A mutation in a certain number of sky balloons made it so they continued to grow much larger than the average sky balloon. This resulted in the balloons falling closer and closer back to the surface and making it harder for them to pull nutrients out of the higher-atmosphere environment they had adapted for. This resulted in the dormant root system to return, allowing sky trees to gather nutrients from the water and aid in the production of more hydrogen within the balloon, letting them return to the skies until they became too weighed down with water or their "bodies" became too heavy. The original single circular pedal has split into multiple layers of individual sturdy leaves that aid in photosynthesis but aren't as good at collecting water, not that it matters with the re-emerged root system. Sky trees also naturally "feel" moisture in the air through their roots, and when the humidity is high enough, they lower themselves automatically into the water. This doesn't always work,however. In cases such as very heavy fog, a sky tree may attempt to gather water but actually lower itself onto land, and without water or nutrients to get, and no way to produce more hydrogen, it will slowly die.