Bad start for me this month.
Seems all my Google search has led to nothing, so here is my train of thoughts at least, for all that its worth:
The definition of "landscape Features is:
The land, water, vegetation, and structures that compose the characteristic landscape.
As the question says: once-numerous, now declining" I guess that points to
vegetation.
Based on that, singular, special
trees were a landscape feature that were used for orientation and surveying over the centuries, as this site states:
Landmark - A survey mark made on a 'permanent' feature of the land such as a tree, pile of stones, etc.
http://foxsurveying.com/services/surveying-terms
Now going with trees, the connection to Civil War sites might be that there still are trees that already stood there during the Civil War. Once numerous, now declining in numbers. And there is a popular name for these trees...
So my answer is "
Witness Trees".
https://www.urbandictionary.com/define.php?term=Witness Tree&=true
This was meant as the weak attempt to save my reputation as serious Trivia player 

My respect to all members who have found out about the right answer!
Edit - Your reputation is safe for at least another day.
hoosier