Ecological dynamics

MLRA-42, SD-1: Loamy

Overview

The loamy site occurs largely on relatively level, lower portions of piedmont slopes. Soils classified as loamy vary continuously in texture and soil moisture regime such that different potential communities occupy different soil series and different landscape positions within soil series. Loamy soils intergrade with sandy soils and clayey soils.

The historic community type is usually dominated by black grama (Bouteloua eriopoda). In lower positions and depressions blue grama (Bouteloua gracilis) may dominate. Dropseeds (Sporobolus cryptandrus), galleta (Pleuraphis jamesii), and burrograss (Scleropogon brevifolius) are commonly subdominant. Continuous heavy grazing leads to reductions of black grama. If black grama cover is reduced to trace levels, it may not recover leaving a galleta-dominated state. Continued poor grazing management may eliminate most galleta and dropseeds, leaving a burrograss-dominated state. On soils where blue grama is common, it may also be driven to a burrograss state. Although soil-sealing may occur in some loamy soils (especially in the burrograss state), shrub invasion is not usually observed.

Catalog of states and community pathways

State Containing Historic Plant Community

Grama Grassland: Black grama, followed by galleta, and dropseeds, was likely to have dominated most areas included within this site. In slight depressions and shallow swales (Turney loam) or areas near to mountain footslopes (e.g., Latene loam) that receive more rainfall, blue grama may have been co-dominant or dominant. On the ungrazed (for 35 yr) Sevilleta NWR, soils where blue grama was dominant within areas mapped as Turney loam or loamy sand were more heavy textured than where black grama occurred5. Shifts between black and blue grama dominance may also occur through time on some soils due to variation in the seasonality of soil moisture6. Blue grama tends to recover more quickly from disturbances such as grazing and fire and may be favored by disturbance4. Where blue grama does not occur, galleta and dropseeds increase in representation as black grama declines with heavy grazing pressure. With prolonged heavy grazing, burrograss may become dominant.

Diagnosis: Grass cover is uniform with bare patches typically > 30 cm in width. Black grama and/or blue grama is dominant. Litter covers much of interspaces such that raindrop impact on the soil surface is low.

Additional States:

Galleta-dominated: Black grama abundance is persistently depressed or eliminated, leaving galleta, dropseeds, and burrograss as dominants. Blue grama may also be present. Burrograss tends to increase with grazing pressure. Overall grass cover may be somewhat reduced, but galleta and other grass growth may compensate black grama’s absence. This state is not frequently observed.

Diagnosis: Black grama is very rare or absent. Galleta, dropseeds, or burrograss are dominant. Bare ground patches up to 1 m across may be observed, with patchy evidence of soil sealing on some soils.

Transition to Galleta-dominated (1a) Loss of black grama grass cover due to continuous heavy grazing through drought periods causes the transition. Once most plants are eliminated, recovery of black grama cover via stolons would require periods of high summer rainfall, and be very slow even with grazing rest. Establishment by seed appears to be limited.

Key indicators of approach to transition:
? Overutilization, decadence, and mortality of black grama.
? Absence of stolons
? Increasing size of bare ground patches

Transition back to Grama Grassland (1b) The recolonization of black grama may be facilitated by prescribed grazing and seeding in SD-1, although establishment rates are low.

Burrograss fluctuates from 15-28%cover or 5-14% without other species colonizing

Bare ground returns slowly to burrograss in extreme cases (35-40 yrs)

Burrograss-Dominated: Burrograss is dominant and interspaces between plants are sealed. Erosion may have stripped away surface soils. Galleta, dropseeds, and other perennial grasses are rare or absent. Burrograss seeds are well adapted to establishment in soils that have developed physical crusts due to raindrop impact1,3. These plants also reproduce by stolons. Long-term monitoring within the Sevilleta NWR indicates that the burrograss-dominated state may be quite stable. Over 20 years in ungrazed conditions on the Turney loam map unit, burrograss canopy cover fluctuated between 15-28% in one transect and 5-14% in another, with very little or no recruitment of other species 5, 6. Burrograss cover may decline to very low levels with drought and continuous grazing and take decades to recover. Burrograss is relatively tolerant of drought3.

Diagnosis: Burrograss is dominant, other grasses are usually rare, and soil sealing in exposed soil is extensive.

Transition to Burrograss state (2a, 3) Intense, continuous heavy grazing, possibly with drought, drives blue grama, galleta and other grasses to low overall cover. Exposed soils (loam and sandy clay loam surface) seal due to raindrop impact or surface horizons are eroded away during this low-cover period. Persisting burrograss recovers over time and other species do not.

Key indicators of approach to transition:
? Overutilization, decadence, and mortality of perennial grasses
? Increase in size and frequency of bare patches
? Evidence of soil physical crusts, lack of cryptobiotic crust.

Transition back to Grama Grassland (2b) Seeding with grazing deferment and soil-surface disturbance would be necessary. If erosion has stripped away part or all of the A horizon, then soil addition would be necessary.

Contributors. Data and ideas were provided by Darrel Reasner, Gary Garrison, George Chavez, Elizabeth Wright, Will Hooper, and David Trujillo.