SIGNIFICANT

• The subject tree is listed as a Heritage Item under the Local Environment Plan (LEP) with a local, state or national level of significance; or
• The subject tree forms part of the curtilage of a Heritage Item (building /structure /artifact as defined under the LEP) and has a known or documented association with that item; or
• The subject tree is a Commemorative Planting having been planted by an important historical person (s) or to commemorate an important historical event; or
• The subject tree is scheduled as a Threatened Species or is a key indicator species of an Endangered Ecological Community as defined under the Threatened Species Conservation Act 1995 (NSW) or the Environmental Protection and Biodiversity Conservation Act 1999; or
• The tree is a locally indigenous species, representative of the original vegetation of the area and is known as an important food, shelter or nesting tree for endangered or threatened fauna species; or
• The subject tree is a Remnant Tree, being a tree in existence prior to development of the area; or
• The subject tree has a very large live crown size exceeding 300m² with normal to dense foliage cover, is located in a visually prominent in the landscape, exhibits very good form and habit typical of the species and makes a significant contribution to the amenity and visual character of the area by creating a sense of place or creating a sense of identity; or
• The tree is visually prominent in view from surrounding areas, being a landmark or visible from a considerable distance.

Very High

• The tree has a strong historical association with a heritage item (building/structure/artifact/garden etc) within or adjacent the property and/or exemplifies a particular era or style of landscape design associated with the original development of the site; or
• The subject tree is listed on Council’s Significant Tree Register; or
• The tree is a locally-indigenous species and representative of the original vegetation of the area and the tree is located within a defined Vegetation Link / Wildlife Corridor or has known wildlife habitat value;
• The subject tree has a very large live crown size exceeding 200m²; a crown density exceeding 70% Crown Cover (normal-dense), is a very good representative of the species in terms of its form and branching habit or is aesthetically distinctive and makes a positive contribution to the visual character and the amenity of the area.

High

• The tree has a suspected historical association with a heritage item or landscape supported by anecdotal or visual evidence; or
• The tree is a locally-indigenous species and representative of the original vegetation of the area; or
• The subject tree has a large live crown size exceeding 100m²; and
• The tree is a good representative of the species in terms of its form and branching habit with minor deviations from normal (eg crown distortion/suppression) with a crown density of at least 70% Crown Cover (normal); and
• The subject tree is visible from the street and surrounding properties and makes a positive contribution to the visual character and the amenity of the area.

MODERATE

• The subject tree has a medium live crown size exceeding 40m²; and
• The tree is a fair representative of the species, exhibiting moderate deviations from typical form (distortion/suppression etc) with a crown density of more than 50% Crown Cover (thinning to normal); and
• The tree makes a fair contribution to the visual character and amenity of the area; and
• The tree is visible from surrounding properties, but is not visually prominent – view may be partially obscured by other vegetation or built forms.
• The tree has no known or suspected historical association

Low

• The subject tree has a small live crown size of less than 40m² and can be replaced within the short term with new tree planting; or
• The tree is a poor representative of the species, showing significant deviations from the typical form and branching habit with a crown density of less than 50% Crown Cover (sparse); and
• The subject tree is not visible from surrounding properties (visibility obscured) and makes a negligible contribution or has a negative impact on the amenity and visual character of the area.

Very Low

• The subject tree is listed as an Environment Weed Species in the relevant Local Government Area, being invasive, or a nuisance species.
• The subject tree is scheduled as exempt (not protected) under the provisions of the local Council’s Tree Preservation Order due to its species, nuisance or position relative to buildings or other structures.

INSIGNIFICANT

• The tree is a declared Noxious Weed under the Noxious Weeds Act (NSW) 1993