Emerging restoration technique: Using mesquite branch mulch to increase success of native grasses
Mesquite (Prosopis spp.) is a controversial plant. Depending on who you ask, it can be either friend or foe. On one hand, mesquite is an incredibly resilient plant that can provide habitat, food, forage, and lumber in challenging drought conditions1–4. On the other hand, it is also considered by many to be an invasive plant that is encroaching into historical grasslands and degrading the condition of Southwestern rangelands5–7.
Friend or foe, one thing is sure: mesquite is here to stay. Restoration practitioners use diverse methods to manage mesquite populations. The goal is to keep mesquite populations low enough to minimize potential harms that come with dense mesquite cover, such as reduced grass productivity, while maintaining the benefits that mesquite trees provide, such as forage during dry times of the year, shade for cattle, and habitat for wildlife.
To control mesquite populations, land managers have a wide array of tools to choose from. Herbicide applications, mechanical or manual removal, and use of fire have all received much attention and have had varied success at controlling mesquite populations8. Many of these management options result in large amounts of mesquite biomass being left on the land. Sometimes the thicker diameter wood can be used as lumber or whole trees placed into eroded washes to slow the flow of water and reduce erosion. However, small diameter branches are not often used for restoration purposes.
A recent study in the Altar Valley tried using small diameter mesquite branches as mulch in degraded sections of a ranch with lots of bare soil. Using branches as mulch is different from masticating with heavy machinery, or chipping mesquite and spreading woodchips. Both mastication and chipping break the mesquite branches into small pieces. Conversely, mulching with whole branches involves simply placing whole branches on the soil surface. When combined with mechanical or manual mesquite removal strategies this may simply involve placing branches from cut mesquites onto bare soil. When used alone, it can be as simple as cutting branches from standing trees with handheld loppers or chainsaws, and laying these branches on nearby exposed sections of soil.
Leaving branches whole can shade the soil, reduce temperatures, and increase soil moisture without smothering the soil and preventing emergence of species that cannot germinate through a heavy woodchip mulch cover. Using whole mesquite branches also has the benefit of not requiring the use of heavy machinery to cut or spread the mulch. A small crew with hand tools (loppers or chainsaws) can quickly and effectively cut and spread mesquite branches as mulch.
The study showed that a 6-8” deep cover of mesquite branches helped plants establish in bare sections of a rangeland where seeding alone failed. The soils of this area were sandy and very gravel-rich, and the plant community was mainly cacti (cholla and prickly pear) and shrubs (mesquites, palo verdes, and snake weed), surrounded by large patches of bare ground. Simply broadcasting native grass seeds and laying branches cut from mesquites on site increased the presence of the seeded native grasses, and the grasses and forbs already in the soil seedbank.
There are several important considerations when using mesquite branch mulch to help plants re-establish in bare areas. First and foremost, it is critical to ensure that the mesquites do not have viable seedpods when they are cut for use as mulch to prevent the spread of mesquites. It is also important to consider the abundance of mesquites on site and distance from patches of bare ground. Cutting mesquite in one area and carrying great distances to bare areas might not make logistical sense. And since mesquite branch mulch has shown limited effectiveness in areas that already have grass cover9, it will be most effective in bare areas. Where the mulch is placed on the landscape is also important. If placed in areas of concentrated water flow, smaller branches may simply be washed away.
The potential of using mesquites, an abundant rangeland plant that is often already being managed in large areas of the Southwest, to increase grass cover is a promising win-win management strategy. However, using mesquite branch mulch in the Southwest to help restore native grasses is not a very well documented restoration technique. Apart from the recent study in the Altar Valley, we have heard of mesquite mulch being effective in the Altar Valley along a restored pipeline right of way and when used along contour as “media lunas” to reduce erosion from overland flow. These are all encouraging examples that indicate the potential for mesquite mulch to help restore degraded areas of rangelands.
But there are still many exciting questions to answer and much to learn! Most of these experiments were in coarse gravel-rich soils. Will mesquite branch mulch be effective in finer textured soils? What about at different elevation grasslands being encroached upon by pinon pine and juniper? Or in years with especially high or low rainfall? If you have experience using whole branches from mesquites and other woody plants as mulch that you would like to share, or want to learn more about the recent study in the Altar Valley, please contact Ariel Leger at arielleger@email.arizona.edu for more information.
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