Spatial patch patterns and altered forest structure in middle elevation versus upper ecotonal mixed-conifer forests, Grand Canyon National Park, Arizona, USA

Abstract

In the American Southwest, mixed-conifer forest experienced altered disturbance regimes with the exclusion of fire since the early 1900s. This research analyzes patch development and tree spatial patterns in the middle versus upper mixed-conifer forests at Grand Canyon National Park in Arizona (USA). The methods used include: (1) size–structure analyses, to compare species patch development; (2) dendrochronological dating of tree establishment and fire history; (3) tree ring master chronology, to determine periods of suppressed growth, compared to a palmer drought severity index; (4) spatial analyses by size and age, with univariate and bivariate analyses of spatial association as well as spatial autocorrelation. Results show that unlike the lower ecotone of the mixed-conifer zone, both the middle elevation and upper ecotone were mixed-conifer forests before Euro-American settlement. At the upper ecotone, two decades (1870s and 1880s) had no successful conifer establishment but instead aspen cohorts, corresponding to the fire history of synchronized fires. Overall, the upper ecotone has shifted in composition in the absence of surface fires from mixed conifer to encroachment of subalpine species, particularly Engelmann spruce. Spatial patterns of tree sizes and tree ages imply development of a size hierarchy in an aging patch. In addition, shifts in species composition from ponderosa pine and white fir overstory to Engelmann spruce and Douglas-fir understory affected within-patch spatial patterns. These results provide quantitative evidence of past and present forest conditions for the development of restoration strategies for Southwestern mixed-conifer forests.