It is always intriguing to find the outlier in a plant family. As noted in the Angiosperm Phylogeny Group summary, the Aristolochiaceae, or birthwort family, “…are more or less herbaceous plants (they are quite often vines)”. The epithet arborea, though, reveals the habit of this species: a (small) tree.
I imagine if I had first seen this in person, my reaction would have been similar to Matt’s initial bewilderment, as documented on the In Defense of Plants weblog. No matter how much one thinks one might know, there is always room to learn more.
There are 500 or so species of Aristolochia, with most species either found in the tropics or subtropics. Only a few are shrub or tree-like. Aristolochia arborea is native to moist mountainsides of El Salvador, Guatemala, and adjacent Mexico.
In Revision of the North and Central American Hexandrous Species of Aristolochia (Aristolochiaceae), the author notes that a collector (Broadway) had described the flowers as “[resembling] bats clinging to the plant on old wood”. Others (including Matt above) have been struck by the appearance as being similar to mushrooms. This resemblance is also present in the floral scent (like fungi) and where the flowers are borne on the tree (near the ground). Apparently, the mimicking even occurs at the microscopic level. The Botanischer Garten München-Nymphenburg reveals why (deceiving pollinators) in their audioguide tour: Aristolochia arborea.
Botany / photography resource link: the fungi photographs of Bernd Rügemer. Spectacular, and brought to our attention by one of the UBC Botanical Garden fungi forum participants, allelopath, in this thread.