The specimen in the photo is actually a younger example of B. murrayana (there are also a few other young specimens in the same area); there is a much older one located elsewhere in the forest, located some distance away from the road. At the end of the day, it's good to know that the forest -- although it's changed over the years -- holds gems that will be useful for plant IDers in years to come.
Below is a photo of the older specimen of B. murrayana in Saginaw Forest. (Photo taken by Anton Reznicek) Note that it is much larger than the one in the photo above. The birch in this photo was the original one described by Barnes and Dancik in 1985. The photo of that tree (in winter) can be found here.
The original paper, "Characteristics and origin of a new birch species, Betula murrayana, from southeastern Michigan" was published in the Canadian Journal of Botany. The abstract for which can be found here, and briefly describes the species thusly:
A new taxon, Betula murrayana, is described from southeastern Michigan. This birch appears to be an oetoploid (2n = 112) derivative of an unreduced gamete of Purpus birch (B. × purpusii Schneid. = B. alleghaniensis Britt. × B. pumila L., 2n = 70) and a reduced gamete of yellow birch (B. alleghaniensis, 2n = 84). Betula murrayana has relatively uniform, good, large pollen grains and leaf stomata larger than its putative ancestors; this multistemmed plant has larger leaves and fruits than the hybrid.There aren't a lot of other papers written about B. murrayana that I could find, and the notation on the National Collection of Imperiled Plants website indicates that the species was initially described in Saginaw Forest, from two individuals, one of which died in the 1980s. Clippings were made and distributed to the Matthei Botanical Gardens in Ann Arbor, as well as the Holden Arboretum in Kirtland, OH (but the clipping at Holdren was infested by bronze birch borer, and may be extirpated from there).
Therefore, other than the remaining specimen, and the specimens made from the clippings of that specimen -- found on the edge of the lawn area -- the Murray Birch doesn't really appear to be described as being outside of Saginaw Forest.