INTRODUCTION
The
evaluation of urban trees has been a challenge for many years.
Trees are difficult to evaluate objectively because they
are living and dynamic organisms. Various assessment criteria
have been developed over time, each with a slightly different
focus depending on the desired outcome.
These
include assessment criteria to determine:-
• the monetary value of trees
• Asset valuation
• the retention value of trees in relation to proposed development
• health, condition and hazard potential
• Cultural significance
MONETARY
VALUE
Methodologies
for assessing the monetary value of ‘amenity’ trees
have been progressively developed since 1929, and have included
the Helliwell system (1967) further revised 1990, Australian
Institute of Horticulture (1977), Thyer Method (1986), Burnley
Method (1986), Draft Australian Standard (version 1) 1992,
Draft Australian Standard (version 2) 1999 to name a few.
This
includes the latest version of the draft Australian Standard,
which has been under development for a number of years still
under review.
There
has been little agreement or consensus on a particular method,
primarily because most methods use highly subjective criteria.
The implication of this is that even trained professionals
evaluating the same tree with the same method can arrive
at considerably different results. The inconsistency in monetary
valuation makes most systems unreliable and therefore their
validity is often called into question.
Monetary
valuations usually use a range of criteria to assess the
amenity value of a tree or group of trees in the landscape
and include similar fields to those highlighted in The Burra
Charter
Initially,
the main purpose of such systems probably arose from the
issues of nuisance, negligence and trespass in Common Law,
in order to arrive at an acceptable value for the purpose
of determining appropriate levels of compensation (for the
loss, injury or damage to trees).
These
systems have been applied to land development, in order to
establish the value of compensation for trees injured or
destroyed as part of development works, or to establish monetary
figures for bonds and bank guarantees in the event of non-compliance
with conditions of development consent relating to the protection
of trees.
More
recently the methodologies have been applied to establishing
the value of tree, for the purpose of strategic asset management.
RETENTION
VALUE
Various
methodologies have been developed to establish retention
values or priorities for trees in the context of proposed
development. Essentially these are aimed at establishing
the relative value of trees using a systematic approach to
determine which are most desirable for retention. Early systems
derived in the UK were centred on determining the trees’ ‘amenity
value’ as provided for under Town and Country Planning
regulations.
These
have included the criteria for assessment in British Standard
5837:1991 – Guide for Trees in Relation to Construction.
These criteria focus on issues of health and condition, life
expectancy and suitability in the context of the proposed
development as well as visual amenity (size, form and location
relative to existing features). From this assessment, trees
are divided into four categories relating to their desirability
and appropriateness for retention.
In
1996, a new system derived from the methodology in BS 5837
was developed by arborist Jeremy Barrell. This system became
known as SULE (acronym for Safe Useful Life Expectancy) and
was promoted as the ‘cutting edge’ of pre-development
tree assessment. This system centres on safety as priority,
with secondary considerations of sustaining amenity and cost/benefit
of tree retention. The intent was to come up with a systematic
way of categorising trees in pre-development tree assessments.
A
new system was developed from SULE in 2005 after field testing
by Jeremy Barrell known as Tree AZ. Again, this uses a systematic
approach to categorise trees and establish retention values
for development planning purposes. Like SULE, the focus is
on retaining those trees that are in the best health and
condition, with the greatest life expectancy and the highest
amenity value.
Both
systems have been widely used in Australia particularly since
the introduction of the Environmental Planning and Assessment
Act (1979), which requires consent authorities to have due
consideration for the environmental impact of a proposed
development (including its impact on existing trees).
This
system has been challenged in recent times by a greater focus
on the role of trees in environmental processes and particularly
biological diversity. In this case, the values may be diametrically
opposed to the values of SULE, for example, those trees which
are locally-indigenous, contain hollows for habitat of arboreal
animals and birds and a range of species and ages are more
important values than general ‘amenity’ that
trees provide.
ASSET
VALUE
The
introduction of Australian Accounting Standard 27 (AAS 27)
in the early 1990’s has had significant implications
for both local and state government in terms of tree management.
