Practice Description
Soil exposure, sometimes referred to as “soil scarification,” refers to the purposeful act of exposing mineral soil to create invertebrate habitat, reengage the native seed bank, and/or diversify vegetative ground cover. This practice is best suited for fire-influenced or disturbance-dependent landscapes that have experienced a deep buildup of organic duff and litter on top of the parent material. A large suite of invertebrates associated with fire-influenced communities–including most bees, solitary wasps, and other taxa–rely on access to bare mineral soil for nesting, and variation in abundance of these species can partially be attributed to a lack of access to adequate nesting sites. Additionally, the seeds of most fire-adapted plant species will not germinate in duff, and instead depend on exposure to mineral soil for germination. The seeds of many of these species are very long-lived and can remain dormant below the duff layer for decades, ready to germinate immediately following a disturbance event. Similarly, oak regeneration can be greatly enhanced by soil exposure for the same reasons. In most cases, restoration activities in fire-influenced communities will not require seeding because the existing seed bank will provide the desired species composition and coverage. Soil exposure can often be achieved mechanically, as well as through the application of prescribed fire, where appropriate.
Goals and ecological processes
Soil exposure is important in disturbance-dependent natural communities where appropriate disturbance events were suppressed or lacking in the recent past. The primary example of this is found in fire-influenced landscapes where fire exclusion resulted in the establishment of a substantial duff layer above soil parent material. The goal of soil exposure is to remove or break up the duff layer to expose the parent material, thereby permitting the ecological benefits that exposed mineral soil provides.
Target habitats and species
All fire-influenced upland natural communities may benefit from soil exposure when duff/thatch layers have built up. Mechanical disturbance is generally inappropriate in fire-dependent wetland communities (e.g., calcareous wetland communities), in which prescribed fire may be preferable. In contrast, prescribed fire or light scraping (as opposed to tilling or plowing) should be the preferred method in terrestrial communities without a history of ground disturbance in order to maintain the undisturbed soil horizons.
A wide suite of disturbance-dependent species benefit from soil exposure, including:
- Invertebrates: species dependent on open patches of bare sand or other mineral soil for nesting or foraging include bees (Anthophila), ants (Formicidae), antlions (Myrmeleontidae), tiger beetles (Cicindelidae), ground beetles (Carabidae), scarab beetles (Scarabaeidae), thread-waisted wasps (Sphecidae), spider wasps (Pompilidae), velvet ants (Mutillidae), bee flies (Bombyliidae), and robber flies (Asilidae) and hunting spiders.
- Forbs: a long list of fire-adapted plant species depend upon simultaneous contact with sunlight and mineral soil for germination (ex. Ceanothus, Lupine, Chamaecrista, specialized graminoids, etc,). This increase in flowering-plant diversity can, in turn, further benefit native pollinators, many of which nest in the exposed soil.
- Game Species: bobwhite quail and wild turkey use the thick herbaceous vegetation that regenerates in scarified areas as key habitat for chicks to forage.
- Turtles: scarification can provide critical nesting sites for turtles in otherwise undisturbed landscapes.
Associated practices
- Selective tree removal: Tree cutting is often the initial restoration action taken in the restoration of dry barrens and oak woodland communities that have been degraded by fire exclusion and cutting operations can be an effective mechanism to expose soil (see Methods, below).
- Prescribed fire: Prescribed fire is generally the preferred maintenance approach in areas appropriate for purposeful soil exposure. The reduction of the duff layer in dry barrens, oak woodlands, and calcareous wetland communities should be considered as an objective in any prescribed fire plan. Duff reduction is often difficult in these communities because burning into the duff produces more smoke than a typical surface fire, and persistent burning material in the duff layer can create difficulties during post-fire mop-up. Prescribed fires are often designed to gradually reduce the duff later over many prescribed fire events to avoid a single, large-scale smoke and smoldering event.
- Native vegetation planting: The practice of native vegetation planting refers to establishing desired vegetation as a restoration action through direct seeding or transplanting. In many cases, the duff layer will need to be completely removed prior to seeding in order for the seeds to germinate. Further, in situations where plugs are used to establish vegetation, it is often appropriate to scarify the site prior to planting so that once established, the seeds of the planted stock are able to germinate.
- Invasive plant control: In very specific circumstances—for example discrete sandplain grassland areas supporting invasive or generalist monocultures—removing the topsoil may be a highly effective way of dealing with invasive plants while also setting up the site to be recolonized by desirable plant species through revegetation. The benefit of removing topsoil is that it instantly removes undesirable vegetation, the invasive seed bank, thatch, and other organic surface layers. Additional considerations when using this approach include: securing an appropriate location for disposal of topsoil and other organic material, follow-up invasive treatment, and a plan for the establishment of the desired vegetative community.
