I agree that the course setter should avoid using any objects buried in medium or dark green areas. Sometimes, however, the feature, as mapped, overlaps the edge of a green area, and would be obscured if we didn't mask out the green (and sometimes yellow) areas.
The most obvious example is a giant lone pine in an area of deciduous shrubs. It can be seen from some distance away, and might be on the edge of a thicket.
Here in Illinois, we suffer from a dearth of navigable objects in our woods, and an abundance of undergrowth. For many years, our mappers would include all distinct rootstocks (brown Xs at the time). Of course, after just a few years, many of the old rootstocks would have rotted away, and new trees might have fallen, leading to the situation where course setters and participants learned to ignore the rootstocks, which defeats the purpose of mapping them.
Starting about 10 years ago, we set the standard that rootstocks would only be mapped if there were roots raised at least 2 m off the ground. These largest of rootstocks could be expected to last for many more years, and would be less often mistaken for your run-of-the-mill fallen tree.
Also about 10 years ago, we noticed that many of the fallen trees and rootstocks included very distinct trunks and canopies that could be mapped so that the competitors could use their orientation and length for navigation. Controls could be placed to make it more likely that the feature would be seen before the control flag, which is usually more fun.
Of course, ISOM doesn't include distinct trunks for either fallen trees or rootstocks. So, we had to make our own symbol.
Here is the results, from OOM 0.9.0, which also includes the white mask under the distinct tree and fallen tree symbols. I modified the symbols to include the stem, and the underlying white mask for the roostock symbol.https://www.chicago-orienteering.org/mapping/OOM_s...
As you can see, the white mask covers contour lines, form lines, and both green and yellow areas. It also covers paved areas and undergrowth symbols, but this is not shown in the example. Notice, too, that the distinct bush symbol has been updated to include a tiny white dot in the center to make it stand out a little more.
If anyone would like a copy of the symbol definitions, please let me know. The color scheme is a little more complicated than normal, as there are now 2 Browns ("Brown 100%" for contour lines and "Top Brown" for the rootstock over the white mask) and 2 Greens (the regular green between yellow and blue, and a "Green 100%" above the white mask above the regular colors).
I'd love to see the trunk option added to both the green X and brown triangle symbol in the ISOM, but I don't want to deal with the politics of the mapping committee. ;)