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 similar to 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 o f elevations: (1) the sites on which it is found, although separated by >10 00 m elevation, may not be very different with respect to environmental fac tors that affect tree growth, and (2) limber pine growth is insensitive to environmental factors that change with elevation. We compared site characte ristics 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 degrees to 12.6 degreesC. The growth and morphology of limb er pine leaves. shoots, and trees were, in general, not related to the elev ation or July mean air temperature of the sites. There was, however, a sign ificant decrease in stomatal density with increasing elevation, which may b e an acclimational response to restrict water loss at high elevations. Our data suggest that the fundamental and realized niche of limber pine is broa d with respect to air temperature. In light of the high gene flow and only slight genetic differentiation among populations of species with bird-dispe rsed seeds, such as limber pine, it is especially unusual to see similar gr owth throughout an environmental gradient. Physiological and anatomical pla sticity or wide physiological tolerance ranges may enable limber pine to un couple its growth from its environment.