Southwestern Association of Naturalists

Rock Outcrop Communities (Glades) in the Ozarks: A Synthesis Author(s): Stewart Ware Reviewed work(s): Source: The Southwestern Naturalist, Vol. 47, No. 4 (Dec., 2002), pp. 585-597 Published by: Southwestern Association of Naturalists Stable URL: http://www.jstor.org/stable/3672662 . Accessed: 15/08/2012 14:22

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp

. JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected].

.

Southwestern Association of Naturalists is collaborating with JSTOR to digitize, preserve and extend access to The Southwestern Naturalist.

http://www.jstor.org THE SOUTHWESTERNNATURALIST 47(4):585-597 DECEMBER 2002

ROCK OUTCROP PLANT COMMUNITIES (GLADES) IN THE OZARKS: A SYNTHESIS

STEWARTWARE*

Departmentof Biology, College of Williamand Mary, Williamsburg,VA 23187-8795 * Correspondent:[email protected]

ABSTRACT-Whenbedrock is at or near the surface in forested regions, moisture stress limits tree establishment,and herbaceous dominate the shallow soil areas (glades). Despite flo- risticand environmentalsimilarities (full sunlight,winter moisture, summer drought,importance of soil depth, substrate-controlledsoil chemistry),rock outcrop communitiesin the Ozarks region and the southeasternUnited States are vegetationallydifferent. In the Southeast, level terrainand level rock surfaces with consequent moderate erosion allow a linear change in depth of soil out- ward frombare rock, producing several vegetational zones on shallow soil maintained by moisture stress.Perennial grasses are largelyconfined to edges of openings adjacent to surroundingforest. The distinctvegetational zones of shallow soil are dominated by annual dicots, annual grasses, and perennial dicots, including some endemic species; these are called "glades" in the Southeast. In the Ozarks, glade areas are typicallystrongly sloping, so that soil erosion frequentlyexceeds weathering of bedrock. Soil removal rates vary greatlyover short distances, producing a mosaic of soil depths and, thus, a mosaic of plant species with differentdepth requirements.Plants with differentsoil depth requirements are intermixed or distributedin small patches, rather than in distinctzones, so fire-supportingperennial grasses are interspersedamong shallow soil herbs and patches of deeper soil that support cedar trees (Juniperus).Perennial grasses support periodic fires that kill most woody plants and promote continued grass abundance. Because shallow soil plants are interspersedamong deeper soil plants in the glade mosaic, studies of vegetation of "glades" (the total vegetational mosaic) in the Ozarks find perennial grasses and cedars dominant, with shallow-soilplants, including endemics, as minor associates.

RESUMEN-Donde el lecho de roca se asoma a la superficie de la tierra o esti pr6ximo a ella en areas forestales,la falta de humedad elimina los airboles,y las plantas herbaiceasdominan las areas de tierra poco profunda (los claros, o en ingles, glades). A pesar de semejanzas de flora y de entornos (iluminaci6n completa, humedad invernal, sequias del verano, importancia de la profundidad del suelo, quimica del suelo controlada por el lecho de roca), las comunidades de roca expuesta en la regi6n de los Ozarks y en el sudeste de los Estados Unidos son distintasen su vegetaci6n. En el sudeste de los Estados Unidos, el terreno plano y la superficie plana de la roca en combinaci6n con un indice de erosi6n moderada fomentan un cambio lineal en la pro- fundidad de suelo con la distancia de la roca descubierta. Este cambio lineal produce varias zonas de vegetaci6n en suelo poco profundo mantenidas por la falta de humedad. Los pastos perennes en gran parte se confinan a las orillas de los claros adyacentes al bosque circundante. Las distintas zonas vegetales de suelo poco profundo se dominan por los dicotiled6neos y pastos anuales y los dicotiled6neos perennes, incluyendo algunas especies end6micas; estas zonas son las comunidades de los "claros" del sudeste de los Estados Unidos. En los Ozarks, los claros normalmente ostentan fuertesvertientes de manera que la erosi6n generalmente excede la formaci6n de suelo. Las tasas de la erosi6n de tierra varian mucho dentro de distancias cortas de manera que producen un mosaico de profundidades del suelo y, por consiguiente, un mosaico de especies de plantas con diversos requisitos de profundidad de suelo. Las plantas con diferentesrequisitos de profundidad de suelo estin mezcladas o distribuidasen parcelas pequefias, en vez de zonas distintas.Los pastos perennes (que pueden soportar los incendios) se entremezclan con las hierbas de suelo poco profundo y con pequefios terrenosmas profundos que sostienen los cedros (Juniperus).Los pastos perennes soportan fuegos peri6dicos que matan las plantas lefiosas y promueven la continuaci6n de la abundancia de los pastos. Porque las plantas del suelo poco profundo se entremezclan con plantas del suelo mas profundo en el mosaico de vegetaci6n en los Ozarks, los estudios de los 586 TheSouthwestern Naturalist vol.47, no. 4

"claros" (el mosaico entero de vegetaci6n) en esta regi6n encuentran los pastos perennes y los cedros como dominantes. Las plantas del suelo poco profundo, las endemicas incluidas, son com- ponentes menos importantes.

