Ecological dynamics

Dolomite Colluvium Bluff Prairies are open areas on steep, exposed slopes in an otherwise forested landscape. Herbaceous plants that can tolerate droughty, rocky soil conditions dominate the site. Bluff prairie plants in general possess many adaptations enabling them to survive in a harsh environment, often subject to widely fluctuating extremes of temperature and moisture. Also, having aspects exposed to the full drying effect of the sun as well the full brunt of prevailing winds combine to enhance evapotranspiration compared to adjacent ecological sites. Bluff prairie plants are both pyrogenic (fire adapted) and heliophytic (sun-loving).

Prior to European settlement, open prairies persisted on these sites since the Holocene Thermal Maximum, 9000-5000 BP (i.e., the hypsithermal; Anderson 1954). Fire, drought, and native grazers were the primary natural disturbances on pre-settlement bluff prairies. It is likely that these sites burned at least three to five times per decade (Jones and Bowles 2013). These periodic fires removed litter and stimulated the growth and flowering of grasses and forbs. They also limited the growth and dominance of trees, especially eastern redcedar, which is extremely fire intolerant. In landscapes with predominant south to west facing valley sides, there was a complex pattern of open bluff prairie, savanna, and open oak woodlands (Figure 4). Fire tolerant bur oak (Quercus macrocarpa) and black oak (Quercus velutina) occupied the edges of bluff prairie landforms. Only during long, fire-free intervals in times of higher than average precipitation, would tree and shrub species increase on bluff prairies. In comparison, in landscapes with predominately north to east facing valley sides, ravines favored development of true forests, due primarily to the change in aspect.

Bluff prairies greatly increase the diversity of plants and animals in the landscape (see Table 6, later in document). Prairie grasses and prairie forbs not usually found on other landform settings in the region occur on bluff prairies. Common native prairie grasses that are present include little bluestem, prairie dropseed, sideoats grama, big bluestem, Indian grass, switchgrass, prairie Junegrass (Koeleria macrantha), and porcupinegrass (Hesperostipa spartea). The mid height grasses, little bluestem and prairie dropseed, dominated the site. Likewise, a significant presence of prairie forbs like: compassplant, candle anemone (Anemone cylindrica), fewleaf sunflower, grooved flax (Linum sulcatum), and white prairie clover (Dalea candida), are indicators of DCBP. Some of these prairie forbs found on bluff prairies are the sole host to certain insect species. There are many invertebrate species, including some 29 species of land snails that are found only on prairies and savannas (Theler 1997). Several vertebrate species also rely heavily on bluff prairie habitat. These include the six-lined racerunner (Cnemidophorus sexlineatus), prairie ringneck snake (Diadophis punctatus), and the timber rattlesnake (Crotalus horridus). For example, it is the open bluff prairie habitat that allows timber rattlesnakes to successfully reproduce at the northern limit of their range. Also, golden eagles (Aquila chrysaetos) wintering in the region are often seen hunting on bluff prairies. It is possible that the recent expansion of this species into the MLRA is due in part to restoration of DCBP, and related ecological sites (Craig Maier, Ecologist with the Tallgrass Prairie and Oak Savanna Fire Science Consortium, personal communication).

The last 300 years can be divided into three disturbance eras that relate to bluff prairies: 1) before 1866, prior to European settlement; 2) from 1866 to 1960, post settlement; and 3) 1960 to present. The first period ended with European settlement, no later than 1865. In this pre-settlement time period, DCBPs were in the Reference State (State 1; Figure 5), fluctuating between the High Diversity Prairie community (Phase 1.1) and the Reduced Diversity Prairie community (Phase 1.2) depending on the frequency of fire. Prior to European settlement, indigenous cultures used fire liberally for a multitude of purposes, over thousands of years (Abrams and Nowacki 2008). During this time, drought-driven landscape fires were common, as many as five per decade, and were not suppressed (Jones and Bowles 2013). Native species on bluff prairies were in a dynamic equilibrium related to the frequency of landscape wide fires. Native grazers, like elk (Cervus canadensis), bison (Bison bison), and white-tailed deer (Odocoileus virginianus) were present, and undoubtedly had an effect on bluff prairie dynamics. However, it is generally unclear to what level native grazers may have affected disturbance dynamics of bluff prairies.

The second period started immediately post European settlement, roughly between 1866 and 1960 (Figure 6). Many, if not most DCBP sites were completely transitioning from the Phase 1.1 to Phase 1.2 at this time. However, fire continued to be commonplace. Tree ring data at Fults Hill Prairie Nature Preserve in Illinois indicate that fires occurred three to five times per decade from the late 1800s, all the way through the middle 1900s (Jones and Bowles 2013). This confirms many of the observations of local residents, land managers and conservation professionals – in that land owners commonly burned bluff prairie sites up until about 1960. These fires were set for various reasons. In parts of the Root River Valley in Minnesota, for example, neighbors would gather and set fires to pastured areas in and around bluff prairies (Valiree Green, MN DNR Forester, personal communication). Also, people in Lansing, Iowa recall seeing fires burning on bluff prairies across the Mississippi River that were set by Wisconsin citizens, in an attempt to control timber rattlesnake populations (Armund Bartz, WI DNR Ecologist, personal communication). During this time period, bluff prairies were used intermittently for grazing purposes. Specifically, a mix of grazers would forage on these sites, including cattle, sheep, and goats (Jaime Edwards, MN DNR Nongame Biologist, personal communication). Livestock numbers increased from the early 1900s through the middle 1900s. Grazing on bluff prairies became intensive during the severe drought years of the 1930s, when farmers were desperate for forage (Dr. Stanley Trimble, UCLA Professor, personal communication). Many bluff prairies were overgrazed, and became degraded by soil erosion. However, in some cases, this varied and diverse mix of grazing systems limited significant encroachment of trees and shrubs, which undoubtedly helped maintain bluff prairies in the Reference State.

