Ecological dynamics

The plant communities occupy a diverse landscape and climatic regime and are influenced by aspect, climate, elevation, livestock grazing, and fire frequency and intensity. The site can be found on near level terrain, rolling hills and knolls, and steep and rocky limestone hills. Aspect has a significant effect on plant composition, species dominance, and fire behavior. Some of the plant communities identified in the state-and-transition model of this ESD are estimated from collected data. For the estimated plant communities, composition and production of plant species and other related numerical values are reconstructed from either similar plant communities or historical accounts and based on ecological principles, historical records, or anecdotal evidence.

The typical climax community is a savannah of oneseed juniper with pinyon pine. Under a normal fire regime, tree spacing would be quite open with dominant mature trees. A dense and vigorous understory of cool- and warm-season plants would be maintained by frequent fires. A low fire-frequency would increase tree and shrub density with commensurate decreases in herbaceous density.

The site varies from a warmer, drier climatic regime to a colder, wetter regime. The warmer, drier type is dominated by oneseed juniper with a subdominant two-needle pinyon pine (hereafter referred to as pinyon) tree component. This type is generally found at lower elevations, on gentler terrain and occurring adjacent to grassland sites. The plant community would be characteristic of a juniper or juniper-pinyon savanna with a canopy cover ranging between 20% and 30%. An example of this is the rolling terrain at lower elevations within map units 828 and 840 in eastern Socorro County. South slopes consist of widely spaced trees, predominantly oneseed juniper. Recurrent fires would maintain the savanna appearance with canopy cover between 20% and 25%. Crown fires would not be prevalent except in isolated instances where sufficient ground fuels exist (such as pack rat nests) that can carry fire into the canopy with the help of strong winds.

At lower elevations, precipitation can be infrequent, making pinyon readily susceptible to drought-induced mortality. On north aspects at lower elevations, the plant community may have a high frequency of standing dead or down pinyon carcasses, leaving an almost pure oneseed juniper stand. This site would be prone to natural fires if the herbaceous understory is dense enough to carry a fire. Livestock grazing can sufficiently reduce fine fuels (grass, forbs, and small shrubs), reducing the natural fire frequency or eliminating fire altogether in some localities.

The colder, wetter locations would occupy higher elevations with aspect strongly influencing the plant community. On north aspects, this site would be dominated by pinyon with subdominant oneseed juniper and scattered alligator juniper. South slopes would be dominated by oneseed juniper with scattered pinyon and alligator juniper. Occasional occurrences of Rocky Mountain juniper and ponderosa pine are found on north slopes. These species are not prominent in the plant community except in the transition between pinyon-juniper woodland and ponderosa pine forest.

In fire-prone landscapes, alligator juniper may outcompete oneseed juniper at the upper extent of the pinyon-juniper zone. In addition, wavyleaf oak may also become the dominant shrub species since it resprouts after fire. Tree density would be substantially less on south aspects resulting in more understory fires which maintains the open tree spacing.

On steeper slopes and at the higher elevations, livestock grazing may be limited to toeslopes, ridgetops, saddles, and less steep slopes making the fire frequency quite variable. The steep slopes have substantial rock outcrops and other surface coarse fragments (stone and cobble) that impede fire spread and momentum. Burning patterns may be irregular, resulting in a mix of surface and crown fire effects on the same slope. Crown fires would decrease oneseed juniper density and favor alligator juniper dominance. Reductions in pinyon may also occur, with an open stand persisting for several decades. Wavyleaf oak, along with hairy mountain mahogany and skunkbush sumac would dominate the understory along with cool-season species. Reestablishment of pinyon will be slow, perhaps requiring a century for pinyon to dominate.

Natural fire would have been part of the historical climax plant community (HCPC) development. Based on the frequency of burned stumps in the warmer, drier zone, the fire return interval may have been short. The fire return interval may have been less than 25 years and as low as 10 years, maintaining a savannah. Livestock grazing can reduce fine fuels significantly making natural fire occurrences less frequent and it may also impeded the use of prescribed fire. In the HCPC within the wetter, colder zone, the fire return interval could have been many decades, with the fuel buildup likely producing a catastrophic fire. Occasional small, low-intensity fires may have consumed understory fuels in localized areas with little effect on stand structure and species composition.

