Southern Rockies Juniper Woodland

This ecological system occupies the lower and warmest elevations, growing from 1370 to 1830 m in a semi-arid climate, primarily along the east and south slopes of the Southern Rockies and Arizona-New Mexico mountains. It is best represented just below the lower elevational range of ponderosa pine and often intermingles with grasslands and shrublands. This system is best described as a savanna that has widely spaced, mature (>150 years old) juniper trees and occasionally Pinus edulis. Juniperus monosperma and Juniperus scopulorum (at higher elevations) are the dominant tall shrubs or short trees. These savannas may have inclusions of denser juniper woodlands and they have expanded into adjacent grasslands during the last century. Graminoid species are similar to those found in Western Great Plains Shortgrass Prairie (CES303.672), with Bouteloua gracilis and Pleuraphis jamesii being most common. In addition, succulents such as species of Yucca and Opuntia are typically present.

Vegetation structure is typically a savanna with widely spaced, mature (>150 years old) juniper trees and moderately dense perennial grasses in between trees but includes inclusions (patches) of denser juniper woodlands with less herbaceous cover. Vegetation is dominated by an open tree canopy of 2- to 10-m tall Juniperus monosperma. Juniperus scopulorum may be present or dominant at higher elevations. Occasional Pinus edulis trees may be present but have low cover and are typically restricted to mesic microsites. The open to dense herbaceous layer is dominated by perennial grasses that vary with environments. Grass species are similar to those found in adjacent shortgrass prairie and piedmont grasslands. Bouteloua gracilis, Bouteloua curtipendula, and Pleuraphis jamesii are most common with Koeleria macrantha, Lycurus phleoides, Muhlenbergia torreyi, and Piptatheropsis micrantha often present. Midgrasses such as Achnatherum hymenoides, Hesperostipa comata, or Hesperostipa neomexicana are more common in foothills and piedmont stands. Bouteloua eriopoda and Bouteloua hirsuta are more common grass in the southern extent, while Andropogon hallii and Muhlenbergia pungens are characteristic of deep sandy sites. Forbs such as Astragalus spp., Cryptantha cinerea var. jamesii (= Cryptantha jamesii), Eriogonum jamesii, Erigeron divergens, Hymenopappus filifolius, Ipomopsis multiflora, Mentzelia spp., and Penstemon spp. are also common. Shrubs are poorly represented or absent except the ruderal subshrub Gutierrezia sarothrae and succulents such as Cylindropuntia imbricata, Opuntia phaeacantha, Opuntia polyacantha, Yucca baccata, and Yucca glauca. Other occasional shrubs may include Artemisia bigelovii, Rhus trilobata, or Cercocarpus montanus.

This ecological system occupies the lower and warmest elevations, growing from 1370 to 1830 m primarily along the east and south slopes of the Southern Rockies and Arizona-New Mexico mountains. It is best represented just below the lower elevational range of ponderosa pine and often intermingles with grasslands. In the canyons and tablelands of the southern Great Plains, this system forms extensive cover at some distance from the mountain front.

Climate: Climate is cool-temperate, continental, and semi-arid. Precipitation ranges from approximately 33-46 cm (13-18 inches) annually and has a bimodal distribution with moisture peaking in winter and summer. However, most precipitation generally occurs during the summer growing season.

Physiography/landform: Stands occur on gentle upland and transitional valley locations, where soil conditions favor grasses (or other grass-like plants) but can support at least some tree cover. Some savannas apparently have sparse tree cover because of edaphic or climatic limitations on woody plant growth (Romme et al. 2009).

Soil/substrate/hydrology: Savannas are found on moderately deep to deep, coarse- to fine-textured soils that readily support a variety of growth forms, including trees, grasses, and other herbaceous plants, and in regions that receive reliable summer rainfall that fosters growth of warm-season grasses (Romme et al. 2009). This type appears to be especially prevalent in the basins and foothills of northeastern New Mexico, where a large portion of annual precipitation comes in the summer via monsoon rains (Romme et al. 2009).

Juniperus monosperma is a long-lived, slow-growing, drought-tolerant small tree (3-12 m in height) that also occurs as a tall shrub (Johnson 2002). It is more drought tolerant than Pinus edulis and often occurs without pinyon on more xeric, lower elevation sites (Johnson 2002). It is also non-sprouting and may be killed by fire (Wright et al. 1979). Juniper stands at cooler, higher elevation sites typically occur on xeric microsites that are too arid for pinyon or on post-disturbance sites such as where extended drought or ips beetle (Ips confusus) epidemics have eliminated pinyon from mixed pinyon-juniper stands. In this situation junipers and shrubs may act a nurse plants providing shade for pinyon germination and re-establishment, converting a juniper woodland to pinyon-juniper woodland.

