Historical forest structure, composition, and spatial pattern in dry conifer forests of the western Blue Mountains, Oregon

TL;DR: In this paper, the authors quantified the historical size, density, composition, and spatial patterns of dry mixed-conifer forests in the Blue Mountains of Oregon to establish reference conditions that could be used for ongoing forest restoration efforts.

Abstract: Churchill, Derek J.; Carnwath, Gunnar C.; Larson, Andrew J.; Jeronimo, Sean A. 2017. Historical forest structure, composition, and spatial pattern in dry conifer forests of the western Blue Mountains, Oregon. Gen. Tech. Rep. PNWGTR-956. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 93 p. In frequent-fire forests of the interior Western United States, historical (prefire suppression) conditions are often used as a reference to set management objectives, guide prescriptions, and monitor treatment effectiveness. We quantified the historical size, density, composition, and spatial patterns of dry mixed-conifer forests in the Blue Mountains of Oregon to establish reference conditions that could be used for ongoing forest-restoration efforts. In total, 14 reconstruction plots ranging from 2.3 to 5.0 ha were established, with plots located on ponderosa pine (Pinus ponderosa Lawson & C. Lawson), Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), and grand fir (Abies grandis (Douglas ex D. Don) Lindl.) potential vegetation sites. Within each plot, all historical structures—live trees, snags, and logs that were alive in 1890—were mapped, and 1890 diameters were reconstructed. Historical structure and composition on the 14 plots consisted of an open canopy, uneven-aged mosaic of widely spaced individual trees, tree clumps, and openings dominated by large trees. Mean values for historical tree density of trees greater than 15 cm diameter at breast height (d.b.h.) ranged from 53 to 197 trees per hectare, and basal area ranged from 11 to 24 m ∙ ha. Basal area varied widely across each plot, and the mean value was not representative of the density in the majority of the plot in almost all cases. Mean tree diameter ranged from 35.2 to 57.8 cm, with over 50 percent of the basal area composed of trees greater than 50 cm d.b.h. Species composition was dominated by ponderosa pine on plots within the ponderosa pine and Douglas-fir series. Grand fir was abundant on three of the four plots within the grand fir series, comprising 44 to 48 percent of the basal area on these three plots. The proportion of historical trees occurring as widely spaced individuals (no neighbors within 6 m) ranged from 0.53 to 0.10. Ten of 14 plots had large tree clumps (10 to 15 trees) or very large clumps (16 to 30 trees), with the proportion of trees in these clumps ranging from 0.05 to 0.43. The proportion of the plot area in large openings ranged from 0.15 to 0.72, with most openings being sinuous and linear in shape and ranging from 18 to 45 m across. The range of conditions presented in this reference dataset provides managers flexible targets that can be used to guide restoration of variable conditions within and across treatment units. When using the dataset to inform prescription development, we recommend first determining the target stem density appropriate for the site and then selecting a clumping level to help guide the spatial pattern to be created through restoration treatment. Management objectives, current conditions, biophysical conditions, and anticipated future disturbance and climate change can then be considered when setting targets for density and clumping. This reference information can also be used to monitor the extent to which treated stands are within the ranges of size, density, composition, and pattern quantified in this study.