Evolutionary significance of a flat-leaved Pinus in Vietnamese rainforest.

Pines are generally absent from tropical rainforests. An important exception, Pinus krempfii, is a unique tree that bears flattened needles and competes with evergreen angiosperm trees in southern Vietnam. Here, the photosynthetic and hydraulic physiology of P. krempfii leaves were examined to determine whether this species departs from the widespread pattern of high-light-demanding photosynthetic physiology displayed in needle-leaved Pinus species. Maximum photosynthesis and light saturation of photosynthesis, as well as stem and leaf hydraulic efficiencies, were all very low in P. krempfii compared with other Pinus species. These characteristics were consistent with our observations of P. krempfii seedling regeneration under the forest canopy. By possessing shade tolerance coupled with the production of flattened leaves, P. krempfii has converged morphologically and physiologically with many genera of the southern hemisphere conifer family Podocarpaceae. This convergence extends to a key feature of leaf anatomy, the production of tubular sclereids in the leaf for radial transport of water from the vein to the margin. These observations suggest that few adaptive possibilities are open to conifers when moving into tropical rainforest, meaning that Pinus is forced into direct competition with southern hemisphere conifers for a narrow niche in the equatorial zone.