1995 Midwest Oak Savanna and Woodland Ecosystems Conferences
"In biotic communities a limitation in [system] structure means each species is confined to a definite community position in relation to other species with the unlikehood [sic] that all combinations of species can occur. It is the limited number of stand compositions which permits the application of quantitative methods to community classification."
(Bray and Curtis 1957, p. 345)
The dynamic, and perhaps transitional, nature of oak savannas and woodlands has made their classification contentious. Problems of terminology and selection of criteria, coupled with the lack of extensive sites in good condition, have hindered progress in our understanding. Yet the term "savanna" and "woodland" are inherently vegetational terms, that is they center around concerns over the assemblage of species patterns and their shared structural features. Ongoing discussion over the past few years has led to some consensus in our interpretation of these systems. Some will agree that the term "savanna" may not be the best term to describe the open grasslands with scattered trees (many formal classifications avoid the term, whether in the tropics or temperate zone), but the concepts the term conveys, the ecological context, and the floristic composition of the types remain as definable units. To that end, the vegetation classification presented here offers one way of organizing the ecological patterns of these systems.
A word of caution is still in order for any attempt at classification. Classification typically results in the identification of discrete and definable units, despite the continuous variation present within a unit. Thus, it is far too easy to design a classification that fixes on a point in time in evaluation and administration of programs of restoration. Private landowners and agencies and others will need to appreciate the dynamic nature of savanna and woodlands to enable restoration to take an appropriate course. We may visit an area and decide, because trees are abundant, that the site is a woodland; it is difficult to make the choice to reduce tree cover even though the system historically functioned with much less tree cover. Nevertheless, there is a need to establish a vegetation-based classification of natural communities that documents the existing condition of stands, noting their floristic composition and structure, and response to management efforts. This has become more critical as efforts are made to coordinate restoration efforts across state borders.
A vegetation-based approach will need to be used in conjunction with other methods. The spatial relations of sites, where one area influences the behavior of adjacent areas, or the movement of species through a landscape, are better understood through landscape ecology methods. Ecosystem-based classifications integrate many factors operating at a site, whether climate, geology, soils, or vegetation, and provide an interpretation of the critical ecological processes operating at different scales. The many different questions and objectives that are needed for conservation and restoration planning will require a variety of methods.
A major obstacle to using communities in general as conservation units at the national level was the lack of a consistent national vegetation classification system. To overcome this problem, The Nature Conservancy, in conjunction with the Association for Biodiversity Information (a network of state Heritage programs) and federal agencies, has developed a standardized hierarchical system to facilitate the identification and classification of vegetated terrestrial communities across the United States (Jenkins 1976, Hunter 1991, Grossman et al. 1994).
The classification system was primarily developed for the purposes of conservation planning and biodiversity protection. The intent of the classification system is to provide a complete, standardized listing of all communities that represent the variation in biological diversity and to identify communities that require protection. The classification will be consistent throughout the U.S. at appropriate scales for conservation planning and the management and long-term monitoring of ecological communities and ecosystems.
Because The Nature Conservancy's mission is to protect biological diversity, the classification system emphasizes existing biota as the major attribute. More specifically, the vegetation is the primary attribute used to classify terrestrial communities.
The classification emphasizes existing vegetation rather than potential natural vegetation, climax vegetation, or physical habitats. The vegetation types described in the classification range from the ephemeral to the stable and persistent. Recognizing and accommodating this variation is fundamental to protecting biodiversity. Not restricting the classification only to stable vegetation types ensures that the units are useful both for inventory/site description and as the basis for building dynamic ecological models.
For purposes of prioritizing classification research, it is useful to partition the existing vegetation into groups reflecting their relative "naturalness". Broadly speaking, the classification emphasizes "natural vegetation," including "semi-natural" and "modified" vegetation, but also can include categories for "cultural" vegetation or "planted/cultivated" landcover (Kuchler 1988). These distinctions, while somewhat arbitrary, have been used to help focus conservation efforts primarily on the natural and semi-natural types. However, when necessary, modified and cultural types are identified with the Conservancy's classification system, especially for the purpose of vegetation mapping.
The basic unit of inventory, the plant community, or plant association, is more or less uniform in structure, composition and habitat. The uniformity of the plant community makes the comparison and identification of protection priorities more objective than would be possible at more heterogeneous scales. Typical conservation sites, however, tend to include many different examples of uniform vegetation types. When community elements combine in predictable ways to create repeatable landscape mosaics across large regions, comparisons can be made at larger scales in a way analogous to the smaller scale contrasts. Thus the particular mosaic of community elements present at a site and their distribution across the landscape provide information that is fundamental to any type of ecological land management. For example, in the Anoka Sandplains of central Minnesota, Bur Oak-Northern Pin Oak Barrens and Northern Pin Oak Woodlands can be distinguished based on structure and composition. Occurrences of these types can be recognized elsewhere in the state as well. However, on the sandplain these types occur in a mosaic of Tamarack Swamps and Fens, Sand Prairies and Wet Meadows that are interrelated by ecological processes and must be managed together to ensure maintenance of the individual communities.
