Is this sort of life cycle on a megamangrove feasible?

Question

Point of departure--56 million years ago, in which the Paleocene-Eocene Thermal Maximum lasted three to four times longer than it did in our timeline. Today, on this alternate Earth, the twilight zone of the tropical and temperate seas (200-1000 meters below the surface), a genus of trees has evolved to germinate in the dim light of the twilight zone. They grow in three different stages:

1. An infancy in which they develop red leaves in which they can photosynthesize on blue light.
2. Once they've grown tall enough, they shed their red leaves and grow dark green leaves to photosynthesize on the brighter but still dim sunlight.
3. Upon maturity, they replace dark leaves with light leaves.

Each tree has the following parameters:

1. An average height of 492 feet above the water surface
2. An average diameter of 38 feet
3. Canopy and prop roots supporting the tree at an average area of 3.5 square miles.
4. A single prop root has an average width of 13 feet.

Could such a tree with such a lifestyle be feasible?

Answer 1

492 meters is too tall.

488.5 meters is also too tall. 120 meters is pretty much the max. Trees are limited by the negative pressures that occur in the xylem, which are the tubes that conduct water to the top. The water must in essence be sucked to the top by negative pressures generated by transpirations. If the negative pressure is too high, the column of water breaks (cavitation) and suction is lost.

The Limit to Tree Height

According to the cohesion-tension theory, water transport in plants occurs along a gradient of negative pressure (tension) in the dead, tube-like cells of the xylem, with transpiration, water adhesion to cell walls, and surface tension providing the forces necessary to lift water against gravity . Height growth may slow if the xylem tension and therefore leaf water potential (W) predicted for great heights, &22 MPa , reduces sufficiently the positive pressure (turgor) necessary for expansion of living cells or increases the risk of xylem cavitation—cavitation is the formation of embolisms that reduce hydraulic conductivity and can cause branch dieback and plant death...

471¾ meter tall trees is a no go with the existing tree setup. But the idea of mangroves muscling in on kelp and forming an underwater forest is very sweet. As far as I know only grasses among the vascular plants can pull off being completely underwater all the time. I am not sure why there are not more underwater vascular plants. Maybe the vascular mechanisms do not work right if there is not air to evaporate the water into and pull it up. Could you "evaporate" freshwater into seawater? Hmmm...

Answer 2

Cogongrass

As AlexP has pointed out in the opening comment, the different leaf colors are quite common. When this hot circumstance will remain for millions of years, your plants will adjust their chemistry to climate and light conditions.

The length is indeed enormous ! but suppose you'd call it a leaf rather than a tree ? If you take grass species with a very sturdy cylindrical stem, like Imperata cylindrica you got a stem that is 2cm wide, and a leaf can get to 3m in size. That is 1/5 of the thickness/length ratio you present (38/3)/492. (note: the question was edited meters to feet, so we have a factor 15 times !)

So this could be some grass-like species ? Scaled up a bit..

Must be a beautiful sight to see its giant flowers, waving in the wind, 492 meters up

Note: square cube law will make it quite heavy.

As a grass, the stem would be relatively thick.. as pointed out, your stem is 5x (no, 15x) thicker compared to Cogongrass. but I wonder if that factor in extra sturdyness would compensate for the square cube law in action. When your plant is 13 meters thick and 492 high, it would have a giant mass and catch a lot of wind. The roots it would need..