Effects of mowing on vegetation largely depend on mowing seasonality and frequency. Mowing at a high frequency during the growing season reduces resource allocation to the roots and prevents regeneration of aboveground vegetation, which exhausts stored root energies (K. Fauteux, Interview). Some evidence shows that areas mowed at a high frequency over a long period of time exhibit an open structure compared with un-mowed areas (R. Freeman, Interview). However, too much mowing, and particularly mowing in a homogenous way (e.g., the same time and areas each year) could have undesirable consequences. While frequent mowing to reduce woody growth is necessary, implementing mowing annually or several times each year can have unintended consequences, such as creating a less biodiverse system. Such management could favor a few native species such as little bluestem (Schizachyrium scoparium) but reduce diversity of rarer forbs (Greller et al. 2000; M. Mello, Interview.). One suggested method to promote a more heterogeneous plant community is mowing irregular portions of the landscape over space and time to create a patchy mosaic that fosters higher biodiversity of flora and fauna. K. Beattie (Interview) suggested targeting only the portions of the landscape that have the highest shrub cover and leaving the grassy patches untreated. Further, mowing intensively multiple times per year and then relaxing management for several years to allow recovery of impacted rare species might be effective.

Mowing during the growing season and dormant season may reduce the abundance of rare plants by preventing seed set or burying seeds under mowing debris, inhibiting favorable germination conditions. Therefore, when managing for rare species, mowing is often conducted in the fall after seed production (Clarke and Patterson III 2007). Another practice to maintain habitat for rare species was implemented in Manuel F. Correllus State Forest, where annual or biennial fall mowing conducted in the grassy fire lanes created before 1938 was sufficient to maintain habitat for five rare grass and forb species. Effect on soils and fuels

Relatively little work has examined effects of mowing on soils and the relationships of soil characteristics to plants. However, increasing soil organic matter, nutrients and moisture are pretty major changes caused by mowing that could have potentially dramatic impacts on habitat suitability for different plant species (K. Beattie, Interview). While evidence from the northeast US is mostly anecdotal, effects on soils potentially include an increase in soil organic matter, slightly elevated soil nutrients, and higher soil moisture. A detrimental effect of mowing is that litter created by mowing serves as a mulch layer that reduces bare soil patches that are important sites for recruitment of desired plant species from seed (K. Beattie, Interview). The buildup of duff over time could change the microhabitat and potentially favor non-native or invasive species, or native species that are not affiliated with sandplain grassland habitats, but we found no studies highlighting this potential effect.

Even where soils have been studied in more detail, patterns are not especially clear. Martin (2008) compared an area of Nantucket heathland that was unmowed and burned with an area that had been mowed and burned and found different trends in nutrients and plant cover. For example, burned, unmowed areas were higher in soil nutrients and had a positive relationship with black huckleberry (Gaylussacia baccata) cover and a negative relationship with Pennsylvania sedge (Carex pensylvanica) cover.

The increase in litter associated with mowing increases the amount of fine fuel available to burn, likely increasing wildfire risk in sandplain grassland. However, we found no studies that quantified these effects.

Initial species assemblages, their life history characteristics and timing determine how they will respond to mowing disturbance. For example, black huckleberry (Gaylussacia baccata) is fire-tolerant and clonal, therefore prescribed fire could stimulate growth more than mowing depending on the applied fire regime (Matlack 1997). Physical crushing, maceration, and mechanical impacts during the summer growing season have also been found to kill this species (T. Simmons, Interview).

There is evidence that mowing during the summer growing season is effective at reducing woody shrub cover and increasing plant biodiversity (Fig. 3). Dunwiddie (1998) studied various mow treatments at 14 sites across Cape Cod and the islands of Martha’s Vineyard and Nantucket and found that cover decreased for some functional groups. With August mowing, frequency of forbs and graminoids increased but shrub growth was not reduced. Dunwiddie et al. (1995) found that frequency of forbs species increased 83 percent in a summer mowed treatment compared with an unmowed control and that small bayberry (Morella caroliniensis) declined eight-fold in frequency with mowing. The increase in the frequency of forbs with mowing was greater than the 62 percent increase in forbs documented in a paired fire treatment (Dunwiddie et al. 1995). Dunwiddie concluded that August burn and mow treatments reduced shrub cover and frequency and increased the frequency of graminoids and forbs compared with both spring burning and an unburned control. In a separate study, Dunwiddie and Caljouw (1990) found that mowing in August increased forb cover, decreased shrub cover, and increased graminoid cover. Dunwiddie (1990) found frequency of herbaceous species increased more after burning than mowing. There is additional evidence that dormant season mowing is not very effective at reducing cover of woody plants compared with summer mowing. Dunwiddie (1998) examined mowing during the dormant season at Katama on Martha’s Vineyard and found that frequency of shrubs was not reduced and forb frequency decreased, but there was an increase in graminoid frequency.

On Naushon Island, C. Neill (Interview) found that mowing grassland edges once per year in June for three successive years during summer had relatively small effects on the cover of common greenbrier (Smilax rotundifolia) and black huckleberry (Gaylussacia baccata) but mowing twice per year in June and August reduced shrub cover and increased graminoid cover.

Overall, there is consistent evidence that dormant season mowing increases graminoid cover but does not necessarily achieve other goals such as reducing woody shrub cover, increasing forbs, or affect the frequency of target plant species. Summer mowing, in contrast, increases the number of species and cover of forbs and reduces cover of woody shrubs, although it appears that even more aggressive measures are needed to completely remove shrubs.

