Velvet Mesquite Thinning on San Pedro River

Introduction

Image Details

Mesquite bosques are characterized by stands of mature mesquite trees with low-stem density and a dense, closed canopy. These habitats are known to support a diversity of native plants in the understory and wildlife. In an effort to restore a bosque structure structure
Something temporarily or permanently constructed, built, or placed; and constructed of natural or manufactured parts including, but not limited to, a building, shed, cabin, porch, bridge, walkway, stair steps, sign, landing, platform, dock, rack, fence, telecommunication device, antennae, fish cleaning table, satellite dish/mount, or well head.

Learn more about structure to a velvet mesquite community, scientists with The Nature Conservancy implemented a tree-thinning experiment in 1998 at Bingham Cienega Natural Preserve (the Preserve), a 115-ha site on the San Pedro River owned by the Pima County Flood Control District. The site was revisited for monitoring in 2017, 19 years after the thinning occurred. 

Key Issues Addressed

Mesquite bosques were historically abundant along low-elevation (below 1200m) rivers in the southwestern U.S. and northwestern Mexico, but have been reduced to small remnant stands due to declining water tables and land clearing for agriculture. Virtually none remain in pristine condition, suggesting a need for both protection and restoration. In some cases, abandoned farm fields have been rapidly recolonized by dense stands of small mesquites, all competing for water and sunlight. Research on other tree species has demonstrated that individual trees in dense stands will grow faster after thinning reduces resource competition. Thinning velvet mesquite in a riparian riparian
Definition of riparian habitat or riparian areas.

Learn more about riparian setting may help promote  a bosque-like forest structure. 

Project Goals

• Determine whether thinning of a dense stand of young velvet mesquite can speed the development of mature mesquite bosque community characteristics

Project Highlights

• Experimental Design: Study plots were established in 1998, with a 0.34 ha (0.83 ac) treatment plot and an adjacent 0.18 ha (0.43 ac) control. Initial density in the control plot was 2,683 trees/ha (1,086 trees/ac).
• Treatment and Monitoring: Thinning occurred on the treatment plot in April 1998 so that adjacent individuals were 5-6 m apart, for a density of 542 trees/ha; this value was informed by literature densities for mature mesquite forests. Trees were cut by hand, the cut stumps were treated with herbicide (Garlon), and the remaining trees were counted. Stem diameter and maximum height were measured in June 1998 and two years after treatment in May 2000.
• 19 Years Later: Plots were revisited 19-years after treatment in December 2017. For 20 randomly chosen mesquite trees on each plot, researchers measured basal diameter at ground level, diameter at breast height (1.37 m/4.5 ft above the ground surface), and counted stems. Measurements were compared using two-sample t-tests assuming unequal variances
• Height Measurements with Lidar: To compare current tree heights, Lidar data for the area were obtained from Pima County, with data acquired from flights on Feb 20-26, 2015. There was no field verification of individual tree height and no attempt to correlate with basal diameter for a weighted canopy average. Therefore, these height measurements allow for comparison between plots at this treatment site, but cannot be used to compare to other locations. 

Lessons Learned

Image Details

Quality Habitat: This study suggests that thinning can accelerate development of higher-quality wildlife habitat in young mesquite stands by establishing a bosque-like structure preferred by a diversity of birds and wildlife species. 

• The results demonstrate that thinning a dense stand of young velvet mesquite can promote a bosque-like structure. Thinning resulted in faster growth rates with 2.5cm diameter increase compared to 1.5cm increase in control plots during the first two years following treatment. Thinned plots also had lower tree density compared to the untreated plots 19 years after treatment occurred with 1,362 trees/ha on thinned plots and 1,975 trees/ha on control plots in 2017.
• Resprouted stems were conspicuous and significantly more abundant on the thinned plot. Trees on the thinned plot had twice the average density of live stems at breast height (5.2 stems) than on the control plot (2.7 stems), indicating resprouted trees developed a more shrub-like growth form.
• It is possible that the benefits of thinning would have been stronger if the area had been retreated at least once to eliminate root sprouts from cut trees. Retreatment may have led to reduced stem density, increased growth rates, and lower basal area more typical of a mesquite bosque.
• This study suggests that thinning can accelerate development of higher-quality wildlife habitat in young mesquite stands by establishing a bosque-like structure. While mesquite trees in any context provide some habitat value for wildlife, the high value of mature mesquite bosques is largely due to the composition and structure of the plant community. High foliage density in multiple layers of the canopy supports diversity and abundance among birds, while understory shrubs support fruit-eating birds.

Next Steps

• These results may be useful for informing management decisions at Bingham Cienega, other sites with similar conditions along the San Pedro River, and elsewhere
• Implement a bird monitoring program in the experimental plots to test for differences in habitat use

Funding Partner

• Arizona Water Protection Fund

Resources

• Overview of the Nature Conservancy’s Work on the San Pedro River
• Library of Reports and Data Available for the San Pedro River
• Proceedings of the 2013 Conference on Biodiversity and Management of the Madrean Archipelago

Contacts

• Dale Turner, Conservation Scientist, The Nature Conservancy: dturner@tnc.org
• David Gori, The Nature Conservancy

Case Study Lead Author

• Ashlee Simpson, CART Graduate Research Assistant, University of Arizona

Suggested Citation

Simpson, A., C., and Grabau, M., R. (2018). “Velvet Mesquite Thinning to Improve Riparian Forests along the San Pedro River.” CART. Retrieved from https://www.fws.gov/project/velvet-mesquite-thinning-san-pedro-river.