Ecological Impacts

Grading and vegetation removal at the project site are likely to affect surrounding plant and wildlife populations.

Biological Soil Crusts: Soil Structure and Stability
Grading, erosion control measures, and the movement of workers and vehicles during the construction and operation of a solar facility will result in substantial disturbances to soil crusts. Biological soil crusts play a significant role in stabilizing soil in a water limited and erosion prone environment. Unfortunately, biological soil crusts are incredibly fragile and can be crushed by a single human footstep. When crusts are crushed, the aggregate crust structure is destroyed and soil stabilizing capabilities will be significantly compromised.¹

Biological Soil Crusts: Dust Emission
The scraping and grading of sites, particularly during facility construction, will likely have immediate impacts on dust emission. Scraping removes vegetation and biological soil crusts, which increases soil surface vulnerability to wind erosion and exposes layers of dust that may have been trapped for decades or longer.² In some areas, dust layers can be several meters thick, and trapped just below the desert surface.³ Thus, sites that have large areas of soil crusts may become large sources of dust emission during construction (Figure 1). Neither dust sequestration nor biological soil crusts have been mapped in the California desert. Therefore, without doing a survey on site, it may be difficult to predict an area’s potential for dust emission prior to construction.

Biological Soil Crusts: Nutrient Cycling
Solar development that directly destroys biological soil crusts could alter nutrient cycles by eliminating the main source of nitrogen and carbon fixation. In the California desert, biological soil crusts are the main source of nitrogen and carbon fixation — in many cases they are “the only show in town” for this important ecosystem function.⁴ In addition to storage and release of nutrients by the crusts directly, the fine soil particles trapped by biological soil crusts also bind nutrients, increasing soil fertility.⁵ The rough surface in Figure 2 is created by a black soil crust (a dime is used for scale). If crusts are destroyed as a result of solar development, nutrient availability will be drastically reduced and the re- establishment and/or growth of vegetation may be limited.

Biological Soil Crusts: Water Infiltration
Solar development that destroys soil crusts can alter water runoff patterns, changing the path of water across the landscape.⁶ Additionally, disturbance of crusts that have been enhancing water infiltration on site may lead to increased runoff. This runoff can also erode the soil, transport nutrients away from the project area and deliver it elsewhere, and change the nutrient balance of the surrounding ecosystem. Fluctuations in the nutrient balance may be difficult for the ecosystem to tolerate.

Biological Soil Crusts: Recovery and Long-term Consequences
Siting decisions should be made in light of the potential long-term consequences that a solar facility might have for the recovery of biological soil crusts. The true recovery potential for soil crusts on solar facility sites remains largely unknown due to the sheer number of acres that could be disturbed. Estimates for post-disturbance soil crust recovery time in the Mojave Desert range from several decades to over 1,000 years.^7,8,9,10 The recovery rate and recovery potential of soil crusts are extremely dependent on the characteristics of a particular site.¹¹ Sometimes it may not be possible to regain species diversity of biological crust organisms after a disturbance because conditions that allowed the establishment of a particular species in the past may no longer be present.¹²

Invasive Plants: Soil Disturbance
The land disturbed within the solar project site boundaries could become a source of propagules and allow for the establishment of invasive plants because habitat disturbances can facilitate the colonization of natural areas by invasive plants.¹³ Construction machinery and other earth-moving equipment could carry invasive plant material and seeds to the solar facility site from other construction sites. The disturbed soils created by grading and vehicle traffic could create sites that facilitate the growth of invasive plants. Invasive plants will also likely benefit from water used to suppress dust during facility construction.

