Ecological dynamics

MLRA 36 occurs on the higher elevation portion of the Colorado Plateau. The Colorado Plateau is a physiographic province which exists throughout eastern Utah, western Colorado, western New Mexico and northern Arizona. It is characterized by uplifted plateaus, canyons and eroded features. The Colorado Plateau lies south of the Uintah Mountains, north of the Mogollon transition area, west of the Rocky Mountains, and east of the central Utah highlands. The higher elevation portion of the Colorado Plateau which is represented by MLRA 36 is characterize by broken topography, and lack of perennial water sources. This area has a long history of past prehistoric human use for thousands of years. MLRA 36 shows archaeological evidence indicating that pinyon-juniper woodlands where modified by prehistoric humans and not pristine and thus where altered at the time of European settlement (Cartledge & Propper, 1993). This area also included natural influences of herbivory, fire, and climate. This area rarely served as habitat for large herds of native herbivores or large frequent historic fires due to the broken topography. This site is extremely variable and plant community composition will vary with the water fluctuations on this site.

There is a winter-summer bimodal precipitation pattern on this part of the Colorado Plateau. Meaning that this site developed under climatic conditions that include wet, cold winters, and hot, dry summers with summer rains. This area has climatic fluctuations and prolonged droughts are common occurrences. Between an above average year and a drought year, forbs are the most dynamic (Passey et.al. 1982) and can vary up to 4 fold. The precipitation and climate of MLRA 36 are conducive to producing Pinyon/juniper, and sagebrush complexes with high productive sites in the bottoms of the canyons. Predominant species on the Colorado Plateau are Wyoming big sagebrush (Artemisia tridentata var. wyomingensis), mountain big sagebrush (A. tridentata var. vaseyana), and black sagebrush (A. nova), Basin Big Sagebrush (A. tridentata var. tridentata), Utah Juniper (Juniperus utahensis) and Pinyon (Pinus edulis).

This ecological site is located in draws, small valley bottoms and drainage-ways on the Colorado Plateau. Herbivory and fire are the dominant disturbance factors in the sagebrush biome (Boyd et. al., 2014). Removal or repeatedly detrimental application of either of these disturbances on the system and/or the landscape can result in a shift of the ecological dynamics of this system. The ecological dynamics of the associated uplands are influential to the dynamics of this site. Excessive runoff and erosion from degraded uplands can increase concentrated flow and increased potential erosion towards gully formation. This site is located in the bottoms where fire occurred more frequently, removing brush and keeping it in a grassland state.

The plant community on this site is very productive and relatively resilient compared to its surrounding uplands. It has deep soils with good water holding capacity that receive a large amount of moisture from run-in and overflow water. Because of these factors many of these sites are now or once were cleared and converted to agricultural lands depending on their accessibility.

This site is dominated by perennial grasses but shrub species are present with Basin Big Sagebrush. Although, this site evolved with grazing and is relatively durable, it will be affected negatively with continuous heavy grazing pressure. Livestock are attracted to these sites because they are so productive and these sites lie in low areas with gentle slopes. Proper distribution of livestock is critical. Appropriate placement of fencing, water, and salt/supplement is necessary. Heavy grazing combined with drought and/or lack of fire, will cause this site to become shrub dominant. As shrubs increase and perennials are weakened due to heavy grazing and/or drought, undesirable annuals such as cheatgrass may invade the site. As perennial grasses with fibrous root systems are replaced by weaker annuals, erosion may occur more rapidly. Runoff from the uplands can cause channels to form and incise dropping the water table. As soils dry out, greasewood may begin to invade into these sites. This seems to occur more frequently at lower elevations.

A greasewood site occurs in the similar ecological site (Alkali Bottom) in MLRA 35. The 2 bottom sites have not been broken out very well in published soil surveys and this will be need to be done in the future. The greasewood site occurs on soils that are high in salinity and alkalinity. Loamy bottom can transition in to an alkali bottom on the landscape. This typically occurs in the lower ends of the drainages, possibly because of salt accumulations from upstream or salt layers that are exposed through erosion and where the landscape has transitioned into the warmer portion of mesic temperature regime.

Soils, topographic location, climate, periodic droughts and fire influenced the stabilization of the Reference State on this site as was the case on most drainage-ways and draws ecological sites. The Reference State is presumed to be as found by European settlers in the early 1800's developed under the prevailing climate over time along with the soils in their topographic location. Grazing and/or browsing by wildlife influenced the plant community as well. Sagebrush below 8500 feet has been slower to recover from settlement of the west (Winward, 2004). This site has been found to have basin big sagebrush.