On the warmer, drier areas, the HCPC canopy cover is about 20% on south slopes and 30% on north slopes. In the potential natural community (PNC), canopy cover would range between 25% and 45%. In the colder, wetter zone, the canopy cover for HCPC would range between 20% and 50%, with the PNC having a canopy cover ranging between 35% and 75%. The state-and-transition model depicts an average of these variations for the entire moisture regime.

Tree spacing in the HCPC ranges from 18 to 22 feet in the colder, wetter areas with warmer, drier areas ranging from 18 to 38 feet. In the PNC, a well-managed site including prescribed fire may have a tree spacing ranging from 30 to 45 feet. Where heavy grazing removes fine fuels and fire frequency is substantially reduced, the tree spacing will drop below 18 feet, producing a very dense stand of trees.

Photographs herein may depict plant communities of similar structure and function to those described but with minor differences in species composition and density.

Description of State and Transition Model
State 1:
Plant Community 1 (PC1) is the historical climax plant community (HCPC), self-sustaining through periodic natural fires with minimal susceptibility to insect and disease.

HCPC is generally comprised of an old-age class of pinyon and oneseed juniper trees dominating the plant community. In the wetter moisture regime, alligator juniper may be present but not in significant quantity, possibly with incidental occurrences of Rocky mountain juniper and ponderosa pine. Pinyon and oneseed juniper may coexist in about equal stand density.

In the wetter moisture regime, pinyon may exert dominance, comprising up to 70% of the tree composition. At the drier end of the moisture regime, oneseed juniper comprises between 70 and 95% of the tree composition.

Pinyon will be more common on north aspects and infrequently encountered on south aspects. Recurrent understory burns will reduce the mid-age class trees, leaving remnants as recruitment trees. Young-age class trees will be minimal occurrences in the plant community. Pinyon growth and reproduction decrease with decreasing elevation and moisture. Pinyon will likely be much smaller than on more productive sites, seldom exceeding 12 to 14 inches in diameter at the root collar.

On the colder, wetter areas, especially on north slopes at higher elevations, the sites would appear heavily wooded. At the lower elevations, warmer and drier areas, the plant community would appear more savanna-like, and alligator juniper, Rocky Mountain juniper, and ponderosa pine would not be encountered.

At the lower elevations, frequent fires maintain an open stand. At the wetter moisture regime on steeper slopes, fire frequency would be lower due to abundant, large, surface coarse fragments. The fire return interval may span many decades or more than a century. Lower elevation fires would tend to be understory burns, whereas upper elevation fires would tend to be catastrophic and consume portions of the overstory.

The herbaceous component is comprised of a mixture of warm- and cool-season species, with slight differences in species composition between the drier, lower elevations and the wetter, upper elevations. Shrubs species and densities would also differ by moisture regime.

The HCPC is comprised of 70 to 80% old trees, 15-25% or less mid-age trees, and 0-10% young trees (seedlings and saplings). Tree density ranges from 60 to 130 trees per acre. (Note: The HCPC projections are derived from reconstructed stand structure analysis using existing data and an average description of the drier and wetter moisture regimes.)

Plant Community 2 (PC2) results from overstory tree mortality caused by a crown fire. (Note: The result is from a shift in plant species dominance, not stand replacement.) Overstory fires would be rare, and usually small, at the drier, lower elevations. At the wetter, higher elevations, crown fires are more likely to occur in short up-slope runs reducing tree density and accelerating shrub and grass growth. PC2 will revert to PC1 through succession.

State 2:
State 2 occurs where livestock grazing at light to moderate levels reduces fine fuels (herbaceous plants), thereby reducing fire frequency and increasing the shrub and tree components. With grazing eliminated and land treatments applied (thinning, burning, seeding herbaceous species) State #2 can revert to State #1 (HCPC) and a natural fire regime.

Plant Community 3 (PC3) is the potential natural community (PNC) dominated by pinyon with oneseed juniper as codominant on gentler (less than 30%) slopes. Shrub density decreases due to overstory competition in the absence of fire. Oneseed juniper and pinyon density and canopy cover is greater than in PC1, comprised mostly of mid-age and young-age class trees. Intensity of natural or prescribed fire will be greater because of the high tree density, and therefore, have more of an impact on stand density and composition. A high stand density will add competition for space and nutrients among pinyon trees which will exacerbate drought stress. Drought and disease can cause widespread pinyon mortality, which will move PC3 to PC4. Canopy removal (fuelwooding, thinning) or herbicide treatments can move PC3 to PC5.

