Monoterpenoid and morphological differentiation within Pinus cembroides

Wood of Pinus cembroides from 232 trees sampled in 23 localities throughout its range in the United States and Mexico has been analysed for monoterpenes. With a few exceptions the trees of the continental populations from the state of Hidalgo northward produced α-pinene as the main monoterpene. The majority of trees of the southern populations (subsp. orizabensis) from the state of Puebla and adjoining regions of the neighboring Veracruz and Tlaxcala produced 3-carene in appreciable amounts. The population from Baja California (subsp. lagunae) produced monoterpenes of the sabinene family (thujene, sabinene, γ-terpinene, terpinolene and p-cymene) as the main constituents. The variations in morphological characters, including number of needles per fascicle, number of stomatal rows in needles, number of resin canals in needles, as well as other characters of the internal needle morphology, needle length and thickness, needle retention, thickness of seed shells, bark and cone characteristics, rate of growth and cotyledon number either paralleled or did not contradict the chemical variations. The evolution of the southern populations (subsp. orizabensis) into a separate group most likely resulted from geographic separation and climatic differences, while the evolutionary separation of the Baja California trees (subsp. lagunae) was connected with northwestern movement of the California coastal region from Miocene onward and the formation of Sierra Madre Occidental resulting in separation of this population from the mainland populations by the gap of the Gulf of California and the coastal mountain ranges.