Ecological dynamics
This ephemeral stream ecological site occurs in headwaters and first order drainageways on channels and stream terraces on upper fan piedmonts. There is a rarely flooded (1 to 5 times in 100 years) narrow active channel with an associated very rarely flooded (surface flooding may occur 1 in 100 years) broad stream terrace or inset fans. This site has developed in response to infrequent flash flood events, which form sheet flow across the broad flat areas. Headwater streams are hydrologically connected with lower stream reaches, which may be the mid-sized Thermic to Hyperthemic Ephemeral Stream (R030XY186CA) or the Large, Sandy, Thermic Ephemeral Stream (R030XY167CA). The headwater and upper watershed first order drainageways may accumulate nutrients and organic matter during moderate storm events. During dry periods these areas may accumulate organic matter. The organic matter and nutrients are mobilized during large storm events to lower reaches of the drainageway. These nutrients can influence downstream productivity (Levick et al. 2008). The soils are very deep and sandy with very little cohesive structure. They are easily redistributed with flood events. The soil moisture regime is aridic or aridic bordering on xeric, with a thermic soil temperature regime. This ecological site has cooler temperatures and more moisture than the other ephemeral stream ecological sites within Joshua Tree National Park. Drought and fire are also important drivers of plant community succession and hydrological dynamics.
This site is in broad, low lying, headwater positions which accumulate surface and groundwater flow. The drought-tolerant vegetation that exists on ephemeral streams is referred to as xeroriparian vegetation. It is distinct from the surrounding landforms due to a difference in species composition, size, and production (Johnson et al. 1984, Levick et al. 2008). Xeroriparian vegetation is present because of the increased availability of water and flood disturbances in these drainageways. This site does not provide a deep water source for phreatophytes that rely primarily on deep water source, so species like desert willow (Chilopsis linearis) are absent from this site. The active channel is generally bare of vegetation. Along the channel margins, California jointfir, peach thorn (Lycium cooperi), and desert almond (Prunus fasciculata) can be relatively productive. California jointfir, Mojave indigo bush, burrobrush, peachthorn, desert almond, Joshua tree (Yucca brevifolia), and California juniper (California juniper) form an open mosaic on broad terraces. Collectively, all of these plant communities are part of the xeroriparian vegetation, and provide xeroriparian habitat.
California jointfir is a long lived shrub, often associated with ephemeral drainages in arid and semi-arid regions. It is a gymnosperm, reproducing by wind dispersed pollen and bearing seed in cones (Anderson 2004). It only produces abundant seeds during years of ample precipitation. The seeds have a short dormancy period, and will germinate only after the dormancy period and sufficient moisture. Once established, California jointfir is fairly drought tolerant. Desert almond and peachthorn are also long lived shrubs. They produce seeds that are dispersed by animals, by gravity, or by water. Sheet flow disperses the heavier seeds of these long lived shrubs, which germinate in moist soil after floods. Burrobrush is a shorter lived shrub that will establish with any disturbance and brief moisture availability. Mojave indigobush is most abundant in habitats where additional run-on is available.
Other disturbances such as drought, fire, and human hydrologic alterations can affect the community composition and/or hydrologic process of this site. Drought is common in the desert, and can cause mortality or die-back of vegetation. Decreased vegetative cover can lead to increased erosion and change sediment deposition patterns.
Historically fire was very uncommon in these ephemeral drainages; however the presence of continuous and flashy fuels from non-native grasses can increase the possibility of fire. Invasion by non-native annual grasses has increased the flammability of desert vegetation communities (Brooks 1999, Brooks et al. 2004), and after fire, Mojave Desert ecosystems appear to be more susceptible to invasion by exotic grasses, leading to a grass-fire cycle (D'Antonio and Vitousek 1992). The abundance and biomass of these grasses is highest on sandy soils where nitrogen deposition from air pollution from adjacent urban areas is high (Rao et al. 2010). This ecological site is susceptible to high biomass loads of non-native annual grasses due to its sandy soils and proximity to the greater Los Angeles area. Very wet (El Nino) years followed by severe drought produce conditions where large areas of desert scrub may burn (Brown and Minnich 1986).
