"Trees were so rare in that country, and they had to make such a hard fight to grow, that we used to feel anxious about them, and visit them as if they were persons. It must have been the scarcity of detail in that tawny landscape that made detail so precious."Willa Cather in My Antonia
Photo by Peter Kohn
L'Arbre du Ténéré (or Tree of Ténéré) - the most isolated tree on the planet (before it was unceremoniously mowed down by a drunk driver in '73).
Intriguing: Does extreme rarity confer mythic status?
Also interesting if you have time to kill: Wikipedia's Famous Trees Page.