Updated GTR on: 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

The USFS Pacific Southwest Research station has released an updated version of the PSW-GTR-256.

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Safford, Hugh D.: Stevens, Jens T. 2017. 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. Gen. Tech. Rep. PSWGTR-
256. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific
Southwest Research Station. 229 p.

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