Biogeography of Two Southwest American Oaks in Relation to Atmospheric Dynamics

A causal relationship between Holocene patterns of plant distributions in the American Southwest and shifts in atmospheric flow structure has been inferred, but has yet to be demonstrated. To investigate this inferred relationship, seedlings of Quercus gambelii were transplanted along various local microhabitat gradients and regional air mass gradients and assessed for patterns of seedling establishment. The primary cause of seedling mortality appeared to be summer drought stress, which paralleled the latitudinal gradient of the `Arizona monsoon'. The `Arizona monsoon' air mass gradient produces an increasing probability of summer drought stress with increasing latitude, while the `polar front' air mass gradient produces an increasing probability of winter and spring cold stress with increasing latitude. We hypothesize that the two gradients produce, as a function of increasing latitude, convergent upper and lower elevational limits of Q. gambelii. The results of this elevational convergence appear to be a virtual absence of oak seedling establishment near the species' northern limits and a gradual senescence of long-lived clones from spring freeze stress when followed by summer drought stress. These conclusions have been extended to the biogeography of Q. turbinella through growth chamber experiments on the comparative drought physiology of seedlings of the two species and through previous work on their associated biogeographic patterns. Thus, the northern ecotones of Q. gambelii and Q. turbinella, appear to be primarily caused by the combination of spring freeze stress and summer moisture stress, rather than freeze stress alone, as previously proposed. We report systematic temporal and spatial shifts in the two air mass gradients, between warming and cooling trends, which can account for the Holocene biogeographic history of the two oak species and, in part, the observed spatial and temporal variations in oak morphology and oak/pine community structure.