Also during this second time period, non-native plants were first introduced. The impact of non-native, invasive species gradually increased over time, and is continuing to increase today. In addition, some native species, such as common pricklyash (Zanthoxylum americanum), smooth sumac, and gray dogwood (Cornus racemosa) gradually became invasive; historically, their abundance was controlled by fire. Most bluff prairies were not transitioning to the Eastern Redcedar/Prairie State (State 2) and probably not to the Scrub Deciduous Tree and Shrub/Prairie State (State 3) in the second time period, as is evidenced by aerial photos from the 1930s thru 1950s, showing little significant change in eastern redcedar cover.

The third period started in 1960 and continues to present day, when fire intervals on bluff prairies generally decreased to less than two per decade, with fire free intervals of ten to twenty years, or more (Jones and Bowles 2013). Historically, there was a direct relationship between fire frequency and climate, in that fire occurrence was more likely during times of drought (Stambaugh and Guyette 2006, Jones and Bowles 2012). With the advent of fire suppression, which was used as a mechanism for public safety, the relationship between fire and climate became negated. This fire suppression coincided with an increase in non-farm rural population, along with fragmentation of land ownership, both of which contributed to the reduction in fire frequency. Old, cultural ways of land management through burning were abandoned, which contributed to the reduction in landscape wide fires. This period of local fire suppression is embedded within a larger geographical area spanning much of the eastern United States, whereby fire has been ubiquitously suppressed, leading to forest encroachment and “mesophication” (Nowacki and Abrams 2008). There was also a dramatic change in farming practices in this third time period. The diversity of grazing animals found on farms decreased, and grazing on bluff prairies ceased altogether in many areas. Most of the remaining Reference State DCBPs transitioned to Phase 1.2, the Reduced Diversity Prairie, as fire return intervals decreased. It was during this most recent time period that many DCBPs transitioned to States 2 and 3. The introduction and spread of non-native, invasive plants continued in this time period. Specifically, the introduction of common buckthorn (Rhamnus cathartica) has degraded many native habitats, including bluff prairies, and is the hallmark invasive species of the Non-Native Invaded State (State 4).

In general, xeric site conditions contribute to extended persistence of these ecosystems, due in large part to the fact that few species can tolerate such extreme sites (Evers 1955). Still, fire is an essential process, needed to maintain even the most xeric prairies (Jones and Bowles 2013; Greg Nowacki, USFS Ecologist, personal communication). The exact frequency of fire that is required to maintain the DCBP Reference State, or restore alternative states, depends on physical setting and the unique history of a given bluff prairie site. The fire intervals stated in the state-and-transition model (Figure 4) are not based on empirical data. Rather, they are based on data published from similar ecosystems, and most importantly, input from numerous land managers and local conservation experts. Today, many bluff prairies exist as small, fragmented relics, surrounded by encroaching woody species. Restoration requires a combination of management techniques, including mechanical removal of invading trees and shrubs, herbicide application to tree and shrub species with propensity to stump sprout, and prescribed fire. Management should always start in areas near the highest quality prairie openings of a site (i.e., the steepest, most convex slopes, with south to southwest aspect). These areas are most likely to respond to restoration treatments. Once at least some herbaceous cover is restored (e.g., after one or more growing seasons), prescribed fire can be introduced, and repeated up to every other year, if possible (Jones and Bowles 2013). Prescribed fires will further stimulate dormant prairie seed. In subsequent years, gradual expansion of management to the entirety of the site can be considered. In cases of resulting low species diversity, supplemental seed or plugs are sometimes introduced, ideally from local sources and similar ecological sites. The initial stages of bluff prairie restoration could be impacted if sites are located near common buckthorn seed sources. When any bluff prairie restoration project is considered, an evaluation of potential common buckthorn seed sources should be completed. Adjacent woodlands with common buckthorn will be a likely seed source.

The presence of restricted plants and animals presents a challenge when using prescribed burning as a management tool. Fires may impose greater stress on certain plant and animal populations in contemporary, confined landscapes that lack connectivity (Nelson et al. 2013). Although fire is essential for restoring and maintaining bluff prairies, burns should be designed so they do not have unintended impacts. Burning only a portion of a bluff prairie unit in any given year may reduce the likelihood of these impacts.

Some variability in DCBP sites can be described, often depending on differences in physical landform features, disturbance and management history, and type of adjacent plant communities. The driest, most xeric locations of any given DCBP site is on shoulder slopes, or upper most backslopes of large, doubly convex nose slopes. These south and southwest facing slopes are the most exposed to climatic extremes, and are the most likely locations to persist in the absence of fire. In contrast, locations on a bluff prairie landform that are even slightly concave, can provide comparatively better growing conditions for plants, In some cases allowing trees to persist, even in the presence of fire.

Small areas of rock outcrops do occur on this ecological site. When this occurs on shoulder slopes or narrow ridges, plant communities are drier, and less productive. Other areas of rock outcrop include small ledges and cliffs, which often have a higher density of shrubs and trees immediately adjacent to them. These areas provide protection from fire, as well as cooler, shaded microsites where woody species can become established. This complexity adds to the diversity of a site by providing unique habitat for plants like purple-stemmed cliff brake (Pallaea atropurpurea) and smooth cliffbrake (Pallaea glabella), that are not found anywhere else on the DCBP ecological site. Even large boulders can produce similar dynamics. In fact, historically these cliff areas were likely the historic refugia for eastern redcedar, ultimately becoming the primary seed source fueling the invasion of this species onto DCBP sites during the third time period.