Natural variability in forests of the Grand Canyon, USA

Aim: Compare contemporary with pre-fire-disruption forest structures, assessing the influence of factors that caused ecological change and evaluating remote sites as relatively natural areas. Location: Grand Canyon National Park contains the largest never-harvested and long-term ungrazed forest ecosystem in Arizona, providing valuable sites for measuring natural variability. However, anthropogenic disruption of natural fire regimes since Euro-American settlement c. 1880 has led to changes in forest structure. Methods: We compared species composition, tree structure, regeneration, and canopy cover on large (135-603 ha) ponderosa pine-dominated study sites: (1) isolated points on the North Rim where some surface fires continued after 1880, (2) a higher-elevation North Rim site where fire has been excluded and (3) a South Rim site, also without recent fire, with a paired Kaibab National Forest site. Forest tree structure prior to fire-regime disruption was reconstructed with dendroecological techniques. Results: Before fire exclusion, all sites had relatively low tree density (140-246 trees ha^-1) dominated by large trees (basal area 9.1-28.5 m² ha^-1), primarily ponderosa pine or pine/Gambel oak on the South Rim. Currently all sites are relatively dense (389-955 trees ha^-1, 14.1-41.3 m² ha^-1) but patterns of species composition and regeneration differed substantially with fire regime and elevation. Regeneration at continued-fire sites was primarily through sprouting species, Gambel oak and New Mexican locust, forming a shrubby midstorey under a relatively open pine canopy. In contrast, all fire-excluded sites were dense with seed-reproducing conifer species. Main conclusions: Comparison of change caused by climate fluctuation, tree cutting, fire exclusion, livestock herbivory, and wildlife herbivory, suggests that fire regime alteration appears to have played the greatest role. The remote North Rim sites provide a close analogue to conditions prior to fire regime disruption, a contemporary example of the forest characteristics that might have been extant had recent human-caused disruption of disturbance regimes and heavy resource extraction not occurred. They merit broader study of natural variability on a range of ecological variables in ponderosa pine ecosystems.