**2. Methodology and analysis**

#### **2.1 The review of upper level ecoregions of the United States**

The ecosystem can be a complex system more than we thought, which is changed and varied along with longitude, latitude, and elevation on the earth's surface, and constantly adapted to the slope, aspect, environmental variables in macroscales [1, 2, 7, 9, 15, 17, 24]. Bailey had contributed to the ecological classification framework and application, which represented his scientific collections of mapping on ecosystem classification of the United States (**Figure 1A**).

Theoretically, Bailey's Ecosystem Classification of Land had explained the ecoregions and their nested structures in the upper levels of Domain, Division, and province. However, these advantages had not been fully applied and examined as ECL's bases for Terrestrial Ecozones and Ecoregions of Canada in [31, 36–39], even though technically Bailey's ECL polygons in the upper three levels can be easily retrieved in GIS spatial model in [14] when the ECL project was conducted.

#### **Figure 1.**

*(A) Upper level ecoregions of the United State. https://www.environment.fhwa.dot.gov/env\_topics/ecosystems/ veg\_mgmt\_rpt/images/vegmgmt\_ecoregional\_approach\_fig 03.png (more detail, refer to the web link). (B) Terrestrial ecozones and ecoregions of Canada. Data source: Environment Canada, Terrestrial Ecozones and Ecoregions of Canada 1995. https://mspace.lib.umanitoba.ca/bitstream/handle/1993/24087/cad\_map. jpg?sequence=1&isAllowed=y (more detail, refer to the web link).*
