What are some of the models for calculating soil volume and how do they differ?
A. There are several available soil volume models (listed below). As an example, let’s look at a 24″ dbh Zelkova serrata with a crown spread of 50’. Using the Lindsey & Bassuk model, this tree would require about 2200 cu. ft. in a typical soil in Blacksburg, Virginia. The Urban et al. model would predict about 1500 cu. ft. would be required. The Natural Forest method based on upland hardwoods in the Eastern U.S., would predict that between 2700 and 4100 cu. ft. would be required, depending on soil quality. The rule of thumb mentioned above (each inch of dbh requires about 20-25 ft2 of open ground with uncompacted soil) would estimate 960 to 1200 cu. ft. To some extent, results will depend upon the species, as some species, such as Zelkova, are better able to exploit soil resources under pavement or in compacted areas. The larger soil volumes will support a healthier, more vigorous tree, but these volumes may be unattainable in restricted downtown areas.