Mowing can select for some plants over others (Raleigh et al. 2003), especially when mowing is repeatedly applied at the same time of year. Mower blades do not typically cut low-lying vegetation. For this reason, seasonality is important to consider (Raleigh et al. 2003). For example, late-flowering species are less susceptible to mowing during spring when most of the biomass is in short basal rosettes or underground. In contrast, mowing in the late growing season would negatively impact these same species considerably, as they are typically tall and flowering (Raleigh et al. 2003). Chris Buelow (Interview) suggested that mowing works well for converting an area from woody vegetation or clonal herbs such as some goldenrod species (Solidago spp.) to grassland when mowing is conducted during summer.

We found two key gaps in understanding the effects of mowing on existing northeastern U.S. sandplain grassland vegetation. First, little information on key aspects of life history exists for many of the infrequent species that are sandplain grassland conservation targets that might be managed by mowing. This parallels the lack of similar information about effects of burning. Farnsworth (2007) compared the characteristics of infrequent sandplain grassland species with their more abundant close relatives and found that they typically have distinct life-history traits such as: (1) higher habitat specialization, (2) larger seed size, (3) smaller plant height, (4) less reliance on vegetative (colonial) reproduction, and (5) tendency toward annual or biennial life history. There is very little information on how the phenology and abundance of most of these infrequent species respond to mowing. Second, despite the potential importance of the invasion of non-native woody shrubs and vines in existing northeastern U.S. sandplain grasslands, widespread concerns by managers about their control, and the potential of mowing as a management method, we found no studies that specifically examined invasive species responses to mowing within areas of existing sandplain grasslands. There is the strong perception by managers that the list of potentially important non-native and invasive species that now threaten northeastern U.S. sandplain grasslands is growing. For example, Amur peppervine (Ampelopsis glandulosa) and black swallow-wort (Cynanchum louiseae) appear to be spreading rapidly on both Long Island and southeastern Massachusetts (P. Weigand and C. Neill, Interviews).

The mechanical disturbance of mowing can be harmful to nesting birds and immobile animals. Atwood et al. (2017) suggest that mowing hay during breeding season is the leading threat to grassland-nesting birds in New England, and that adjusting mowing schedules could have drastic effects. Rather, they suggest avoiding mowing in New England from May 15 to August 15, and to collect and cut hay at least every three years.

Mowing blades and tractor tires can crush and injure some animals (M. Jones, Interview). Long-lived species such as box turtles (Terrapene carolina) can be especially vulnerable. To combat turtle mortality due to mowing, it is recommended that mowing should be rotated on a multi-year basis, and that no more than 25 to 50 percent of areas greater than 4 hectares (10 acres) should be mowed in any given year (Mowing Advisory Guidelines 2009).

Mowing can also kill larval stages of invertebrates such as moths and butterflies (M. Mello, Interview). In addition, thatch build up can negatively affect grassland birds that require bare patches between vegetation for travel corridors and nesting (Rudnickey et al. 1997). Zuckerberg and Vickery (2006) compared the effects of burning and mowing on the response of bird species. They found that mowing was more effective than burning for restoring grassland bird habitat in shrublands and affecting abundances of shrubland birds and vegetation structure. However, the effects were species-specific: Eastern Towhee (Pipilo erythrophthalmus) and Common Yellowthroat (Geothlypis trichas) abundance decreased as the frequency of mowing increased at sites on Nantucket, mowed several times throughout the season. Further, Savannah Sparrow (Passerculus sandwichensis) abundance showed no response to mowing, while Song Sparrows (Melospiza melodia) preferred unmanaged habitat.

The effects of mowing on grassland animals can be reduced by limiting the size of the area mowed and by leaving patches of un-mowed habitat each year as refugia. One method, “drunken mowing,” leaves a mosaic of mowed and un-mowed patches. Avoiding mowing between May 15 and July 15, when birds are nesting, can limit the negative impact of mowing on birds (D. Vitz, Interview). In many cases, mowing patterns can more easily be controlled than fire, and properly staged management can provide escape routes for mobile wildlife. For example, instead of mowing from the outer edge and spiraling inward, mowing which is initiated in the middle of a field and travels in an outward spiral pattern may allow mobile animals to escape (Raleigh et al. 2003). Increasing the height of the mowing blade can also help protect wildlife especially turtles, snakes and ground-dwelling insects and reptiles (M. Jones and P. Goldstein, Interviews). Lower invasion by non-native invasive species may be a secondary benefit associated with the ability to raise the mower deck and reduce soil disturbance in places where invasive species occur and can spread. Regardless, it is important to thoroughly clean mowing equipment each time it is transported between sites to prevent spread (K. Beattie, Interview).

Because reducing the spread of shrubs and trees, especially non-native invasive plants, is most effective when mowing is done frequently and during the growing season, the timing of management needs to be carefully planned to minimize conflicts with nesting birds and other wildlife. Woody growth should be mowed immediately following the nesting season, which generally concludes around July 15 in this region. Mowing during late summer (July 15 to August 30) will affect woody species just after flowering or seed drop.

The nesting season is shifted (to about 2 weeks later) on the coastal Massachusetts islands and perhaps even Long Island) as the maritime influence results in cool and damp springs; thus, August 1 is a better date to aim for at these sites (K. Beattie, Interview) (Fig. 5).

These timeframes are likely to achieve the greatest reduction in woody plant biomass because it is the time when woody plants have lowest belowground energy reserves. Mowing during fall will reduce the impact to nesting birds but will be less effective at reducing woody regrowth because plants have greater belowground reserves at this time. A second mowing during March following a late summer mowing can reduce additional shrub growth that occurred after the initial cut during fall.