Within a given region, the density of trees, both historically and currently, is strongly related to topo-edaphic gradients. Less steep sites, especially those with finer-textured soils, are where savannas, grasslands, and shrub-steppes have occurred in the past. Juniper stands on these gentler slopes may have been larger but more savanna-like, with very open upper canopy and high grass production. Expansion of juniper into previously non-wooded areas occurred prior to European settlement on some sites, although this expansion may have been more extensive in the 20th century versus the previous. However, loss of juniper from marginal sites also occurred historically and recently in some areas (Romme et al. 2009). Especially in areas in which trees were historically rare or absent, there have been type conversions such that the historical condition is unidentifiable/replaced today. An important result of expansion into formerly non-wooded areas in many regions is that formerly heterogeneous mosaics of small patches of woodland, shrubland, and grassland are becoming more homogeneous as trees become established in the shrubland and grassland patches (Romme et al. 2009).

Past fire regimes in southwestern juniper woodlands were mixed, having both surface and crown fires, reflecting variable intensity and frequency depending on site productivity. "Productive sites" could sustain patchy fires at intervals of 10-50 years and could have attained densities sufficient to carry crown fires at intervals of 200-300 years. In open stands, where grass cover was continuous, fire intervals might have been 10 years or less, and probably maintained grasslands and savannas (Gottfried et al. 1999). Romme et al. (2009) state that low-severity fires were probably uncommon except in savannas and in small patches in persistent woodlands.

Soil texture drives the fire regime. Sites with higher potential for graminoid understory will have higher fine-fuel loading and create the spread component for more frequent and lower intensity fires. Sites with shallow, gravelly soils produce less grass and more shrub components, less fire frequency, more lethal when wind-driven events occur (LANDFIRE 2007a).

LANDFIRE developed a state-and-transition vegetation dynamics VDDT model for this system which has five classes in total (LANDFIRE 2007a, BpS 2711190). The model was reviewed and reference to pinyon were removed then summarized as:
A) Early Development 1 All Structures (10% of type in this stage): Grass/forb/shrub/seedling - usually post-fire. Cover is 0-30%. Shrub height is 0-5 m. This class succeeds to B, a mid-open stage after approximately 70 years; however, it could be much longer depending on size of burn. Recruitment is even more episodic in response to optimal climate conditions than in ponderosa. An alternate successional pathway could take this class to class C, a mid-development closed stage, with a probability of 0.015. Replacement fire occurs infrequently, every 400 years. Competition/maintenance can maintain this stage, with a probability of 0.01.

B) Mid Development 1 Open (10% of type in this stage): Tree cover is 11-40%. Tree height is 5.1-10 m. Mid-development, open (<40% cover) juniper stand with mixed shrub/herbaceous community in understory. Review for MZ27 suggested this might even be lower canopy cover to 20%. This class succeeds to class E, a late-open stage after approximately 170 years. An alternate successional pathway could take this class to class D, a late closed stage, with a low probability of 0.002. Replacement fire occurs infrequently, every 500 years. Surface fire occurs every 25 years. Mixed fire occurs every 300 years. Competition/maintenance can maintain this class in class B, with a probability of 0.007.

C) Mid Development 1 Closed (10% of type in this stage): Tree cover is 41-70%. Tree height is 5 m. Mid-development, dense (>40% cover) pinyon-juniper woodland; understory being lost. Review for Map zone 27 suggested this might even be lower canopy cover to 30%. This class succeeds to D, a late-closed stage after 100 years. Mixed fire in this stage either causes no transition (every 1000 years) or brings it to an open mid stage (every 200 years). Surface fire occurs infrequently (every 1000 years) and causes no transition. Replacement fire also occurs infrequently (every 500 years).

D) Late Development 1 Closed (5% of type in this stage): Tree cover is 41-70%. Tree height is 10.1-25 m. Dense, old-growth stands with multiple layers. Late-development, closed pinyon-juniper forest. May have all-aged, multi-storied structure. Moderate mortality within stand. Occasional shrubs with few grasses and forbs and often much rock. Review for MZ27 suggested this might even be lower canopy cover to 11-35%. This class can persist. Mixed fire can cause this class to move to a late open stage, class E, but very infrequently - every 200 years. Replacement fire occurs very rarely (6-700 years), and surface fire also occurs very, very rarely. Insect/disease can also open this class and cause a transition to the late-open stage, class E, every 200 years. This interval may be even longer. Also, drought likely plays a major role, but it was not modeled here.