The Conservancy's terrestrial vegetation system is hierarchical and combines physiognomy at the highest levels of the hierarchy and floristics at the lowest levels. This type of system allows the geographic orientation of physiognomic characters to be tied to the more local site specific information of the floristic characters. In combination, these systems can satisfy a broader range of objectives for use of the classification system.
Physiognomic systems are easily recognized in the field, can be applied with little knowledge of the flora, permit generalizations of vegetation patterns over large areas, and can be linked to remote sensing signals to facilitate vegetation mapping. These attributes allow the identification of patterns where little is known about an area, or more detailed survey is impractical. Floristic information, however, is almost always used for detailed site analyses, whether for studying environmental gradients, ecological site factors, or forming classification units. Furthermore patterns of succession, disturbance, history (including paleoecology) and natural assemblages are better assessed through floristic composition than physiognomy.
The rationale for coupling physiognomic and floristic systems has been developed over the years (e.g., Rubel 1930, Whittaker 1962, Webb et al. 1970, Werger and Sprangers 1982, Borhidi, 1991). These studies have found a good fit between floristic and physiognomic classifications. In the United States, Driscoll et al. (1984) recommended the development of a joint system using the physiognomic units of UNESCO (1973) and the floristic units of habitat types, of which an example has recently been provided by Dick-Peddie (1993) in New Mexico. Similar methods have been developed in both Canada and Australia (Specht et al. 1974, Strong et al. 1990).
The physiognomic portion of the Conservancy's classification hierarchy is a modification of the UNESCO world physiognomic classification of vegetation (1973) and incorporates some of the revisions made by Driscoll et al. (1984) for the United States. There are four physiognomic levels (class, subclass, group and formation).
Below the physiognomic levels are two floristic levels, the alliance and the association, or "community element." The alliance level is a physiognomically uniform group of plant associations (see community element below) sharing one or more diagnostic species (dominant, differential, indicator, or character), which, as a rule, are found in the uppermost strata of the vegetation (see Mueller-Dombois & Ellenberg 1974). The alliance level includes existing (not just climax or potential) vegetation types. The concept of an alliance is similar to a "cover type". An alliance is equivalent to a "cover type" when the dominant species also have diagnostic value. The alliance may be finer than a cover type when the dominant species extend over large geographic areas and varied environmental conditions. The concept for the alliance is also similar to the concept of the "series". Alliances, however, are described by the diagnostic species for all existing vegetation types whereas series are restricted to climax types and are described by the primary dominant species (see Pfister and Arno, 1980). The alliance level has proven to be a useful conservation planning tool (Margules 1989).
The basic unit of the classification is the "community element" which is defined as a plant association of definite floristic composition, having uniform physiognomic and uniform habitat (Flahault and Schroter 1910). The community element concept is quite similar to the plant association concept used in the Zurich-Montpelier tradition (Braun-Blanquet 1965, Westhoff and van der Maarel 1973). These floristic units are characterized as patterns of co-occurring species that recur either in space or time under similar environmental conditions.
On the ground, community elements are recognized as stands of vegetation with relatively uniform structure and floristics that occur in a relatively uniform environmental setting. As expected from the individual distribution of species (the continuum concept), the community element or type varies along environmental gradients (Curtis 1959). The consequences are that repeating plant associations can be identified on the ground based on specified criteria for homogeneity, but individual occurrences of a particular plant association will have variable species composition and structure.
Based on discussions at previous oak savanna conferences, the proposal is made that the term savanna be limited to stands where tree canopies cover between 5 and 30% of the ground. The term savanna will be used in the popular sense as a physiognomic category, but in the classification below will technically be called either "wooded grassland" or "shrub grassland." Barrens and oak openings may be classified as either savanna or woodland, depending on their structure. Barrens, as used in the Midwest, is neither synonymous with, nor a subset of, the term savanna.
Following Eiten (1986) and others, savanna is defined as follows: "A savanna is any area where scattered trees and/or shrubs and other large persistent plants occur over a continuous and permanent groundlayer visually dominated by herbs, usually graminoids."
Woodlands, by contrast have an open to partially closed tree canopy where shrubs, forbs and other non-graminoid plants may dominate or codominate with the graminoids. The groundlayer is no longer the dominant layer. The term woodland has not been widely used in the Midwest prior to more recent efforts to resolve the definition of the term savannas.
