It'll die, the leaves would turn into a slushie. The trunk would keep it's appearance underwater (logs are sometimes kept under water for processing.) And your whole aquarium would turn green from algae and mold pretty quick and you can't look inside anymore.
Some quick diagrams
Source: https://www.evolvingsciences.com/Transpiration%20Stream.html
Source: https://opentextbc.ca/biology/chapter/5-1-overview-of-photosynthesis/
Second one is a little simpler, but the source link is to a site with open access educational materials. Pretty cool
What would happen then if you took a tree which only has leaves at the top and submerged it to just under the leaves? Does the bark need to be exposed to air as well or would it just rot out like timber does?
It would die. The roots need to do gas exchange too. Ideal soil composition is generally 45% mineral, ~25% water, ~25% air, and 5% organic matter.
Tree bark also does gas exchange.
Depends on the tree and type of water I assume
https://youtube.com/watch?v=twjPo2Luk5A
Mangrove trees live submerged in saltwater for example. This video also goes over some of the adaptations it has to do that (ex. Special roots to allow it to stand upright in soft unstable ground)
This is a mangrove, right?
Also aren’t there those alligator forests in America with lots of flooded trees.
Bayou is the word but those trees are usually only covered the first few feet by water unless it's flooding.
Alligator Forest is the new word, calling it.
I mean everyone knew exactly what he was talking about... and clearly not everyone knows what a bayou is, making "alligator forrest" the more universal term
So I know it’s not a tree, but how does algae survive underwater?
Basically the same way, but instead of absorbing gasses through the air, it's through the water. They don't have stomata (air holes) that are found on plant leaves that exist on land and I believe they can absorb carbon directly through their cell walls.
Algae can get by with dissolved gases in the water