Management Methods: Physical Methods

Impacts of Physical Methods

Most physical methods cause either direct or indirect damage to plants, whereas other methods alter growing conditions.

In nature, plants are subjected to physical injury, such as girdling by porcupine. Photo credit: J O’Brien/USFS, www.forestryimages.org

In nature, plants are regularly subjected to physical injury.

• Floods uproot plants, break stems at the soil surface, and temporarily decrease oxygen availability to plants.
• Winds can stress or break limbs and stems.
• Wave action can break and dislodge aquatic plants.
• Grazing and foraging animals defoliate and trample plants, dig roots, consume seeds, and girdle tree trunks.

While the effects of physical injury may be lethal to some plants, others have characteristics that allow them to persist in the plant community:

• Some plants tolerate injury and stress, or avoid it entirely, by protecting growing points and propagules structurally (e.g., thick bark or seed coat), spatially (e.g., underground buds), and/or temporally (e.g., dormancy).
• Many plants can recover from damage by resprouting from growing points or plant fragments.
• Plants that are susceptible to physical injury and stress, such as annual forbs, often produce large numbers of seeds and establish seedbanks, thus allowing populations to persist over time.

How plants withstand or recover from stress and injury depends on the availability of carbohydrate reserves and the continued ability to grow and reproduce. Plant response is determined by the following traits, mechanisms, and factors:

• life history strategy (e.g., annual, biennial, perennial)
• life cycle stage (e.g., seed, seedling, rosette, bolt, bud, flower)
• growth form (e.g., grass, forb, shrub, vine, tree, aquatic)
• growth status (e.g., dormant or actively growing, stressed or healthy)
• reproductive mechanisms (e.g., production and dispersal of seed and vegetative propagules)
• specific adaptations to disturbance (e.g., flood tolerance)
• ecological interactions before and after injury (e.g., community dynamics, environmental stress)

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The effect of a physical method on a plant depends on the interactions between plant biology, the characteristics of the physical method, and how it is applied.

Some plants, such as Himalayan knotweed (Polygonum polystachyum), can resprout from stem fragments left behind after physical methods are used. Photo credit: www.nwcb.wa.gov

Physical methods may control some invasive plant species when applied at an optimal time and intensity that target vulnerable plant structures and processes, and at a level of selectivity that minimizes damage to desirable plants. To control an invasive plant population completely (i.e., to eradicate it), all perennating structures and propagules must be eliminated. In established invasive plant populations, where mature plants often have substantial carbohydrate reserves and/or extensive seedbanks, eradication is rarely achieved with a single application.

Typically, physical methods are combined with another method to control regrowth, sprouting, and seed germination, or are administered several times to exhaust carbohydrate reserves and deplete seedbanks (Radosevich et al. 1997). For newly established populations where plants are young or have not produced seeds, a simple and selective physical method such as pulling may be particularly well suited.

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The effects of physical methods may extend beyond the invasive plants they are intended to control. The range of tools and techniques employed may result in nontarget impacts on biological communities and the environment. As with all natural resource management decisions, land managers must weigh the value of reducing or eliminating invasive plants against the risk of nontarget impacts. The following examples describe some general impacts of physical methods in both terrestrial and aquatic habitats.

Flora

Selective methods of physical management generally do not affect nontarget plants, although laborers and equipment can inadvertently trample desirable vegetation. Nonselective methods such as tilling, soil solarization, flooding, and water drawdowns and dredging disturb soils and substrates and affect all vegetation in the treated area.

Fauna

Sinuous channels were developed to maintain fish passages during water drawdown treatments to manage reed canarygrass at Toppenish NWR in Washington. Photo credit: USFWS

Most physical methods can be timed or applied selectively to avoid direct impact on fauna. However, any changes in vegetative structure, such as removing shrubs or trees, can affect wildlife by changing plant cover and food resources (McPherson and DeStefano 2003). The effect can be positive; for example, when the girdling technique results in standing dead trees that may be useful for cavity-dwelling wildlife species. But nonselective physical management such as mowing or tilling may affect ground-nesting birds, small mammals, and arthropods, as well as biological processes such as feeding, pollination, and predation (Vickery et al. 2000, 2001).

While erecting benthic barriers can control aquatic invasive plants, it also prevents establishment of desirable aquatic vegetation required for fish and wildlife habitat. Similarly, harvesting removes the canopy and shade-producing portion of aquatic plants, which has implications for aquatic organisms at all depths. Water drawdowns to dry out invasive aquatic or wetland plants can have severe direct effects on fish and other organisms. At Toppenish NWR in Washington, where water drawdowns were used to eliminate monotypic stands of reed canarygrass (Phalaris arundinacea), sinuous channels were developed to maintain passages for fish.

Water

In terrestrial plant communities, physical methods such as tilling that destroy the root systems anchoring soils may cause increased erosion and runoff into waterbodies. In aquatic plant communities, physical management of invasives directly affects water quality and flow. For example, harvesting aquatic plant biomass can resuspend sediments, thereby altering nutrient supply in the water column. Dredging alters water depth and creates a substrate devoid of vegetation, thus damaging the habitat and food supply of aquatic organisms.

Soil

Hand pulling can create small areas of disturbed soil. Photo credit: J Miller, www.invasive.org

Many physical methods impact soils positively and negatively.

• Organic mulches may decrease erosion, reduce evaporation, and improve soil structure as they decompose over time (Bronick and Lal 2004). However, their decomposition may also temporarily reduce soil mineral nitrogen available for plants (Bond and Grundy 2001).
• Tilling can break up compacted soil, aerate the soil, and prepare a seedbed for planting desirable species (Zimdahl 1999). On the other hand, it can decrease soil moisture and increase soil erosion (Venner 2006), and may decrease microbial populations in the soil.
• Heavy equipment such as tractors or large mowers can compact soils, as can laborers conducting manual techniques such as cutting, girdling, or hoeing (Tu et al. 2001). Cutting, however, leaves roots intact to help stabilize soil (Venner 2006).
• Soil solarization can significantly change biological, physical, and chemical properties of soils, thus affecting subsequent plant growth. It also results in an open substrate that can be readily occupied by new plant species, both desired and undesired (Tu et al. 2001).
• In some cases, invasive aquatic plant species may serve to stabilize sediment and dampen wave action. Removing invasive aquatic plants can therefore result in increased shoreline erosion.