Sierra Nevada Jeffrey Pine Forest

These forests are found on relatively xeric sites in mountains and plateaus from southern Oregon (600-1830 m [1800-5000 feet] elevation) south into the Sierra Nevada, throughout the Transverse Ranges of California, and into northern Baja California (1200-2740 m [4000-8300 feet]), Mexico. While the two dominant pines tend to segregate by soil fertility and temperature regimes, they may co-occur in certain areas (e.g., Modoc Plateau). These stands are more common on the east side of the Sierra Nevada, although they do occur on the west side. Stands are pure Pinus jeffreyi, Pinus ponderosa, or a mix of the two. Ponderosa pine and/or Jeffrey pine on the west slope of the Sierras with other conifer species are part of Mediterranean California Dry-Mesic Mixed Conifer Forest and Woodland (CES206.916). This system includes sites where Pinus ponderosa and/or Pinus jeffreyi are the predominant conifers and other tree species do not occur in high abundance, if at all. The exception to this is in southern California on the edges of the Mojave Desert where Pinus monophylla or Juniperus californica might occur in a subcanopy under Pinus ponderosa or Pinus jeffreyi. Pinus jeffreyi is more tolerant of colder, drier and poorer sites and replaces Pinus ponderosa as the dominant at higher elevations. In the north, Pinus jeffreyi may be replaced by Pinus ponderosa var. washoensis (Carson Range and Warner Mountains). Throughout California, pure stands of ponderosa pine are relatively uncommon. Only on the Modoc Plateau do these pines co-occur in mixed stands. Juniperus grandis [in the south] and Juniperus occidentalis can co-occur in these stands but typically are not dominant. On moister and cooler sites, Abies lowiana can be present in some stands. There can be well-developed shrub understories with strong Great Basin affinities; species can include Artemisia tridentata, Purshia tridentata, Symphoricarpos rotundifolius var. parishii, Arctostaphylos patula, Ceanothus cordulatus, Ceanothus prostratus, Ceanothus integerrimus, Chrysolepis sempervirens, Eriogonum wrightii, Quercus vacciniifolia, and Lupinus elatus. Cercocarpus ledifolius is common on steeper slopes throughout the range. Historically, frequent localized surface fires maintained these systems. Stands of ponderosa pine on the east side of the Cascades transition into East Cascades Oak-Ponderosa Pine Forest and Woodland (CES204.085), or Northern Rocky Mountain Ponderosa Pine Woodland and Savanna (CES306.030) north of the Warm Springs Reservation of central Oregon.

This system includes sites where Pinus ponderosa and/or Pinus jeffreyi are the predominant conifers and other tree species do not occur in high abundance, if at all. The exception to this is in southern California on the edges of the Mojave Desert where Pinus monophylla or Juniperus californica might occur in a subcanopy under Pinus ponderosa or Pinus jeffreyi. Pinus jeffreyi is more tolerant of colder, drier and poorer sites and replaces Pinus ponderosa as the dominant at higher elevations. In the north, Pinus jeffreyi may be replaced by Pinus ponderosa var. washoensis (= Pinus washoensis) (Carson Range and Warner Mountains). Throughout California, pure stands of ponderosa pine are relatively uncommon. Only on the Modoc Plateau do these pines co-occur in mixed stands. Juniperus grandis (= Juniperus occidentalis var. australis) [in the south] and Juniperus occidentalis can co-occur in these stands but typically are not dominant. On moister and cooler sites, Abies lowiana (= Abies concolor var. lowiana) can be present in some stands. There can be well-developed shrub understories with strong Great Basin affinities; species can include Artemisia tridentata, Purshia tridentata, Symphoricarpos rotundifolius var. parishii (= Symphoricarpos parishii), Arctostaphylos patula, Ceanothus cordulatus, Ceanothus prostratus, Ceanothus integerrimus, Chrysolepis sempervirens, Eriogonum wrightii, Quercus vacciniifolia, and Lupinus elatus. Cercocarpus ledifolius is common on steeper slopes throughout the range.

This system occupies xeric (mean annual rainfall 200-430 mm, as winter snow), cool (cold winters; January minimums range from -13° to -5°C), and nutrient-poor sites in mountains and plateaus (600-2740 m elevation), in the rainshadow of the Sierra Nevada. Frequent (8-10 years) low-intensity and moderately frequent (44 years) mixed-intensity fires maintain this system. Greater moisture increases tree diversity (Abies lowiana at higher altitudes).

Pinus jeffreyi and Pinus ponderosa trees are structurally and physiologically fire-adapted (Habeck 1992a, d, Gucker 2007). Both species have thick, insulating bark, insulating bud scales that protect terminal buds, self-pruning branches, open crowns, and high moisture content of needles, which make them moderately fire-resistant as saplings and highly fire-resistant as mature trees (Habeck 1992a, d, Gucker 2007). Historically, frequent localized surface fires maintained open canopy woodland stands in this system.

