State 1
Reference

Community 1.1
Savannah (HCPC)

Figure 5. Volcanic Hills 16-20" pz., Savannah (HCPC)

The historic native state includes the native plant communities that occur on the site, including the historic climax plant community. This state includes other plant communities that naturally occupy the site following fire, drought, flooding, herbivores, and other natural disturbances. The historic climax plant community represents the natural climax community that eventually reoccupies the site with proper management. The potential plant community on this site is a diverse mixture of warm and cool season perennial grasses, ferns, forbs, succulents, shrubs and trees. A tree canopy of Mexican live-oak species and juniper ranges from 5-15% and lends the site a savannah appearance. Most perennial herbaceous species are well dispersed throughout the plant community. A few species occur only under tree canopies. Other species like herbaceous sage and amole occur in patches of various sizes on this site. With continuous grazing, mid-grasses decline and are replaced by a continuous turf of species like blue grama, purple grama and curly mesquite. Naturally occurring wildfires in June-August were an important factor shaping this plant community. Fire-free intervals ranged from 10-20 years. In the absence of fire and/or with continuous grazing, species like amole, mimosa, mesquite, prickly pear and one-seed juniper can increase. Oak species on the site are very tolerant of fire. Stones, cobbles and gravels protect the soil from erosion after fire or heavy grazing use. Trees range from 5-50 stems per acre. Periodic drought can occur in this MLRA and can cause significant grass mortality. Significant drought has occurred in the past in the 30s, mid 50s, 1975-76, 1988-89, 1995-96 and 2002. Grasshopper infestations can cause considerable grass mortality, especially in drought years. Bare areas can be taken over by annuals like goldeneye, red brome, little barley and bitterweed, in years with wet winters following drought, or in grasshopper out-break years.

Community 1.2
Shrubs, Succulents & Grass

Shrubs and succulents dominate the plant community with lesser amounts of perennial grass canopies. With the return of favorable climate and fire the site returns to a savannah aspect.

Pathway 1.1a
Community 1.1 to 1.2

Extended periods with no disturbance (fire or grazing) allow the build-up of perennial grass biomass that result in plant decadence while shrubs and succulents flourish. Periodic drought also causes significant grass mortality.

Pathway 1.2a
Community 1.2 to 1.1

With managed grazing, favorable climate and fire this site recovers moderately well in 2 to 3 years.

State 2
Exotic grasses

Community 2.1
Exotic grassland

This state occurs where non-native grass species like Lehmann, Boer and weeping lovegrass, and yellow bluestem have invaded the plant community from adjacent areas with a seed source or from roads and trails running through the site. As these species increase to dominate the site native perennial grasses and forbs are reduced to minor amounts.

State 3
Shrub Invaded

Community 3.1
Mixed Shrubland

Figure 8. Volcanic Hills 16-20" pz., Mixed Shrubland w/ mimosa and juniper

This state occurs where shrubby species like mesquite, juniper, mimosa, whitethorn, ocotillo and prickly pear have invaded or increased to dominate the plant community in the absence of fire for long periods. Sufficient fine fuels still exist to carry fires, but shrubs are well established and re-sprout and quickly re-assume dominance.

State 4
Eroded

Community 4.1
Eroded Dense Shrubland

Figure 9. Volcanic Hills 16-20" pz., Eroded w/ annual goldeneye

This state occurs where sheet and rill erosion is accelerated due to severe trailing, soil compaction and lack of perennial grass cover. In some places it is due to poorly designed road and trail construction. As the soil surface is lost to erosion, clayey (argillic) horizons, and next, hard volcanic bedrock is exposed at the surface. This greatly alters surface hydrology (more, faster runoff) and reduces potential productivity of the site.

Transition T1A
State 1 to 2

Non-native bunchgrass seed is purposely planted or inadvertently introduced into the plant community (wind-blown or mechanical transport). Disturbances such as fire or drought can disrupt the native perennials allowing the non-native grasses an opportunity to expand their range from disturbed or planted areas. Long term events such as continuous unmanaged grazing or community phase pathway 1.1a allow non-native bunchgrasses a competitive advantage over natives.

Transition T1B
State 1 to 3

Continuous unmanaged grazing with heavy to severe utilization reduces perennial bunchgrasses. Fire continuity, intensity and frequency are reduced.

Restoration pathway R2A
State 2 to 1

No restoration pathway known at this time. Perhaps future development of herbicide or biological treatment to remove perennial exotics will occur.

Transition T2A
State 2 to 3

Continuous unmanaged grazing with heavy to severe utilization results in persistently low perennial grass cover and extended fire free periods. Remnant native perennial grasses cannot re-colonize areas with shrub competition.

Restoration pathway R3A
State 3 to 1

Woody species control, native species seeding (as needed) supported by managed grazing. Shrub control maintained with herbicide and/or prescribed burning.

Restoration pathway R3B
State 3 to 4

Restoration activities conducted when a non-native seed bank is present on site (African lovegrasses or yellow bluestem present along trails, roads or in disturbed areas) can result in an exotic grassland community. Native species seeding may enhance the native grass component. Restoration practices are woody species control and native species seeding (as needed) supported by managed grazing. Shrub control maintained with herbicide may favor the native grasses while prescribed burning may favor non-natives. Burning the mixed shrub community with a non-native grass seed source present can result in an exotic grassland co-dominant with shrubs.

Transition T3A
State 3 to 4

Long-term, continuous, unmanaged grazing with heavy to severe utilization affects soil site stability and hydrologic functioning. Reduced soil cover, compaction, and A Horizon loss compound the effect of plant community changes (increased shrub/decreased perennial grass community) to increase surface water run-off rather than infiltration. Drought conditions accelerate this transition. Persistent reduced infiltration severely limits perennial grass recruitment.