Identifying Restoration Opportunities Under Mesquite Canopies

Introduction

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Grasslands in southeastern Arizona and northern Mexico are threatened by climate change climate change
Climate change includes both global warming driven by human-induced emissions of greenhouse gases and the resulting large-scale shifts in weather patterns. Though there have been previous periods of climatic change, since the mid-20th century humans have had an unprecedented impact on Earth's climate system and caused change on a global scale.

Learn more about climate change , drought, human development, overgrazing, and encroachment of invasive species invasive species
An invasive species is any plant or animal that has spread or been introduced into a new area where they are, or could, cause harm to the environment, economy, or human, animal, or plant health. Their unwelcome presence can destroy ecosystems and cost millions of dollars.

Learn more about invasive species . These threats are leading to the loss of biodiversity and the degradation of grassland ecosystems as a whole. Native bunch grasses are especially valued for their role in reducing erosion and providing forage for cattle. Mesquite (Prosopis spp.) and Lehmann lovegrass (Eragrostis lehmanniana) compete with native grasses for space to grow and for water resources, which further expedites the loss of native grasses in these historical grasslands (Gornish et. al. 2020). 

Although mesquite can reduce grass cover when measured at a large spatial scale, at small scales mesquite canopies can create a beneficial microclimate for native plant seedlings in grasslands. Mesquites create nutrient rich soils under their canopies by increasing nitrogen and soil organic matter (Frost and Edinger 1991) and reduce soil temperatures and evaporation (Gornish et. al. 2020). Land managers and restoration practitioners have observed the success of certain native grasses and other grassland plants under mesquite canopies but they currently lack empirical data to inform proper management and restoration techniques.

Researchers at the University of Arizona analyzed long-term monitoring data from the Santa Rita Experimental Range (SRER) in southern Arizona to learn which native species fared better under mesquite canopies. They also wanted to assess the prevalence of non-native grasses like Lehmann lovegrass under mesquite canopies. To complete these objectives, they looked at transect sampling data from 2011, 2014, and 2017 to identify how managers could leverage the beneficial microclimate beneath mesquite to restore native grasses and other grassland plants that are decreasing in abundance.

Key Issues Addressed

Despite ecological theory and observational data that would support mesquite canopies being able to facilitate the growth of native grasses, there is little quantitative data to help managers effectively use mesquite canopies to restore cover of native forbs and grasses. Analyzing long-term plant data could improve understanding of how mesquite canopies influence native plants, and allow restoration practitioners to better manage grasslands.  

Managers currently lack information about how disturbance from grazing and drought may impact opportunities to leverage mesquite canopies for grassland restoration. Previous research shows land uses like grazing can impact which native grasses thrive under mesquite and which do not (Smith and Schutz 1975). Mesquite may alleviate environmental stresses on short rooted plants like native grasses and forbs (Barron-Gaffod et. al. 2017). It is uncertain if mesquite canopies can continue to facilitate growth of native plants during drought, and if so, which plants would benefit. A more nuanced picture is needed to understand how grazing and precipitation changes impact the dynamics between grasslands and mesquite canopies. 

Mesquite canopies may reduce competition between invasive grasses and native grasses, but more information is necessary to fully contextualize this dynamic. Mesquite canopies seem effective at reducing the presence of Lehmann lovegrass under canopies, but there is a lack of data to support this claim. Decreased presence of Lehmann lovegrass under mesquite canopies might lead to less competition for the space and resources that native grasses need to grow. Restoration practitioners need more information to understand how invasive species impact the interplay between mesquite canopies and native grasses. 

Project Goals

• Analyze long-term vegetation data to improve understanding of how mesquite cover impacts grasses, shrubs, and forbs 
• Identify opportunities to leverage the effects of mesquite canopies on native plants for restoration purposes
• Compare the presence of Lehmann lovegrass under and outside of mesquite canopies to see if mesquite canopies reduce competition from this invasive grass on native plants

Project Highlights

Low Grazing Impact: While other studies found that grazing impacts native grass growth, this study did not find a difference between grazed and ungrazed areas, possibly due to the moderate numbers of cattle on the land. 

• Lovegrass Does Not Love Mesquite: Researchers found that Lehmann lovegrass is found in lower densities under mesquite canopies than outside of it. Shading and nutrients from mesquite seem to decrease the prevalence of Lehmann lovegrass. 
• Rare Species Find Refuge Under Mesquite: As a result of less competition from Lehmann lovegrass and other herbaceous plants, rare plant species tend to be found in greater proportion under mesquite canopies than outside them. 
• Not Every Plant Likes Mesquite: Researchers found more individuals of some plants, such as large spike bristlegrass (Setarai macrostachya) and burrow goldenweed (Haplipappus tenuisectus), in greater quantities under mesquite. However, they decreased once they reached a density of 35% under mesquite. There is a threshold of helpfulness for plants under mesquites.
• Grasses and Shrubs Benefit the Most: Native grasses and shrubs tend to benefit from the increased canopy cover of mesquite. Native grasses were found under 95% of mesquite while shrubs were found under 83.7% of mesquite. In comparison, forbs were found under just 3.5% of mesquites. 
• Mesquite Increases Diversity, Decreases Biomass: Although mesquite canopies can provide a beneficial environment for rare plants and native grasses, total herbaceous production decreased as canopy cover increased. This relationship is caused by competition among mesquite, native herbaceous plants, and Lehmann lovegrass. Short rooted annuals may fare better under mesquite canopies, especially as plant density decreases with mesquite growth over time. 

