A new classification of marginal resin ducts improves understanding of hard pine (Pinaceae) diversity in Taiwan

Abstract

Resin ducts provide important characters for classifying the Pinaceae. Here we study Pinus massoniana and P. taiwanensis and show that the generally-used term marginal (=external) resin duct, applied to ducts in needle leaves, needs to be further differentiated into marginal (strongly attaching to the dermal tissue, and lacking the complete ring structure formed by the sheath cells) and submarginal ducts (adjacent to hypodermal cells, with a complete ring structure formed by the sheath cells). On this basis P. massoniana and P. taiwanensis, which are nearly indistinguishable based on external morphology, are clearly differentiated. Their similar morphology has led to a long standing debate on where P. massoniana occurs in Taiwan. Based on this new classification of resin ducts, we examined old herbarium specimens previously identified as P. massoniana, surveyed current hard pines in Taiwan, and checked historical documents. Needle leaves of these two taxa were studied and compared with the material of P. massoniana from mainland China as a reference. Pinus massoniana shows almost exclusively marginal resin ducts, with extremely rare changes along the length of the duct to submarginal positions. In contrast, P. taiwanensis shows a mixture of medial, submarginal, septal, and rarely endonal ducts, with occasional changes of the duct between various types. Re-identification of specimens showed that only 5 specimens are possible native P. massoniana collected from northern Taiwan and the others are all P. taiwanensis. The hard pine of Junjianyan is unexpectedly found to be the only currently known P. massoniana in Taiwan, which is likely a relic of historical afforestation. The hard pine from Huoyansan and the Coastal Range, which is widely accepted as P. massoniana, is not this species. In addition to the taxonomic value of the new definition applied to these two studied taxa, we expect that this approach can generally be applied distinguishing the respective characters in the genus Pinus and in other conifers.