In North America east of the prairies,natu- the unglaciated portions of the forested east- ral vegetation in the absence of disturbance is ern United States, there usuallyhas developed forest. However, within that forested region a specialized flora and vegetation adapted to there are areas where bedrock is at or near the the environmentalextremes of the shallow soil surface, and around these rock outcrops the habitat. The flora often includes endemic or soil is too shallow to support trees. In these near-endemic species and varieties, and the areas, herbaceous plants dominate the natural dominant species usually are differentfrom vegetation. In the Ozark Highlands Province those of disturbance-createdopen areas in the (Fig. 1), such rock-outcropherbaceous plant same region. It is because of this specialized communities surrounded by forestare called flora and vegetation that rock-outcropglade glades (Steyermark,1940). Size of these glades communitieshave attractedso much attention ranges frommany hectares to less than 100 m2, from ecologists in the Ozarks and areas to the but most are I to 3 ha (Ladd and Nelson, 1982; east. Skinner et al., 1983). This paper is a synthetic In unglaciated areas east of the Mississippi overviewof Ozark glades based on a reviewof River,5 major systemsof rock outcrop glades literatureand the author's 3 decades of expe- have several to manyendemic or near-endemic rience working in glades in the Ozarks and species or varieties.The granite "flatrocks' of elsewhere. the Piedmont from Georgia to Virginia (Mc- Bedrock exposed in glades of the Ozarks Vaugh, 1943; Burbanck and Platt,1964; Murdy, may be calcareous (limestone or dolomite) or 1968; Wyatt and Fowler, 1977; Shure, 1999) non-calcareous (sandstone, graniteor other ig- have more than a dozen endemic or near-en- neous rock, chert,or shale). Whateverthe sub- demic species, varieties,or ecological races as- strate,the shallow soils around these outcrops sociated with them. The number of endemics have a number of physical properties that dis- attributedto any glade systemvaries according tinguish them from the surrounding forests to which taxonomic treatmentsof the various (Erickson et al., 1942; Ware, 1990; Quarterman genera one is following. But in all glade sys- et al., 1993): 1) full insolation all year,versus tems, the number of endemic and near-enl- only in winterin forests;2) much bare soil with demic taxa is usually highestin the area where little leaf litter,instead of a continuous litter the outcrops are most numerous and exten- layer of decaying leaves; 3) extremelydry soil sive, and the number decreases awayfrom that in summer, because shallow soil dries out center,as shown dramaticallyfor the Piedmont quickly after rains; 4) saturated soil much of granite outcrops by Murdy (1968). the winter,because of the impermeabilityof The limestone "cedar glades" of middle the rock; 5) great daily extremes of tempera- Tennessee and adjacent Alabama, Kentucky, ture at the soil surface in both summer and and northwesternGeorgia have more than a winter because the lack of a canopy or litter dozen endemic and near-endemic species layerallows free exchange of radiationwith the (Harper, 1926; Quarterman, 1950; Baskin and sky; 6) much frost-heavingof the saturated, Baskin, 1986, 1999), as do the mid-Appala- shallow soil in winter; and 7) extreme soil chian "shale barrens" of Virginia,West Virgin- chemistrybecause of close contact with the ia, and Pennsylvania(Platt, 1951; Keener, 1970, bedrock. 1983; Braunschweiget al., 1999). The "serpen- Most of the weedy flora of a forestregion is tine barrens" of the Piedmont of Pennsylvania adapted to the high lightintensity of disturbed and Maryland (Pennell, 1910; Miller, 1977; areas, but not to the pronounced drought Tyndall and Hull, 1999) and the sandstone stress, extreme soil temperatures,frost heav- outcrop glades of northern Alabama (Ware, ing, or extremes of chemical composition of 1969a; Quarterman et al., 1993) are much less soils derived directly from bedrock. Thus, extensivesystems and have only a few near-en- where there are frequentbedrock exposures in demic taxa. The Tennessee cedar glades, the December 2002 Ware-Ozark rock outcrop communities 587

IOWA eral floristicsimilarities among rock outcrop communitiesin Tennessee, the Piedmont, and MSSOURI the Ozarks. Therefore, both calcareous and non-calcareous glades are included in the fol- lowing discussion. "- a_9 11 "" SOIL DEPTH ZONATION-Soil depth strongly controls available soil moisture,and thus is the main factordetermining the distributionaland ST.FRANCIoIS dominance relationshipsamong glade species. MrsKY, One or more of the vegetational zones or cat- egories described in thispaper (Fig. 2) maybe AKANSAS missingfrom a particularglade, but at least sev- 13STOsrmN eral will be on true AIT1VsTENN . present every glade. On exposed bedrock, crustose pro- vide most of the cover (Nelson and Ladd, OUACHIT MO9UNT$MI 1983; Nelson, 1985), although they are more ARKANSASabundant and conspicuous (including foliose forms) on non-calcareous than on calcareous glades (Keeland, 1975). On the very shallow soils adjacent to exposed rock, most vegetative LOUISANA cover is provided by 1) (); FIG.1-Map of the Ozarks(modified from Bryant 2) mosses and lichens, particularlythe mosses et al., 1993). Grimmia(more often on bare rock), Pleurochae- te (on shallow soil of calcareous glades), and Polytrichum(on rock, soil, or organic matteron Piedmont graniticflatrocks, and the sandstone non-calcareous glades), and the lichens Der- outcrops of northernAlabama have the great- matocarpon(especially on shallow soil of calcar- est ecological and floristicsimilarities to Ozark eous glades) and Cladonia sensu lato (several glades. squamulose and fruticosespecies, the latteres- The Ozarks are so geologically complex that pecially on non-calcareous glades); 3) the withina small region theycontain suitable sub- small succulent perennial Talinum; and 4) strate for the development of glades on lime- small winter annual dicots (Redfearn, 1983; stone, dolomite, sandstone, granite, rhyolite, Quarterman et al., 1993). chert, and shale (Palmer, 1910; Steyermark, In the shallowestsoil, the winterannual di- 1940; Erickson et al., 1942; Kucera and Martin, cots present include species of Sedum,Arenaria, 1957; Keeland, 1975; Hawker, 1983; Nelson and Leavenworthia (the last on calcareous and Ladd, 1983; Nelson, 1985; Jeffries,1985, glades only), and in slightlydeeper soil these 1987; Heikens, 1999). Substrate chemistryis give way to taller,later-maturing winter annu- considered an important factor for glade als. In the slightlydeeper portion of this shal- plants, but most genera of cryptogams and low zone on non-calcareous glades of the small dicots occur on both calcareous and non- Ozarks, the winter annuals Oenotheralinifolia, calcareous glades. Although these genera often Seleniaaurea, Saxifragaspp., and Krigiavirginica are represented by differentspecies on differ- and the rosette perennial Satureja arkansana ent substrates,in some cases the same species may be abundant (Palmer, 1910; Steyermark, occurs on both calcareous and non-calcareous 1963; Quarterman et al., 1993). Winterannual soils, though perhaps represented by different species are presumed to be especially success- ecological races (Reinhard and Ware, 1989; ful in this shallowest soil zone because they Ware, 1990). Thus, despite differencesin soil complete theirentire life cycle during the cool- chemistryamong substratesand the presence er, moister portion of the year, avoiding the of differentendemic species in each rock out- extreme drought and heat of summer by pass- crop system,the similar physical environment ing this season in the seed stage. in all glade habitats means that there are gen- A list of small herbaceous plants and cryp- 588 The SouthwesternNaturalist vol. 47, no. 4

