Post-1935 changes in forest vegetation of Grand Canyon National Park, Arizona, USA: Part 1 - ponderosa pine forest

Vegetation plots originally sampled in Grand Canyon National Park (GCNP), Arizona, USA in 1935 are the earliest-known, sample-intensive, quantitative documentation of forest vegetation over a Southwest USA landscape. These historical plots were located as accurately as possible and resampled in 2004 to document multi-decadal changes in never-harvested Southwestern forests. Findings for ponderosa pine forest (PPF) differed among three forest subtypes (dry, mesic, and moist PPF), indicating that understanding the ecology of PPF subtypes is essential for development of ecologically based management practices. Dry PPF, which is transitional with pinyon-juniper vegetation at low elevation, exhibited no changes from 1935 to 2004. Mesic PPF, the core subtype of PPF, had increased densities of total trees, ponderosa pine (Pinus ponderosa), and white fir (Abies concolor) in the 10-29.9 cm diameter class from 1935 to 2004 that may have induced decreased densities of larger ponderosa pines and total tree and ponderosa pine basal areas. Moist PPF, which is transitional with mixed conifer forest at high elevation, was the most dynamic PPF subtype with decreases from 1935 to 2004 in total density and total basal area that are largely attributable to decreases in quaking aspen (Populus tremuloides). Graphical synthesis of datasets with historical and modern values for density and basal area indicates that overall PPF (all subtypes combined) increased in sapling density of all species combined and conifers with canopy potential and decreased in density of quaking aspen trees since the late 19th century. PPF of GCNP has passed through an accretion phase of forest development with increases in density and, depending on PPF subtype and variable being examined, is at or past the point of inflection to recession of density and basal area. Increases in small diameter ponderosa pine and white fir from 1935 to 2004 portend potential additional accretion, but decreases in total basal area, density and basal area of quaking aspen, basal area of ponderosa pine, and density of larger diameter ponderosa pine indicate PPF has passed the inflection point from accretion to recession. Uncertainties about 19th-century PPF structure and composition and about future ecological and societal environments lead to the conclusion that resource managers of GCNP and other natural areas should consider a change in focus from the objective of achieving desired future conditions to an objective of avoiding undesired future conditions.