In an alternate Earth, early or midway in the Eocene Epoch, there debuted a family of angiosperm trees whose roots need to be completely submerged. As a result, the limit is that they can't germinate in waters that dry up during low tides, nor can they at depths deeper than 250 feet. Such trees with roots that need to be submerged 24/7 could have easier, more efficient water transportation, allowing them to grow taller even than the redwoods. The tallest tree on this alternate Earth, the ash-like yggdrasil, averages 150 meters tall and 12 wide, with crowns averaging 180 feet wide and branches 13 feet wide.

To compensate for this large size, the yggdrasil's roots extend as far out as 400 feet. As mangroves, they are expected to be supported by "aerial roots"--that is, roots that grow above the soil. There are many different kinds of aerial roots--stilt, knee, pencil, cone, buttress and spreading. Of those shapes, which ones would be best at supporting a megamangrove tree, particularly one as large as an yggdrasil?

  • $\begingroup$ Unrelated to question, but how would submerged roots allow higher crown height? I thought that limit had to do with lifting a column of water with vacuum. $\endgroup$
    – Willk
    May 27, 2020 at 16:36
  • $\begingroup$ " averages 150 meters tall and 12 wide, with crowns averaging 180 feet wide and branches 13 feet wide." can you pick a single unit system? $\endgroup$
    – L.Dutch
    May 27, 2020 at 16:39
  • $\begingroup$ @Willk Redwoods don't have 24/7 access to water. $\endgroup$ May 27, 2020 at 20:17

1 Answer 1


Prop roots, banyan style.

prop roots


The Banyan Tree is a species of fig tree that grows aerial prop roots; thick supporting trunks. Prop roots allow the Banyan Tree to provide additional anchorage and support. The circling of prop roots allows these enormous trees to not only grow taller but also wider. A large Banyan Tree can be over 100 feet( 30 meters) wide.

Your megamangroves take a page from the fig playbook and use prop roots, dropping vertically down from large branches. Rather than a tall redwood, the resulting tree is a hemisphere with large branches in all directions, each branch buttressed by many vertical columns. Large branches can produce their own accessory trunks which is a trick the mangroves do not have.

Fig trees develop prop roots by dangling rootlets down from branches until they touch something they can grab. This lends itself to deep water at some distance from the shore as sustained by its branch a (weighted!) prop root can continually descend through many meters of water until it reaches the bottom.

This mode of spread out into the water allows figlike mangroves to even more aggressively make land than current mangroves do. Bays and inlets will quickly be converted to trees.


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