White Trees are a very rare species of tree in Gielinor. They have been driven to near extinction due to the desirability of its pure white wood.[1]
During the Garden of Tranquillityquest, players cut a shoot from the tree with secateurs, effectively killing it. However, they then plant it in the garden outside Varrock Palace, where it grows into a new, fruit-bearing tree. This action saves the tree from extinction.
^The planting experience is awarded upon checking the health or fully harvesting the patch, or upon clearing the patch if it died before becoming fully grown.
After the quest Garden of Tranquillity, players are able to pick up to four fruit from the white tree, which restores 5-10% energy. More fruit will appear on the tree after some time, like fruit trees grown in the Farming skill.
Players cannot grow a white tree of their own; attempting to cut off another shoot a message comes up saying "You've finished the quest-Leave it alone!" However, it is possible to obtain another shoot by interacting with the tree on Ice Mountain after planting a shoot in the garden but not before it finishes growing.
References
^Dantaera. Old School RuneScape. "It is the White Tree, last survivor of a race of Trees that have disappeared from our landscape. Once they covered the land; now there is only one. [What happened to them?] Man desired the wood of the White Tree, because of its wood of pure white. Need I tell you more?"