Methods
Practice details
The majority of active soil exposure projects are achieved through mechanical means. Below are examples of approaches to mechanical scarification and situations where scarification may be appropriate:
- Selective tree removal: Ground conditions in degraded fire-influenced communities often include the accumulation of deep duff layers and the formation of organic surface soils; both of which are detrimental in these settings. Planning a tree removal operation that disturbs the surface layer in these communities is a highly effective method of exposing the parent material below accumulated duff layers, which will immediately reengage the latent seed bank below, and provide critical access to sandy substrates for nesting invertebrates. In addition, this practice promotes oak regeneration. Actions designed to disturb upper organic layers–like choosing to move harvested material with skidders instead of forwarders–should be considered. This method achieves scarification at scale, resulting in near complete coverage of a restoration unit while providing heterogeneity in depth of scarification. Other mechanical techniques, such as scraping, are often not practical at large scales. Care to reduce soil erosion around wetlands and on steep slopes is always a priority, and efforts should be taken not to disturb established desirable groundcover characteristic of barrens and woodland communities, for example aggregations of lowbush blueberry and native grasses.
- Scraping: Scraping areas can be a great way to accelerate sandy soil exposure in dry, fire-influenced sites for both the benefit of ground nesting invertebrates and to engage the dormant seed bank. Scraping is usually only practical and desirable at a relatively small scale (a quarter acre or less), but multiple scrapes can be made to create a mosaic of bare soil patches within a larger unit. Over time, as previously scraped areas naturally become revegetated, creating additional scrapes within the larger landscape will result in a desirable diversity of ground conditions across a site. This type of scraping is usually done with a blade on a machine (e.g a decking blade on a bobcat, skidder, bulldozer, etc.). An important consideration with scraping is the disposal of the duff and topsoil. Removing this material from the site is recommended because windrows of organic soil and duff will persist for many years, suppressing the regeneration of desirable vegetation, supporting the continued spread undesirable vegetation and interfere with ongoing prescribed fire activities.
- Harrowing/disking: Shallow harrowing or disking are effective techniques to break up the duff/thatch layer, as well as areas of dense turf and undesirable shrubs to permit more specialized plants to establish in newly exposed soil. This treatment provides excellent foraging areas for birds, mammals, and predatory invertebrates. Having small harrowed or disked plots nested within the larger matrix of a fire-influenced landscape provides important habitat diversity.
Planning
Scarification, when coupled with a timber harvest as a part of an initial restoration effort in a site degraded by fire exclusion, can incorporate the entire restoration unit. This scale is desirable because it maximizes the short-term reengagement of the native seed bank across the entire unit. This same approach can be taken when using prescribed fire to reduce duff and expose mineral soil. Other mechanical scraping activities should generally be relatively small (<1/4 acre), and not covering in aggregate more than 5% of a restoration site at a time (though each site is different and needs to be evaluated individually). When creating invertebrate habitat, there really is no minimum scale: even small patches of exposed soil will be used.
The frequency of mechanical scarification on a landscape is dependent on how long it takes newly scraped areas to revegetate beyond the desired condition. Rotational scarification (scraping relatively small areas within a larger landscape in a staggered sequence) provides a mosaic of soil exposure that increases the diversity of invertebrate usage at a site.
With any form of soil disturbance, care should always be taken to avoid the establishment of unwanted species. Generalist and invasive species are especially proficient in exploiting areas of soil disturbance, and therefore all equipment should be thoroughly cleaned before entering disturbed sites (to avoid introducing unwanted seeds and propagules) and soil exposure should never occur at sites where invasive plants are established without an invasive control plan. Further, even desirable native plants may become overabundant in disturbed areas, with pitch pine being especially adept at creating dense stands of seedlings.
Examples
- Muddy Brook Wildlife Management Area, Hardwick, MA
Scarification through timber harvest and smaller scale scraping - Frances Crane Wildlife Management Area, Falmouth, MA
Scarification through timber harvest - Blackington Barrens, New Salem, MA (DCR Quabbin Watershed)
Scarification through timber harvest - Birch Hill Dam (Rifle Range)-Winchendon, MA (Army Corps of Engineers)
Sandplain Grassland Restoration/Invasive Removal through topsoil removal - Montague Plains Wildlife Management Area, Montague, MA
Scarification through timber harvest and scraping of patches - Southwick Wildlife Management Area, Southwick, MA
Sandplain Grassland Restoration by tree stump removal and root rake