------INCREASING SOIL DEPTH------"GLADES" in the Ozarks

"GLADES" east of the Mississippi <50% >50% ["barrens"]

BARE SMALL MEDIUM ANNUAL TALLER PERENNIAL CEDAR HARDWOOD ROCK HERBACEOUS HERBACEOUSand GRAMINOIDS (PERENNIAL) GRASSES WOODLAND FOREST DICOT and DICOTS DICOTS CRYPTOGAMS

Sedum Heliotropium Sporobolus Coreopis Schizachyrium Juniperus Quercus

Arenaria Trichostema Rudbeckia Andropogon Pinus Carya (= Minuartia) (incl. Isanthus) Acer Solidago Bouteloua Talinum Croton/ Crotonopis Echinacea Nostoc Aster Hypericum Grimmia

Cladonia

FIG. 2-Representation of general vegetation patterns of rock-outcrop glade vegetation along the soil- depth gradient from bare rock to regional forestvegetation in the Ozarks and east of the Mississippi River. Genera listed for each zone are examples of those commonly encountered on calcareous and non-calcareous glades in the Ozarks and usually also furthereast. A narrowerportion of the vegetational gradient is included in the term "glade" in the east; deeper soil areas with greater than 50% coverage of perennial grasses are excluded. In the Ozarks the term "glade" encompasses more of the gradient,but emphasis in the literature is on the deeper soil end, dominated by perennial grasses, perennial dicots, and cedars. togams likely to be dominant in the shallowest pling plot in this shallow soil zone, and cryp- soil zone of Ozark glades is given in Table 1. togams, such as cyanobacteria, mosses, or li- For these plants, "dominant" does not neces- chens, often have greater cover than vascular sarily mean they have high absolute cover. Bare plants. Small winter annuals, such as Leaven- soil often is the largest cover category in a sam- worthia uniflora of calcareous glades (wide-

TABLE1-Small herbaceous dicots and cryptogamsthat often dominate the shallow soil zone of calcareous and non-calcareous Ozark glades. Species are listed in approximate order of their firstrise to importance along a gradient of increasing soil depth. All dicots are winterannuals except species of Talinum,extremely drought-tolerantsucculent perennials, and Satureja arkansana, a rosette perennial. Nomenclature generally follows Smith (1994), but other widely used names are given in parentheses.

Taxon Habitat

Crustose and foliose lichens on exposed rock, all glades Grimmiaspp. (moss) on exposed rock, all glades Selaginellarupestris (club-moss) on exposed rock, non-calcareous Nostoc(cyanobacteria) on very shallow soil, especially calcareous Dermatocarpon() on soil, especially calcareous Talinum calycinum especially non-calcareous Sedum nuttallianum non-calcareous Sedumpulchellum especially calcareous Leavenworthiauniflora calcareous, mostlyslight depressions and seeps Pleurochaetesquarrosa (moss) calcareous Talinumparviflorum non-calcareous Arenariapatula (= Minuartia patula) all glades Satureja arkansana (= Calaminthaarkan- sana) especially calcareous, often in drainage paths Oenotheralinifolia non-calcareous Cladonia spp. (incl. Cladina) (lichens) especially non-calcareous Selenia aurea non-calcareous Saxifragaspp. non-calcareous Polytrichum(moss) non-calcareous December2002 Ware-Ozarkrock outcrop communities 589

TABLE2-Some medium-heightherbaceous dicots and annualgraminoids likely to be dominantsin Ozark glades. Species are listedin the approximateorder of theirfirst rise to importancealong the soil depth gradienton theirpreferred substrate, but are placedwith less confidence than in Table 1. Mostare summer annuals, but Delphinium,Allium, and Hedyotisare perennials.

Taxon Habitat

Heliotropiumtenellum calcareous Trichostema(= Isanthus) brachiata calcareous Hypericumgentianoides non-calcareous Sporobolusneglectus (and S. arkansana) mostlycalcareous Crotonopsiselliptica non-calcareous Crotonmonanthogynus mostlycalcareous Palafoxia callosa calcareous, southwesternOzarks Delphiniumtreleasii calcareous, southwesternOzarks Alliumspp. calcareous Diodia teres non-calcareous Hedyotisnigricans calcareous

spread but never abundant like its congeners taxa of great floristicinterest are omitted from in eastern glades) or the rare near-endemic the tables because theyhave limited geograph- Geocarponminimum of sandstone glades, may ical distributionsand are not dominants (or have only 5 to 10% cover and still have the even present) on most Ozark glades, even highest cover of any in that por- though they may be abundant when present tion of the shallow soil zone. (Steyermark,1963; Smith, 1988, 1994). There are few studies of soil depth distribu- In somewhat deeper soil, small to medium- tion of shallow soil plants in relation to one tall summer annual dicots are usually conspic- another in the Ozarks, but taxa are listed in uous (Table 2). On calcareous glades these in- Table 1 in the approximate order of theirfirst clude Heliotropiumtenellum and Trichostema rise to importance along the increasing gradi- (=Isanthus) brachiataand, in slightlydeeper ent of soil depth of their respective substrate soil, the weedy Crotonmonanthogynus. These type (calcareous or non-calcareous). There is species give way to denser stands of annual much overlap of distribution,however, and grasses, such as Sporobolusneglecta, S. ozarkana, species may reach their greatestabundance in and S. vaginiflorus,which usually extend into deeper soil than where firstlisted in the table. the deeper soil occupied by tall forbs.On non- Distribution of species along the gradient of calcareous glades, dicot dominants include the soil depth resultsfrom a balance between their fall-fruitingwinter annual Hypericumgentiano- drought tolerance in the shallowerend of their ides, the summer annual Crotonopsiselliptica, soil depth range and abilityto compete with and, in slightlydeeper soil, the weedy Diodia other species in the deeper portion of that teres(Palmer, 1910; Steyermark,1963; Jeffries, range (Ware, 1969b; Baskin et al., 1972; Sharitz 1983; Nelson and Ladd, 1983; Nelson, 1985; and McCormick, 1973). Quarterman et al., 1993). There is some over- Species likelyto be prominent in somewhat lap in the distributionof the winterannuals of deeper soil are listed in Table 2. Like Table 1, Table 1 and the taller summer annuals of Ta- this table is meant to be illustrative,not ex- ble 2, as the latter replace the former along haustive.On any particularglade, taxa listed in the soil depth gradient. Despite the spatial the table, chosen because of strongfidelity or overlap, temporal overlap is relativelyshort in constancy to glades, may be equaled or ex- most cases. Though fall-fruitingHypericum gen- ceeded in abundance or cover by more wide- tianoidesis an exception, winter annuals gen- spread but drought-tolerantplants that can ex- erally mature their fruitand die in the spring ploit the glade habitat. Further,the taxa listed when summer annual seedlings are beginning may be absent from any particular glade on their period of rapid growth,and the reverse their preferredsubstrate type. Several endem- relationship exists in the fall, with even less ic, near-endemic, edge-of-range,or disjunct temporal overlap. The small spring annual 590 The SouthwesternNaturalist vol. 47, no. 4

