potatoguy

From what I searched, cellulose from kelp is different from cellulose from trees, from a properties perspective, so it seems it only acts this way on land is because it's on land. Maybe the need for a stronger material built trees for them to be the way they are, not having a need for a strong trunk can make an inpact on the evolutionary path of those beings.

It could be that the environment primed trees to need this structure, while ocean beings never experienced the evolutionary force to build some structures in the same way.

Idk, I'm not a biologist.

Edit: So the chemical and physical environment not encouraging the evolutions teps needed for trees to form in the ocean.

The chemical structures that make a tree a tree would be very different, lignin and cellulose will act different on alkalyne environments, pulping for paper uses very alkalyne (like pH 13) solutions to change the chemical bonds that make the wood "hard", turning them into loose fibers. Also water infiltrate cellulose, so osmotic pressure, ion pressure, etc, would change how the tree structure would form.

Water pH (water in the ocean is alkalyne, different from the varied soils we can have), water dynamics and currents (whater moves, soils do not), temperature delta (the deeper it goes, there are different temperatures), source of nutrients (carbon cycle on the different soils we have vs the ocean nutrient cycles), sunlight availability (sunlight might change based on the particulates present on the ocean, in different seasons, etc, in a higher variation than above soil), etc. Ocean dynamics is very different than continental dynamics (or island dynamics).