State 1
Historic State
State 1 represents the historic-natural condition for this ecological site. This state no longer exists due to the ubiquitous naturalization of non-native species in the Mojave Desert. If we were to include dynamics for this state it would be the same as displayed in State 2. The presence of non-native species is significant in State 2, but has not yet altered the hydrology or fire frequency.
Historically, stand-replacing fire was probably very rare in this ecological site due to the absence of non-native grasses to fuel fires (Sawyer et al. 2009). When fire did occur, it was probably low severity surface fire that promoted regeneration of short-lived species (Sawyer et al. 2009).
State 2
Reference State
This state represents the most common and most ecologically intact condition for this ecological site at the present time.
Community 2.1
Reference Phase
Figure 5. CT 2.1 California jointfir - burrobrush
Figure 6. CT 2.1 California jointfir - burrobrush
This community phase is dependent upon unimpaired hydrologic function and average to above average precipitation. There may be spatially distinct plant communities within these ephemeral drainages, but they have not been identified and data was not collected for different communities.
Two community components are present, including:
Community Component 1 (CC1):
Barren active channel.
Community Component 2 (CC2):
This community dominates, and is present on broad, very rarely flooded stream terraces and inset fans. The increased run-on and rare floods provide additional moisture and surface disturbance, which enable species such as California jointfir, burrobrush, peachthorn, desert almond, and Mojave indigo bush to establish. Joshua tree and California juniper (Juniperus californica) are present in some areas.
Forbs are abundant in years of high rainfall, but may be patchily present every year due to run-on. Common forbs include woolly easterbonnets (Antheropeas wallacei), pincushion flower (Chaenactis fremontii), cryptantha (Cryptantha spp.), miniature woollystar (Eriastrum diffusum), whitedaisy tidytips (Layia glandulosa), golden linanthus (Leptosiphon aureus ssp. aureus), distant phacelia (Phacelia distans), and chia (Salvia columbariae).
One or multiple years of heavy winter precipitation such as occurs during El Niño events (Hereford et al. 2006) leads to a heavy standing crop of non-native and native annuals in intershrub spaces, providing a continuous fine fuel layer that puts this community at high risk of fire. Native annuals may fuel fire (Brown and Minnich 1986, Minnich 2003), but pose a threat only in the first dry year following a wet year (Minnich 2003). The thatch created from non-native annual grasses is much slower to break down, and can create high-risk fire conditions for several years following heavy precipitation (Minnich 2003, Brooks and Minnich 2006, Brooks et al. 2007, Rao et al. 2010). Unlike the historic state, where fire return intervals were long enough to allow for recovery of burned communities, fire in the reference state may trigger a cycle of increased fire frequency, resulting in a transition to a new state characterized by non-native annual grasslands, with scattered native species.
Table 5. Annual production by plant type
| Plant type |
Low
(kg/hectare) |
Representative value
(kg/hectare) |
High
(kg/hectare) |
| Grass/Grasslike |
56 |
404 |
560 |
| Shrub/Vine |
179 |
269 |
359 |
| Forb |
– |
28 |
39 |
| Total |
235 |
701 |
958 |
Community 2.2
Drought Response
This community develops with prolonged or severe drought. It is difficult to determine the exact duration or intensity of drought that will cause this change, but a one to two year severe drought (of approximately 60 percent or less of average precipitation) can cause severe mortality in short lived perennials (Bowers 2005, Hereford et al. 2006, Miriti et al. 2007). Long-lived shrubs may have severe branch die back, or mortality depending upon severity and duration of the drought. Long lived shrubs, affected by previous drought are more likely to succumb during a second drought (Hereford et al. 2006).