weed Galiumvirgatum is another interestingex- in its range in the southern Ozarks. On non- ception; it germinatesin the spring,as do oth- calcareous glades, blackjack oak (Quercusmar- er summer annuals, but grows rapidly,flowers ilandica) often occurs with J. virginianaas a early, produces seeds by early summer, and dominant tree around the glades. Shortleaf dies about the same time as do the winteran- pine (Pinus echinata) does not display the nuals of shallow soil among which it grows. abundance on non-calcareous glades in the In even deeper soil, taller perennial dicots Ozarks that it and other pines do in sirmilar usually dominate, particularlyspecies of Core- non-calcareous glades east of Mississippi, opsis, Rudbeckia,Solidago, Aster, and Echinacea where pines may be the most abundant tree (Fig. 2). Although these are common genera (Steyermark, 1963; Nelson, 1985; pers. ob- of any open area, such as old fields,pastures, serv.). roadsides, and prairies, most are represented on Ozark glades by at least 1 species or variety THE TERM "GLADE" IN THE OZARKS VERSUS whose geographical distributionis confined to THE SOUTHEAST-In the Ozarks, when people the Ozarks or whose ecological distributionin talk about glades they are often visualizing a the Ozarks is stronglyglade-centered (Steyer- mosaic of cedar woodland, perennial grass- mark, 1963; Smith, 1988). Characteristicpe- land, and shallow soil herb communities on rennial dicots of calcareous Ozark glades in- hillside glades (Fig. 2). Further,most quanti- clude Oenotheramacrocarpa var. macrocarpa,Dal- tativestudies of Ozarks glades have been con- ea (=Petalostemnon)purpurea, Rudbeckia missour- ducted on a scale that considered this entire iensis, Solidago gattingeri,Aster oblongifolius, A. vegetational complex. While shallower soil sericeus,and Echinacea paradoxa var. paradoxa. zones are certainlyincluded, quantitativestud- On Ozark sandstone glades, Coreopsisgrandiflo- ies have emphasized the dominance of peren- ra var. saxicola is often a dominant perennial, nial grasses (Heikens, 1999) and dwarfed ce- as it also can be on granite glades in Georgia dars, because these are the conspicuous and (Smith, 1973). Distributionof these tall,peren- dominant plants when the entire vegetational nial dicot species overlaps both spatially and complex is considered. temporallywith that of summer annuals. This In the Southeast, biologists use the term is because perennial forbsoften get established "glade" in a more restrictedsense to include in cracks, crevices,and pockets of deeper soil, only the shallower soil, up to the depths that while the prevailing soil depth between crev- are dominated by tall forbs,but not soils deep ices and pockets supports only summer annual enough to be dominated by perennial grasses. dicots or graminoids (Quarterman et al., 1993; In fact,when as much as 50% of the vegetative pers. observ.). Perennial dicots of the tall-forb coverage is perennial grasses,eastern glade bi- zone also continue to be conspicuous, though ologists consider these to be grasslands, or less abundant, in even deeper soil, where pe- "barrens", rather than glades, and leave these rennial grasses like Schizachyriumscoparium and for a differentset of ecologists to study (Fig. Bouteloua curtipendulacontribute most of the 2) (DeSelm and Murdock, 1993; Quarterman cover and biomass. Typical prairie forbs (es- et al., 1993). Baskin and Baskin (2000) have pecially calciphiles) may occur in this zone concluded that, at least for calcareous sub- also. This perennial grass-tallforb zone has the strates, the vegetational differences between aspect of a rockyprairie, and only the contin- Ozarks "glades" and southeastern "glades" ued presence of species of forbs normallyas- justifythe use of differentnames for the 2 veg- sociated with glades distinguishessuch vegeta- etation types.They have proposed that the fire- tion from the rockyprairies of the grasslands maintained, perennial grass-dominatedopen to the west and southwestof the Ozarks (Stey- areas around rock exposures in the Ozarks be ermark,1963; Nelson, 1985; Quarterman et al., called "xeric limestone (or dolomite) prai- 1993; Baskin and Baskin, 1999, 2000). ries." They suggested that the term "cedar In the larger cracks, crevices,and soil pock- glades" should be reserved for natural open- ets, and in areas of deeper soil around the mar- ings not maintained by firethat are dominated gins of open glades in the Ozarks, cedars (Jun- by annual dicots and grasses, such as those iperusvirginiana) are always present (Fig. 2). around rock outcrops in the southeastern Juniperusashei is also on limestone glades with- United States. December 2002 Ware-Ozark rock outcrop communities 591

Medium Tall Herb-Graminoid Zone Small Herb-Cryptogam Zone

Small Herb-Cryptogam Zone Perennial

cBare Rock

Soil

S- MAT COMMUNITY

FIG.3-Diagram of a cross-sectionof an "island" vegetationmat on a flatrockglade, withdeepest soil near thecenter of the mat.This kindof successionalmat community has receivedmuch study on Piedmont graniteoutcrops from Georgia to Virginia.

