Large trees provide exponentially larger environmental benefits

In 2017 Auckland Council’s Research and Evaluation Unit published a report on the state of Auckland’s urban forests, in the Monitoring Research Quarterly newsletter.

They said that “Large trees provide exponentially larger environmental benefits – such as carbon storage, area shaded by canopy and building cooling effect – compared to small trees of the same species; a typical 20m tall street tree may provide three to seven times the benefit of a 10m tall one, and these benefits continue to rise rapidly above 20m.”

and also that “Only about seven per cent of Auckland’s urban forest canopy
is greater than 20m in height”
.

If you look at their map (from 2013), you see that the Western Springs Forest is the only 30+ forest in the whole of the Waitemata Local Board, with only scattered trees of similar heights in the Auckland Domain or in the shattered forest fragment to the east of the Western Springs stadium. That fragment alongside the Bullock Track has since then already been clear-felled and is now a struggling, weed-infested patch of regenerating scrub – a far cry from the lush, diverse area of native bush that exists under the Western Springs Forest’s pines.

Figure 9: Urban forest cover within the suburban zone of Waitematā Local Board showing the maximum height of urban forest patches (in metres)

Even in the 20-30m category the board region only has Western Springs park, the Domain, Cox’s Bay and Western Park as providing any significant contribution.

“The majority of trees greater than 20m are concentrated in a small number of public parks, including The Domain, the Western Springs area including the steep escarpment below Old Mill Road, Western Springs Park and the zoo, Cox’s Bay Reserve, small pockets within Ayr Reserve, and Western Park (Figures 9 and 11). There are also scattered examples taller trees in some smaller parks, including Alberon Reserve, Scarborough Reserve and Dove Myer-Robinson Park (Parnell) and Point Erin Park.”

The Council’s own analysis states that “International research has shown that many of the benefits attributed to urban forest are disproportionally provided by larger trees.”

“This is particularly true for environmental-ecosystems benefits such as providing shade, sequestering carbon, trapping pollutants and reducing water run-off. It seems intuitively correct that larger trees will cast more shade, have higher wood volume, greater total leaf area to trap pollutants and higher water requirements, and this is backed up by experimental evidence.”

“This data suggests it is critical to retain the larger trees”

And yet they plan to cut down the entire, and only major, stand of 20+m trees we have.

View or download the newsletter here: http://knowledgeauckland.org.nz/assets/publications/MRQ-08August-2017.pdf

The article is largely based on data from this report from 2013: http://knowledgeauckland.org.nz/assets/publications/TR2017-006-Urban-forest-of-Waitemata-Local-Board-in-2013.pdf