Introduction

Eastern Prairie–Forest Transition

Midwestern savannas occupied a transitional area between eastern deciduous forest and tallgrass prairie. These savannas were part of the eastern prairie–forest transition (Curtis 1959; Anderson 1983; Nuzzo 1986) that extended as a broad arc along the eastern edge of the northern mixed and tallgrass prairies from the Canadian provinces of Alberta, Saskatchewan, and Manitoba southward into Texas (Figure 9.1). We discuss deep-soil savannas (also called black soil, mesic, and tallgrass savannas and barrens) that occurred in the glaciated landscapes of Minnesota, southern Michigan and Wisconsin, Ohio, Indiana, and Illinois. These deep-soil savannas occurred on sites with fine-textured soils, where growth of trees was not severely limited by edaphic factors. Savannas with shallow soil profiles over bedrock, and those with sandy soil with low fertility and water-holding capacity, are considered in other chapters (see Chapters 8 and 21). Essentially all of the original Midwest, mesic, deep-soil savanna vegetation was lost to fire protection and agricultural activities, including overgrazing (Curtis 1959; Nuzzo 1986). These savannas are among the rarest natural vegetation types in the world.

Savannas of the Midwest occupied the eastern edge of a large, triangular-shaped grassland that extended from the Rocky Mountains into the Midwest (Risser et al. 1981; Anderson 1990). The grassland narrowed eastward, producing the well-known prairie peninsula (Transeau 1935).

Introduction

Despite the lack of a climatic tree line, the southern Appalachian mountains support several treeless, high-elevation (> 1,200 m) communities. The three principal open communities are heath balds, grassy balds, and rock outcrops. Other open communities include frequent, but small, rocky, steep streamsides and seeps, and, very rarely, mountain bogs (Schafale and Weakley 1990). The information on heath and grassy balds will be briefly reviewed, but the focus will be on outcrops because they support a highly distinctive flora and are the least studied of the three primary open communities. Notably, they support one of the richest floras of rare species of any regional habitat, including both rare endemics and northern alpine disjuncts.

Heath Balds

Heath balds are species-poor communities with only 10–20 vascular plant species on a site (White and Renfro 1984) and are dominated by ericaceous evergreen shrubs. They occur within a restricted elevation range (1,220–1,525 m) on narrow ridges and adjacent south and west slopes, and become larger and more frequent in localities disturbed by logging activity (White, Wilds and Stratton, unpublished data). In Great Smoky Mountains National Park, heath balds are more frequent (>400) than grassy balds (~30) but dominate less than 50% of apparently suitable topographic sites.

Because of dense shade, thick leaf litter, and high soil acidity (aluminum may reach levels toxic to tree roots), heath balds are stable or only slowly invaded by trees. Some heath balds created by logging remain treeless one hundred years later.

Introduction

Sand shinnery oak communities are some of the least known and most poorly described communities in the southwestern United States and untill recently were given little attention. Historically they have been subjected to degradation and eradication, specifically destructive grazing, herbiciding, and other large-scale disturbances. Early observations by biologists and naturalists suggested that shinnery oak communities may be refugia for a variety of plants, mammals, insects, and birds. More recent studies indicate a rich flora and fauna, including three federally endangered and one locally threatened species. Today, there is a strong need for conservation and restoration, especially because eradication schemes are used widely (e.g., Test 1972; Pettit 1979; Sears et al. 1986). Unfortunately, no conservation or formal restoration is planned for these threatened communities. This is the first review of the ecology and future conservation of the sand shinnery oak communities.

Historical Background

Little has been written about shinnery oak communities. Before the late 1800s, the Llano Estacado (panhandle of Texas and high plains of Texas and New Mexico) was occupied by Native Americans, causing Europeans to avoid settling the area until the late 1870s (Biggers 1991). Early explorers described the area as “exceedingly monotonous and uninteresting, being a continuous succession of barren sandhills” covered with a “dense growth of dwarf oak bushes” (Marcy 1854, quoted in Rowell 1967).

Savannas, barrens, and rock outcrop plant communities are the topic of numerous research and technical articles. Rock outcrop plant communities and serpentine barrens are of interest because they are refugia for endemic species adapted to extreme environmental conditions. Savannas and barrens were major components of the historic landscape before it was extensively altered by agricultural and urban development during the past century. Many of these communities were reduced to less than one percent of their original area and are imperiled ecosystems. There has been relatively little synthesis of information about these ecosystems from papers published in scientific journals, conference proceedings, or technical reports from state and federal governmental agencies and private organizations. Our book synthesizes this technical knowledge and will increase awareness of these vegetation types and communities and aid in their conservation and restoration.

