Development of species dominance along an elevational gradient: Populationdynamics of Pinus edulis and Juniperus monosperma

We evaluated species-environment relationships within pinon-juniper woodlan ds in northern New Mexico (United States) using canonical correspondence an alysis (CCA). The first CCA axis was associated primarily with elevation. O ur results showed separation between pinon and juniper along the elevation gradient, as expected: pinon is relatively more dominant at higher sites, w hereas juniper is relatively more dominant at lower sites. To examine how t his pattern of dominance might emerge with time, we plotted the position of centroids for three pinon and juniper size classes along the first CCA axi s. We found that small pinons and junipers were distributed relatively unif ormly across the gradient, whereas large pinons and junipers were strongly segregated along the gradient, with intermediate-sized pinons and junipers intermediate on the CCA axis between small and large. This produced a patte rn of increased divergence between the two species that increased with size . We suggest that this pattern emerges as a result of differential mortalit y between the species rather than as a result of differences in seedling es tablishment along the gradient. These differences between the species could result from differences in resource use (i.e., physiology) and resource ac quisition (i.e., rooting patterns relative to plant available water). We pr esent a conceptual model of how differences between the species in resource acquisition increase with size (age). We suggest that differences in resou rce acquisition between species, which increase as individuals mature, may play a greater role in determining species dominance along resource gradien ts than has been previously appreciated.