On official USGS topo maps, vegetation is represented by green, and non vegetation by white as you can see below.
How much vegetation does an area need to be colored green? Are short bushes enough or does it require trees?
Vegetation is recorded when it exceeds 20% cover of the landscape for an area greater than one acre (0.4 ha), and 6 ft (2 m) or more in height - tall enough to conceal troops or fugitives - with the same consideration for clearings within vegetated areas.
The mesomorphic tree canopy is typically >10% cover and often exceeds 5 m in height
(I was not able to find a good reference for a definition to "Shrubland" for the spotted irregular green to get a second notion of how green is used.)
In the map you have given us, I assume that the vast majority of the ecosystems in the green fall under the forest definition. Since your map doesn't have a key, I have to assume they are referencing the solid green as a standard.