James,

The two parks are adjacent, but Lake St. Mary is completely within Glacier.  Here is photo I took of the same island from about the same place:

The National Park website talks quite a bit about the forests, but I am not sure about what species are what.  http://www.nps.gov/glac/naturescience/treesandshrubs.htm  http://www.nps.gov/glac/naturescience/forests.htm  There are some really nice distressed trees - Krummholtz from above the pass at the Visitors center on the "Going to the Sun Highway":

This is a common form for whitebark pine at the tree line, but as I said I am not sure about the species present.  I did not visit Waterton on my last trip.

Ed

Duck Island, St. Mary's Lake

Behind Visitors Center at Going to the Sun Pass