Fire size prior to 1850 were most likely a large number of small to medium size mosaic burns and since 1980 can be typified by a few very large fires due to human caused changes (Evers, et al, 2011). This change in fire return intervals and intensities was cause by fire suppression and reduced fine-fuels from livestock grazing practices around the late 1800’s and early 1900’s. Since fire is not always available to be applied, then other shrub management may necessary from time to time to help keep the community in balance. Treatment response will vary among sites due to differences in vegetation composition and abundance, soils, elevation, aspect, slope and climate (McIver, et al, 2010).

Natural fire played an important role in the function of most upland sites. Fire keeps sagebrush stands from getting too dense, while invigorating other sprouting shrubs. Fire helped to keep a balance between the grasses, forbs and shrubs. Plant community dynamics were improved by opening up canopies and stimulating forb growth creating a mosaic of different age classes and species composition. Shrubs which re-sprout (yellow rabbitbrush, rubber rabbitbrush), are suppressed for a time allowing grasses to dominate. If periodic fire or some other method of brush control is not used, then sagebrush will slowly increase and can begin to dominate the site.

There has been shrub die-off in several sagebrush taxa in the past 10-15 years due to several factors. The two dominant factors are disease/pathogens and drought. Die-off due to disease/pathogens is believed to be tied to disease or stem/root pathogens occurring in dense over-mature sagebrush stands throughout the west. While in some areas, when the factors of drought and heavy browsing occurring in conjunction with disease/pathogens complete areas are dying.

Small wet areas can occur occasionally when the bottoms are very narrow and small, allowing for more water accumulation. This may support willows, sedges and rushes and may be a small inclusion of a wet meadow or salt meadow ecological site. Cottonwoods (Fremont and/or Narrowleaf) can occasionally be found in these inclusion areas. Cottonwood are reported to be a facultative wetland species which are tolerant of frequent and prolonged flooding and are not drought resistant (Simonin, 2001). This site is typically too dry to sustain cottonwoods.

A weedy community that has not been depicted on the model can arise if domestic grazing is completely excluded or if there is repetitive, season-long, heavy domestic or wild ungulate use. With grazing exclusion, a thick thatch of residual litter build-up results in shading and a reduction of health, vigor and production of all plants. Improper domestic and wild ungulate grazing use results in preferred species being repeatedly clipped which decreases their health, vigor and production. Both make a site susceptible to invasion by noxious and/ unsuited species.

Few, of any fire histories have been conducted on basin big sagebrush. It is suggests that fire return intervals in basin big sagebrush are intermediate between mountain big sagebrush and Wyoming big sagebrush (Tirmenstein, 1999). Wyoming big sagebrush communities are less prone to fire than mountain big sagebrush. Wyoming big sagebrush communities in the western United States have fire return intervals ranging from 10-115 years (West and Hassan 1985, Evers, et al, 2011, Johnson, 2000). Fire return intervals for Wyoming big sagebrush vary greatly depending on locale differences in precipitation and temperature. The fire return interval habitats with mountain big sagebrush is 10 to 40 years (Boyd et. al., 2014, Johnson, 2000). Johnson (2000) states that basin big sagebrush fire regime is 12 to 43 years. LANDFIRE models fire regimes of Basin and Wyoming Big Sagebrush communities have been found to have a fire interval of 33 to 116 years (USDA, 2012). LANDFIRE data is for general vegetation types in the United States: no specific data for basin big sagebrush dominated sites exist on Colorado Plateau at available at this time.

Variability in climate, soils, aspect and complex biological processes will cause the plant communities to differ. These factors contributing to annual production variability include wildlife use, drought, and insects. Factors contributing to special variability include soil texture, depth, rock fragments, slope, aspect, and micro-topography. The species lists are representative and not a complete list of all occurring or potentially occurring species on this site. The species lists are not intended to cover the full range of conditions, species and responses of the site. The State & Transition model depicted for this site is based on available research, field observations and interpretations by experts and could change as knowledge increases. As more data is collected, some of these plant communities may be revised or removed, and new ones may be added. The following diagram does not necessarily depict all the transitions and states that this site may exhibit, but it does show some of the most common plant communities.