Better urban forest planning - Perth and Peel

The urban tree canopy data is available on the Department of Planning, Lands and Heritage website. State and local government agencies can access related data shape files through Landgate’s SLIP system (DataWA).

Local governments can use the data to track and plan their tree canopies over time.

The CSIRO sources the data for the Western Australian Planning Commission from high resolution (0.2 metre cell size) aerial digital images of the Perth and Peel regions. It is collated through the CSIRO's Urban Monitor program before being transferred to Geographic Information System (GIS) data files, for analysis by the Department of Planning, Lands and Heritage.

The aerial images are captured in late January each year. The tree canopy data is captured, processed and updated every second year.

Most of the tree canopy data falls under three broad land use categories that include specific land use zonings:

• ‘Street blocks’ – includes a combination of lots zoned for residential, commercial, industrial, hospital/medical, educational, and some agricultural and transport uses
• ‘Roads’ – includes the reserved verges of roads and streets managed by local or state governments
• ‘Parks’ – includes public parks, open space, private recreation/sporting grounds and State forest.

Some land use categories are too complex to include in the dashboard but can be found in the tree canopy metadata.

Data percentages are rounded to the nearest whole number. The data error margin is believed to be very minimal and most likely due to expected variances between technologies and methods of capture.

Large complex data sets can have data gaps or occasional anomalies due to human error or technical glitches, such as small areas not being captured properly in aerial imagery. Measures are in place to eliminate or identify errors in the tree canopy data. Any detected anomalies in the published data can be reported to SpatialAnalyst@dplh.wa.gov.au

Tree canopy in parks and on road reserves is less likely to be affected by development. It is also easily improved in areas with underground power and tree planting programs in place.

Trees on street blocks (lots) may be affected depending on the design of a development and approval requirements. Under the State Government’s DesignWA policy, future precincts are required to have more detailed considerations for trees on public land, including deep soil areas to support new tree canopy in medium and high density development.

Canopy growth rates can differ within the land use categories. For example, data can change rapidly when trees on road verges are trimmed, while land owner decisions can affect trees on street blocks.

Among the land use categories, canopy growth rates are found to be highest in parks and streets with underground power; and local governments with active tree planting regimes show up in data trends over time.

Tree canopy changes constantly. Factors that influence canopy loss and gain may include particular land use activities occurring at the point of data capture. For example, subdivision, street tree trimming, climate conditions (high rainfall years can affect canopy growth), fires in bush reserves, development policy settings or implementation of a tree planning program.

Tree canopy can also increase even where trees have been lost because the retained trees will continue to grow.