Biocrusts, grass establishment, and restoration of working rangelands
Biocrusts (biological soil crusts) provide key ecosystem services such as erosion resistance and nitrogen fixation to western rangelands. There is also evidence that biocrusts may inhibit cheatgrass (Bromus tectorum) establishment. This possibility has ramifications for sustainable livestock production given the threats posed by cheatgrass and other invasive plants that decrease the quality and yield of perennial forages, increase production costs for the livestock industry and exacerbate wildfire threats. We, therefore, are using a combination of field experiments and observations to investigate the influence of biocrusts on the establishment of native grasses and non-native invasive grasses (cheatgrass, red brome [Bromus rubens], and buffelgrass [Pennisetum ciliare]) and determine the conditions under which biocrusts might make rangelands more resistant to exotic grass invasion, and, thereby, serve as a potential restoration tool for producers.
Our goal is 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. Our objectives are to: 1) quantify the influence of biocrust type and integrity on native vs. non-native grass germination and establishment; 2) determine if grass seed morphology affects grass germination and establishment on biocrusts; 3) determine if biocrust type and integrity affect grass establishment; and 4) quantify the effect of biocrust restoration on the re-establishment of native grasses and the reinvasion of non-native grasses (cheatgrass, buffelgrass, and red brome). We utilize a comparative approach to test our hypothesis that the type and integrity of biocrusts interacts with grass seed attributes to differentially influence native and exotic grass establishment. Our experiments and field observations are being conducted in contrasting bioclimatic regions to assess the robustness of our findings.
Field experiments on the Colorado Plateau and in the Sonoran Desert will determine how the type/integrity of biocrusts affects the establishment of cheatgrass, red brome, buffelgrass, and native grasses with contrasting seed architectures. Seeds are manipulated by removing appendages or leaving them intact. Manipulated and non-manipulated seeds are placed on contrasting soil surfaces; including intact, disturbed and restored biocrust communities, and germination and establishment are quantified.
If biocrusts limit non-native grass establishment, management of rangelands to resist invasion could develop and incorporate strategies for retaining or re-establishing biocrusts. Along these lines, the research complements ongoing, broad-scale biocrust restoration research and existing and future research on livestock management and biocrusts being conducted by other groups. Knowledge of how biocrust properties and seed morphologies interact to influence grass establishment will provide criteria for formulating species mixtures in re-seeding efforts. Information will be shared with producers, scientists and the public through several forums. We will host a field day for producers and managers and collaborate with the University of Arizona’s Cooperative Extension program to disseminate results to producers and the public. In addition, we will work with Grand Canyon Trust, V Bar V Ranch and other educational organizations to incorporate our results into education/outreach programs and materials. We will also disseminate results via refereed publications.
Our goal is 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 are 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, buffelgrass, and red brome).
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. Rationale: We suppose that biocrusts constitute a mechanical barrier that reduces seed contact with the soil surface. The high evaporation rates in deserts make soil contact crucial for seeds to obtain sufficient water for germination. We predict that large seeds or seeds having large appendages (e.g., long awns) would be less likely to achieve soil contact when biocrusts are present compared to small seeds and seeds lacking appendages.
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. Rationale: Lichen/moss biocrusts are epedaphic and can form mats above the soil surface. Therefore, we expect lichen/moss crusts to form a more impenetrable barrier than cyanobacterial crusts, whose biomass is concentrated just below the surface and occurring as individual filaments with spaces between them. As such, it should be more difficult for seeds to achieve soil contact and burial on the lichen biocrusts. Biocrusts of all types should no longer pose a physical barrier once disrupted.
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. Rationale: Establishment of biocrusts reforms the barriers that inhibit soil contact by the exotic plant seed (Hypothesis 2), but effects will vary depending on plant seed traits (Hypothesis 1).
We initiated experiments in field and semi-controlled settings on the Colorado Plateau and in the Sonoran Desert. Due to permitting issues and logistical constraints, the location of the Colorado Plateau field experiments shifted to BLM-managed sites near Moab, Utah rather than the originally envisioned Forest Service-managed sites in northern Arizona. Procurement of additional funds from a USDA grant (leveraged in part from this Western SARE project) has allowed us to expand our experiments to the Sonoran Desert (near Tucson, AZ) and investigate additional invasive and native grass species. Potted soil/biocrusts trials are now being conducted in settings that control precipitation. These trials are designed to compliment our field plot experiments and also allowing us to investigate a wider range of native (6 species) and invasive (3 species) grasses.
