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What type of assessment do you want? What is the difference between a top-down and bottom-up assessment?

Example of bottom-up field data collection. Credit: Jennifer Wig

Example of bottom-up data collection. Credit: Jennifer Wig

There are two basic ways of assessing an urban forest. The top-down approach involves assessing aerial or satellite images of canopy and other land cover, while the bottom-up approach involves collecting field data on the composition and physical structure of the trees.

A top-down approach (as depicted in the image above) is relatively easy to conduct, is comprehensive across a landscape and is quite accurate, but it is limited in the structural information that is obtained and can be costly depending on what tools are used. Top-down approaches can produce a good estimation for tree cover and planting potential by using high-resolution imagery and detailed maps to zoom-in on specific tree locations. However, the top-down approach is limited to only assessing quantity and distribution, whereas tree species and height is difficult to determine using this approach.

A bottom-up approach involves collecting field data and may provide more detailed information needed to assess urban forest structure and ecosystem services. Species composition, number of trees, tree locations, tree sizes and tree health can all be determined using the bottom-up approach. This is also a good method for assessing and projecting ecosystem services and values, such as carbon storage and air pollution removal. However, there are limits to this approach as well, including a potential limited geographical area of data since it takes more time and resources to assess a large area.

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Section II: How to Choose an Assessment Tool                    Section III: Urban Forest Assessment ToolsNext Arrow