“Apparent Available Soil” A Good Predictor of Tree Size

There’s a nice little study about tree mortality in the most recent issue of the Casey Trees newsletter, Leaflet. Dr. Jessica Sanders, Casey’s Director of Technical Services and Research, examined both tree mortality and the role of apparent available soil in tree size. Her findings were really interesting.

First, tree mortality. For this part of the study, Dr. Sanders looked at trees in New York City. She found that urban trees had a 91.4 percent survival rate after two years, and a 75.8 percent survival rate eight to nine years after initial planting. Unsurprisingly, the highest mortality rates were in downtown areas, while rates dropped in more residential areas.

The second study, to examine the role available soil plays in tree size, looked at parking lot trees 20 years after planting. From Dr. Sanders:

To determine if size reduction occurred because of the amount of apparent available soil, I observed specimens in tree pits, planting strips and trees on the edge of the lot.
The findings suggested that increases in apparent available soil provided by the design of the planting space resulted in larger canopy trees. The largest trees were found on the edge of the lot for all species, showing a correlation between access to soil and canopy size. There was a reduction in canopy area seen across all five species measured as apparent available soil decreased. Specifically, there is an extreme reduction in canopy growth when there is less than 20 square meters of soil. There is a dramatic increase in canopy size when trees are planted in linear strips of at least 40 square meters as opposed to planting pits measuring six square meters.

Overall, this study suggests that better planting design will help accomplish the goal of successful tree establishment. By providing a wider soil zone around trees, we can increase canopy coverage.

[Emphasis is added].

Finally, Dr. Sanders looked at the maximum size diameter at breast height (DBH) of trees in municipal inventories. She found that maximum size is closely related to terminal (trunk) diameter based on the type of the planting site. Again, in her words:

Maximum terminal size was different in all three planting site types (tree pit, planting strip, and nonlimited soil), irrespective of size class. Overall a reduced planting space resulted in a reduced maximum size.

If you look at the chart at the top of this post, you’ll see that the difference in maximum terminal size between the three planting conditions is significant. For example, looking just at the Acer rubrum trees from the study, after 20 years trees in a typical pit had an average terminal size of 11.9, trees in a planting strip had an average terminal size of 31.0, and trees in unlimited soil volume had an average terminal size of 32.3. These are very real differences.

Dr. Sanders suggests that we use more “proven performers” in city plantings — in other words, species that are more resilient in harsh conditions — to help increase canopy in urban areas. And she’s right that these trees have their places. But we can’t rely on super species to compensate for a lack of suitable planting spaces. We can’t just plant the right tree in the right place. We also need to consider more solutions to make the place right for the tree.

Image: Casey Trees


  1. May I advise that you clarify what Green Infrastructure (GI) means here and how it is being applied. In some regions GI is simply “green infrastructure”- as opposed to gray infrastructure. In other regions like NYC, GI strictly means rain gardens and bioswales and other types of infrastructure fabrications that address Storm Water Overflow (SWO). And that the bioswales and the soils within are viewed as intended to address the flow and percolation rates than providing the optimum growing conditions and/or minimizing mortality of any long-lived trees installed therein.

    • Great point, because there is no universally accepted definitely of Green Infrastructure (GI). When I use that term on this blog, I’m referring to the definition put forth by the US E.P.A and the Conservation Fund:

      Green infrastructure is an approach to wet weather management that is cost-effective, sustainable, and environmentally friendly. As defined by the non-profit Conservation Fund, “Green infrastructure is strategically planned and managed networks of natural lands, working landscapes and other open spaces that conserve ecosystem values and functions and provide associated benefits to human populations. The foundation of green infrastructure networks are their natural elements – woodlands, wetlands, rivers, grasslands – that work together as a whole to sustain ecological values and functions. Healthy functioning natural or restored ecological systems are essential to ensure the availability of the network’s ecological services.”

      You’re right that some “green infrastructure” solutions are basically only intended to address the rate and volume of water, rather than truly supporting and sustaining vegetative growth. Those are not the examples of GI I’m referring to here. I think there’s a middle ground that acknowledges the importance of those things but that also creates an environment that is truly hospitable to green and growing things. My understanding is that water treatment calculations for GI tend to only address rate and volume. As we see more data about the impact that plants can have on water quality in addition to rate and volume, I hope we’ll see more and more solutions that consider both halves of the system.

        • I don’t interpret the EPA’s definition to mean that all the networks must be interconnected – for example, I believe a block-long rain garden would count. Clearly there is not an accepted industry-wide definition of this term, though!

  2. I agree Leda – just trying to ensure we don’t get caught up in the large scale, where green infrastructure has the federal laws to help it out. At smaller scales, such as at the parcel scale, green infra. exists there too, and delivers different benefits.


Leave a Reply to Dan Staley Cancel reply