Morphological variation of Pinus flexilis (Pinaceae), a bird-dispersed pine, across a range of elevations.

Limber pine (Pinus flexilis James) grows across a wider range of elevations than any other tree species in the central Rockies, from ∼1600 m at Pawnee Buttes to >3300 m at Rollins Pass. In this study we investigated two possible explanations for limber pine's success across a broad range of elevations: (1) the sites on which it is found, although separated by >1000 m elevation, may not be very different with respect to environmental factors that affect tree growth, and (2) limber pine growth is insensitive to environmental factors that change with elevation. We compared site characteristics of 12 limber pine stands at elevations ranging from 1630 to 3328 m as well as the growth and morphology of trees in each of these stands. Mean daily air temperature in July decreased linearly with the elevation of the site from 22.8° to 12.6°C. The growth and morphology of limber pine leaves, shoots, and trees were, in general, not related to the elevation or July mean air temperature of the sites. There was, however, a significant decrease in stomatal density with increasing elevation, which may be an acclimational response to restrict water loss at high elevations. Our data suggest that the fundamental and realized niche of limber pine is broad with respect to air temperature. In light of the high gene flow and only slight genetic differentiation among populations of species with bird-dispersed seeds, such as limber pine, it is especially unusual to see similar growth throughout an environmental gradient. Physiological and anatomical plasticity or wide physiological tolerance ranges may enable limber pine to uncouple its growth from its environment.