The highest tree line on Earth is found in the Himalayan Mountains, specifically in the Tibetan Plateau. This is where the Juniper tree species, known as Juniperus tibetica, thrives at an altitude of around 5,100 meters (16,700 feet).

It's worth noting that "tree line" can be a bit tricky to define. It's not a sharp line, but rather a gradual transition zone where the conditions become too harsh for trees to grow. So, while the highest recorded tree is a Juniper at 5,100 meters, there might be other species surviving even higher up, albeit with lower density and smaller size.