From Monkey Island wiki
Rubber trees are a particularly bouncy variety of tree that make several appearances in the Monkey Island series.
In the first game Guybrush can fall off the highest cliff on Monkey Island (where the primitive catapult can be used). A message box will appear onscreen, telling the player that their game is over and asking if they'd like to start over. However, Guybrush will suddenly fly back up onto the ledge, exclaiming "Rubber tree!"
At the Field of Competition on Plunder Island there are a few rubber trees, including a unique Sumatran rubber tree that is a gift from Puerto Pollo's East Indian sister-city. Guybrush knocks a keg next to this tree and blows it up, turning it into a caber; with this bouncy pole he is able to best Haggis McMutton in the caber toss and win him over to his crew.
In "The Siege of Spinner Cay" Guybrush finds a rubber tree on Brillig Island which he can have McGillicutty's pirates dig up for him by telling them that he thinks they buried their treasure near its roots. When McGillicutty wrecks the Screaming Narwhal's mast, Guybrush can have Anemone take this tree for his mast. With a rubber mast, McGillicutty's cannonballs will bounce right off the Narwhal, sinking his own ship instead.