Development of Species Dominance along an Elevational Gradient: Population Dynamics of Pinus edulis and Juniperus monosperma

We evaluated species-environment relationships within piñon-juniper woodlands in northern New Mexico (United States) using canonical correspondence analysis (CCA). The first CCA axis was associated primarily with elevation. Our results showed separation between piñon and juniper along the elevation gradient, as expected: piñon is relatively more dominant at higher sites, whereas juniper is relatively more dominant at lower sites. To examine how this pattern of dominance might emerge with time, we plotted the position of centroids for three piñon and juniper size classes along the first CCA axis. We found that small piñons and junipers were distributed relatively uniformly across the gradient, whereas large piñons and junipers were strongly segregated along the gradient, with intermediate-sized piñons and junipers intermediate on the CCA axis between small and large. This produced a pattern of increased divergence between the two species that increased with size. We suggest that this pattern emerges as a result of differential mortality between the species rather than as a result of differences in seedling establishment along the gradient. These differences between the species could result from differences in resource use (i.e., physiology) and resource acquisition (i.e., rooting patterns relative to plant available water). We present a conceptual model of how differences between the species in resource acquisition increase with size (age). We suggest that differences in resource acquisition between species, which increase as individuals mature, may play a greater role in determining species dominance along resource gradients than has been previously appreciated.