Ecological dynamics

Abiotic Factors

Occasional ponding that may be of long duration, a shallow water table, and salic soils in an extremely arid and warm climate on the edges of soft playa margins are the dominant features driving this ecological site. These harsh conditions prevent most plants from surviving. Saltgrass (Distichlis spicata) is the dominant, and generally the only, vascular plant species present. This ecological site is known from the western shore of Soda Lake, California, a discharge playa where water is at or near the surface due to a surficial spring. To the east are barren playa surfaces where the groundwater declines, and evaporation causes heavy accumulations of salts to form on the playa surface, conditions that are restrictive to plant growth. Towards the ephemeral river fan delta, sand deposits cover the playa surface, creating less saline and less anoxic conditions that may be colonized by deeper-rooted species.

Saltgrass is a warm-season, rhizomatous, native perennial grass (Baldwin et al. 2002, Hauser 2006). In the desert southwest and Great Basin, it is considered a facultative wetland species (Lichvar and Dixon 2007), and is most often found in saline habitats where the water table is shallow to the surface (Cluff et al. 1983, Groeneveld and Or 1994, Elmore et al. 2006, Hauser 2006, Lichvar and Dixon 2007, Mata-Gonzalez et al. 2012). With adaptations for high salinity, tolerance of anoxic soil conditions, drought and flooding, and the ability to colonize heavy soils, saltgrass can form very dense meadow communities in habitats where few other species can survive (Cluff et al. 1983). Lacunae tissue allow for gas exchange in inundated, anoxic soils and partial inundation (Hansen et al. 1976, Dahlgren et al. 1997). Salt glands excrete excess salt (Hansen et al. 1976). Saltgrass may also exist where the depth to water table (DTW) is deep; however it does not form monocultural meadows in these situations, and occurs with a mixture of more xeric and less salt tolerant species.

Disturbance dynamics

Seasonal and annual fluctuation in playa surface inundation and accompanying fluctuations in soil surface salinity are the primary drivers influencing vegetation dynamics of this site, with the extent and productivity of the plant community declining with both long duration, deep ponding or excess salinity. Infrequent flooding of ephemeral rivers may cause more extensive flooding on a roughly decadal basis (Enzel 1992). Off-road vehicle (OHV) use and historic cattle grazing are anthropogenic disturbances that may impact this site. Declines in groundwater have the potential to cause this site to cross a threshold to an alternative state, but this has not been observed within the geographic reference of this site.

Saltgrass is tolerant of at least 24 days of inundation (Aldon 1977), most often occurring in habitats subject to intermittent flooding, such as playa margins and tidal marshes (Hauser 2006, Sawyer et al. 2009). Inundation will reduce cover and vigor, and long duration, deep inundation will kill saltgrass. Following inundation, surviving rhizomes are repositories of sediments as floodwaters recede; these incipient hummocks provide sites for new growth and re-colonization of the playa surface (Hansen et al. 1976). Sharp silica coated rhizome tips are especially suited for colonizing heavy playa soils (Hansen et al. 1976). Recently inundated surfaces are characterized by fresh mud deposits, moist soils, and low density saltgrass, with large patches of bare ground between rhizomes. With time, infilling and leaf growth will lead to dense saltgrass meadows.

Soil surface salinity may cause reductions in saltgrass productivity and cover (Warren and Brockelman 1989, Hauser 2006). Surface salinity fluctuates with soil moisture and depth to water table; it is diluted when the soil is wet, i.e. with precipitation or ponding, but as the soil dries capillary action concentrates salts at the surface (Waisel 1972, Rosen 1994, Reid et al. 2000, Brostoff et al. 2001). Patterns of ponding and salinization may be highly dynamic and spatially variable across the playa surface (Rosen 1994), consequently temporal and spatial fluctuations in plant community phases are common in this ecological site. Salt will be drawn to the soil surface only when groundwater remains within the capillary range; if groundwater declines below 3 meters (9.8 feet), surface salinization due to evaporation no longer occurs (Waisel 1972).

Patches occupied by saltgrass may accumulate sands, with deposition leading to hummocks that are higher than the playa surface (Hansen et al. 1976, Cluff et al. 1983). These areas have lower salinity, and are more hospitable for saltgrass growth. Reductions in surface salinity also make these patches more favorable for colonization by more xeric and less salt tolerant species, and when these hummocks occur closer to the sand source at higher elevation on the playa margin, they are likely to be transitional to different plant communities, including R030XY229CA, which is composed of alkali goldenbush (Isocoma acradenia) and saltgrass.

Historic cattle grazing has impacted this ecological site. Saltgrass is tolerant of grazing and trampling (Cluff et al. 1983, Reid et al. 2000); however excessive grazing will lead to declines in cover (Reid et al. 2000). Grazing was removed in 2003 and since baseline conditions are not available, it is unknown what the extent of grazing impacts has been. Recreational OHV use has also impacted this ecological site. OHVs are most disruptive in sand accumulation phases, as they destabilize soils and increase vulnerability to erosion, which can lead to saltgrass mortality.

Significant declines in the water table will alter the community composition of this ecological site, transitioning this site to an altered state where a monoculture of saltgrass is not supported. The severity of the decline would lead to different scenarios. Moderate declines with a lack of ponding state would lead to a state dominated by a barren playa surface with heavy salt accumulations. Unvegetated playa surfaces are more prone to wind erosion, and the resulting dust is a major source of air pollution (e.g. Gill 1996, Lancaster and Baas 1998, Pelletier 2006, Reynolds et al. 2007, Groeneveld and Barz 2013). There is a lack of data demonstrating declines in DTW at Soda Lake. However, groundwater drawdown and diversions of the Mojave River, which contributes intermittent flow to Soda Lake, have had profound ecological effects in lower reaches of the hydrological system, causing mortality in riparian vegetation and reactivating eolian surfaces (Laity 2003). Widespread mortality in phreatophytic trees in the delta feeding the playa, and erosion of sand accumulation patches on the playa, probably previously occupied by saltgrass is currently occurring (NRCS observations), and it is likely that reduced flow reaching the playa has reduced groundwater recharge and ponding, increasing the occurrence of a barren salt-crusted surface state. Any anthropogenic modifications that alter the hydrology of the playa system, including groundwater pumping, drainage ditches, roads, or runoff diversion should be considered a management action adverse to the resiliency of this ecological site.

If the water table drops below three meters and surface salinization no longer occurs, invasion by less salt-tolerant deeper-rooted shrubs such as alkalki goldenbush and cattle saltbush (Atriplex polycarpa) could occur. Further declines in the water table will lead to colonization by upland species such as creosote bush (Larrea tridentata), burrobush (Ambrosia dumosa), and big galleta (Pleuraphis rigida). Multiple studies from a Mojave Desert-Great Basin system and from other playa environments have demonstrated shrub invasion into groundwater dominated saline meadows with declines in groundwater. For example, in Colorado playa communities, saltgrass and rush (Juncus arcticus) meadows existed where DTW was 36 inches (0.9m), and a decline in DTW to 8.2 feet (2.5m) over a 19-year period allowed for phreatophytic shrub invasion (greasewood [Sarcobatus vermiculatus] and rabbitbrush [Ericameria nauseosa]) (Cooper et al. 2006). In the Owen’s Valley, groundwater pumping and a 16 foot (5m) drop in DTW led to phreatophytic and xeric shrub invasion (Torrey’s saltbush [Atriplex torreyi], rabbitbrush, and big sagebrush [Artemisia tridentata]) and grass cover decline in alkali meadows (Pritchett and Manning 2012).