TYPES OF GLADES-Why is the term glade Rock, Arkansas, but successional vegetation used so differentlyin the Ozarks and the mats of the type found on eastern granite out- Southeast? Examination of the various land- crops are not common in Ozark glades. forms where shallow soil glades occur helps OutcropMargin Glades-Where nearly level clarifythis difference.Four typesof glade com- rock exposures are surrounded by nearly level munities are discussed below. terrain, soil depth often increases gradually Island Gladesor Island VegetationMats-East of away from the bare rock toward the surround- the Mississippi,where glades usually occur in ing forest,creating a band of shallow (but in- nearly level terrain,a glade is usually firstper- creasinglydeep) soil all around the rock out- ceived as an abrupt opening in the forest,with crop (Fig. 4). Even on the Piedmont flatrocks, much bare rock and shallow soil, and a sudden shallow soil areas around the margins of rock change back to foreston the other side of the exposures are much more extensive than veg- glade. Weathering and pedogenesis equal or etation mats surrounded by rock. It is in these exceed erosion (Quarterman et al., 1993), and shallow soil areas around the rock exposures isolated vegetation mats may be present on the that the majorityof the endemic plants of Pied- nearly level exposed rock (Fig. 3). The granite mont granite glades occur. Likewise, the exten- flatrockoutcrops of the Piedmont from Geor- sive studies of limestone cedar glades in mid- gia to Virginia are well known for the vegeta- dle Tennessee and adjacent states have largely tion mats that develop on the bare rock by ac- been conducted on such outcrop margin cumulation of silt and organic matter and glades. In the usually thin-bedded Tennessee weatheringof the rock beneath the mat. These limestone, vegetation-soil depth relationships mats exhibit a linear zonation of plant and are somewhat more complicated by fissures, cryptogamspecies related to soil depth, which crevices, and mulch-like rock fragmentsthan increases from the bare rock inward toward in the massively bedded granite outcrops of the center of the mat (Burbanck and Platt, the Piedmont (Quarterman, 1989). Neverthe- 1964; Berg, 1974; Quarterman et al., 1993; less, in both areas, effectivesoil depth often Shure, 1999; Fig. 3). Some mats are in depres- increases in a generallylinear fashion fromthe sions called solution pits, which may hold center of an outcrop (bare rock) toward the enough standing water during the winter and surrounding forest.In response, species distri- early spring that the vegetation is sometimes butions and their peaks of abundance along described as a flood community. However, the depth gradient also show a linear pattern. most vegetation mats have enough drainage In the Ozarks, outcrop margin glades with lin- that they are not flooded for long after rains, ear zonation mostlyoccur where terrainis fair- though even well-drained mats often are satu- ly level, as around some sandstone outcrops rated during much of the winter and early (Calico Rock, Arkansas). spring. Vegetation mats surrounded by ex- BlufflopGlades-In the Ozarks, glades also posed rock occur on some of the larger, gen- occur on the tops of bluffs or on shelves of erally level sandstone exposures near Calico cliffs(Fig. 5). Erosion usually has removed all 592 The SouthwesternNaturalist vol. 47, no. 4

Trees (cedars, scrb oaks, pines)

Perennial Grasses

Medium Herb- Annual Graminoid Zone

Small Herb Zone

Bare Rock

soil Bedrock

OUTCROP MARGIN GLADE

FIG. 4-Diagram of a cross-sectionof an outcrop margin glade of rock exposures in level terrain.Outcrop margin glades of this typehave received much studyon granite in the Piedmont, on limestone in Tennessee, and on sandstone in the Ozarks (Arkansas). Pine is rare in the Ozarks. but a shallow layer of soil at the edge of bluffs, have typicalglade plants only if the bluffis nat- so neither perennial grasses nor trees are likely ural rather than artificial. In the Missouri to invade this shallow soil zone, and herba- Ozarks, many level roadbeds have been cut ceous plants dominate. Blufftopglades are fre- through hilly terrain in the past 35 years. This quently found in both calcareous and sand- has produced many high roadside bluffsthat stone areas of the Ozarks, and they also occur resemble natural bluffs.However, unless a nat- in the sandstone areas of northern Alabama. ural glade was already present before these ar- Blufftopglades are rarely encountered in the tificialbluffs were created, the latter lack typi- much less dissected terrain of the Tennessee cal glade species. Instead, the erosion-created limestone and Piedmont granite areas. shallow soil areas above these roadcut bluffs Blufftop glades are usually narrow, but are are occupied by a widespread weedy flora similar in many ways to outcrop margin glades (some species of which also occur on natural and often include many of the characteristic glades). glade plants. However, blufftops and shelves Hillside Glades-The type of glade in the Ozarks that has attractedthe most studyis the hillside glade. From the time Steyermark Trees first a of this cedar, scrub oaks (1940:374) published picture kind of glade, it has represented the typical Ozark glade to most biologists (Erickson et al., 1942:95; Kucera and Martin, 1957:286; Kuch- Perennial Grasses Shallow Nelson, Hillside are of- and Tall Forbs Soil ler, 1964; 1985). glades Area ten visible from some distance away, because they are large, sloping, and high above the Bare general terrain. The conspicuous visual ele- Rock Soil So ments from a distance are scattered cedars, large treeless areas dominated by perennial BLUFFTOP SHALLOW 7 and of bare as hor- SOIL GLADE grasses, strips rock, usually izontal shelves at the edges of rock beds. In an idealized cross-section of an Ozarks hillside glade (Fig. 6), plants of the shallowest soil occur in abundance only near exposed FIG. 5-Diagram of a cross-section of a blufftop rock. The soil is often sufficientlydeep shallow-soil glade. Blufftop glades can occur deeper for dominance and this throughout the Ozarks, but are uncommon in less by perennial grasses, dissected terrain of the Central Basin (Tennessee) greater soil depth and cracks in the bedding and the Piedmont (Georgia to Virginia). planes allow cedars to invade. Once cedars es- December2002 Ware-Ozark rockoutcrop communities 593

