Morphological, molecular, and ecological divergence in Pinus douglasiana and P. maximinoi

Pinus douglasiana and P. maximinoi (Pinus subsection Ponderosae) are closely-related New World pines with vague taxonomic boundaries where their natural ranges overlap in western Mexico. They are distinguished from each other by the width of their leaves and thickness of their cone scale apophyses. They are also sometimes confused with two other close relatives, Pinus pseudostrobus and P. yecorensis. We integrated morphological, molecular, and ecological data to clarify the taxonomic limits among these four species. Following previous studies, we evaluated 16 quantitative leaf and seed cone characters. Pinus douglasiana, P. maximinoi, and P. pseudostrobus formed non-discrete groups in multivariate space. The absence of leaf hypodermal intrusions, a persistent peduncle, and the shape of the seed cone are useful for differentiating P. pseudostrobus and P. yecorensis from P. douglasiana or P. maximinoi, and the latter two can usually be distinguished by needle width or cone scale apophysis...