State 1
Historic Climax Plant Community

Community 1.1
Historic Climax Plant Community

Grassland State: Black grama and sideoats grama are consistent dominants in sustainably-grazed grasslands, with New Mexico feathergrass cover fluctuating with variation in winter-spring precipitation. Grass and forb cover is near 24% and over half of the ground cover is gravel or bare soil. Retrogression caused by heavy grazing leads to increases in bare ground and increasing representation of blue grama, ring muhly (Muhlenbergia torreyi), and wolftail. At high grazing pressures or in harsh conditions, threeawns often dominate. Green sprangletop (Leptochloa dubia), cane bluestem (Bothriochloa barbinodis), plains bristlegrass (Setaria leucopila), and bottlebrush squirreltail (Elymus elymoides) are often eliminated by heavy grazing. Under current climate, fire is probably necessary to maintain this state (thus the grassland may be a disclimax; Johnsen 1962). Diagnosis: Black grama is dominant or co-dominant with sideoats grama and sometimes New Mexico feathergrass. Bare patches are small (> 0.5 m) and infrequent, as are signs of erosion.

State 2
Transition to woody/succulent-enroaching state

Community 2.1
Transition to woody/succulent-enroaching state

ADDITIONAL STATES Transition to woody/succulent-encroaching state (1a): The reduction of grass cover and fine fuels due to grazing, or changes in fire frequency and/or climate independent of changes in grass cover, may facilitate the establishment of juniper or other woody/succulent adults by reducing disturbance rates. The formation of bare ground patches due to grazing may also decrease competition by grass for water and facilitate establishment. These mechanisms probably work in tandem (Johnsen 1962, Gottfreid et al. 1995). Finally, livestock may increase rates of juniper seed dispersal into grasslands, although seed dispersal by native birds and mammals is probably sufficient for juniper establishment (Johnsen 1962). Key indicators of approach to threshold: Decreased fire frequency, increases in bare ground, decreases in litter cover and grass cover, the presence and growth of tree/cactus seedlings. Transition to barren state (3a): Heavy grazing, persistent reductions in grass cover, and associated erosion causes this transition. Key indicators of approach to threshold: Reduction in grass cover, increases in the size and frequency of bare patches, reduction in soil stability, pedestalling of grasses.

State 3
Woody-succulent encroaching

Community 3.1
Woody-succulent encroaching

Woody/succulent encroaching: Grass cover is often reduced and sacahuista plants increase in density or oak or juniper species invade. Mesquite (Prosopis glandulosa) may invade at lower elevations. Cholla may also invade. It is unclear why different species encroach or invade in different situations, but it is likely due to differences in soils, aspect, dispersal pathways, and elevation within this site. West of Silver City, sacahuista encroachment seems to be more common. Vegetation under mature tree canopies is usually sparse. Much of the erosion from bare intercanopy patches is usually intercepted by vegetated patches downslope within this state. Fire must be applied to maintain this state. Diagnosis: Cover of shrubs, succulents, and/or trees exceeds 20% and grass cover is interrupted by large (> 1 m) bare patches associated with shrub or tree-dominated patches. Transition to woody/succulent-encroaching state (2): Continued fire suppression and/or reduced fuel loads allows the development of mature trees. Over time, competitive exclusion of grasses by trees (amplified by grazing disturbance) increases bare ground to a second threshold (that defines the woodland state) beyond which erosion rates may increase rapidly (Davenport et al. 1998). Generally, the probability of crossing this threshold is greater on more erodible soils and steeper slopes. Loss of organic matter, decreased infiltration, and changes to soil structure inhibit the subsequent reestablishment of grasses. Key indicators of approach to threshold: Increasing size and density of trees, reduction in grass cover, increases in the size and frequency of bare patches, reduction in soil stability, pedestalling of grasses. Transition to grass-dominated state (1b): Tree removal (cabling, grubbing, or herbicides) or shrub removal followed by management of grazing to maintain continuous ground cover and maintenance of fuel levels to facilitate fire.