Cedar e BareRock Shallow Soil Area

o Perennal Grasses Shallow Soil Area Bedrock Soil BareRock

Bedrock Soilk BedrockSoil

FIG. 6-Diagram of idealized cross-sectionthrough an Ozark hillside glade (usually facing south or south- west). Patches of exposed rock are interspersed among patches of sparse vegetation on shallower soil and perennial grasses and tall forbs on deeper soil. Soil depth and amount of bare rock depend on steepness of slope, rate of soil erosion, tilt of the bedrock strata, and rate of crumbling (weathering) of bedrock. Where present, trees shade both shallow soil plants and perennial grasses. tablish, they shade both the perennial-grass is regarded as a necessary management tool zone and the shallow-soilzone. If enough ce- for maintaining them in their original form dars become established, the glade is gradually (Guyette and McGinnis, 1982; Skinner et al., converted into a cedar forest,and both peren- 1983; Nelson, 1985). nial grasses and species typical of shallow soil glades are eliminated. However, perennial SOUTHEASTERN GLADES COMPARED WITH grasses provide biomass to support fire, and OZARKSGLADES-On the generallylevel glades fire retards cedar invasion. If fires are suffi- of the Southeast, direction of exposure is not cientlyfrequent, the number of cedars is kept a factor; tree invasion has not been slowed by low, preservingboth the perennial grasses and aspect-caused moisture stress.Instead, all areas the herbaceous dicot zone. of sufficientsoil depth have been occupied by According to Delcourt et al. (1986), Ozark trees; shallow soil and the resulting moisture hillside glades presumablywere more extensive stress have excluded furtherwoody plant in- under warmer, drier climatic conditions dur- vasion. Although fire occurs (or has historical- ing the Hypsithermal interval 8000 to 4000 ly) around glades in the Southeast, there is rel- years ago, when prairie vegetation prevailed in ativelylittle biomass on the glades to support much of the Ozarks. In the cooler, moister cli- a fire. In contrast to the pattern on hillside mate that has prevailed since 4000 years ago, glades, perennial grasses on outcrop margin hillside glades have become largely limited to glades and blufftopglades are confined to a southern and southwesternexposures. The in- narrow band of deeper soil adjacent to sur- creased daily heat load and resulting higher rounding forest (Figs. 4 and 5). On sandstone water stresson southern and southwesternex- outcrops in Alabama, fire moving onto out- posures (compared to other exposures) pre- crops burns through the narrow zone of pe- sumably retard the invasion and growthof ce- rennial grass and tall forbsat the glade margin, dar, so fire-supportingperennial grasses persist and then dies when it reaches the shallow soil longer on these drier aspects. Historicallythis areas (pers. observ.). Vascular plants in the longer persistence of perennial grasses may shallow-soil zone are too far apart to support have provided a longer time frame for natural fire,and the lichens (Cladonia sensu lato) and or anthropogenic fires to occur on these ex- mosses (Polytrichum)that provide more contin- posures. However, with the continued exclu- uous coverage in some areas of that zone burn sion of fire, even the glades on southern and poorly, even when dry (pers. observ.), so the southwestern exposures appear to be slowly fire soon goes out. This latter observation has converting into cedar woodland. Thus, occa- also been made in the Ozarks (P. L. Redfearn, sional burning of these grass-dominatedglades Jr.,pers. comm.). 594 The SouthwesternNaturalist vol. 47, no. 4