LANDFIRE developed a state-and-transition vegetation dynamics VDDT model for this system which has five classes in total (LANDFIRE 2007a, BpS 1210310). These are summarized as:

A) Early Development 1 All Structures (shrub-dominated - 15% of type in this stage): Shrub cover is 0-100%. Fire-dependent shrubs such as greenleaf manzanita and mountain whitethorn resprout and germinate from seed vigorously after fire. Scattered Jeffery pine seedlings sprout but may take several years to dominate over the shrub community. Perennial bunchgrasses and some forbs cover small portions of the area.

B) Mid Development 1 Closed (tree-dominated - 5% of type in this stage): Tree cover is 51-90%. This class has developed after escaping significant fire and it is modeled as an alternative pathway when three fire cycles have been missed. In the absence of fire, a closed forest with a dense stand of multi-layered pole and medium-sized Jeffery pine and white fir trees (5-16 inches dbh) develops. This multi-layered forest is often dominated by Jeffery pine in the overstory with white fir dominant in the mid and regeneration layers. The understory vegetation is almost absent due to the lack of sunlight and heavy litter and woody debris accumulations. In some cases, on the east side of the Sierra Nevada, both white fir and Jeffrey pine are pretty equally stocked and have a number of older individuals present suggesting that there is not always a low cover of white fir of small size classes in such settings (e.g., Buckeye Creek and other drainages northeast of Yosemite National Park). The understory vegetation is generally sparse, but not always due to lack of sunlight. Poa wheeleri and Elymus elymoides can be main understory species.

C) Mid Development 1 Open (tree-dominated - 20% of type in this stage): Tree cover is 0-50%. This class has developed with frequent low-intensity surface fires. Pole to medium-sized (5-21 inches dbh) Jeffery pine has become dominant over the shrub layer. Several conifer species could also be present depending on location. Shrubs are prevalent in the understory with scattered forbs and perennial grasses. East of the Sierra crest (e.g., Truckee Basin north of Tahoe), this class can have substantial amounts of white fir, but usually exists where the shrubs are mostly Purshia tridentata and other Great Basin species.

D) Late Development 1 Open (conifer-dominated - 65% of type in this stage): Tree cover is 0-50%. This class is a continuation of class C which has developed with frequent low-intensity surface fires. Large to very large (>21 inches dbh) Jeffery pine is dominant with an open canopy. Scattered shrubs are found in the canopy openings, with a diversity of forbs such as lupines and woolly mule's-ears. Perennial grasses are also present.

E) Late Development 1 Open (conifer-dominated - 5% of type in this stage): Tree cover is 51-90%. This class has developed in time from class B or class D after escaping significant fire (>3 years fire-return intervals). In the absence of fire a closed forest structure continues to develop with a dense stand of multi-layered medium- to large-sized Jeffery pines and white fir trees (16+ inches dbh). The diameter remains smaller than in the open forest due competition. This overstory canopy is often codominated by Jeffery pine and white fir, with white fir dominating the understory. There is severe competition for sunlight and water. This stress combined with insect and disease infestation create a high level of tree mortality. The understory vegetation is almost absent due to the lack of sunlight and heavy litter and woody debris accumulations. Current conditions where there are large Jeffery pine trees along with multi-age classes of white fir suggest that historically there were low-intensity fires that maintained stands without killing white fir, but more recently white fir has become dominant in the understory.

Where stands are relatively dense and sufficient fuels are available, this type is dependent on relatively frequent low-intensity surface fire intervals of about 30 years (LANDFIRE 2007a, BpS 1210310). The mixed-intensity fire interval is about 130 years, and the stand-replacement fire interval is 250 years. The mean fire interval for all fires is 20 years with a range from 8-28 years. Intervals may be longer for relatively open stands with low understory fuels, as over shallow granitic soils in the Kern Plateau or over serpentine substrate in the Klamath Mountains. The fire regimes in this type are more variable and somewhat longer than the ponderosa pine types, due to slower fuel accumulation rates (LANDFIRE 2007a, BpS 1210310).

This system is characterized by frequent (5-30 years fire-return interval) low-intensity ground fires that maintain the open structure. Fire suppression has increased fire-return intervals resulting in higher density of understory shrubs and canopy trees, increased presence of ladder fuels resulting in high-severity, stand-replacing fires. On a landscape scale, a mixed-severity fire regime occurs in Jeffery pine habitats (Habeck 1992a, d, Gucker 2007).

This system occurs in foothills and mountains from southern Oregon south into the Sierra Nevada, throughout the Transverse Ranges of California and into northern Baja California, Mexico.