Structural diversity and conservation implications of Parrotia subaequalis (Hamamelidaceae), a rare and endangered tree species in China

Parrotia subaequalis (H. T. Chang) R.M. Hao & H.T. Wei is a rare and endangered Tertiary relict tree that is endemic to subtropical China. However, little is known about its growth condition and relationship with associated tree species. Here, for the first time we measured the structural diversity of P. subaequalis communities at three representative sites in eastern China using four structural indices, including mingling, tree-tree distance, and diameter and tree height differences. The results showed that: 1) Collectively, most P. subaequalis and associated tree species were small and mid-sized classes in tree height, and small-sized class in diameter; 2) There were two or more other tree species around most of P. subaequalis individuals across the three sites; 3) Overall, the mean distance between reference trees and their neighbors was mainly 1–2 m. Our results indicated that a strong interspecific competition existed between P. subaequalis and its associated tree species. Meanwhile, although the reference tree P. subaequalis had slight advantages in both horizontal and vertical planes, we think that it is necessary to take some effective measures to reduce the interspecific competition and thereby keep it at a proper successive stage. In addition, we also discuss the protection level of P. subaequalis in China, and propose to keep this species at the First-Grade State Protection.