Elucidating differences between two confusing Shorea contorta S. Vid and Pentacme mindanensis Foxw. species of Dipterocarpaceae via leaf morphometrics: An initial investigation

The Dipterocarpaceae is the only timber-producing taxon in the angiosperm, and it is the most important source of timber in Southeast Asia (Soerianegara and Lemmens, 1993). Dipterocarp trees are influential in their structure and function (Brearley et al., 2017). Dipterocarps are highly productive (Banin et al., 2014) and important storage of above-ground carbon (Slik et al., 2013). The taxonomy of several genera of Dipterocarpaceae is relatively well defined but the genus Shorea remains problematic and has proved most controversial (Maury-Lechon and Curtet, 1998). In the Philippines, there are about 45 species of dipterocarps distributed in 6 genera (Rojo and Aragones Jr, 1997). It has been well-studied but the identities of individual species remain ambiguous (Rojo and Aragones Jr, 1997). In Mindanao, a species locally known as "malacayan blanco" was originally described by Foxworthy (1938) from Zamboanga Province, and named as Pentacme mindanensis. Based on Foxworthy’s description, P. mindanensis has resemblance with Shorea contorta but differs in its large leaves and large fruits. However, Ashton (1978) treated P. mindanensis as a synonym of S. contorta, an island endemic, commonest, and widely distributed dipterocarp species (Foxworthy, 1938; Rojo and Aragones Jr, 1997). Furthermore, there are few detailed morphological, anatomical (Pulan and Buot, 2014), and molecular (Umali, 2016; Villarin et al., 2016) studies on S. contorta. Until recently, Umali (2016) confirmed through molecular characterization that S. contorta and P. mindanensis are two different species. Generally, the recent advancement in biosystematics using DNA sequences has provided evidences to the resolution in the correct classification of many dubious taxon. However, doubts to the reliability of the system arises because many species with very obvious morphological dissimilarities were merged and considered into one species, for instance, Ficus latsonii Elmer and Ficus variegata Blume of Moraceae, the former having a distinct brown-orange trunk while the later has whitish. Also, for the species Melicope monophylla Merr. and Melicope triphylla (Lam.) Merr. of Rutaceae, the former with simple leaf while the latter having a compound one. In this present study, our longstanding knowledge on these species aside from differing fruits and leaf sizes was based mainly on the color of the inner bark with P. mindanensis having a cream-yellow while S. contorta has white. Thus, in support to the work of Umali (2016), we aim to provide more striking differences via leaf morphometrics. Leaf morphometrics has proved to be effective in resolving taxonomic problems and discriminating species.