Lessons Learned

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Leveraging mesquite canopies for grassland restoration is particularly applicable to smaller landscapes because it is easier to manually plant and tend to seeds. Seeding methods used for large areas, like aerial seeding, will not be effective when using mesquite canopy restoration techniques because they lack the necessary precision to see positive results. Aerial seeding and similar methods cannot use a different seed mix under canopies versus in the spaces between mesquites. This reality makes leveraging canopies difficult. Although certain species will do better under mesquite, it is hard to place specific seeds under mesquite canopies when seeding large landscapes - aerial seeding, seed drills, and broadcast seeding tend not to be able to plant one seed mix under mesquite canopies and another in the interspaces between mesquites. 

Factors other than the presence of mesquite canopy cover may impact the success of native grassland restoration. If there is no rain, then plants will not grow. Researchers tried to supplement the observational study with experimental data by planting native seeds under mesquite, but were unable to follow through because drought conditions following seeding prevented seeds from establishing. Restoration under mesquite canopies will only go as far as precipitation will allow. 

While observational data can help managers select species that respond to short-term influences like yearly changes in precipitation, analysis of long-term data sets can help select species that will be successful under mesquite canopies in the long run. The long-term data set at the SRER (and other long-term ecological monitoring sites) are publicly available. Working with partners with data analysis skills can allow managers to effectively interpret and make use of these freely available data to see beyond seasonal trends and develop restoration techniques for long-term results.

Next Steps

• Conduct experiments that test seeds under mesquite to supplement long-term data findings. 
• Test seeding methods using seed balls versus naked seeds in restoration. Canopy conditions could mitigate the need for seed balls, but further testing is needed. 
• Determine optimal density of mesquite to maximize opportunities for grassland restoration success. 

Resources

• Bagchi et al. (2017). “Quantifying long-term plant community dynamics with movement models: implications for ecological resilience.”Ecological Applications 27(5): 1514-1528. 
• Barron-Gafford et al. (2017). “Impacts of hydraulic redistribution on grass–tree competition vs facilitation in a semi-arid savanna.”New Phytologist 215: 1451–1461
• Cruz-Alonso et al. (2019). “Long-term dynamics of shrub facilitation shape the mixing of evergreen and deciduous oaks in Mediterranean abandoned fields.”Journal of Ecology 108(3): 1125-1137. 
• Davies et al. (2017). “Legacy microsite effect on the survival of bitterbrush outplantings after prescribed fire: capitalizing on spatial variability to improve restoration.”Restoration Ecology 25(5): 723-730.  
• Gornish et al. (2020). “Identifying restoration opportunities beneath native mesquite canopies.”Restoration Ecology 29(2): e13334.  
• Liang, M., Feng, X., and Gornish, E. S. (2020). “Rainfall pulses mediate long-term plant community compositional dynamics in a semi-arid rangeland.”Journal of Applied Ecology 58:708-717. 
• McClaran, M.P., and Angell, D.L. (2007). “Mesquite and grass relationships at two spatial resolutions.”Plant Ecology 191:119- 126. 
• Smith, D.A. and Schmutz, E.M. (1975). “Vegetative Changes on Protected versus Grazed Desert Grassland Ranges in Arizona.”Journal of Range Management. 28(6): 452-458. 
• Tiedemann, A.R. and Klemmedson, J.O. (1973). “Nutrient Availability in Desert Grassland Soils Under Mesquite (Prosopis juliflora) Trees and Adjacent Open Areas.” Soil Science Society of America Journal 37(1): 107-111. 
• Throop, H.L. and Archer, S.R. (2008). “Shrub (Prosopis velutina) encroachment in a semidesert grassland: spatial–temporal changes in soil organic carbon and nitrogen pools”Global Change Biology 14(10): 2420-2431.  
• Tielborger, K and Kadmon R. (2000). “Indirect effects in a desert plant community: is competition among annuals more intense under shrub canopies?”Plant Ecology 150: 52-63. 
• Yavitt, J.B., and Smith Jr., E.L. (1983). “Spatial Patterns of Mesquite and Associated Herbaceous Species in an Arizona Desert Grassland”The American Midland Naturalist 109(1): 89-93. 
• Zou, C.B. et al. (2005). “Soil moisture redistribution as a mechanism of facilitation in savanna tree–shrub clusters.”Oecologia 145:32-40.  
• University of Arizona School of Natural Resources & the Environment (SNRE)
• Santa Rita Experimental Range

Contact

Elise Gornish, University of Arizona: egornish@arizona.edu

CART Lead Author

Sam Johnson, Pollinator Conservation in Grasslands - Stories of Adaptive Management Intern, U.S. Fish and Wildlife Service, Hastings College

Suggested Citation

Johnson, S.E. (2022). “Identifying Restoration Opportunities under Mesquite Canopies” CCAST. Retrieved from https://www.fws.gov/project/identifying-restoration-opportunities-under-mesquite-canopies.