E) Late Development 1 Open (conifer-dominated - 65% of type in this stage): Tree cover is 11-40%. Tree height is 10-25 m. Late-development, open juniper-pinyon stand with "savannah-like" appearance; mixed grass/shrub/herbaceous community. This class persists. Replacement fire occurs infrequently - every 500 years. Mixed fire also occurs infrequently - every 200 years, and surface fire every 25 years, but neither cause a transition. Insect/disease occurs every 200 years but causes no transition. This interval may be even longer. Also, drought likely plays a major role, but it was not modeled here.

Other important ecological processes include drought, insect infestations, pathogens, herbivory and seed dispersal by birds and mammals. Juniper berries crops are primarily utilized by birds and small mammals (Johnsen 1962, McCulloch 1969, Short et al. 1977, Salomonson 1978). The most important dispersers of juniper seeds are birds, although many mammals also feed on them. These animals consume juniper berries and excrete viable scarified juniper seeds, which germinate faster than uneaten seeds, over extensive areas (Johnsen 1962, Meeuwig and Bassett 1983). Primary juniper seed dispersers are Bohemian waxwing (Bombycilla garrulus), but cedar waxwing (Bombycilla cedrorum), American robin (Turdus migratorius), turkey (Meleagris gallopavo), and several species of jays are also dispersers (Johnson 2002, Scher 2002).

There are several insects, pathogens, and plant parasites that attack juniper trees (Meeuwig and Bassett 1983, Gottfried et al. 1995, Rogers 1995, Weber et al. 1999). For juniper, there are several insects, plus the fungus blackstain root-rot (Leptographium wageneri) and juniper mistletoe (Phoradendron juniperinum). Mistletoe reduces vigor and causes occasional dieback but rarely causes mortality (Meeuwig and Bassett 1983). The insects are normally present in these woodland stands, and during drought-induced water-stress periods, outbreaks may cause local to regional mortality (Gottfried et al. 1995)

Many juniper savannas and woodlands in the Southwest have high soil erosion potential (Baker et al. 1995). Several studies have measured present-day erosion rates in juniper woodlands, highlighting the importance of herbaceous cover and cryptogamic soil crusts (Baker et al. 1995, Belnap et al. 2001) in minimizing precipitation runoff and soil loss in juniper woodlands.

Although juniper woodlands and savannas are expected to occur naturally on the landscape, the extent and quality of this system have been severely altered since the early 1900s. Numerous studies have shown that juniper has encroached on shrublands and grasslands (e.g., West 1999b). Processes that influence the formation and persistence of juniper savannas include climate, livestock grazing, altered fire regime, tree harvest (fence posts), and insect-pathogen outbreaks (West 1999b, Romme et al. 2009).

The altered fire regime (intensity and frequency) in this savanna system in the form of fire exclusion has also allowed for juniper infill in some stands as well as expansion of juniper trees into the surrounding grasslands (West 1999b, Romme et al. 2009). Heavy grazing by livestock reduces fine fuels and indirectly decreases fire frequency, favoring fire sensitive woody species such as Juniperus monosperma. This may result in uncharacteristically high cover of trees (infilling) that shade out the grassy understory as it transitions from savanna to woodland, as well as tree invasion into adjacent grasslands. Some people confuse these younger juniper woodlands with true woodlands dependent on naturally fire-protected features such as rock outcrops. Lacking understory to carry fire, these woodlands only burn under extreme fire conditions resulting in high-intensity, high-severity stand-replacing fires. With loss of perennial grass cover with tree shading, these stands may have difficulty re-establishing the native perennial grass-dominated juniper savanna. Additionally, these stands are vulnerable to invasion by non-native annual grasses such as Bromus arvensis that can increase fire frequency beyond the natural fire regime.

Juniper savanna is typically invasive in lower valleys, mesas and rolling plains if deep soils, but natural if medium (~shallow) depth soils, e.g., low rises between drainages typically with large seemingly old junipers (LANDFIRE 2007a).

In addition, many stands within this system have been impacted by past range practices of chaining, tilling, and reseeding with exotic forage grasses and prescribed burning to reduce juniper and increase forage production, which have had mixed results. Although the dominant trees appear to regenerate after such disturbances, the effects on understory and soil crust species are poorly known. More study is needed to understand and manage these woodlands ecologically.

This system occupies the lower and warmest elevations, growing from 1370 to 1830 m elevation in a semi-arid climate, primarily along the east and south slopes of the Southern Rockies and central New Mexico mountains. This includes the Sacramento Mountains, especially the east side; the west side has Madrean elements but is mostly southern Rocky Mountains. This system also occurs in the canyons and tablelands of the southwestern Great Plains extending some distance from the mountains. It may occur along the Cimarron River in the panhandle regions of Oklahoma and Texas, and in the very southwestern corner of Kansas.