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I have a clonal colony of trees that are around 150 meters in height and live in a wetland habitat. These trees cover the majority of the wetland area, covering thousands of square miles.

What reasons would the trees have for growing a "wall" of aerial roots (like cypress knees) only at the edge of their range? This would effectively block the wetland area off from the rest of the area except for breaks in the barricade allowing fast moving water (large rivers) through or breaks caused by wildlife.

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this would effectively block the wetland area off from the rest of the area

this is your reason.

Wind carries, among other things, nutrients and dust. While nutrients are vital for trees, wetlands can sometime be peculiar environments from the nutritional standpoint, and a plant adapted to grow 150 meter in such environment is subject to a really delicate equilibrium which can be easily disrupted. For a real life example, think of the carnivore plants being able to grow on Nitrogen poor lands, and how they die if provided with fertilizer. Also sand can be a threat in the long term.

Therefore the outer "wall" takes care that the winds are slowed down or even halted at the border of the wetland, depositing there their load.

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It is one tree. The curtain of aerial roots are how it increases its area.

Fig trees (genus Ficus) drop aerial roots down from the ends of their branches once they get big enough. In some species these roots can form an impenetrable curtain that extends up to the branches. Depicted - the Curtain Fig in Queensland.
https://en.wikipedia.org/wiki/Curtain_Fig_Tree enter image description here

Your tree has aspects of the strangler fig with its curtains of roots and aspects of the banyan, a different fig. Banyan trees are very long lived and grow to become immense. Over time the curtains of roots condense into trunklike "prop roots" in the tree interior.

giant banyan https://www.ebay.com/itm/GIANT-BANYAN-TREE-in-Tropical-Florida-POSTCARD-Unused-Vintage-Free-Ship-/182798796629

If you can make it through the perimeter curtain of thin young roots expanding the domain of this tree, you might think you are in the shade of a forest. Actually, you are under the tree. Every trunk you see is part of the same tree. The Great Banyan is 4.6 acres - 2 city blocks. It does not take much imagination to scale that up.

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Your trees have been selected by evolution such that their aerial roots anchor themselves in the ground, but would rot and decay if the ground has a high content of water. Borrowing from L.Dutch's intuition about the wind, the actual problem is not bringing the nutrients, but rather avoiding excessive evaporation due to the continuous wind.

The process of root rotting is quite simple, and already present in our everyday plants. In fact, in many plants, "excess water makes it very difficult for the roots to get the air that they need, causing them to decay. To avoid root rot, it is best to only water plants when the soil becomes dry[..]" [1]

In this way, not only your trees get their shield at the edge of the wetland, where the soil becomes drier, but also continue to expand their dominion as the wetland increases in area. Trees whose trunk is not however deep in water will dry out, whither, and die, so that the wall will shrink if the wetland retreats.

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