Biocrusts, grass establishment, and restoration of working rangelands

View the project final report

• 2016 annual report

Commodities

• Additional Plants: native plants
• Animals: bovine

Practices

• Animal Production: range improvement
• Education and Training: extension, workshop
• Natural Resources/Environment: habitat enhancement, soil stabilization
• Pest Management: weed ecology
• Soil Management: soil microbiology, soil quality/health

Biological soil crusts (biocrusts) provide many ecosystem services to rangelands, but their role in grass establishment is unclear. We conducted a series of experiments to determine the extent to which nativity and seed attributes (e.g., size, presence/absence of awns) influence grass establishment outcomes on biocrusted and non-crusted soils and whether a restored biocrust community can confer resistance to invasion by non-native grasses.  The experiments were conducted in contrasting bioclimatic regions to assess the robustness of our findings. Placing seeds in cracks of biocrusts facilitated seed-soil contact and resulted in high germination/emergence in semi-controlled environments. This supports the notion that species with small seeds and or seeds with small or no appendages may be better suited to exploit cracks in biocrusts. In addition, native species with small seeds emerged at higher rates than native species with larger seeds on intact biocrusts in a Sonoran Desert semi-controlled environment. Larger seeds also had longer awns, but contrary to predictions, removing awns decreased emergence. In field settings, higher rates of germination and early-establishment on broken biocrusts may be due to enhanced seed capture/retention, seed burial, or biocrust nutrient release. 

Introduction

Biocrusts (biological soil crusts) generally cover open spaces on soils in rangelands of the American West. Communities of cyanobacteria, lichens, mosses, and other organisms that live on and in the soil surface, biocrusts contribute to ecosystem services such as soil stability, water infiltration, and carbon-nitrogen and fixation (Belnap 2003). Biocrust microtopography, lichen, and bryophyte abundance increase with decreasing potential evapotranspiration (Belnap 2003). In hot deserts, biocrusts typically have up to 3cm of microtopography (Belnap 2003) with up to 5cm of relief observed in the Mojave Desert. (Williams et al. 2012). In cold deserts, biocrusts are pinnacled and have up to 15cm of relief, likely due to freezing of soils (Belnap and Gardner 1993). The microtopography generated by biocrusts may affect vascular plant composition via its influence on seed capture (relatively low on smooth biocrusts; higher on biocrusts with more microtopography (Belnap et al. 2003).

Biocrusts have variable effects on native plant germination and establishment (Belnap et al. 2003). Observational field studies and experiments in controlled environments often show a negative effect of biocrusts on the germination and abundance of cheatgrass (Bromus tectorum) and other annual exotic grasses (Belnap et al. 2003). Biocrust and cheatgrass abundance in the Great Basin are negatively correlated (Peterson 2013). Conversely, in the Mojave Desert Bromus spp. density is higher on biocrusts compared to bare soil (DeFalco et al. 2001) suggesting that biocrust microtopography and species composition may affect establishment outcomes.  Some Great Basin biocrusts can inhibit cheatgrass establishment in greenhouse (Howell 1998) and growth chamber (Deines et al. 2007) settings. However, the effect depends on biocrust community composition with some lichens inhibiting establishment and others being neutral (Deines et al. 2007). Disturbance of biocrusts can increase Bromus spp. germination (Kaltenecker et al. 1999, Belnap et al. 2003).

Several authors speculate that the species-specific outcomes are influenced by seed morphology (Zhang and Belnap 2015), seed size (Briggs and Morgan 2011, Kitajima 2007) and biocrust species composition (Deines et al. 2007). Along these lines, we seek to quantify how biocrust characteristics (dominant species, microtopography, integrity) and seed morphology (size, appendages) interact to determine grass germination and establishment. 

Our goal was to determine the extent to which nativity and seed attributes (e.g., size, presence/absence of awns) influence grass establishment on biocrusted and non-crusted soils and whether a restored biocrust community can confer resistance to future invasions. Our objectives were to: 1) quantify biocrust influence on native vs. non-native grass germination and establishment; 2) determine if grass seed morphology affects establishment on biocrusts; 3) determine if biocrust type and integrity affect grass establishment; and 4) quantify the effect of biocrust restoration on establishment of native grasses and reinvasion of non-native grasses (cheatgrass (Bromus tectorum), buffelgrass (Pennisetum ciliare), and red brome (Bromus rubens)).

Hypothesis 1 (Objectives 1 and 2): The influence of biocrusts on grass germination and establishment (positive, neutral or negative) varies according to seed characteristics. Specifically, (a) biocrusts will reduce recruitment of plants whose seeds have large awns or appendages compared to smoother seeds; (b) biocrusts will reduce recruitment of large-seeded species compared to small-seeded species; and (c) biocrusts will reduce recruitment of exotic grasses compared to native grasses.

Hypothesis 2 (Objective 3): Grass germination and establishment varies with biocrust type and integrity. Specifically, (a) lichen/moss biocrusts are a more effective barrier than cyanobacterial biocrusts when of comparable roughness; (b) biocrusts form a physical, not biological barrier to seeds; and (c) intact biocrusts are a more effective barrier against exotic grasses than broken biocrusts.

Hypothesis 3 (Objective 4): Restoration of biocrust communities following removal of non-native grasses  (a) decreases the probability of their reinvasion and (b) increases or has a neutral effect on re-establishment of native grasses.