Metadata: 
https://www.fs.fed.us/nrs/atlas/littlefia/species_table.html#

All GIS files are zipped. If you don't have an unzip utility on your computer, you can download freely available utilities (zip, unzip and wiz) from http://www.info-zip.org/pub/infozip/.

The Little's range maps (little.shp) are vector based and are provided as shape files. These are compatible with ArcView, ArcGIS and ArcInfo software from ESRI. These maps can span "U.S." or "U.S. and Canada" in extent depending on the species. The shape files can be imported into ArcInfo using the command "shapearc".

The Importance Value (IV), based on Forest Inventory Analysis (FIA) data, are raster maps (asciigrid) in asciigrid format. This is an ascii file with header information that can be used to import data into ArcInfo GRID or ArcView's Spatial Analyst GIS software. The spatial resolution is 20km. These raster maps span the eastern U.S. (east of the 100th meridian) in extent. The command used to import the file to ArcInfo grid is "asciigrid".

Header Information:
no. of columns: 150
no. of rows: 140
LowerLeft X-coordinate (m): -1793092
LowerLeft Y-coordinate (m): -1650894
cellsize (m): 20000
NODATA_value: -9999

All maps (Little's and IV) are in Albers equal area projection as follows (this would be useful only to ArcInfo users):

Projection: ALBERS
Units: METERS
Spheroid: CLARKE1866
Parameters:
1st standard parallel: 38 0 0.000
2nd standard parallel: 42 0 0.000
central meridian: -82 0 0.000
latitude of projection's origin: 40 0 0.000
false easting (meters): 0.00000
false northing (meters): 0.00000

The Little's range maps were digitized by our GIS lab or the U.S. Geological Survey. We have in some cases edited the USGS digitized maps to correct errors.
You can find out if a species has been originally digitized by the USGS by bringing the mouse over the species number (1st column in the species table). A popup box will appear if it is USGS digitized map; no popup box appears if we have digitized the map.

All Little's maps have a field called "CODE", whose value and meaning are as follows:

0 = water bodies
1 = species present polygon
2 = "species absent" island within a "species present" polygon

The IV raster maps based on FIA data have values ranging from 0 to 100. The IV is based on the basal area and the number of stems of both the understory and the overstory and are derived from plot-level FIA data and aggregated to 20 km cells.

We generated importance values (IV) for each species as follows:
IV(x) = 50*BA(x) / BA(all species) + 50*NS(x) / NS(all species)
where:

x is a particular species on a plot
BA is basal area
NS is number of stems (summed for overstory and understory trees).
In monotypic stands, the IV would reach the maximum of 100. The IVs were rounded to decimal numbers with one exception. If the IV was greater than zero but less than one, it was assigned to one. This decision was made since rounding would have falsely turned species-present counties to species-absent counties.