Because southeastern glades are not main- Thus, patches of soil of the same depth may tained by fire, neither perennial grasses nor have differentmoisture availabilities over time, trees are likely to spread onto these glades. depending on their location in relation to a Therefore, the herbaceous dicots and annual seepage pathway. grasses of shallow soil remain the conspicuous Island glades and outcrop margin glades of elements of southeasternglade vegetation,and the Southeast are characterizedby distinctveg- it is on these plants, not perennial grasses or etation zones, each dominated by 1 to a few cedars, that southeastern ecologists focus in herbaceous plants. In contrast,on Ozark hill- theirquantitative sampling of glade vegetation. side glades the 3 categories of vegetation on There are also some glades in the Ozarks shallow soil (small herb and cryptogam, mne- that persist in the absence of fire. Outcrop dium-tall herbaceous dicot and annual grass, margin glades, such as those on nearly level taller perennial dicot) have a greater mix of sandstone outcrops, do not require fire, and species because of more varied soil depth and neither do most blufftopglades. Even on some drought stress. More importantly,because of hillside glades, where the thin rock beds tilt differentialerosion in the past, the 3 shallower downhill,strong erosion has removed so much soil zones of Ozark hillside glades are distrib- soil that perennial grasses and cedars are large- uted in a mixed or mosaic patternwith respect ly absent, occurring only occasionally in crev- to one another, and also are scattered among ices. These glades have shallow-soilvegetation patches of deeper soil supporting perennial that presumably will persist without periodic grasses and cedars. This mosaic pattern of fires. However, such glades contrast with the patches of herbaceous dicots intermixedwith fire-dependent hillside glades that are the perennial grasses and cedars contributes fttr- characteristicglade of the Ozarks, both in ex- ther to viewing the entire glade complex as a tent and amount of ecological studyreceived. single, complicated vegetational entity. Soil erosion has had a much greater effect on the complexityof the vegetationon Ozark CONCLUSIONs--Thedifferent use of the term glades compared with those of the Southeast. "glade" in the Ozarks and in the Southeast is As noted earlier, erosion is generally equaled not entirelya semantic choice. There are many or exceeded by soil formation in the level floristic similarities between the calcareous glades of the Southeast, presumably because glades of the Ozarks and those in Tennessee, rainwater moving slowly across the relatively both at the generic and species level (Baskin level surface of the glade does not remove and Baskin, 1986; Bridges and Orzell, 1986). much sediment. Thus, the linear gradient of However, the differences in topography, as- soil depth is essentiallystable over time (Quart- pect, effectsof erosion rates,tendency to burn, erman et al., 1993). In contrast,in the sloping extent of perennial grasses, and resultingveg- hillside glades and blufftop glades of the etational complexitydistinguish Ozark hillside Ozarks, rapid drainage across the outcrops af- glades fromthe level limestone glades and oth- ter a rain causes mild to severe erosion, and er flatrockglades of the Southeast. erosion intensityvaries over shortdistances de- Experience with hillside openings carved by pending on the drainage patterns. Thus, soil erosion and maintained by fire has led Ozark depths also vary considerably over short dis- glade ecologists to treatthe entirevegetational tances, changing from bare rock to several complex as a unit, rather than concentrating decimeters deep over a distance of half a me- on plants of the shallowestsoil. Thus, theyuse ter,and back to bare rock again over the same the term "glade" in a much broader, more in- distance. Because of differingerosion rates in clusive sense than do ecologists workingin the various portions of the glade, soil depth on an Southeast, who largelylimit the term to areas Ozark hillside glade usually varies in a mosaic of shallow soil dominated by dicots and annual pattern rather than a generallylinear pattern. grasses. It is importantto recognize the differ- Thus, plants of differentsoil-depth tolerances ence in the vegetational meaning of the term also occur in a mixed or mosaic pattern.Down- "glade" when used by biologistsin the Ozarks hill seepage of water across portions of the and the Southeast. Preservationof these 2 veg- shallow-soilzone of hillside or blufftopglades etation typesand theirendemic and otherwise also may occur for several days after a rain. characteristicfloras will require differentman- December2002 Ware-Ozarkrock outcrop communities 595 agement strategies based on an appreciation of M. A. thesis, College of William and Mary,Wil- their differences. liamsburg,Virginia. BRAUNSCHWEIG, S. H., E. T. NILSEN, AND T. F. WIE- This paper is expanded from an invited presen- BOLDT.1999. The mid-Appalachianshale barrens. tation at a symposiumon "Vegetationof the Ozarks" In: Anderson, R. C.,J. S. Fralish,andJ. M. Baskin, at the 1997 annual meeting of the SouthwesternAs- editors. Savannas, barrens, and rock outcrop sociation of Naturalists in Fayetteville,Arkansas. plant communitiesof NorthAmerica. Cambridge Many field observations, initial review of literature, UniversityPress, Cambridge, United Kingdom. and preparation of the firstdraft took place during Pp. 83-98. 1996-1997, while the author was a VisitingScientist BRIDGES, E. L., AND S. L. ORZELL. 1986. Distribution at the Department of Biological Sciences, University patternsof the non-endemic flora of middle Ten- of Arkansas, Fayetteville,supported by a year-long nessee limestone glades. Association of South- Faculty Research Grant from the College of William eastern Biologists Bulletin 33:155-165. and Mary. Williamsburg,Virginia. Thanks are ex- BRYANT, W. S., W. C. MCCOMB, AND J. S. FRAIISH. tended to D. Talburt, Chair of Biological Sciences, 1993. Oak-hickoryforests (western mesophytic/ Universityof Arkansas,for generously providing of- oak-hickory forests). In: Martin, W. H., S. G. fice, greenhouse, and library facilities,and to K. Boyce, and A. C. Echternacht, editors. Biodiver- C. E. E. Smith, Sagers, Dale, Jr.,and E. B. Smith of sityof the southeasternUnited States: upland ter- that same institutionfor their cooperation and en- restrialcommunites. John Wiley and Sons, New I coiuragement. also thank D. M. E. Ware of the Col- York. Pp. 143-201. of William and for much with the lege Mary help BURBANCK, M. P., AND R. P. PLATT. 1964. Granite out- field work,and M. Baskin and C. C. Baskin of the J. crop communities of the Piedmont plateau in of for available their University Kentucky making Georgia. 45:292-306. then-unpublished manuscripts on floristics and DELCOURT,H. R., P. A DELCOURT,G. R. WILKINS,AND of southeastern cedar plant community ecology E. N. SMITH,JR. 1986. Vegetational historyof the glades and of those with of the comparison glades cedar glades regions of Tennessee, Kentucky,and Midwestand the Ozarks. P. L. Redfearn, of South- Jr. Missouri during the past 30,000 years.Association west Missouri State contributed University helpful of Southeastern Biologists Bulletin 33:128-137. conversations and distribution of about ecology DESELM,H. R., ANDN. MURDOCK.1993. Grass-domi- Ozarks mosses; G. Greenia of the of William College nated communities. In: Martin, W. H., S. G. and Mary with the resumen; provided help Spanish Boyce, and A. C. Echternacht, editors. Biodiver- and C. Lauver of the Universityof Kansas and an sityof the southeasternUnited States: upland ter- anonymous reviewer provided helpful suggestions restrialcommunities. John Wiley and Sons, New for improvementof the manuscript. York. Pp. 87-142. ERICKSON,R. O., L. G. BRENNER, AND J. WRAIGHT. LITERATURECITED 1942. Dolomite glades of east-centralMissouri. Annals of the Missouri Botanical Garden 29:89- BASKIN, C. C., M. BASKIN, AND E. J. QUARTERMAN. 101. 1972. Observations on the ecology of Astragalus AND E. A. 1982. Fire his- tennesseensis.American Midland Naturalist 88: GUYETTE, R., MCGINNES, JR. of an Ozark in Missouri. 167-182. tory glade Transactions of the Missouri of Science 16:85-93. BASKIN,J. M., AND C. C. BASKIN. 1986. Distribution Academy HARPER, R. M. 1926. The cedar glades of middle Ten- and geographical/evolutionary relationships of nessee. 7:48-54. cedar glade endemics in southeastern United Ecology HAWKER, L. 1983. The hardest to live. Mis- States. Association of SoutheasternBiologists Bul- J. place letin 33:138-154. souri Conservationist40:(6):18-21. A. L. 1999. and BASKIN, J. M., AND C. C. BASKIN. 1999. Cedar glades -IEIKENS, Savannas, barrens, glade of southeastern United States. In: Anderson, R. communities of the Ozark Plateau Province. In: R. C., J. S. Fralish, and J. M. Baskin, editors. Savan- Anderson, C., J. S. Fralish, and J. M. Baskin, editors. nas, barrens, and rock outcrop plant communi- Savannas, barrens, and rock outcrop ties of North America. Cambridge University plant communitiesof North America. Cambridge Press, Cambridge, United Kingdom. Pp. 206-219. UniversityPress, Cambridge, United Kingdom. BASKIN,J. M., AND C. C. BASKIN. 2000. Vegetation of Pp. 220-230. limestone and dolomite glades in the Ozarks and JEFFRIES,D. L. 1983. The vegetationand soils of sand- Midwest regions of the United States. Annals of stone glades of northernArkansas. Unpublished the Missouri Botanical Garden 87:286-294. M. S. thesis,University of Arkansas,Fayetteville. BERG, J. D. 1974. Vegetation and succession on Pied- JEFFRIES,D. L. 1985. Analysis of the vegetation and mont graniticoutcrops of Virginia. Unpublished soils of glades on Calico Rock Sandstone in 596 The SouthwesternNaturalist vol. 47, no. 4