The savannas covered in this volume occur in diverse and geographically distant regions of the continent. They include pine savannas of the southeastern Gulf Coastal Plain; aspen parklands of the Canadian provinces of Alberta, Manitoba, and Saskatchewan; California oak savannas; juniper/piñon savannas; subarctic lichen woodland of northwestern Canada; and others. Some of the savanna types cover broad geographical areas, such as the cross timbers that extended from Kansas into Texas and the southern Gulf Coastal pine savannas that occurred from North Carolina to Texas along the coastal plain. All of these communities have unique ecological features. However, they share a common feature in having an environment that restricts tree growth and prevents development of closed-canopy forests.

Introduction

Between the tundra and the closed-canopied boreal forest is a vast region of largely undisturbed, open-canopied conifers with a ground cover of lichens (Figure 26.1). This region has been called various names in the North American literature: subarctic forest, lichen woodland, hemiarctic, and spruce woodland. Refer to Löve (1970) and Blüthgen (1970) for the semantics of these terms.

The subarctic woodlands are primarily climatic in origin, occupying the transitional areas between the summer location of Arctic airstreams and more southern airstreams. As in all transition zones, small variation in the primary forcing variable changes the vegetation composition and structure. This change in airstream position interacts with the available plants, landforms, elevations, substrates, and fire regime to create a vegetation mosaic reflecting past and present interactions.

The North American subarctic woodland is one of the last remaining extensive and continuous ecosystems (Figure 26.2a). It covers approximately 2 million square kilometers, most of which is without roads or permanent settlements. Hunting, trapping, and some mineral extraction are the primary land uses. The subarctic has a population of indigenous people whose way of life is still strongly related to the land. Furthermore, during part of the year it is the home of caribou (Rangifer tarandus), the last large migratory ungulate herd in North America.

Distribution

The subarctic woodland in North America (Figure 26.2a) occurs from the northern interior of Alaska, cutting through the Richardson Mountains and north of the Mackenzie Mountains, to the south end of Hudson Bay.

Introduction

The term barren is a historical one, used by settlers to refer to a landscape or landscape feature with little, if any, timber-sized trees, although tree species may have been present in smaller size classes. We use the term serpentine barren to refer to a serpentine outcrop and its associated vegetation. Prior to European settlement, serpentine barrens vegetation in much of eastern North America was composed predominately of fire-maintained communities of grassland and savanna (grassland with trees). This chapter considers the history, flora, vegetation, and physiological ecology of the serpentine barrens of eastern North America, with emphasis on the Mid-Atlantic states of Maryland and Pennsylvania. In this region, afforestation of nearly all undeveloped barrens, and more than 90% of undeveloped historic communities, has occurred during the past 50 years. The invading trees are relatively fire intolerant species such as Pinus virginiana (Virginia pine) and Juniperus virginiana (redcedar) (nomenclature follows Gleason and Cronquist [1991] unless authorities are given).

Serpentine soils are derived from ultramafic rocks, which occur in a discontinuous band along the eastern edge of the Appalachian mountain system from Newfoundland and Quebec, Canada, through New England, U.S.A., to Alabama on the Piedmont Plateau (Reed 1986; Brooks 1987). Ultramafic rocks are thermally altered and largely plutonic.

They are ferro-magnesium silicates high in magnesium and iron and low in aluminum, calcium, and silica. Serpentinite is a hydrated ultramafic rock, and it may be ultrabasic and contain heavy metals such as chromium and nickel (Reed 1986).

Introduction

Florida scrub exhibits a multitude of contradictions. It is a shrubland dominated by xeromorphic plants in a region of subtropical temperatures, abundant rainfall, and luxuriant primary productivity. Florida scrub is resilient to fire, but sensitive to short fire return intervals and fire suppression. Although scrub has low species diversity, its level of endemism is among the highest of any North American plant community. To many, Florida scrub appears harsh and unappealing, but it inspires rhapsody among its defenders. Finally, although scrub plants and animals have persisted for millennia of drought, fire, and infertility, the unique biota of Florida scrub is vulnerable to continuing human development.

Scientific knowledge of Florida scrub is as patchy as the remaining distribution of this endangered ecosystem. Many studies on community responses to fire have been completed or are ongoing, but data suggesting the normal range of fire return intervals are sketchy. Information on nutrient cycling, herbivory, and belowground competition is scarce. The distributions and habitat requirements of several endemic plants are well known, although the reasons for high endemism remain controversial. New species of invertebrates, and even new trophic interactions, are being described continually. A sense of urgency pervades the basic scientific study of Florida scrub because (1) habitat destruction is proceeding rapidly, and (2) it is clear that unanswered land management questions will be crucial to conserving the biodiversity and ecological integrity of the remnant scrub.

