Site characterization and regeneration attributes of managed and unmanaged ponderosa pine sites in the southwestern United States

These data consist of environmental covariates, measured plot-level and tree characteristics for seven coniferous tree species across the southwestern United States. The objectives of the study were to assess how growth characteristics of conifer tree species vary across environmental gradients and across the different tree species. These data represent conifer growth under a variety of stand and site characteristics. These data were collected in the summer of 2019, from sites across Nevada, Arizona, New Mexico and Colorado, and collected by field crews directed by Matt Petrie (University of Nevada Las Vegas), Rob Hubbard (USDA Forest Service), Tom Kolb (Northern Arizona University) and John Bradford (U.S. Geological Survey). We selected six locations to encompass a wide range of regional climate conditions. Within each location, we selected sites to capture diversity in local factors expected to influence regeneration including topography, adult tree density, vegetation characteristics, management action, and disturbance. To include more and less sheltered forest microsites in each plot, we located the center of each plot at the boundary between a moderately sized forest interspace and a higher density area, using a spherical densiometer to estimate the midpoint of this boundary for each site based on canopy cover. Interspace sizes differed for each forest site, such that in a dense forest stand a moderate interspace was smaller (∼ 10-100 m−2) than that of a thinned forest stand, where a moderate interspace was in some cases > 1.0 ha in area (10,000 m2). Sites were located on shallow slopes when possible (< 10◦). Recognizing the important influence of soil properties and soil parent material on regeneration, we selected sites with similar texture (sandy loams, loamy sands) at each study location to minimize edaphic influence between our regional study locations. We did not pre-evaluate regeneration in the field prior to setting plot boundaries. We note that some of our sites were located in different environments (differing canopy covers, understory vegetation and debris, adult tree densities, etc.) within the same forest management unit, and other sites were located in nearby managed and unmanaged forest stands. These data can be used to assess how environmental conditions and site characteristics may influence conifer tree regeneration.