Fig. 4: Spatial variation in the % forest stability index (%FSI) of eight most abundant species across their ranges in the western US over the period 2001–2018.
From: Over half of western United States' most abundant tree species in decline
Mean %FSI values are mapped at two spatial scales: ecoregion subsection a and ecoregion division b. Boundaries of the study region are outlined in black, and white areas within the study area indicate absence of a species in the FIA plot network (i.e., colored regions represent species' ranges). For reference, ecoregion division boundaries (as seen in Fig. 1) are outlined as gray dotted lines, and maps of the %FSI within ecoregion divisions (bottom) have been clipped to the extent of each species' range (i.e., defined by ecoregion subsections where the species was detected on an FIA plot). Tree populations have been observed to expand in areas characterized by positive FSI estimates (blue), and decline in areas characterized by negative %FSI estimates (red) during the inventory period (approximately two decades). For reference, %FSI values −2% indicate a 36% decline in relative density over the duration of the study period. Source data are provided as a Source Data file.