We used 6 native and 3 non-native grasses, a mix of warm- and cool-season grasses, seeds with a range of sizes, and manipulated seeds by clipping awns or bristles to address our Objective (2) of determining the extent to which seed attributes influence germination and establishment outcomes. We investigated cyanobacteria-dominated and lichen-dominated biocrusts and simulated trampling of biocrusts to determine if the integrity of biocrusts influence grass germination and establishment outcomes (Objective 3). In the semi-controlled settings, we excluded natural precipitation and hand-watered pots (equivalent of 6 mm rainfall for 3 consecutive days or every other day), but seeds were subject to relatively ambient temperature and wind conditions.
We utilized linear mixed-models to analyze our germination (emergence) data from our experiments in semi-controlled environments. Species, seed manipulation (awns/bristles intact or removed), surface type (bare soil, polyacrylamide [PAM] crust, cyanobacteria-dominated biocrust, and lichen-dominated biocrust), surface disturbance (intact or broken), and seed placement (on the surface or in fissures) were the main effects in the analysis (without interactions) and we included pot location as a random effect. In the fall of 2015 we investigated cheatgrass (Bromus tectorum) on the Colorado Plateau (the native grasses in the study typically germinate in the spring). Preliminary analysis of autumnal cheatgrass germination/emergence on biocrusts and soils from the Colorado Plateau shows that seeds placed in fissures emerged at a higher rate than those placed on the surface (mean ± SE of 0.13 ± 0.01 vs 0.04 ± 0.01; F89=34, p<0.01). Considering only seeds placed on the surface, the type of soil surface was significant (F60=14.6, p<0.01) with cheatgrass emerging being reduced on PAM crusts. Surface disturbance and awn removal did not significantly affect cheatgrass germination in the semi-controlled setting (F60=0.65, p=0.42 and F60=0.11, p=0.74, respectively). In the spring of 2016, we investigated native grasses and cheatgrass on the Colorado Plateau. Preliminary analysis of vernal native grass germination/emergence on biocrusts and soils from the Colorado Plateau shows that the type of surface is significant (F469=12.2, p<0.01), but seed placement and surface disturbance are not. Native grass seeds were more likely to germinate on cyanobacteria or lichen biocrusts than on bare soil or PAM crusts (p<0.05). The potential role of surface soil moisture is currently under investigation.
Preliminary analysis show that seed placement, surface disturbance, soil surface type, and species influence emergence buffelgrass (Pennisetum ciliare) and warm-season native grasses in the Sonoran Desert. Seeds placed in fissures of biocrusts or polyacrylamide (PAM) crusts emerged at a significantly higher rate (0.17 ± 0.02) than those placed on the surface (0.09 ± 0.01; F514=27, p<0.01). Seeds in fissures of lichen-dominated biocrusts (0.12 ± 0.02) emerged at lower rates than those in fissures of PAM crusts (0.23 ± 0.03; p=0.001), suggesting that fissure-placement effects may be biotic, abiotic (related to dry-down rates), or a combination of the two. Considering only seeds placed on the surface, emergence of warm-season grasses is significantly higher for broken crusts (0.09 ± 0.01) compared to intact crusts (0.04 ± 0.01; F255=6.34, p=0.01). When emergence was standardized as a fraction of seed viability, species was significant for both intact and manipulated seed (F16=21.98, p<0. 01), with buffelgrass having lower emergence rates than the native grasses. Results for emergence of cool-season grasses in the Sonoran Desert follow those of warm-season grasses.