On very steep slopes in the wetter moisture regime, the plant community is dominated by pinyon with oneseed juniper as subdominant. Alligator juniper can also be found in the stands with occasional Rocky Mountain juniper and ponderosa pine.

In the drier zone, oneseed juniper would dominate with pinyon as subdominant. Livestock grazing reduces fine fuels (herbaceous plants), thereby reducing fire frequency and allowing young-age class oneseed junipers to increase in density. Thinning and prescribed burning would be required to mimic natural fire regime. Canopy cover would be substantially greater than in the HCPC. This plant community could maintain itself in a state of equilibrium until a catastrophic fire results from long-term woody fuel accumulation (drought, disease, insect) and severe fire weather conditions (high temperature, low humidity, strong winds). A high-intensity fire would shift the plant community to PC5.

Depending on the steepness of the slope, cattle grazing would have a different effect on tree recruitment and fuel accumulation than sheep or goat grazing (browsing). Ridgetops and adjoining upper sideslopes along with toeslopes would be used more by cattle with the mid-slopes less impacted and prone to severe fires.

Moderate sheep or goat browsing on the hillsides would reduce the shrub component and may contribute to increases in grass production; however, heavy browsing on the hillsides may increase sheet erosion and improve conditions for oneseed juniper expansion on the slopes. With drought stress and insect infestations, pinyon mortality would be substantial and would move PC3 to PC4. This plant community would revert to PC3 through succession. Pinyon mortality would create a substantial fuel source and the site would be susceptible to a high-intensity fire, which would result in a shift from PC4 to PC5.

On gentler slopes, livestock grazing would decrease herbaceous plants and allow trees and shrubs to increase. The natural fire frequency may be reduced enough to require prescribed fire to mimic natural fire events, or mechanical and herbicidal treatments could be used to reduce tree densities. This would shift PC3 to PC4.

Plant Community 4 (PC4) is dominated by oneseed juniper with shrub and herbaceous components increasing in density. Grazing can reduce herbage and shrub production, extending the natural fire interval by reducing fine fuels which are also required for prescribed fire. Reduced fire frequency promotes pinyon establishment. As pinyon matures, PC4 reverts to PC3. Prescribed fire or thinning can move PC4 to PC5.

On gentler slopes, wavyleaf oak would increase with a commensurate decrease in herbaceous plants due to shading by shrubs and livestock grazing. Tree canopy reduction by prescribed fire, combined with thinning or herbicidal treatment of the shrubs, may increase grass production, thereby increasing fine fuels and the likelihood of naturally occurring ground fire.

Mechanical treatments can also be used to reduce tree density, and combined with prescribed fire, it will reduce the canopy enough to allow shrubs to dominate for a period of time, moving PC4 to PC5.

PC4 may not occur on steep slopes where fire frequency is extended to several decades or even over a century. A fire on steep slopes would likely be catastrophic and lead to PC5 rather than PC4.

Plant Community 5 (PC5) varies by elevation. At lower elevations (warmer, drier) it may be grassland that can be sustained with some level of natural or prescribed recurrent fires. Four-wing saltbush, winterfat, Mormon tea, and skunkbush sumac would be more common in lightly to moderately grazed areas. Bunchgrasses such as sideoats grama, New Mexico feathergrass, curlyleaf muhly, and galleta would be present with minor amounts of prairie junegrass and littleseed ricegrass. Surface fires would maintain an open stand, with oneseed juniper establishing before pinyon.

At the upper elevations, (colder, wetter) the plant community would be dominated by shrubs such as wavyleaf oak, skunkbush sumac, winterfat, and hairy mountain mahogany. Herbaceous plants would increase for a period of time and recurrent fires would not likely occur. Sideoats grama, New Mexico muhly, prairie junegrass, plains lovegrass, mutton bluegrass, and little bluestem would be present. The site would take a substantial amount of time to become populated with trees, especially on steeper slopes. The shrub-dominated plant community could persist for a long time on steep, rocky hillsides such as in MU830 (Soil Survey of Socorro County Area) or MU91 (Soil Survey of Lincoln County Area). With reduced fire frequency, PC5 eventually reverts to PC3 as trees become established.