northernArkansas. Bulletin of the TorreyBotan- demic to them. Ecological Monographs 21:269- ical Club 112:70-73. 300. JEFFRIES,D. L. 1987. Vegetationanalysis of sandstone QUARTERMAN,E. 1950. Major plant communites of glades in Devil's Den State Park, Arkansas. Cas- the Tennessee cedar glades. Ecology 31:234-254. tanea 52:9-15. QUARTERMAN,E. 1989. Structureand dynamicsof the KEELAND,B. D. 1975. Vegetation and soils of calcar- limestone cedar glade communities in Tennes- eous glades in northwestArkansas. Unpublished see. Journal of the Tennessee Academy of Sci- M. S. thesis,University of Arkansas,Fayetteville. ence 64:155-158. KEENER,C. S. 1970. The natural historyof the mid- QUARTERMAN,E., M. B. BURBANCK,AND D. J. SHURE. Appalachian shale barren flora. In: Holt, P. C., 1993. Rock outcrop communities: limestone, editor. The distributionalhistory of the biota of sandstone, and granite. In: Martin, W. H., S. G. the southern Appalachians. Part II, Flora. Virgin- Boyce, and A. C. Echternacht,editors. Biodiver- ia Tech Research Division Monograph Number sityof the southeasternUnited States: upland ter- 2, Blacksburg. Pp. 215-248. restrialcommunities. John Wiley and Sons, New KEENER,C. S. 1983. Distribution and biohistoryof York. Pp. 35-86. the endemic flora of the mid-Appalachian shale REDFEARN,P. L.,JR. 1983. Bryophytesof cedar glades. barrens. Botanical Review 49:65-115. In: Kucera, C. L., editor, Proceedings, Seventh KUCERA,C. L., AND C. S. MARTIN.1957. Vegetation North American Prairie Conference, August and soil relationshipsin the glade regions of the 1980. SouthwestMissouri State University,Spring- southwestMissouri Ozarks. Ecology 38:285-291. field. Pp. 77-83. KUCHLER,A. W. 1964. Potential natural vegetationof REINHARD, R. T., AND S. WARE.1989. Adaptation to the coterminous United States. Special Publica- substrate in rock outcrop plants: Interior High- tion Number 36. The American Geographical So- lands Talinum(Portulacaceae). Botanical Gazette ciety,New York. 150:449-453. LADD, D., ANDP. NELSON. 1982. Ecological synopsis SHARITZ, R. R., ANDJ. F. MCCORMICK.1973. Popula- of Missouri glades. In: Davis, W., editor. Proceed- tion dynamics of two competing annual plant ings of the cedar glade symposium,School of the species. Ecology 54:723-740. Ozarks. Missouri Academy of Science Occasional SHURE,D. J. 1999. Granite outcrop communities of Paper 7. Pp. 1-20. the southeastern United States. In: Anderson, R. MCVAUGH,R. 1943. The vegetation of the granitic C., J. S. Fralish, and J. M. Baskin, editors. Savan- flatrocksof the southeastern United States. Eco- nas, barrens, and rock outcrop plant communi- logical Monographs 13:121-166. ties of North America. Cambridge University MILLER,G. L. 1977. An ecological study of the ser- Press, Cambridge, United Kingdom. Pp. 99-118. pentine barrens in Lancaster County,Pennsylva- SKINNER, B. R., G. E. PROBASCO, AND F. B. SAMSON. nia. Proceedings of the PennsylvaniaAcademy of 1983. Environmental requirements of three Sciences 51:169-176. threatened plants on limestone glades in south- MURDY,W. H. 1968. Plant speciation associated with ern Missouri. Biological Conservation 25:63-73. granite outcrop communitiesof the southeastern SMITH, E. B. 1973. A biosystematicstudy of Coreopsis Piedmont. Rhodora 70:394-407. grandifloravar. saxicola (Compositae). Brittonia NELSON,P. W. 1985. The terrestrialnatural commu- 25:200-208. nities of Missouri. Missouri Natural Areas Com- SMITH, E. B. 1988. An atlas and annotated list of the mission,Jefferson City. vascular plants of Arkansas,second edition. Print- NELSON,P. W., AND D. LADD. 1983. Preliminaryre- ed by the author. Fayetteville,Arkansas. port on the identification,distribution, and clas- SMITH,E. B. 1994. Keys to the flora of Arkansas. Uni- sification of Missouri glades. In: Kucera, C. L., versityof Arkansas Press, Fayetteville. editor. Proceedings, Seventh North American STEYERMARK,J. A. 1940. Studies of the vegetation of Prairie Conference, August 1980. SouthwestMis- Missouri. I. Natural plant associations and suc- souri State University,Springfield. Pp. 59-76. cession in the Ozarks of Missouri. Fieldiana: Bot- PALMER,E. J. 1910. Flora of the Grand Falls chert any 9:349-475. barrens. Transactions of the St. Louis Academy STEYERMARK,J. A. 1963. Flora of Missouri. Iowa State of Science 19:97-112. UniversityPress, Ames. PENNELL,F. W. 1910. Flora of Conowingo Barrens of TYNDALL, R. W., AND J. C. HULL. 1999. Vegetation, southeastern Pennsylvania. Proceedings, Acade- flora, and plant physiological ecology of serpen- my of Natural Science (Philadelphia) 62:541- tine barrens of eastern North America. In: An- 584. derson, R. C., J. S. Fralish, and J. M. Baskin, ed- PLATT,R. B. 1951. An ecological study of the mid- itors. Savannas, barrens, and rock outcrop plant Appalachian shale barrens and of the plants en- communities of North America. Cambridge Uni- December2002 Ware-Ozark rockoutcrop communities 597

versityPress, Cambridge, United Kingdom. Pp. it-in rock outcrop plants: Sedum and Arenaria. 66-82. American Journal of Botany 77:1095-1100. WARE,S. 1969a. On the ecology of Talinummengesii WYATT,R., ANDN. FOWLER.1977. The vascular flora (Portulacaceae). Bulletin of the TorreyBotanical and vegetationof the North Carolina graniteout- Club 96:4-10. crops. Bulletin of the TorreyBotanical Club 104: WARE,S. 1969b. Ecological role of Talinum (Portu- 245-253. lacaceae) in cedar glade vegetation. Bulletin of the Torrey Botanical Club 96:163-175. Submitted2 April2001. Accepted27 November2001. WARE,S. 1990. Adaptation to substrate-and lack of AssociateEditor was ChrisLauver