As Christchurch baked in a series of 30° days in February, its City Council put out an Urban Forest Plan as one means of mitigating temperature rise in the city.  Was the plan any good?

The area of tree canopy cover within Christchurch City (as measured by aerial photography and LIDAR) decreased 2% to 13.56% in the three years between 2015 and 2018.  That compares with Auckland at 18% and Wellington at 30% canopy cover.  Much of that difference can be pinned to the different landscapes of the cities, particularly at the time of European settlement; Auckland and Wellington were forested while Christchurch was largely swamp and wetland.  

Whatever, we do need to increase canopy cover within the city to help with two things: mitigating the rise in temperature for city residents and mitigating temperature rise itself due to global warming.  The Urban Forest Plan seeks to address both issues by planting significantly more trees within the city to create an “urban forest”; it’s not really a forest – it’s just a lot more big trees within the city boundaries.

What is in the plan?  In essence, the Ōtautahi Christchurch Urban Forest Plan is a simple one with a long timeframe.  Plant lots of the right species of tree in the right location and for the right function, work collectively to manage trees in public and private spaces, protect trees so that they grow to maturity for future generations and make sure that the distribution of trees is across the entire city, not just in the leafy suburbs. Will it work?

A plan is just a plan. How well it will work depends on whether it offers a practical solution that gets community-wide buy-in.  The Urban Forest Plan, while a little simplistic, is reasonably practical.  Its usefulness will depend on the buy-in at all levels, public and private.

Housing Density is a problem Plan Change 14 which brings in Medium Density Residential Standards to the Christchurch District Plan is currently out for public consultation.  Concerns about increasing housing density focus on shading from neighbouring buildings and reduction in the number of trees.  It is hard to intensify housing (which is good for climate change mitigation) and retain trees at the same time; that is just the reality.  However, planting more trees within streetscapes, and adapting streetscapes to better suit increased tree coverage IS possible.  The complication is with infrastructure: power lines above and below ground as well as pipes.

Trees as infrastructure The Urban Forest Plan promotes the concept of trees needing to be elevated to the position of essential urban infrastructure that MUST be accommodated alongside these other infrastructural components so that they are all designed to fit together and accommodated. This will require altered design standards that will change the way roads, trees, power and drainage are laid out.  Some changes, like inserting trees in islands within roads may be able to happen without too much disruption, but generally, accommodating trees as essential infrastructure is a long-term action. The plan envisages only a 1% gain in canopy cover within streets by 2030 and 7% by 2070 while residential property canopy cover is forecast to rise 2% by 2030 and 7% by 2070.  

Trees in open spaces Greater canopy cover gains can be achieved by increasing the planting of trees in open spaces like parks and reserves. While many urban parks accommodate sports and recreational activities in their central spaces, there is plenty of space for more dense tree planting on the edges of these parks.  The plan does not state it, but Council staff have indicated that they propose placing trees to match park-edge recession planes thereby minimising shading effects of trees on properties that neighbour open spaces.  An increase of canopy cover from open spaces of 17% is expected by 2070.

Waterways the big winner The biggest gains in canopy cover are planned for waterways where a quick increase of 9% by 2030 and 54% by 2070 is envisaged. Canopy cover over waterways benefits not only people through its cooling effect, but it also benefits waterway health. Shading water prevents temperature rise which increases dissolved oxygen, increasing abundance within microbiological and fish ecosystems.  Planting trees to this density along our waterways will also require some change, however, although the urban Forest Plan is silent about the mechanism by which waterway plantings can be increased.  It is also a bit too quiet about the ecological basis for waterway environment planting.

 

What the Urban Forest Plan also avoids discussing is that, as communities, we will have to stop wanting our riverbanks to be mown grass. The riparian strip will need to be densely planted with native grasses, shrubs and trees although regular view corridors to the river must continue to be available. Reducing the width of riverside streets, or removing them entirely in places, will need to be considered not only as a means of increasing the area for tree planting but as part of managed retreat from sea level rise and future flooding.

Planting on the Port Hills Also not mentioned, other than in passing reference, is the opportunity and benefits of planting trees on the Port Hills.  Sediment reduction would be the greatest long-term benefit for the Ōpāwaho Heathcote River, but it would come with the downside of increased risk of fire.  However, having witnessed the destructive power of rushing silt-laden water in the North Island recently, perhaps plains dwellers would prefer a slightly raised risk of fire in the distant future to a much higher risk of flooding in the near term.

Plan is already making change There is recent evidence that, even before it is adopted, the Urban Forest Plan is causing changes to decision-making within the City Council. Sell or keep decisions relating to the sale of road reserve lands, which previously were guided largely by transport requirements, must now factor in opportunities for extending the urban forest.  It may well be that the day of the Urban Forest Plan has arrived.  It is worth a read.

The Ōpāwaho Heathcote River Network has made a submission of the Urban Forest Plan highlighting areas where we believe it can be strengthened.  That also is worth a read.