We also utilized linear mixed-models to analyze establishment data from our autumn 2015 experiment conducted in field plots on the Colorado Plateau (as above, the experiment only investigated cheatgrass as our native study grasses typically germinate in the spring). Species, seed manipulation (awns/bristles intact or removed), surface type (bare soil, PAM crust, cyanobacteria-dominated biocrust, or lichen-dominated biocrust), and surface disturbance (intact or broken) were main effects with site and sub-site included as random effects. Establishment of cheatgrass varied significantly with surface type (F17=4.69, p=0.02) and surface disturbance (F17=21.79, p<0.01) with cheatgrass being more likely to establish in disturbed crusts (0.16 ± 0.02) than on intact crusts (0.04 ± 0.01). Awn presence or absence had no effect on establishment rates (F3=0.99, p=0.39). Cheatgrass was more likely (p<0.01) to establish on bare soil and PAM crusts when compared to cyanobacteria or lichen biocrusts. However, cheatgrass establishment on bare soil and PAM crusts were statistically comparable (p=0.22). Contrary to our hypothesis, establishment rates on cyanobacteria (0.09 ± 0.02) and lichen biocrusts (0.11 ± 0.02) were not statistically different (p=0.6). In the spring of 2016, we conducted establishment experiments on cheatgrass and native grasses on the Colorado Plateau. However, insufficient rainfall caused experiments to fail (only 86 of 2,520 cheatgrass seeds and only three of 7,560 native seeds established).
In the Sonoran Desert, we quantified establishment of two warm-season grasses (buffelgrass [Pennisetum ciliare; invasive] and purple threeawn [Aristida purpurea; native]) and cool-season native grasses (sixweeks fescue [Vulpia octoflora] and needle-and-thread [Hesperstipa comata]). As in the semi-controlled environment, seed manipulation (awn presence/absence) did not significantly influence establishment of warm-season grasses (F82=0.73, p=0.40). However, surface disturbance stimulated establishment of warm-season seedlings (0.31 ± 0.09 disturbed vs. 0.08 ± 0.02 undisturbed; F82=5.65, p=0.02). Overall, the warm-season grasses were most likely to establish on broken lichen biocrusts. Buffelgrass (0.32 ± 0.08) established at a significantly higher rate than purple threeawn (0.05 ± 0.01) after accounting for seed viability (F9=10.62, p<0.01). Establishment rates among cool season species was very low overall (<10% of potential based on seed viability) and we did not find any significant factors.
To address the conditions under which a restored environment can resist future invasion (Objective 4) we removed invasive buffelgrass (Sonoran Desert sites) or cheatgrass (Colorado Plateau sites) from one 100 m2 plot at each of the 6 filed sites (3 per desert). Subsequently, biocrust restoration treatments were applied to 0.4 x 0.4 m subplots at varying ratios. During the upcoming field seasons, we will test for establishment of native and invasive grasses on these restored biocrusts. This, combined with repetition of the experiments described above, will enable us to develop more comprehensive education and outreach materials.
Impacts and Contributions/Outcomes
Early results were presented in a special biocrusts session at the 13th Biennial Conference of Science & Management on the Colorado Plateau & Southwest Region in Flagstaff, Arizona. The session was well attended and included scientists and land management professionals from the National Park Service, U.S. Geological Survey, University of Arizona, Northern Arizona University, Arizona State University, University of Colorado at Boulder, Oregon State University, and several non-governmental organizations. Additionally, The Nature Conservancy’s Canyonlands Research Center highlighted our research in their fall newsletter and has featured our study on their Facebook page.
In the Sonoran Desert, we presented our preliminary results during the poster session of the Annual Research Insights in Semiarid Ecosystems (RISE) Symposium in Tucson, Arizona. Cheryl McIntyre, the graduate student on the project gave an invited presentation to the Buffelgrass Working Group in Tucson, Arizona on November 18, 2015. The Buffelgrass Working Group consists of local, state, and federal managers and scientists, university scientists, and non-profit scientists and advocates who coordinate management of buffelgrass in the Tucson basin under the umbrella of the Southern Arizona Buffelgrass Coordination Center. Additionally, Cheryl McIntyre presented our research at the Santa Rita Experimental Range’s “Discovery Saturday” on April 30, 2016. An abstract to present our work at the 3rd International Workshop on Biological Soil Crusts in Moab, UT (September 2016) was submitted and has been accepted.
Preparation of additional outreach materials (e.g., website, slideshow, factsheets) and workshop content are underway and are pending the results of our next field seasons.
University of Arizona
School of Natural Resources & The Environment
Bio Science East 325
Tucson, AZ 85721-0043
Office Phone: 5207307617