The standard focuses on ensuring the appropriate reporting
of the current value and rate of depreciation of infrastructure
and other public owned assets, including trees.
This
has required the use of appropriate methodologies for:-
• Establishing the current health and condition of an asset;
• Its current material value (in monetary terms); and
• Its rate of depreciation and time period for replacement (life cycle
cost).
This
has forced a more strategic view of asset management aimed
at optimising the useful life of assets and ensuring that
appropriate funding is available for maintenance and capital
replacement. This total assessment management view included
a closer reign on risk management. It also re-sparked the
debate on monetary valuation methods.
In
most instances, methodologies have been drawn from existing
valuation methods and available assessment criteria for establishing
current health and condition and estimated life expectancy.
One
of the problems facing managers is that trees as assets tend
to increase in value before they diminish, not in a straight
line like many other forms of infrastructure.
CULTURAL
SIGNIFICANCE
The
Burra Charter was adopted by the Australian National Committee
of the International Charter of the Conservation and Restoration
of Monuments and Sites (ICOMOS) in August 1979.
The
Burra Charter provides guidance for the conservation and
management of places of cultural significance (Cultural Heritage
Places), with a view that conservation is an integral part
of the management of Places of Cultural Significances. This
includes natural, indigenous and historic places with cultural
values. A Place is further defined as a “site, area,
land, landscape, building or other work” including
the components thereof (e.g. trees, gardens and parks may
be considered components of “Places”).
“Cultural
Significance” is defined as “aesthetic, historic,
scientific or social value for past, present or future generations”
Guidelines
for Assessing Heritage Significance have been further developed
by the NSW Heritage Office (2001) using criteria encompassing
the values outlined in the Burra Charter.
The
recognition of trees as important components of places of
cultural significance has lead to the development of Guidelines
for the Conservation and Management of Street Trees in 1990
by the then Department of Planning. In particular this was
in response to increasing development and trees being considered
expendable, that is their importance and value to the community
not recognised or appreciated.
The
higher awareness of environmental issues and the lack of
proper management and conservation has also lead to the development
of numerous “Significant Tree Registers” across
Sydney, commencing with Blue Mountains City Council in 1986.
However, few of these provide any legislative protection
of trees over and above existing Tree Preservation Orders.
Only a few (to my knowledge) are linked to Local Environment
Plans, for instance.
HEALTH,
CONDITION AND HAZARD ASSESSMENT.
Numerous
methodologies have been developed, mainly by arborists and
arborists associations for the systematic evaluation of trees
to provide some informed judgement on safety and potential
hazard. The most well know of these is the Visual Tree Assessment
(VTA) Procedure developed by Claus Mattheck and Helge Breloer.
Other Systems include David Lonsdale’s Tree Assessment
Strategy.
These
methods rely initially on fairly subjective assessment, but
become increasingly objective as issues are identified and
warrant further investigation. Hazard Assessment Methodologies
focus on levels of risk and more recently, probability of
failure, based on criteria relating to tree size, structural
integrity, proximity to ‘targets’ and the type
and intensity of use of the areas surrounding the tree within
the target area. Essentially all these methodologies are
aimed at reducing risks associated with urban trees.
Since
the physical health and condition has a strong relationship
to its value, these methodologies are in some ways precursors
to all other valuation methods and criteria.
SUMMARY
In
summary, a number of evaluation methods exist in the industry
to aid in the evaluation of urban trees. The type of methodology
used is largely dependent on the outcome required and its
purpose. Many of these methodologies are still undergoing
change and refinement. Monetary valuation systems in particular
are the subject of continual debate. The overall problem
with most systems is the lack of strongly objective criteria.
The subjective nature of most assessment methods leads to
a large differential in results, calling the validity of
the system into question. Objective criteria are difficult
to formulate, particularly in monetary systems, because ultimately
the result cannot be easily tested. Trees may be regarded
and accepted as an asset, but (at least as mature specimens)
they are not usually a commodity that can be bought and sold.
additional
tables: One and Two (pdfs)
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