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Pacific Southwest Research Station

 
Pacific Southwest
Research Station

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General Technical Report

Title: Natural range of variation for yellow pine and mixed-conifer forests in northwestern California and southwestern Oregon

Author: Bohlman, Gabrielle N.; Safford, Hugh D.; Skinner, Carl N.

Date: 2021

Source: Gen. Tech. Rep. PSW-GTR-273. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station

Station ID: GTR-PSW-273

Description: Yellow pine and mixed-conifer (YPMC) forests are common forest types in northwestern California and southwestern Oregon (the “assessment area”). YPMC forests occur above oak woodland and mixed-evergreen forests and below red fir forests, and are dominated by diverse combinations of ponderosa pine (Pinus ponderosa Lawson & C. Lawson), Jeffrey pine (P. jeffreyi Balf.), incense cedar (Calocedrus decurrens (Torr.) Florin), Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), white fir (Abies concolor (Gord. & Glend.) Lindl. ex Hildebr.), and sugar pine (P. lambertiana Douglas) with a notable component of hardwood species, including a number of oaks (e.g., California black oak (Quercus kelloggii Newberry), canyon live oak (Q. chrysolepis Liebm.), Oregon white oak (Q. garryana Douglas ex Hook.), and various other species. We conducted an indepth assessment of the natural range of variation (NRV) of YPMC forests for the assessment area, focusing on ecosystem processes and forest structure from historical data sources from pre-Euro-American colonial settlement (presettlement) times (16th through mid-19th centuries) and current reference forests (YPMC forests that have retained frequent fire and have suffered little human degradation), and then compared current conditions to the NRV. The Mediterranean climate of the assessment area, modified by strong latitudinal and elevational gradients, combined with high geological and topographic complexity in the assessment area, strongly influence the distribution of forest types and the structure and composition of YPMC forests. Historically, fire was a key functional process in YPMC forests that helped maintain relatively open canopies, limit fuel accumulation, decompose biomass, recycle nutrients, promote the dominance of mostly shade-intolerant species, and create structural heterogeneity on the landscape. Forest structure in presettlement YPMC forests was highly variable at larger spatial scales, but was characterized by relatively low tree densities, large tree sizes, high seedling mortality as a result of recurrent fire, and highly heterogeneous understory structure. Following Euro-American colonial settlement, widespread changes occurred in YPMC forests in the assessment area, principally because of extensive logging accompanied by a century of highly effective, ubiquitously applied fire suppression. Modern YPMC forests have departed from NRV conditions for a wide range of ecosystem processes and structural attributes. There is strong consensus among published studies that, on average, modern YPMC stands have much higher densities dominated by smaller trees (often of shade-tolerant species) and much longer fire return intervals compared to reference YPMC forests. Additionally, fires that escape initial attack can be much larger and generally have larger proportions of high-severity areas than typical pre-Euro-American settlement fires. There is more moderate consensus among published studies that the average modern YPMC stand in the assessment area supports greater fuels and deeper forest litter, higher canopy cover and fewer canopy gaps, higher tree basal area, more coarse woody debris, a higher density of snags, lower grass and forb cover, less area burned across the landscape, and experiences a longer fire season compared to reference YPMC forests. Among the variables assessed, overall plant species richness and total percent cover of shrubs appear to be within or near the NRV.

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Citation

  • Bohlman, Gabrielle N.; Safford, Hugh D.; Skinner, Carl N. 2021. Natural range of variation for yellow pine and mixed-conifer forests in northwestern California and southwestern Oregon. Gen. Tech. Rep. PSW-GTR-273. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. 146 p.