Seed development, germination, and seedling survival are negatively affected during drought years. In the absence of flood events, seeds are not dispersed by sheet flow, and moisture is lacking in the soil to initiate germination. Some shrubs, such as California jointfir, only produce seed in years of normal or higher than average precipitation. Seedling survival is dependent upon adequate moisture during the early stages of development.
The plant community components remain similar to those described in Community Phase 2.1, but show a decline in overall health, cover, regeneration, and production due to drought. Shorter lived species such as Cooper's dogweed (Adenophyllum cooperi), Wiggins' cholla (Cylindropuntia echinocarpa), Mojave rabbitbrush (Ericameria paniculata), yucca buckwheat, (Eriogonum plumatella), burrobrush (Hymenoclea salsola), may suffer high mortality. Longer lived species may have branch die back, or mortality depending upon the severity of drought.
With prolonged or severe drought, canopy cover declines leaving more open spaces susceptible to erosion during the next flood event. Over time, drought shift species dominance to the long-lived drought tolerant species. Since this site is in areas of water accumulation, it may be slower to respond to drought than the surrounding drier terrain. During moderate droughts it could harbor species for seed dispersal to more drought prone areas when average precipitation returns.
Community 2.3
Post-fire Regeneration
Figure 8. 11 years post-fire
There may be high variability in the species response in the post-fire regeneration phase. The composition of the post-fire regeneration community depends upon the species present before the fire, fire intensity, precipitation patterns, and time since last fire. Initially annuals and short-lived perennials will dominate. The longer lived shrubs, California jointfir, peachthorn, and desert almond, may resprout vigorously after low to moderate intensity fires. Joshua tree may also, resprout, but still typically suffers high mortality after fire due to increased herbivory and other factors. Short-lived shrubs capable of quickly colonizing after fire include burrobrush, Cooper's dogweed, Mojave rabbitbrush, and yucca buckwheat, among others. The photo above was taken 11 years after the Juniper Complex fire in Joshua Tree National Park. It is in area with less run on, compared to other locations where this ecological site is found. This location is heavily dominated by peachthorn, desert almond, and giant woollystar (Eriastrum densifolium).
Community component 4 (CC4) represents CC2 post-fire. This data is from one plot, 11 years post fire. Data from before the burn is not available. It is assumed California jointfir would be a part of this community in other sites. This was a high intensity fire, with high mortality of all species (Minnich 2003). Recovery after fire, to pre-fire conditions, will take decades.
Table 6. Annual production by plant type
| Plant type |
Low
(kg/hectare) |
Representative value
(kg/hectare) |
High
(kg/hectare) |
| Shrub/Vine |
448 |
504 |
583 |
| Grass/Grasslike |
135 |
213 |
291 |
| Forb |
56 |
78 |
95 |
| Total |
639 |
795 |
969 |
Pathway 2.1a
Community 2.1 to 2.2
This pathway is caused by a prolonged or severe drought.
Pathway 2.1b
Community 2.1 to 2.3
This pathway is caused by moderate to severe fire
Pathway 2.2a
Community 2.2 to 2.1
This pathway occurs with the return of average to above average precipitation and associated flood events.
Pathway 2.2b
Community 2.2 to 2.3
This pathway occurs as a result of fire. Given low cover of annuals during drought, this pathway is unlikely except in periods immediately following heavy precipitation years.
Pathway 2.3a
Community 2.3 to 2.1
This pathway occurs in response to the passing of time with average to above average precipitation and associated flood events.
Pathway 2.3b
Community 2.3 to 2.2
This pathway occurs in response to the passing of time with drought conditions and absence of flooding
State 3
Frequent Fire
This state develops when the fire return interval is less than 20 years. This state has been significantly altered from the natural range of variability found in States 1 and 2. Frequent fire intervals favor the non-native annual grasses, and reduce the cover and regeneration of longer lived shrubs, which need more time to reestablish after fire. Competition with non-native grasses for water, nutrients and sunlight reduces seedling survival of native species. Data was not collected for this state, it is based on literature review and the observation that the high cover of non-native grasses present in extensive areas of this site create a high risk for a conversion to a grass-fire cycle.
