The alvera tree was a woody plant, native to Kreetassa. It was considered by the Kreetassans to be a cultural treasure. A stand of alvera trees outside the planet's Hall of Diplomacy was, by 2152, over 300 years old.
When in 2152 Enterprise NX-01 visited Kreetassa to negotiate for a new plasma injector, Captain Archer's pet beagle Porthos urinated on one of the 300-year-old alvera trees outside the Hall of Diplomacy. The Kreetassans were insulted, and demanded Archer and his crew return to their ship.
Archer was ultimately required to perform a Kreetassan reconciliation ritual in order to apologize for defacing the tree. Ironically, the ritual itself involved the cutting of Alvera trees into pieces with a chainsaw. (ENT: "A Night in Sickbay")