Vegetation Transition Model

Pinyon-Juniper Woodland

      Description.-Pinyon-juniper woodland is dominated by pinyon pine (Pinus edulis) and Utah juniper (Juniperus osteosperma) trees. Pinyon pine and Utah juniper trees, combined, make up the largest component within a stand (comprise a plurality in association with other species). They both are present in most stands, the former usually more abundant on cooler, moister sites, the latter more abundant on warmer, drier sites. Consequently, juniper tends to be more abundant at the lower elevations, pinyon tends to be more abundant at the higher elevations (Woodin and Lindsey 1954, Heil et al. 1993). At the highest elevations, Utah juniper may be replaced by Rocky mountain juniper (Juniperus scopulorum). Tree density is highly variable among sites, ranging from dense, closed-canopy stands to sparse, open-canopy stands with widely scattered trees, although tree cover tends to increase with elevation due to greater precipitation and reduced evapotranspiration at the higher elevations (Spencer and Romme 1996). Ponderosa pine (Pinus ponderosa), or Douglas fir (Pseudotsuga menziesii) trees may be present in some stands, but usually as minor components (< 10% canopy cover).

      The understory is variable in composition, but is distinguished from other pinyon-juniper woodland types by the consistently low cover of short, non-sprouting shrubs and herbs. Common shrub species include bitterbrush (Purshia tridentata), Mormon tea (Ephedra viridis), and black sagebrush (Seriphidium novum or Artemisia nova). Herb composition is highly variable and may include the cool-season grasses muttongrass (Poa fendleriana), Poa secunda, Agropyron smithii, the warm-season grasses galleta (Hilaria jamesii), Bouteloua gracilis, Sitanion hystrix, and Oryzopsis hymenoides, and the forbs hairy goldenaster (Heterotheca villosa) and Penstemon linarioides.

      Terrestrial cryptogams composed of cyanobacteria, algae, lichen, moss, fungi, or liverwort are often significant components in pinyon-juniper woodlands forming crusts on the ground surface that contribute to soil stability, nutrient supply (organic matter and nitrogen), and biodiversity.

      Distribution.–Pinyon-juniper woodland is widespread at lower elevations in the semi-arid climate zone, especially on sandstone substrates (Figure-distribution map; Figure-elevation chart; Table-areal coverage). It is usually intermixed with other pinyon-juniper woodlands, ponderosa pine forests, warm-dry mixed conifer forests, mountain shrublands, and xeric sagebrush shrublands. Similar communities are found throughout the Colorado Plateau and southern Rocky Mountain regions (Loope 1977, West 1983, 1988; Peet 1988, Heil et al. 1993, Romme et al. 1993, Floyd-Hanna et al. 1996).

      Stand conditions.–We recognized four separate stand conditions:

      1.   Herb dominated (HD) – This is the initial short-lived successional stage following a severe disturbance. Stands are dominated by a mixture of short-lived native herbaceous species, including Chenopodium fremontii and the fire-dependent Nicotiana attenuata, as well as re-sprouting native grasses including Hilaria jamesii and Bouteloua gracilis. Non-native annuals and biennials such as Bromus tectorum, Lactuca serriola, and Alyssum minor may dominate in highly disturbed sites.

      2.   Herbs - Shrubs (HS) – Within approximately a decade after a severe disturbance, shrub seedlings become established and co-dominate with the early successional herbs which are still abundant.

      3.   Shrubs - Trees (ST) – The low shrubs dominate the stand, though the grasses and perennial forbs of earlier successional stages are still well represented, and young Pinus edulis and/or Juniperus osteosperma are poking through the shrub canopy. Note, this condition also includes stands in the aftermath of a pinyon decline outbreak in which percent tree cover and the proportion that are pinyon pine are reduced substantially, or eliminated altogether, although the junipers persist. Stands in this stage that transitioned from the tree-dominated stage following a pinyon decline disturbance may include a low percent tree cover of old junipers.

      4.   Tree Dominated (TD) – The stand is dominated by a mature canopy of Pinus edulis and/or Juniperus osteosperma. The low shrubs and perennial herbs are still present, but have reduced cover if tree cover is high.

      Succession Transitions.–Succession transitions occur as follows (Figure-model):

      1.   HD - persists from stand age 0 until age 10, after which all stands transition to the HS condition.

      2.   HS - persists until age 50, after which stands begin transitioning to the ST condition. Stands may persist in the HS condition until age 70, after which all stands will have transitioned to ST contingent upon having been in the HS condition for at least 30 years.

      3.   ST - persists until age 150, after which stands begin transitioning to the TD condition. Stands may persist in the ST condition until age 200, after which all stands will have transitioned to TD contingent upon having been in the ST condition for at least 80 years.

      4.   TD - persists until the next stand-replacing disturbance.

*Note, see Succession Rules for the formal implementation of these rules in RMLANDS.

      Wildfire Disturbance Transitions.-Fire kills the trees and shrubs, most of which must re-establish from seed. Herbaceous plants lose above-ground biomass to fire, but most promptly re-sprout from surviving below-ground organs. Wildfires invoke the following potential transitions (Figure-model):

      1.   HD – high mortality wildfire recycles the stand through the HD condition while a low mortality wildfire maintains this condition.

      2.   HS – high mortality wildfire returns the stand to the HD condition while a low mortality wildfire maintains the stand in this condition.

      3.   ST – high mortality wildfire returns the stand back to the HD condition while a low mortality wildfire maintains the stand in this condition.

      4.   TD – high mortality wildfire returns the stand to the HD condition while a low mortality wildfire maintains the stand in this condition.

*Note, see Disturbance Rules for the formal implementation of these rules in RMLANDS.

      Insect/Pathogen Disturbance Transitions.–Pinyon decline kills pinyon trees of all sizes, but does not directly affect junipers, shrubs, or herbs, and invokes the following potential transitions (Figure-model):

      1.   HD – not susceptible to pinyon decline.

      2.   HS – high or low mortality pinyon decline maintains the stand in this condition because the juniper component survives the outbreak, after which the stand resumes its usual successional trajectory.

      3.   ST – high mortality pinyon decline returns the stand to the HS condition while a low mortality pinyon decline recycles the stand through this condition. Note, high mortality pinyon decline returns the stand to the HS condition only when the tree component is relatively pure pinyon pine; otherwise, the juniper trees would survive and maintain the stand in the ST condition. Thus, for purposes of RMLANDS modeling, since we did not attempt to map the distribution of pure pinyon pine, we estimated the proportion of this cover type that was pure pinyon (and therefore subject to a high mortality outbreak) and used this to estimate the high versus low mortality response following an epidemic.

      4.   TD – high mortality pinyon decline returns the stand to the HS condition while low mortality pinyon decline returns the stand to the ST condition. Similar to the ST condition, the high mortality response was based on an estimate of the proportion of this cover type that was pure pinyon.

*Note, see Disturbance Rules for the formal implementation of these rules in RMLANDS.