MIDDLE AND HIGH-ELEVATION CONIFEROUS FOREST COMMUNITIES OF THE NORTH RIM REGION OF GRAND-CANYON NATIONAL-PARK, ARIZONA, USA

We examined the composition and structure of forest communities in a 3 700 ha watershed in relation to environmental gradients and changes in land management practices. We identified four mixed-conifer forest ty pes dominated by different combinations of Abies concolor, Picea punge ns, Pinus ponderosa, Populus tremuloides, and Pseudotsuga menziesii, a nd a spruce-fir type dominated by Picea engelmannii and Abies lasiocar pa. The forest types occur in a complex pattern related to elevation a nd topographic-moisture gradients and variations in past fire regimes. However, widespread regeneration of A. concolor following possible ch anges in the fire regime in the late 19th century and continuing with institution of a fire suppression policy early in the 20th century is producing a more homogenous mixed-conifer forest with greater horizont al and vertical continuity of fuel. This shift toward landscape homoge neity not only may adversely affect biodiversity, but also may be perp etuated as the probability of large, high-severity fires increases wit h continued fire suppression.