Clayey Upland 20-24

Description

The historic climax plant community (HCPC) description was derived from a literature review and assessment of historic photos by The Nature Conservancy (Historical Range of Variation for Potential Natural Vegetation Types of the Southwest. June 2007. TNC). The best potential natural vegetation for this ecological site was sampled for point data to describe the reference community. This reference plant community can be described as belonging to the vegetation grouping by TNC as the mixed native grass type of the semi-desert grassland grouping. Within this type, the species composition varies greatly with soil type and topography. Historically, this vegetation type was found to be an open grassland with low shrub canopy cover (<10%) occurring in an upland position (Gori and Enquist, 2003). According to the TNC, frequent fires maintained this grassland by killing young shrub and tree regeneration with fire return intervals of 2.5 to 0 years. Other studies have surmised that grasslands FRI corresponded to the surrounding woodland or forest. In the USFS-FEIS (Fire Effects Information System) the FRI for Pinyon-Juniper systems was less than 35 years, and for interior ponderosa pine was 2 to 46 years. Dan Robinett suggested a FRI of 10 to 15 years in the Clay Loam Upland Ecological Site Description. Fire return intervals are difficult to determine in non-woody plant communities, due to lack of tree ring evidence, but likely occurred frequently and maintained a grassland versus a shrubland. The TNC used photo interpretation to show that historically the semi-desert grassland area was in an open grassland state, with very little in the shrub or tree state. TNC did a current regional assessment and found that only 17% of extant and former grasslands within the region can now be classified as open (<10% shrub cover) native grasslands. It is thought that missed fire cycles, drought, and grazing have affected the rate of shrub increase, but the exact interactions are unclear (Brown and others, 1997; Cable, 1971; McPherson, 1995; Robinett, 1994).
Fires within the southwest are mainly lightning ignitions, covered large areas and occurred in June and July (Swetnam and Betancourt, 1998). This timing occurred because of cool-season moisture allowing some greenup, and arid foresummer; providing dry conditions followed by pre-monsoonal lightening storms providing ignitions. The dominant shrubs in the semi-desert grasslands are easily killed by fire as seedlings or young plants and do not produce seeds until they are at least 10 years old. Therefore, frequent fires would avoid shrub invasion into grasslands. Many studies show large reductions in shrub cover, such as broom snakeweed and cacti after fires (Bock and Bock,1997; Humphrey, 1949; Reynolds and Bohning, 1956). Studies have also shown that most native perennial grasses show no negative effect after fire, recovering after one to two seasons, or with drought three to four seasons (Bock and Bock, 1992; Gosz and Gosz, 1996; Cable, 1972; Martin, 1983; Wright, 1980).
Normal drought coupled with other disturbance events such as unmanaged grazing can decrease perennial grass cover and increase sub-shrub cover. This is because perennial grasses are generally intensive water exploiters that have a dense network of shallow roots that can easily exploit water found in the shallow soil layers. Therefore, they are able to extract water from these shallow layers during light seasonal rains, where precipitation penetrates most frequently. The sub-shrub, snakeweed, is also an intensive exploiter, and occupies the same soil layers and therefore competes with perennial grasses (Weaver, 1920; Campbell and Bomberger, 1934; Jameson, 1966). They differ in their shoot dieback during dormancy in that grasses usually dieback nearly to the surface whereas subshrubs only in part. This may allow subshrubs to dominate over perennial grass in areas with lower cover. The presence of broom snakeweed and shrubby buckwheat indicates a strong drought influence in the area. As well, Broom snakeweed increases due to lack of fire and has been shown to be an increaser species with grazing. Therefore, within the reference state there are two communities that can cycle naturally between each other.
The TNC did an intense literature review of native perennial grassland succession and developed the following model. It is thought that 2 years post-burn, grass would regenerate with 0 to 5 percent shrub canopy cover and this community would be maintained for a couple of years. In the absence of a stand replacing fire, shrub canopy cover would increase to 6 to 10 percent and this would be stable for 3 to 40 years. This is based on the average frequency of low, moderate, and extreme wet winter precipitation events (every 20 years) and the time (about 20 years) it takes for shrubs to show large cover increases (3-fold) following these wet periods. Drought could also effectively maintain this community with low shrub covers, by killing young shrubs, for about 37 years. Prolonged drought has been shown to cause declines in shrub density and cover within these grasslands. They used moderate drought events (equal to 1950’s drought) in the model to transition vegetation back to its starting conditions within the state, using the average historic moderate drought frequency (from 1,000 years of reconstructed winter precipitation data) (Ni and other, 2001;Bock and Bock, 1997; McClaren, 2003; Turner and other, 2003). Fire would return this community to one with 0 to 5 percent shrub cover. After 40 more years, shrub cover could increase to 11 to 30 percent cover, again maintained for about 37 years with drought cycles. Stand replacing fire would return this community to one with 6 to 11 percent shrub cover. With increasing grazing management intensity, each community can transition to the next with higher shrub cover. When drought conditions are followed by wet winters, shrub cover can be exponentially increased leading to an altered shrub state.
The reference plant community for this ecological site is a western wheatgrass grassland.