Northern rātā (Metrosideros robusta) is a huge forest tree endemic to New Zealand. It grows up to 25 metres (82 ft) or taller, and usually begins its life as a hemiepiphyte high in the branches of a mature forest tree; over centuries the young tree sends descending and girdling roots down and around the trunk of its host, eventually forming a massive, frequently hollow pseudotrunk composed of fused roots. In disturbed ground, or where there are gaps...