Natural range of variation for yellow pine and mixed-conifer forests in the Sierra Nevada, southern Cascades, and Modoc and Inyo National Forests, California, USA
Yellow pine and mixed-conifer (YPMC) forests are the predominant montane forest type in the Sierra Nevada, southern Cascade Range, and neighboring forested areas on the Modoc and Inyo National Forests (the "assessment area"). YPMC forests occur above the oak woodland belt and below red fir forests, and are dominated by the yellow pines (ponderosa pine [Pinus ponderosa Lawson & C. Lawson] and Jeffrey pine [Pinus jeffreyi Balf.]); white fir (Abies concolor (Gord. & Glend.) Lindl. ex Hildebr.); sugar pine (P. lambertiana Douglas); incense cedar (Calocedrus decurrens (Torr.) Florin); and black oak (Quercus kelloggii Newberry), along with a number of other hardwood and conifer 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 settlement times (16th through mid-19th centuries) and current reference forests (YPMC forests that have retained frequent fire and have suffered little human degradation), and comparing current conditions to the NRV. The Mediterranean climate of the assessment area, modified by strong latitudinal, topographic, and elevational gradients, plays an important role in shaping the structure and composition of YPMC forests. Fire was an historically important ecosystem process that occurred frequently, generally burned at low to moderate severity, created a heterogeneous forest structure at a fine spatial scale, and maintained pine dominance in many stands that would otherwise undergo succession to more shade-tolerant fir and cedar species. Forest structure at larger spatial scales was highly variable but was characterized mostly by relatively low tree densities, large tree sizes, high seedling mortality as a result of recurrent fire, and highly heterogeneous understory structure that could include locally abundant fire-stimulated shrub species. Following Euro-American settlement, wholesale changes occurred in YPMC forests in the assessment area, principally because of extensive logging followed by a century of highly effective 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), much longer fire-return intervals, and less area burned across the landscape compared to reference YPMC forests. In addition, fires that escape initial attack are much larger and higher severity on average than the average pre-Euro-American settlement fire. There is more moderate consensus among published studies that the average modern YPMC stand in the assessment supports greater fuels and deeper forest litter, higher canopy cover and fewer canopy gaps, more coarse woody debris, a higher density of snags, and experiences a longer fire season compared to reference YPMC forests. Among the variables assessed, only basal area, overall plant species richness, and percentage cover of grass/forbs and shrubs appear to be within or near the NRV.