I maintain that 18" to 36" of vertical space cannot recreate a delineation between terrestrial and arboreal habitats.
Assuming the designer of a vivarium has made the upper reaches of the enclosure accessible through hardscaping (no frog can take advantage airspace), then any animal currently in the dentrobatid hobby is capable of using vertically stacked space. I have 50+mm Robertus that have scaled a 30" tall glass wall to catch a fly and they as well as my Azureus are adept at climbing cork bark and finding roosts in the upper portions of their vivs. Given adequate ledges and back / side wall hardscapes, larger animals will absolutely use all available space in a taller vivarium.
As additional discussion points, I often refer to these two snippets. First, from the document describing the discovery of D. tinctorius Azureus:
http://www.sipaliwinisavanna.com/docs/discovery_of_the_blue_arrow_poison_frog.pdf wrote:After about 20 minutes I saw my first blue Dendrobates in the wild. It was sitting on the forest floor on fallen leaves, and when I moved nearer it hopped away with short, quick movements. Its bright blue colour contrasted beautifully with the brown dead leaves. It was easy to capture. During my trip through the creek valley I saw many more and collected some of them, restraining myself of capturing more than a few because I had no idea about the extent of the forest island and the size of the population of the blue Dendrobates. Also, at the time, I had no idea whether this species would occur in other forest islands in the region or not. Most specimens were on the ground, but a few were moving up the trunks of large trees to…..where?
(Born, M., 1994, MS dissertation) wrote:Dendrobates tinctorius is mainly terrestrial, although we observed frogs climbing tree trunks to heights of 40 m.
Scaling two to three feet off the ground is still well within an expected range for any "terrestrial" animal. I suppose my question is, why do we think we can distinguish between a terrestrial and an arboreal habitat within the confines of a 24" tall glass box? Any dendrobatid, terrestrial or arboreal can make use of an 18" to 36" tall enclosure.
In terms of housing discussions, as with the argument that we should size in terms of square inches per frog and not gallons, the vertical versus horizontal discussion should be framed the same way - usable square inches per frog. In my opinion it does not matter if those square inches are tiered vertically, or if they are one continuous horizontal space. In fact, vertical tiering can help with increasing visual barriers, providing more hides.