Valuing our urban trees

At last, some good news: city trees have been given the same habitat and biodiversity value as their country cousins.

STOP PRESS

Since writing this blog, we have now responded to Defra’s Small Sites Metric (SSM) Consultation. It develops further our critique of the way that urban tree habitats are being undervalued. Perhaps urban trees are now the poor country cousin? Here it is:

Bristol Tree Forum response to the Small Sites Metric consultation.


Our Original blog

The important contribution that urban trees (native and non-native) make to our cities has finally been recognised by Natural England, with their publication of Biodiversity Metric 3.0 (BM3.0) on 7 July. It states that:

Trees in urban areas can, under the right conditions, provide a large range of habitat opportunities, supporting lichens, bryophytes, invertebrates and birds. Tree planting in urban areas has for over two hundred years also introduced non-native species into towns and cities. In the context of biodiversity, native species are the preferred option. However, non-native tree species can contribute positively to biodiversity richness particularly in relation to providing a seasonal food source for nectar feeders and other invertebrates as well as supporting vertebrates that feed on species that are hosted by non-native trees. Examples are early and late flowering species of Prunus and aphids on varieties of Acer providing food for species higher up the food chain.

Trees in urban areas provide opportunistic sites for biodiversity to colonise and re-colonise, increasing connectivity and contributing to biodiversity critical mass between already established patches or sites. This is especially true where transport corridors are populated with mixed native species.

What’s an urban tree?

The new BM3.0 habitat category, urban tree, includes individual trees, lines of street trees and blocks of trees growing within the urban setting.

BM3.0 Guide – TABLE 7-1: Urban tree definitions

The previous urban tree habitat categories, woodland, orchard and street tree, which appeared in the beta test version of Biodiversity Metric 2.0 (BM2.0) have been discarded.

The urban tree habitat calculation has been set to ‘medium’ distinctiveness and ‘low’ difficulty for both habitat creation and enhancement. Urban trees are categorised into small, medium or large. Their condition may also be assessed as poor, moderate or good.

The problem with BM3.0

The three size bands set out in the table below are useful when creating new habitats or enhancing existing ones (for example, nursery-raised standards ready for planting have a stem diameter of around 30 cm and so are Medium). However, these bands are not useful for assessing the baseline habitat of existing urban trees.

This is the size table used in BM3.0:

BM3.0 Guide – TABLE 7-2: Urban tree size by girth and their area equivalent

(NB: the second column of this table is wrongly labelled. It should read Girth (circumference) at Breast Height, not Diameter.)

This table is based on the methodology adopted for calculating the Root Protection Area (RPA) that must be set aside (protected) during the development of sites where trees are growing or likely to be affected. It is set out in BS:5837 2012 Trees in relation to design, demolition and construction, Recommendations. Its purpose is to make sure that the tree is not damaged during development.

The RPA formula used is simple: RPA radius = 12 x DBH (Stem Diameter is also known as DBH – Diameter at Breast Height). This value is then used to calculate the RPA using the formula DBH = PI * RPAr^2.

Every application to develop land where trees will be affected should produce a BS:5837-compliant survey, called an Arboricultural Impact Assessment (AIA). This will report the stem diameters of all the trees growing on and around the site. The AIA also reports several other tree features including species, height, cardinal point canopy radii, condition, life stage and the BS:5837 category – a measure of the quality of the tree.

However, the BM3.0 table above provides no logical way of establishing whether a given surveyed tree with a stem diameter of, say, 15 cm or 40 cm – halfway between categories – is Small, Medium, or Large.

It would be better if the table gave ranges – say Small up to 10 cm, Medium 10-50 cm and Large 50 cm or more – but this has not been done. Also, doing this would distort the habitat calculation with all Small trees set to their upper range and all Large trees set to their lower range.

Our solution

Why use the table at all? It would be far simpler to calculate a tree’s baseline habitat area just by using the calculated RPA provided in the AIA. It would be better still to use its actual measured canopy area, which will have been reported in the AIA and thus be readily available.

In our view, RPA does not reflect the habitat value of a tree. All it does is use a formulaic approach to solving the problem of finding an acceptable way to protect trees. It bears little relationship to the habitat or biodiversity value of a tree.  It would be far better to calculate a tree’s canopy cover (TCC), the standard method of working out the value of a tree. Every AIA reports the canopy radii of the four cardinal compass points of each tree surveyed. These can be averaged and used to calculate TCC.

The Bristol One City Plan adopted TCC as the measure of tree planting success when it set the target to double TCC by 2046. TCC is a standard measure used by the various i-Tree tools and Forest Research uses it in its UK Ward Canopy Cover Map which used i-Tree Canopy. We used it to calculate the TCC of the city’s wards in our 2018 Bristol Tree Canopy Cover Survey and we are using it to update the new city-wide survey for 2021.

We made these observations when Natural England was consulting on its beta test version, but these seem to have been overlooked. We hope they now take note.

Some further thoughts

The introduction of the three new urban tree poor/moderate/good condition criteria, set out in detail in the BM3.0 Technical Supplement, will require all AIA surveys to include this data. Perhaps BS:5837 should be updated to require this to be recorded in the AIA.

Where tree surveys identify mixed urban tree conditions, the person undertaking the BM3.0 calculation will need to record more than one urban tree baseline habitat to capture this information.

BM2.0, which was only published as a beta test to allow for wider public consultation, is still being used by Bristol’s Local Planning Authority (LPA) for pending applications but needs to be abandoned. Pending applications which require a biodiversity net gain report should be required to recast their calculations using BM3.0 rather than still relying on BM2.0. This is particularly true for the Council’s own, direct applications such as the one pending for the Baltic Wharf Caravan Park.

Our initial analysis shows a significant net gain deficit when BM2.0 is used instead of BM3.0. This is especially true for urban street trees, which are significantly undervalued under BM2.0. Furthermore, the LPA is currently allowing applications which propose a zero net gain outcome, even though the Environment Bill (currently being considered in Parliament) will require a net gain of 10% above the baseline valuation.

Given that the Council has declared climate and ecological emergencies and aims to achieve carbon neutrality by 2030, it is surprising that developers continue to be allowed to present biodiversity net gain proposals that either undervalue biodiversity or offer no net gain whatsoever.

Conclusion

We welcome the publication of BM3.0, but its flaws need to be rectified.

The Council’s declaration of climate and ecological emergencies and its commitment to achieve carbon neutrality by 2030 means that it needs to ensure that the latest, most accurate biodiversity net gain calculations are part of all pending and future planning applications.

As Natural England recognises in its recent blog – Biodiversity Metric 3.0 – a milestone moment for biodiversity net gain:

Publishing Biodiversity Metric 3.0 was a landmark moment for biodiversity net gain, it will become the metric used to calculate and evidence whether a project has achieved the biodiversity net gain requirements set out in the Environment Bill. Biodiversity Net Gain (BNG) is:

an approach to development, and/or land management, that leaves nature in a measurably better state than beforehand‘ …

Biodiversity Metric 3.0 ensures that:

all habitats, from street trees to woodlands, green roofs to grasslands are recorded, scored and valued for their importance for wildlife. At the same time, it provides an evidence-based, transparent, consistent and easy to use way of ensuring that nature is considered within the design of developments and in land management practice, leaving nature in a better place than it was before, benefitting wildlife, people and places.