Physiognomy and Structure: This broadly defined evergreen division is composed of a woodland, savanna (or open woodland), and scrub structure. Stands are typically short (2-10 m tall), with an open to closed (10-60% cover), evergreen needle-leaved or scale-leaved or broad-leaved, sclerophyllous tree and/or shrub canopy. The understory is variable with lush grass cover and occasionally scattered shrubs in the savanna stands to a sparse to dense short-shrub layer and/or herbaceous layer in woodland stands. This division encompasses savanna that has widely spaced, short (2-10 m tall), mature (>150-year-old) trees with a moderately dense to dense herbaceous layer dominated by perennial graminoids. On extremely xeric sites, diagnostic juniper and pinyon trees species may only attain 2 m in height and have a more shrub form. However, Juniperus occidentalis-dominated stands have two different tree canopy structures: (1) an old-growth woodland with large, fairly well-spaced trees with rounded crowns, and (2) relatively young, often dense junipers trees with pointed crowns. Cover of understory species sharply declines when tree canopy cover exceeds 40% (Young et al. 1982). Many of the tree savannas have a sparse shrub layer present. Herbaceous layers are variable depending on the density of woody canopy, substrate, landscape position, and disturbance history. Perennial graminoids typically dominate most herbaceous layers with most species individually contributing low cover.

Floristics: This division includes woodland, savanna (or open woodland), and scrub characterized by an open to closed tree and shrub canopy of Pinus monophylla and/or Pinus edulis, Juniperus occidentalis, Juniperus osteosperma, Juniperus monosperma, or Juniperus scopulorum. Typically one, or combinations of two, of these species dominate the canopy. In the Mohave Desert mountains, Yucca brevifolia may be an associate of Pinus monophylla. Shrub species include Arctostaphylos patula, Artemisia arbuscula, Artemisia bigelovii, Artemisia nova, Artemisia tridentata, Atriplex canescens, Cercocarpus intricatus, Cercocarpus ledifolius (in tree or shrub form), Cercocarpus montanus, Chrysothamnus viscidiflorus, Coleogyne ramosissima, Ericameria nauseosa, Fraxinus anomala, Fraxinus cuspidata, Glossopetalon spinescens, Krascheninnikovia lanata, Purshia stansburiana, Purshia tridentata, Quercus gambelii, Quercus turbinella, Quercus x pauciloba, Ribes cereum, Rhus trilobata, Tetradymia spp., Yucca baccata, Yucca brevifolia, and Yucca glauca. Shrub species more characteristic of 1.B.1.Nc ~Californian Forest & Woodland Division (D007)$$, such as Juniperus californica, Quercus chrysolepis, and/or Quercus john-tuckeri, may occur near the southwestern limits of the range. The most frequent succulents are Cylindropuntia imbricata, Opuntia phaeacantha, and Opuntia polyacantha. The herbaceous layer may be sparse to dense depending on overstory density, substrate, landscape position, and disturbance history, with the densest graminoid layer in open-tree or shrub savanna. Common graminoid associates include Bouteloua curtipendula, Bouteloua eriopoda, Bouteloua gracilis, Carex filifolia, Hesperostipa comata, Hesperostipa neomexicana, Festuca idahoensis, Leymus cinereus (= Elymus cinereus), Leymus salinus, Muhlenbergia pauciflora, Pleuraphis jamesii, Poa fendleriana, Poa secunda, Pseudoroegneria spicata, and the non-native invasive annual Bromus tectorum. Forb species may be diverse but typically have low canopy cover values, and can include Astragalus spp., Castilleja integra, Cryptantha cinerea var. jamesii (= Cryptantha jamesii), Eriogonum jamesii, Erigeron divergens, Hymenopappus filifolius, Ipomopsis multiflora, Mentzelia spp., Penstemon spp., and Petradoria pumila.

Dynamics:  Key dynamic processes are drought, fire, herbivory, and insect/disease outbreaks. Characteristic Pinus spp. and Juniperus spp. are relatively short (generally <15 m tall), shade-intolerant, drought-tolerant, slow-growing, long-lived trees (especially Juniperus osteosperma can reach 650 years old). Pinus spp. are non-sprouting and may be killed by fire. The effect of a fire on these stands is largely dependent on the tree height and density, fine fuel load on the ground, weather conditions and season. Large trees generally survive unless the fire gets into the crown due to heavy fuel loads in the understory. Fire acts to open stands, increase diversity and productivity in understory species, and create a mosaic of stands of different sizes and ages across the landscape while maintaining the boundary between woodlands and adjacent shrublands or grasslands.

As modeled by Landfire, this division is generally characterized by a spectrum of fire regimes, including frequent non-lethal fires, mixed-severity mosaic fires (mean FRI of 50-200 years), and very infrequent replacement fires (mean FRI of 200-500 years). Surface fire was likely the most frequent in savannas. Frequently, fire spreads from adjacent vegetation. Severe climatic events occurring during the growing season, such as frosts and drought, are thought to limit the distribution and density of pinyon and/or juniper stands to relatively narrow altitudinal belts on mountainsides and foothills. Weather-related stress thins trees in more closed stands. Insects/disease has a similar effect, but with a greater frequency in closed stands (mean return interval of 100 years) than open ones (mean return interval of 1000 years). Competition from grasses and older trees in late open stands is also included as a disturbance that maintains open woody canopies.