Globally, many Pinus species occupy arid habitats yet rely on a vascular network to supply their leaves with water that is relatively vulnerable to loss of function. The main cause of the high vulnerability of the xylem seems to be relictual pores through the torus of the interconduit pit membranes, which in most species is impermeable to air. Because of their vulnerable xylem, pine species tend to avoid highly negative branch water potentials by closing their stomata at milder leaf water potentials and maintaining a lower leaf area-to-sapwood area ratio than most other conifer species. To mitigate the effects of the low hydraulic supply to their leaves, pines have the greatest leaf hydraulic conductance (Kleaf) across conifers, which allows for a greater ability to effectively replace water at the site of evaporation and maintain mild leaf water potentials. Pines’ efficient Kleaf also permits them to maintain a high photosynthetic rate and grow rapidly. High Kleaf may also have permitted the evolution of very long needles in a few Pinus species that survive in habitats with very high fire return intervals by forming a grass stage as seedlings, in which the needles are thought to insulate the apical meristem during fires. The costs and benefits of the unusual physiology that pines exhibit requires further study to effectively predict the ecological consequences for this important group.