Invasive Plants: Alterations to Fire Regimes
Disturbances from solar development that facilitate the spread of invasive grasses, such as Bromus spp. and Schismus spp., can increase the frequency of fire in the California desert. Thick layers of plant litter can accumulate when invasive annual plant species die off each year.¹⁴ The accumulation of litter can lead to increased size and intensity of fires, and can shorten the amount of time between fire events.¹⁵ Invasive annual grasses increase the frequency of fire by providing a more persistent and uniformly distributed fuel than is normally supplied by native plants.¹⁶ Fires were historically uncommon in the California desert due to limited and sparsely distributed vegetative fuel; consequently native perennial shrubs are poorly adapted to the increasing frequency of natural and anthropogenic fires.¹⁷
The shift from a natural fire regime characterized by small, infrequent fires to a new fire regime characterized by large, frequent fires would be detrimental to native plants, and would also allow invasive species to gain an even greater competitive advantage. Invasive plants are better able to exploit the increased availability of soil nutrients and light after a fire than native plants due to their relatively high growth rates and ability to disperse quickly into burned areas.¹⁸ A shift in the natural fire regime around multiple solar facilities could result in regional changes to the natural fire regime, giving invasive plants an advantage over native plants on a much larger scale. Once a fire regime that favors invasive plants over natives is established, restoration of preinvasion conditions is challenging.¹⁹ Solar development could further contribute to this positive feedback cycle if the potential for fires from power generation or transmission are not adequately dealt with. Shifts in the natural fire regime could result in fundamental changes in native plant community structure and plant and animal food web dynamics.²⁰

Hydrology: Soil Compaction
Soil compaction is detrimental to desert ecosystems because it crushes soil pores, decreases soil permeability and impedes water infiltration into the soil, thereby increasing runoff.²¹ In addition to increasing the volume of runoff, soil compaction can also alter the flow of runoff over the landscape. Moving heavy construction equipment and vehicles over the facility site, industrial-scale soil compaction, as well as foot traffic on the site will likely result in soil compaction and alter water flow across the project area. Soil compaction can result from trampling by people or passage of a vehicle over the soil.

Hydrology: Re-routing Stream Channels
Solar developers plan to redesign hydrologic features on site in order to accommodate the construction of the facility, such as re-routing desert washes that fall within a project area. In general, ephemeral stream flow will be diverted to artificial channels that go around the site and return to the approximate location of the existing wash. Changing water movement across the landscape may divert water away from important water capturing features such as ephemeral stream channels. The diversion of ephemeral stream flow could affect water infiltration and groundwater recharge depending on the length and acreage of stream channel that is covered up by the project site. Ephemeral stream channels are characterized by exceedingly high infiltration rates, caused by the sand and gravel sediments within these channels.²² The infiltration that occurs in stream channels during precipitation events is the primary source of groundwater recharge in desert ecosystems.²³ Diverting water flow away from these natural channels could reduce or eliminate a much-needed source of recharge for an already overdrawn aquifer system.

Hydrology: Vegetation Removal
Removal of vegetation for facility construction and fire prevention, especially perennial vegetation (e.g., shrubs like creosote bush, Larrea tridentata), could result in reduced water infiltration and decreased soil moisture. The removal of all or a large proportion of the vegetation on a several thousand acre solar facility site could increase soil evaporation rates, as well as significantly speed up the flow of water over the land surface, increasing the potential for water erosion of the soil and decreasing the potential for water infiltration into the soil.^{24, 25} Over time, reduced water infiltration and increased evaporation may lead to decreased soil moisture, thereby increasing the likelihood of wind erosion of drier soils and also making recolonization of the site by plants more difficult should vegetation be allowed to return.

1 Jayne Belnap, S.L. Phillips, and J.E. Herrick, “Wind erodibility of soils at Fort Irwin, California (Mojave Desert), USA, before and after trampling disturbance: implications for land management,” Earth Surface Processes and Landforms 32 (2007): 75-84.

2 Richard Reynolds and others, “Aeolian dust in Colorado Plateau soils: Nutrient inputs and recent change in source,” Proceedings of the National Academy of Sciences of the United States of America 98, no. 13 (2001): 7123-7127.

3 Richard L. Reynolds, Research Geologist, U.S. Geological Survey, Earth Surface Dynamics, Denver, CO, personal communication, November 6, 2009.

4 Jayne Belnap, Research Ecologist, U.S. Geological Survey, personal communication, November 4, 2009.

5 Jayne Belnap, “The world at your feet: desert biological soil crusts,” Frontiers in Ecology and the Environment 1, no. 4 (2003): 181-189.

6 Jayne Belnap and others, “Linkages between microbial and hydrologic processes in arid and semiarid watersheds,” Ecology 86, no. 2 (2005): 298-307.

