How can I get a giant banyan tree with mangrove roots?

Question

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.

Answer 1

Your elves live in a forest comprised of two kinds of trees, not one.

Because both the stilt-like roots of banyans and the arched support roots of mangroves serve similar functions, it's unlikely that a single tree would evolve both adaptations. However, it's entirely plausible that a common epiphyte found in the mature mangrove forests where your elves live could have horizontal branches with vertical support roots reaching down to the ground.

For mangrove roots to develop, you effectively need to be not just in a nutrient-low environment, but in an environment where the ground is persistently waterlogged, such as a swamp or estuary. Aerial prop roots in these environments give the root system of the plant the ability to take in oxygen directly from the air in environments where underground conditions are anoxic. The arcing structure of the roots also provides the trees with additional support to resist the force of tides, waves and storms.

The mangroves that your elves call home are also home to a species of fig, similar to a banyan. This fig is an epiphyte, sprouting high in the branches of mature, established mangroves. Using the mangroves for support, the figs don't need the same sort of arcing roots to anchor themselves against waves and storms, instead growing horizontal branches to gather as much sunlight as possible, and then dropping vertical roots down from the canopy to gather water and oxygen.

Eventually, the figs grow to form massive horizontal branches, supported by thick vertical trunks and fed by thousands of smaller vertical branches. These bulky trees can withstand tidal action, but due to their more unyielding nature compared to the mangroves they grown between, aren't as resistant to heavy wind storms. They grow deep within the mangrove forests, where they're protected from the worst of the tropical storms by the trees around them.

The elves primarily build their cities anchored around the banyans, in the most protected part of the forests. Unlike the smaller mangroves, the banyans are large and sturdy enough to serve as effective foundations for buildings. Canals are cut through the roots of the mangrove forests, allowing the elves to transport people and cargo between different banyans.

Answer 2

Does the tree have to have grown in that state? I would recommend you look into grafting trees. Grafting trees is essentially cutting some top sections of one tree and attaching (through various means) a different species on top of it. Both of the tissues are from distinct trees, but to a casual observer, it now appears you have a single tree that is scientifically impossible. It would be theoretically possible without magic and some great effort to get the result you require.