Introduction

The Niagara Escarpment is a 50–100-m high cuesta following the rim of a 450-million-year-old saucer-shaped geological structure known as the Michigan Basin. It extends in a roughly circular shape from Niagara Falls north to Manitoulin Island, across to the junction of Lakes Michigan and Huron at Sault Ste. Marie, then south along the western shore of Lake Michigan. Generally, the Silurian-aged rim is buried by glacial till, but 150 km of exposed near-vertical cliff face is present between Niagara Falls and Manitoulin Island, a straight-line distance of approximately 360 km.

The escarpment lies within the Great Lakes–St. Lawrence Forest (Rowe 1972). The climate of the area is temperate with a strong local influence of the Great Lakes. Mean January air temperature ranges from -5°C in the south to -7.5°C in the north; mean July temperature varies between 21°C in the south to 18°C in the north. Total annual precipitation of 900 mm is evenly distributed throughout the year. Lumbering activities were intense from 1800 to 1920, and the area was almost totally deforested by European settlers. Fire consumed almost all of the forested horizontal landscape of the Bruce Peninsula in 1908 (Gillard and Tooke 1975). However, no evidence of fire was found in cores or cross-sections obtained from trees growing on the exposed vertical cliffs (Larson and Kelly 1991).

Natural History

Cliffs of the Niagara Escarpment support a stable, ancient forest ecosystem whose structure probably has changed little since the melting of the Laurentide ice sheet.

Introduction

Serpentine habitats in western North America have been recognized for their special geological, floristic, and ecological significance since the mid-20th century. Their intimate link with plate tectonics was established in the revolution that remade geological science. The occurrence of serpentines is not uncommon in the three Pacific Coast states (California, Oregon, and Washington) and in adjacent British Columbia, Canada. The uniqueness of biota on serpentines has commanded much attention. Their rich endemic floras and remarkable physiognomy, which contrasts with typical vegetation on adjacent soils, coupled with specialized ecophysiological attributes, have interested a variety of plant biologists from physiologists and taxonomists to population and evolutionary biologists. Comprehensive reviews of serpentine biology can be found in Proctor and Woodell (1975), Kruckeberg (1985), Brooks (1987), Baker, Proctor and Reeves (1992), Roberts and Proctor (1992).

This review focuses on an aspect of Pacific Coast serpentines that is one of the major themes of the present volume, namely barrens. The word barren and its companion word serpentine have taken on diverse meanings. Serpentine has been used adjectivally (and even as a noun) by botanists in a loose generic sense to stand for rock, minerals, soils, flora, vegetation, habitats, and even landscapes. For the geologist, the word embraces a family of ferromagnesian minerals, whereas serpentinite is used for a class of metamorphic rocks that contain serpentine minerals. The geological concept of serpentine will be considered later. However, the loose, all-inclusive meaning of serpentine is retained in this chapter.

Naturally open habitats consisting of exposed granite rock with grassy areas, shrubs, and scattered trees often are referred to as granite barrens. These areas are never entirely barren, but nevertheless, openness and bare rock or rock with only lichen and moss cover is a characteristic feature (Figure 24.1). In Ontario, granite barrens are a restricted and special habitat with unique environmental factors and specialized plants and animals.

The terrain is composed of a ridge and trench system of extensively folded granite rockland. A faulting network (differential displacement of bedrock blocks resulting in long, steep-sided depressions) often exists perpendicular to the folds. The trenches often contain oligotrophic ponds created by beaver, or they may contain bog mats. One of the most striking features is the contrast between adjacent wet and dry land, a consequence of the fact that the granite rock is impervious and holds water in depressions. Lakes, ponds, and other wetlands are often as characteristic of granite rock barren landscapes as are the dry rock exposures. Due to the retention of water in small or shallow depressions, extreme wetness may be followed by extreme drought. In fact, granite rock barrens are characterized by a wide variation in soil depth and in water-holding capacity over a short distance. These variations are responsible for the mosaic patterns of vegetation. The rock is acidic and generally contributes to the formation of acidic soils. The wetlands are generally bogs or acidic lakes.

Introduction

California oak savannas are dominated by blue oak (Quercus douglasii), valley oak (Q. lobata), interior live oak (Q. wislizenii), coast live oak (Q. agrifolia), and Engelmann oak (Q. engelmannii), occurring in mixed or monospecific stands occupying less than 30% cover. Nine tree species of oaks occur in the state (Pavlik et al. 1991), but four are limited to forests or woodlands. Annual grassland is the major understory type, although some savannas may have a shrub understory component. Nomenclature follows Hickman (1993).

The oak savannas of California generally occur at elevations ranging from 60 to 700 m between annual grasslands at lower elevations and mixed conifer or ponderosa pine (Pinus ponderosa) forests at the higher margin. Precipitation occurs primarily during winter to early spring, generally late October through April. Thus, oak savannas persist through a severe, annual drought period lasting 2–11 months.

