When in Drought, Low Compaction is Key

Fall 2014 Ecesis, Volume 24, Issue 3

Clearing brush prior to the MTBA March 15 deadline. The compost remained onsite to compost, and was incorporated into the soil after grading.

Clearing brush prior to the MTBA March 15 deadline. The compost remained onsite to compost, and was incorporated into the soil after grading.

In some habitat restoration projects, due to the location or how the mitigation plan was written, the restoration contractor may not be able to install a temporary irrigation system to provide water to the native plants. During years of drought, this can create even more of a challenge for the contractor. 

This was the case for a riparian mitigation project constructed in southern California for a local school district. The project included the removal of 1,300 non-native trees and shrubs including Arundo donax, Tamarix and Fraxinus (Ash), along with the creation of 3.5 acres of riparian habitat. We reviewed the site grading plans and realized the site would need a cut of about 12 feet in order to achieve the proper hydrology for the riparian plant pallet. 

Due to the deep cut, we surmised that we would be dealing with soils that were compacted and devoid of any nutrients. Along with this, we were in ongoing drought conditions with a project that would not allow for temporary irrigation. Low water and nutrient weak and compacted soils — for a successful project, this would require some careful planning. 
We started with the removal of the Arundo donax and Tamarix to a local green waste facility. But, the Fraxinus and other shrubs were composted and stored in piles onsite while the site was graded. Keeping with our schedule, we cleared the land prior to the Migratory Bird Treaty Act (MBTA) deadline on March 15, and over the summer months graded the new riparian corridor while the school was built. In September, we took over the newly graded mitigation site. 

The first step for this newly graded site was to pull soil samples and send them out to the soils lab. We then crossed-ripped the site to 18 inches with agricultural rippers on our low ground pressure CAT 277. Our soils lab recommended three cubic yards of compost per thousand square feet. We utilized our salvage compost that had been decomposing over the summer, and using the CAT 277 with grapple bucket, we placed chippings over the ripped site. 

Although it would have taken less time to use a CAT 950 loader than the CAT 277, we would have re-compacted the soil — and compaction is the enemy of restoration. With the CAT 277 and the rototiller attachment, we tilled in the amendments approximately 8 inches. 

Walking on this newly amended soil was like walking on a sponge. The 6,300 pole cuttings of various Salix species were installed and watered in with a hose; then we applied 3 inches of our project-generated composted mulch around the cuttings to retain moisture and impede weed growth. Over the next few months we had less than 2 inches of rain and we did not water the cuttings.

The pole cuttings budded out and thrived in the newly-created riparian corridor. At the end of Year One, the average willow height was 6 feet. At the end of Year Two, they reached 12 feet, and by Year Three, we could no longer pull the tape from the transects stations. The site was already exceeding the 5-year coverage criteria. 

Looking back, 12 years later, the willow canopy is forty-plus feet tall and thriving. This project site has survived one El Niño and the current 90-year historical drought. The three keys to this success was creating a healthy soil through de-compaction, agronomy, and utilizing low ground pressure equipment. — by Mark Girard, President of Habitat Restoration Sciences, Inc.