Post-1935 changes in Pinyon-juniper persistent woodland on the South Rim of Grand Canyon National Park, Arizona, USA.
Pinyon-juniper (Pinus-Juniperus) persistent woodland (PJPW) is widespread in western North America. This study examined changes in PJPW on the South Rim of Grand Canyon National Park (GCNP), Arizona, USA based on resampling study plots established in 1935 that are the earliest-known, landscape-level, quantitative documentation of PJPW. Resampling provided data essential to understanding post-1935 dynamics and current conditions, as well as developing ecologically based management practices. Plots were divided into Core PJPW, Transition PJPW-Ponderosa Pine Forest (PJPW-PPF), and Seral PJPW. These differed greatly in their post-1935 vegetation dynamics. Core PJPW experienced high mortality of mid-diameter Juniperus osteosperma (JUOS) and Pinus edulis (PIED), large losses of total basal area, and increases of small-diameter JUOS. In context of other research, these changes indicate drought during 1953-1956, 1959-1964, and 2002-2011 led to increased mortality of JUOS and PIED and that Core PJPW is a dynamic system with phases of tree recruitment and mortality related to climate. In contrast, Transition PJPW-PPF was generally stable, likely as a result of less drought stress at the relatively high elevation where PJPW intergrades with PPF. Seral PJPW plots were early successional in 1935 and provided a uniquely long, quantitative documentation of succession. They changed most among the PJPW subtypes with large increases in total tree density and basal area as plots transitioned into tree-dominated successional stages. Current conditions of Core PJPW in GCNP provide a model for restoration of PJPW in areas of similar environment. Elsewhere, managers should consider using contemporary data from the area's least-disturbed PJ vegetation. In general, the ultimate goal should be to restore to the least-altered present, not attempt to re-establish the past. Likewise, findings suggest ongoing management of PJPW in protected areas should not focus on restoration of historical stand structure and composition, as done in PPF. Instead, management should emphasize leaving naturally occurring processes unimpaired. These include disturbances such as stand-replacing fire and native insect outbreaks that historically led to varied spatial and temporal stand composition and structure. However, active management may be needed when modern anthropogenic factors have potential to substantially alter key processes, e.g., if increasing cover of invasive plants such as cheat grass (Bromus tectorum) is likely to change the historical fire regime. Also, management plans for adjacent vegetation types such as PPF, shrublands, and grasslands need to consider potential short- and long-term impacts on PJPW. Additional challenges to management of PJPW are likely to arise with further climate change.