Mechanical ground disturbance can change the plant composition in fire-dependent pine communities, sometimes permanently degrading or eliminating native plants if they are repeatedly disturbed.
A recently published study by Tall Timbers Fire Ecologists Cinnamon Dixon and Kevin Robertson analyzed data from ground disturbance on plant communities from more than 30 sites in the North American Coastal Plain from North Carolina to Texas.
Their research reviewed data from 29 previous studies and three groundcover studies they conducted on Red Hills properties, covering three pine savanna community types, flatwoods, sandhills, uplands.
A main takeaway from the study was that native pine savanna plant communities are quite vulnerable to mechanical soil disturbance.
Researchers were able to place 311 plant species found in the study sites into three categories of response to soil disturbance: decreasers (negative), increasers (positive) and neutral.
Although some ground disturbance occurs naturally and can be beneficial by providing low competition, sunlit areas for plants to take root, such as tree tip-ups and burrowing and mounding of soil by animals, the study focused on the impact of machinery on ground disturbance which tends to be more thorough and extensive.
Some soils were disturbed just once and others, such as in agricultural fields or annually disked firebreaks, were repeatedly disturbed, both providing long-term and short-term data on the impact to the local plant communities.
What they found was that plants negative affected by repeated disturbance tended to be those that are limited in their distribution to the Coastal Plain and especially included bunchgrasses, native oaks such as bluejack oak, turkey oak, sand post oak, and running oak, and shrubs in the blueberry family
These species are the most flammable in the system, so their loss restricts conditions and number of days in which sites can be burned effectively. They have a perennial life span and complex root systems but have a non-persistent seed bank and limited dispersal to allow them to recolonize areas impacted by intensive ground disturbance.
While repeated disturbance in this plant community may totally eliminate these species, single disturbances usually leave some survivors from which they can  slowly recovered if given enough time without further disturbance. Sites that saw repeated, long-term disturbance were often dominated by herbaceous forbs, non-bunching grasses, off-site woody  plants such as sweet gum, water oak, and live oak, and invasive species like bahiagrass. In general, plants that regrew following major disturbance had a larger percentage with annual life spans, a persistent seed bank, and a widespread geographic distribution. These species are much less flammable than those dominating the undisturbed sites.
A species was categorized as neutral if it showed little change when disturbed, either meaning the species is generally robust, not dependent on soil disturbance, or its response was difficult to broadly predict among particular soil disturbance events and locations.
Of plant species found to have a short term response to single disturbances, there were 49 decreasers (34%), 52 increasers (36%), and 43 neutral responders (30%).
Of plant species studied for their long term response following repeated disturbance, there were 143 decreasers (55%), 77 increasers (29%), and 42 neutral responders (16%).
Ninety-five species were reported in both single disturbance and repeated disturbance studies, of which the the majority (52%) Â showed the same response to disturbance.
Although the different plant species differed in their response to soil disturbance, the results were consistent across all three different pine community types, flatwoods, sandhills and uplands.
Altering the plant composition through soil disturbance can have an impact on habitat structure necessary for food and shelter for some animals, can reduce flammability for prescribed fire by attracting offsite species, and can reduce biodiversity.
Researchers noted that, while soil disturbance can quickly degrade the plant community, recovery is very slow and often never complete. Sometimes soil disturbance is used with the intent of reducing dominant native plants and encouraging early successional weeds for wildlife food, but in case the long-term effects on those management decisions should be considered.
“Given that such changes occurring over decades can be reset by intensive soil disturbance, the decision to apply mechanical disturbance merits careful consideration for its long-term implications in these communities,” they wrote. “Perhaps most important is the threat posed by invasive species, many of which appear to respond positively to soil disturbance and threaten to displace native plants and decrease biodiversity in both native and post-agricultural pine savannas.”
They made recommendations to limit the effects of soil disturbance for a variety of practice.
- It is helpful to keep annually disked firebreaks in the same place each year as long as they do not cause erosion problems.
- In silviculture operations, keep logging roads and skid trails narrow and avoid operating during wet periods that aid in rutting. Place logging decks and staging zones in already disturbed areas. Doing site prep with a V-blade instead of disking can help keep soil clods intact.
- For reducing woody vegetation, use of above-ground methods such as mowing and herbicide application followed by frequent prescribed fire may be effective enough without using intensive soil disturbance which often results in increases in less desirable woody vegetation.
Read the full study here