Here's the set up: in my fantasy world, there is a species of tree similar to Banyan trees. There are several giant versions of these, and smaller ones are quite common.
One of the biggest of these trees serves as a city for elves. There is none of the 'tree-growing' that you commonly see in fantasy novels (no growing houses out of trees), because these elves do not use magic. Fortunately for them, the vertical roots of the Banyan Tree split about ten to eight feet off the ground, forming very handy housing supports. It's basically a Banyan, with Mangrove trunks for its vertical roots.
Here's my question: Since magic isn't a factor and this world is more or less earth-like, how can I create such a plant so that it is plausible? As I understand it, the unique root structures of Mangroves are formed due to low-nutrient and harsh conditions. These don't apply to the cities, which are in largely forested areas. How can I create a need for these roots to develop?
Note: Magic is present. It's just not used in this case. Due to that, and the fact that this in reality a different world, a small amount of theory is permitted.