The range of physiognomic classes for all vegetation can be defined as follows (parentheses show optional limits depending on individual types of vegetation):
Additional categories can be recognized for SHRUBLAND (25-100%, including shrub thickets) and SHRUB GRASSLAND (10-25%).
This classification is offered for several reasons:
The application of these categories to specific sites does not require that a savanna type fit the physiognomic definition exactly. A certain amount of flexibility is required because other criteria are used to define community types, such as dominant plant species and complete floristic data.
Application of these definitions is also complicated by the heterogenous nature of savanna and woodland systems. Depending on the scale, a site could be broken into patches of open prairie grassland and woodland or averaged into one savanna type. The small fragmentary nature of many remnants makes it almost impossible to resolve these issues. Furthermore defining canopy cover is a somewhat subjective procedure and varies with the internal density and volume of the canopy.
The attached table presents an overview of the known savanna and woodland types found in the Midwest. The geographic area covered is roughly the tallgrass prairie region and prairie-forest border from Manitoba to Texas.
It should be stressed that the types proposed here are based on geographic patterns of major dominant species and their responses along ecological gradients. Thus they are intended to serve as initial guides for the diversity of savanna types in the Midwest. However, virtually none of these types have received sufficient documentation. A concerted effort needs to be made to characterize the distribution of these types throughout the Midwest.
Several geographic patterns can be highlighted. In the Great Plains the Bur Oak Woodland Group is most common, with a Post Oak-Blackjack Oak Cross Timbers type in Kansas, Oklahoma and Texas. In the upper Midwest, the more moist woodland types belong to the White Oak-(Black Oak) Woodland Group and the Bur Oak-Mixed Oak Woodland Group. Dry woodland types belong to the Black Oak-Northern Pin Oak Woodland Group. In the central and southern Midwest, more moist woodlands belong to the Post Oak-Black Oak-White Oak (Southern Red Oak) Group, and more xeric types to the Post Oak-Blackjack Oak Woodland Group. Chinquapin Oak Woodlands are found irregularly in xeric, alkaline habitats throughout the region.
For savannas, the more moist types of the Bur Oak Tallgrass Wooded Grassland Group are found throughout the central tallgrass prairie region. The White Oak-Pin Oak Wooded Grassland Group is found in the Great Lakes lakeplain. More xeric savanna types of the Post Oak-Blackjack Oak Wooded Grassland Group are found in the central and southern Midwest, and the Black Oak - (Northern Pin Oak) Wooded Grassland Group is found in the upper midwest.
A shrub type, the Hazelnut Barrens, is listed since it forms part of the gradient from prairie to woodland, and certain black oak barrens may exist in this state after catastrophic burns.
A vegetation classification for oak savanna and woodland natural communities in the Midwest, showing their global rarity rank and state or provincial distribution. The vegetation types (associations) are organized by physiognomic categories (woodland, savanna (wooded grassland), shrubland) and alliances (or groups).
1 This classification has been developed by The Nature Conservancy's Conservation Science Department in cooperation with state Natural Heritage programs in the United States and Conservation Data Centres in Canada (Manitoba -MB and Ontario-ON). Currently, Great Plains states and provinces (IA, KS, MB, MN, MO, NE, ND, OK, SD, TX) have reviewed this document more extensively than other states listed. A complete review is being planned by December of 1995.
2 Woodlands are defined as open stands of trees usually over 5 m tall with crowns not usually touching, generally forming 25-60% cover. Understory can be dominated by forbs, graminoids and/or shrubs. Savannas (Wooded Grasslands) are defined as having trees usually over 5 m with widely spaced crowns, generally forming 10-25% tree cover. The ground layer is dominated by grasses, with forbs and shrubs admixed.
Shrublands are defined as shrubs or scrub trees usually 0.5 to 5 m tall with individuals or clumps not touching to interlocking (generally forming greater than 25% canopy cover.
3 The alliance is defined as a group of plant associations with similar physiognomy that share one or more diagnostic species. The diagnostic species (dominant, differential or character) are, as a rule, found in the dominant and/or uppermost strata of the vegetation (Mueller-Dombois and Ellenberg 1974).
4 A plant association can be defined as "a plant community of definite floristic composition, presenting a uniform physiognomy, and growing in uniform habitat conditions." (Mueller-Dombois and Ellenberg 1974). As used here, the emphasis is on the floristic and physiognomic pattern of existing vegetation. Use of the association concept does not imply that communities are discrete entities. The synonym name is a more informal name for the type.
5 Global ranks have been assigned to the rarest savanna and woodland types in the Midwest (see Ambrose et al. 1994). Communities with no rank are either G3 through G5 or not yet ranked, as they may have been defined after the initial ranks were assigned. Thus, absence of a G-rank at this time should not be construed as an indication that the community is not rare.
6 A question mark following the name of the community indicates that it's status as a separate type needs further review.
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