Ninety percent of the oak savannas in California are privately owned (Ewing et al. 1988). Over 300 species of vertebrates live in oak savannas and woodlands of California (Jensen, Tom and Harte 1990). Livestock are the primary users and the primary economic products of the savannas; firewood, wildlife, and water are secondary products (McClaran and Bartolome 1985; Standiford and Tinnin 1996).

Past and Present Geographic Distribution

Oak distributions have been in flux throughout the geologic record because of changes in climatic patterns, which led to migration and juxtaposition of communities, with each individual savanna species responding according to its own tolerances (Evett 1994).

Introduction

The cross timbers are a mosaic of forest, woodland, savanna, and prairie vegetation located in portions of Kansas, Oklahoma, and Texas (Figure 14.1). Nonetheless, two woody species characterize the cross timbers: post oak (Quercus stellata; Figure 14.2a) and blackjack oak (Quercus marilandica; Figure 14.2b) (Dyksterhuis 1948; Hale 1955; Rice and Penfound 1955). There are an estimated 4.8 million ha of cross timbers located between 38° N latitude in southeastern Kansas and 32° N latitude in north central Texas (Küchler 1964; Engle and Stritzke 1992). Approximately half (2.5 million ha) of the cross timbers are in Oklahoma (Rice and Penfound 1959; D. D. Dwyer and Santelman 1964). The cross timbers form two distinct bands of vegetation in Texas, known as the western and eastern cross timbers (Figure 14.1). These formations were referred to as the upper and lower cross timbers, respectively, by early European settlers due to their location along the Red River (Foreman 1947).

Josiah Gregg, an entrepreneur who was active in the region during the 1840s, left an apt description of the cross timbers that is relevant today:

Introduction

Dry soil oak savanna in the Great Lakes region occurs in Minnesota; Wisconsin; Michigan; northeastern Iowa; the northern portions of Illinois, Indiana, and Ohio; and the Great Lakes plains of southeastern Ontario (Chapman et al. 1995). Dry soil oak savanna, where low moisture availability limits biomass production, includes vegetation with quite varied physiognomy; terms for variants include woodland, barrens, sand savanna, scrub oak savanna, and brush prairie. The term barrens has no standard usage (Curtis 1959; Heikens and Robertson 1994; Hutchinson 1994; J. White 1994); we use this term and other terms only as defined in this chapter.

Variation in savanna vegetation physiognomy and species composition in the region has been related to interaction of disturbance regime, especially fire, with broad edaphic gradients (Grimm 1984; Bowles and McBride 1994). Fire frequency and intensity are in turn influenced by landscape structure (Leitner et al. 1991; Will-Wolf and Montague (1994) interacting with climate (Grimm 1985). Vegetation classification, used to define units for mapping and for management, frequently incorporates the assumption that site environment determines vegetation composition and structure. However, in recent savanna classifications for the upper Midwest, dry soil savanna vegetation units distinguished by structure and species composition are described as a function of different disturbance regimes as much as of different site environments (Homoya 1994; Faber-Langendoen 1995; Haney and Apfelbaum 1995). These relationships are critical for understanding savanna dynamics and for successful site management.

Savanna vegetation associated with dry, low-productivity sites in the Great Lakes region falls into two groups (Figure 8.1).

Introduction

Oak savanna in the southwestern United States and northern Mexico represents a diverse, widely distributed vegetation type. The northern Sierra Madre of Mexico has 41 species of oak (Quercus L.) (Nixon 1993), and in southeastern Arizona, plant species richness is greater in open oak woodland than in any other plant community (Whittaker and Niering 1975). In this chapter we focus on the southwestern United States, because there is little information about oak savanna in Mexico.

Current and Past Geographic Distribution

Oak savanna generally is considered a subset of the more extensive evergreen oak woodland, or encinal (D. E. Brown 1982; Rzedowski 1983). The savanna is characterized by tree canopy cover between 1% and 30% and is restricted to relatively low (1,100–2,200 m) and dry elevations, where encinal grades into desert grassland or mattoral (shrubland) (White 1948; Gentry 1957; Rzedowski 1983; McPherson 1992; McClaran 1995) (Figure 17.1). Savanna tends to be located at lower elevations as the latitude increases. In Arizona, New Mexico, and northern Sonora, the savanna generally has a discontinuous distribution in the foothills of isolated mountain ranges that are separated by lowlands covered by desert grassland and shrubland. In the northern part of the Sierra Madre Occidental, in Chihuahua, Durango, Sinaloa, and central-southern Sonora, the savanna forms a somewhat more continuous distribution in large areas of rolling topography above desert grasslands or mattoral (Rzedowski 1983) (Figure 17.2).