7 Jayne Belnap and S.D. Warren, “Patton’s tracks in the Mojave Desert, USA: An ecological legacy,” Arid Land Research and Management 16, no. 3 (2002): 245-258.

8 Jayne Belnap, “The world at your feet: desert biological soil crusts,” Frontiers in Ecology and the Environment 1, no. 4 (2003): 181-189.

9 Matthew A. Bowker, Jayne Belnap, and Diane W. Davidson, “Microclimate and Propagule Availability are Equally Important for Rehabilitation of Dryland N-Fixing Lichens,” Restoration Ecology 18, no. 1 (2010): 30-33.

10 Robert H. Webb, Jayne Belnap, and K.A. Thomas, “Natural Recovery from Severe Disturbance in the Mojave Desert,” in The Mojave Desert: Ecosystem Processes and Sustainability, eds. R.H. Webb and others, (Reno: The University of Nevada Press, 2009), 343-377.

11 Robert H. Webb, Jayne Belnap, and K.A. Thomas, “Natural Recovery from Severe Disturbance in the Mojave Desert,” in The Mojave Desert: Ecosystem Processes and Sustainability, eds. R.H. Webb and others, (Reno: The University of Nevada Press, 2009), 343-377.

12 Robert H. Webb, Jayne Belnap, and K.A. Thomas, “Natural Recovery from Severe Disturbance in the Mojave Desert,” in The Mojave Desert: Ecosystem Processes and Sustainability, eds. R.H. Webb and others (Reno: The University of Nevada Press, 2009), 343-377.

13 Matthew L. Brooks, “Spatial and Temporal Distribution of Nonnative Plants in Upland Areas of the Mojave Desert,” in The Mojave Desert: Ecosystem Processes and Sustainability, eds. R.H. Webb and others (Reno: The University of Nevada Press, 2009), 343-377.

14 Matthew L. Brooks, “Alien annual grasses and fire in the Mojave Desert,” Madroño 46, no. 1 (1999):13-19.

15 Matthew L. Brooks, and J.R. Matchett, “Spatial and temporal patterns of wildfires in the Mojave Desert, 1980-2004,” Journal of Arid Environments 67 (2006): 148-164.

16 Matthew L. Brooks, and J.R. Matchett, “Spatial and temporal patterns of wildfires in the Mojave Desert, 1980-2004,” Journal of Arid Environments 67 (2006): 148-164.

17 Matthew L. Brooks, “Peak Fire Temperatures and Effects on Annual Plants in the Mojave Desert,” Ecological Applications 12, no. 4 (2002): 1088-1102.

18 Matthew L. Brooks and C. D’Antonio, “The role of fire in promoting plant invasions,” in Proceedings of the California Exotic Pest Plant Council Symposium, Volume 6, ed. M. Kelly, (2003), 29-30.

19 Matthew L. Brooks and others, “Effects of Invasive Alien Plants on Fire Regimes,” BioScience 54, no. 7 (2004): 677-688.

20 Matthew L. Brooks, “Competition between alien annual grasses and native annual plants in the Mojave Desert,” American Midland Naturalist 144 (2000): 92-108.

21 Jayne Belnap, “Surface Disturbances: Their Role in Accelerating Desertification,” Environmental Monitoring and Assessment 37 (1995): 39-57.

22 Walter G. Whitford, Ecology of Desert Systems (San Diego: Academic Press, 2002).

23 L.H. Hekman Jr. and W.R. Berkas. “Modeling Desert Runoff,” in Water in Desert Ecosystems, eds. D.D. Evans and J.L. Thames, (Stroudsburg, PA: Dowden, Hutchinson & Ross Inc., 1981), 244-264.

24 John A. Ludwig and others, “Vegetation Patches and Runoff-Erosion as Interacting Ecohydrological Processes in Semiarid Landscapes,” Ecology 86, no. 2 (2005): 288-297.

25 D.D. Breshears and others, “Effects of woody plants on microclimate in a semiarid woodland: Soil temperature and evaporation in canopy and intercanopy patches,” International Journal of Plant Sciences 159, no. 6 (1998): 1010-1017.