Reconstruction of age structure and spatial arrangement of piñon–juniper woodlands and savannas of Anderson Mesa, northern Arizona

Abstract

We examined age structure and spatial arrangement of piñon–juniper woodlands and savannas on six plots distributed across three different soil types in northern Arizona. These stands, as typical of many others in piñon–juniper ecosystems, have experienced increases in tree densities since the arrival of European settlers. The goal of this study was to reconstruct stand conditions in 1860, prior to livestock grazing, using stem-mapping to determine tree arrangement and tree-ring analysis to examine age structure and density. Ripley's K( t), Ripley's K 12( t), and Moran's I were used to analyze nearest neighbor distances, spatial association, and spatial autocorrelation, respectively. All sites have long term presence of juniper and piñon trees, with a pulse of establishment and survival occurring between 1860 and 1880 on basalt- and sandstone-derived soils. In contrast, limestone-derived soil had no pulse of tree establishment in 1860, but rather a steady increase in tree establishment since ca. 1700. Spatial arrangement of juniper trees in 1860 showed strong clumping patterns from a minimum distance of 15 m to all spatial scales. Piñon pine varied in spatial arrangement from clumping at all spatial scales to random at all spatial scales. Positive spatial autocorrelation was determined for age of juniper trees to a minimum distance of 21 m in current stand conditions, in contrast to no strong trends of spatial autocorrelation in 1860. By comparing the age structure and spatial results for the forest reconstruction of 1860 to current conditions, we were able to show variation among soil types in nurse tree association between piñon and juniper trees and unable to support the density dependent mortality hypothesis for these sites. Presettlement (1860) diameter distributions and basal areas can be used to develop structure control (BDq) prescriptions unique to each soil type to restore either savanna or woodlands condition. However, identifying one blanket prescription for tree reduction in piñon–juniper ecosystems of the southwestern United States, or even the Anderson Mesa landscape, would reduce the range of variability present in the form of woodlands and savannas.