We use Swedish tree climbing ladders, each one 3meters long. They stack one on top of the other, and are chained to the tree bole. Essentially, you could stack these ladders to the moon, if the tree was that tall. I've climbed 180ft tall trees, and stacked ladders 100 ft. It is scary, and if you aren't nervous, you're risky or stupid... and I've climbed with stupid climbers...