Tap trees were plants that grew on Sulon. They grew long roots that siphoned large amounts of water up from aquifers deep below the soil, more than they, or their symbiotes, can use.
The Sulonese farmers made use of this trees' property to bring water up to the surface for their crops, more or less like natural "pumps". For this to function, they had built an irrigation system of pipes and tubes distributing moisture around a farm, accessed by tunnels and maintained by droids.
Like his neighbors, Morgan Katarn made use of tap trees along with the underground network under his farm, with access doors to the tunnels at several points. The tree in the front of the Katarn homestead was over a thousand years old.