State 4
Woody/succulent dominated

Community 4.1
Woody/succulent dominated

Woody/succulent dominated: This state is characterized by severely reduced grass cover (mostly threeawns or blue grama) and moderate trees/acre). Fuel loads may be too low and trees too large to support fire management. Juniper root systems may monopolize soil water additions. Grass tends to be organized as patches separated by large bare areas. Erosion from bare patches may be extreme and resources may not be intercepted by local vegetation patches. Diagnosis: Grasses are isolated as patches, if present, and bare areas are continuous. Shrubs, trees, or succulents are the dominant vegetation. Transition to barren state (4): The removal of junipers via cabling and/or herbicides would produce a barren state if soil degradation inhibited the recovery of grasses.

State 5
Barren

Community 5.1
Barren

Barren: Grass cover is very low, dominated either by fluffgrass (Dasyochloa pulchella) or blue grama. Snakeweed (Xanthocephalum spp.) may be common depending upon climatic conditions. Erosion rates are high and probably similar to that of the Woody- succulent dominated state, although the cover of trees or succulents is low in this state. Stable barren conditions are rare in WP-3 and are more likely to occur at lower elevations near the transition to SD-2. Diagnosis: Bare ground is interconnected, evidence of erosion is abundant. Blue grama or fluffgrass is the dominant perennial vegetation. Diagnosis: Bare ground is interconnected, evidence of erosion is abundant. Blue grama or fluffgrass is the dominant perennial vegetation. Information sources and theoretical background: Communities, states, and transitions are based upon information in the ecological site description and observations by Gene Adkins and Bill Schwebke, NRCS. Several hypotheses are represented in the explanations for transitions at this site. For juniper invasion thresholds, the favored hypothesis is the fire hypothesis. This holds that frequent fires prevent tree establishment or growth to maturity in healthy grasslands. Fire-free periods of 85-90 years may result in the development of mature juniper woodlands (Tirmenstein 1989 and references therein). If fine fuels produced by grass are reduced below a threshold amount, there may be insufficient fuel to carry fire, or insufficient heat from fire to kill trees or shrubs. According to this hypothesis, trees are better competitors than grasses and can come to dominate grasslands without disturbance (Johnsen 1962). The competition, fire and climate hypotheses may be complementary. The competition hypothesis holds that grassland maintenance depends upon the competitive exclusion of tree seedlings due to limitations in water or nutrients (Johnsen 1962). There may be a threshold grass density below which the probability of juniper establishment increases rapidly, leading to a transition to the woodland state. Junipers compete with grasses directly for water in shallow soil layers in intercanopy areas (Breshears et al. 1997) and juniper removal prior to soil loss can result in dramatic recovery of grass cover (e.g. Aro 1971). Dispersal limitation of junipers to grasslands is believed to have been historically unimportant. The climate hypothesis holds that the current advance of junipers throughout the west is a natural part of long-term cycle of advances and retreats due to fluctuating climate. Belsky (1996) points out, however, that the current advance (during a dry phase) is not consistent with recorded advances during wet phases. Changes in the seasonality of precipitation to wetter winters, however, may be an important factor because junipers are C3 plants. The competition, fire and climate hypotheses may be complementary. Once junipers become established, the erosion hypothesis holds that persistent reduction in grass and litter cover, perhaps in conjunction with trampling, will eventually lead to persistent changes in soil fertility or structure that prohibit the capacity of grasses to reestablish (Allen 1989). Furthermore, erosion degrades the ability of grasses to resist erosion due to pedestalling and increasing environmental harshness to grass, leading to accelerated erosion in a positive feedback (Allen 1989, Gottfried et al. 1995). Soil erosion levels may be highly variable within an area due to soil texture, slope, and land use, thus the ability to recover grasslands upon removal of competing junipers is also variable (Davenport et al. 1999).