Community 3.1
Average precipitation
This community phase develops with time without fire (5-20 years). It may be dominated by annual grasses with a diversity of short-lived shrubs, subshrubs, and longer-lived shrubs that have resprouted. The presence and cover of long-lived shrubs such as peachthorn, desert almond, and catclaw acacia are dependent upon the fire history and intensity. California ephedra is most likely reduced in importance, and the mosaic of diverse shrubs will not recover, because the fire return interval is too short for these shrubs to regain former dominance.
With the normal precipitation cycle, very rare to rare flood events will sheet flow across these flats and flow down the channels. Non-native grasses have poor canopy cover protection from wind or rain, and their roots are not as extensive as capable of holding soil as the more deep rooted and laterally extensive roots of many native species. As diversity and cover of native species increases, the more stability they can provide for surface erosion.
Community 3.2
Post-fire Regeneration
This community phase occurs one to five years post-fire. Cheatgrass and red brome are quick to reestablish after fire. Native forbs such as bristly fiddleneck, chia and pincushion flower (many other native forbs could also be present). There may be very sparse cover of resprouting shrubs such as peachtorn, catclaw acacia, or desert almond. Seedlings of short-lived shrubs may be present, and may include burrobrush, Cooper's dogweed, and Mojave rabbitbrush.
This community is at high-risk of repeat burning due to high fine fuel cover. This community is also susceptible to wind and water erosion, due to the loss of stabilizing shrub cover (Bull 1997).
Community 3.3
Drought Response
This community develops with prolonged or severe drought. Drought will cause high mortality to the short lived perennials during this phase. The non-native annual grasses will have low production and cover during drought. This phase is extremely susceptible to wind and water erosion. When the next sheet flood occurs, large volumes of loose sandy sediment will be mobilized downstream.
Pathway 3.1a
Community 3.1 to 3.2
This pathway occurs as a result of fire.
Pathway 3.1b
Community 3.1 to 3.2
This pathway is caused by drought.
Pathway 3.2a
Community 3.2 to 3.1
This pathway occurs with the return to average or above average precipitation and associated flood events.
Pathway 3.2b
Community 3.2 to 3.3
This pathway is caused by drought.
Pathway 3.3a
Community 3.3 to 3.1
This pathway occurs with the return of average to above average precipitation and associated flood events.
Transition 1
State 1 to 2
This transition occurs with the introduction of non-native species. Cheatgrass and red brome were most likely introduced into this area in the late 19th century, during active mining and grazing activities. The introduction of non-native species was localized at first, and may have been kept from spreading by a mid-century dry spell. Since the 1970's precipitation has increased and these annual grasses have been spreading to new areas and increasing in cover and density (Salo, 2005, Brooks et. al., 2003).
Transition 2a
State 2 to 3
This transition occurs when the fire return interval is less than 20 years.
Restoration pathway R3a
State 3 to 2
Restoration of communities severely altered by repeat fire at the landscape scale is difficult. Methods may include aerial seeding of early native colonizers such desert globemallow and burrobrush. Increased native cover may help to reduce non-native plant invasion, helps to stabilize soils, provides a source of food and cover for wildlife, including desert tortoise (Gopherus agassizii), and provides microsites that facilitate creosote bush establishment. However, the amount of seed required for success is often prohibitive. Large-scale planting of both early colonizers and community dominants tends to be more successful in terms of plant survival, especially if outplants receive supplemental watering during the first two years. Creosote bush and burrobush can be successfully propagated and outplanted. Pre-emergent herbicides (Plateau) have been used in the year immediately post-fire to attempt to inhibit or reduce brome invasion. How successful this is on a landscape scale, and the non-target effects have not yet been determined.
