Dyani Lewis

Stressed street trees mapping the urban foreststo save them ndash and usCity trees are under increased threat but research tools show that looking after them will lowertemperatures prevent flooding and reduce pollution

Monday 27 March 2017 2351 BST

S tanding proud at the intersection of Queen and Little Bourke streets in Melbournersquos centralbusiness district is one of the cityrsquos beloved London plane trees Itrsquos in declining health andwill probably need to be replaced in a decade or so I know this not as an arborist or even

as a keen observer but because the City of Melbourne has assiduously assessed mapped andput online all of its more than 70000 street and parkland trees

ldquoItrsquos quite extraordinary the amount of data that we have on our treesrdquo says a councillor CathyOke who chairs the cityrsquos environment portfolio

The Urban Forest Visual is a vital part of the cityrsquos urban forest strategy an ambitiousmanagement plan that will see canopy cover nearly double from its current 22 to 40 by2040

Therersquos mounting evidence that the effort will be worth it Street trees city parks green roofsand gardens keep sweltering summer temperatures down help to prevent flash flooding

reduce air and noise pollution support native ecosystems and ultimately make our cities moreenjoyable ndash and healthy ndash places to live and work

So itrsquos little wonder that projects like this that catalogue and track a cityrsquos leafy assets areblossoming ndash and equally unsurprising is that there is community backlash when large trees arefelled

One of the largest of these projects is Treepedia launched this year Developed at MITrsquosSenseable City Lab in collaboration with the World Economic Forumrsquos Global Agenda Councilon the Future of Cities and Global Shapers community Treepedia makes use of the vast GoogleStreet View database to map and measure the visible greenery in cities across the world

The result is a collection of interactive maps of 16 cities ndash and counting ndash and an overallgreenness rating for each dubbed the green view index

At the top of the green league is Singapore with a green view index of 293 Sydney andVancouver come in equal second with scores of 259 and falling at the opposite end of thespectrum ndash 88 ndash is Paris But packing in a whopping 21000 people for every squarekilometre ndash more than 50 times that of Sydney ndash it is also the most densely populated of thecities

The picture that Treepedia gives isnrsquot a complete one ndash Googlersquos fleet is blind to a cityrsquosparklands riverbanks and backyards for instance Nevertheless the project does allow forbroad brushstroke comparisons between cities and even between neighbourhoods within acity

This makes it a powerful tool for change according to Carlo Ratti the director of the SenseableCity Lab ldquoCitizens themselves will use this information to compare the place where they livewith other places to put pressure on government to plant more treesrdquo he says

Another tool in the form of open-source software package i-Tree uses the birdrsquos eye view ofGoogle Earth to assess a cityrsquos canopy cover both on public as well as private land It also givesa dollar value to the many benefits ndash or ldquoecosystem servicesrdquo ndash that trees provide

As climate change brings warmer temperatures and unpredictable drenching downpoursthese benefits are taking on added significance Studies conducted in Sydney and Melbournehave shown that a 10 increase in vegetation cover lowers land surface temperatures by morethan a degree With cities already up to 7C warmer than surrounding rural areas ndash aphenomenon known as the urban heat island effect ndash trees are an important tool councils canwield against high temperatures that drive cooling costs up and increase mortality duringheatwaves

During storms urban vegetation is a cityrsquos sponge Tree roots green roofs and rain gardenshelp to sop up the deluge easing pressure on stormwater and sewerage systems and filteringheavy metals nitrogen and phosphorous from the runoff that does make it into waterways

Climate change has a sting in the tail for a cityrsquos trees though In Melbourne an extendedperiod of drought has set many of its trees on a slow but inevitable path to the grave

ldquoThe combination of urban heat island plus climate change means that street trees are going tobe put under stressrdquo says Nick Williams an urban ecologist from the University of MelbourneldquoYoursquore suddenly five degrees outside of the natural envelope for these plants in a lot of casesand thatrsquos going to put them under increasing stressrdquo

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Melbourne will need to plant 3000 new trees a year ndash comprising an evolving palette ofdrought- and heat-tolerant species ndash to replace dying trees and meet its 40 canopy-covertarget Knowing the specifics of each tree in its care is a key ingredient says Oke

But this type ofdetail is the exception says CSIROrsquos Guy Barnett and is usually limited to citycentres ldquoAcross large metropolitan areas like Sydney or Melbourne our understanding ispatchyrdquo he says

This is especially problematic because the hottest parts of a city are often the places with morevulnerable populations such as elderly people who are particularly sensitive to extreme heatldquoThatrsquos where we need our urban vegetation the mostrdquo Barnett says

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