Successional Theory and the Desired Plant Commun- Ity Approach

Successional Theory and the Desired Plant Community Approach

Item Type text; Article

Authors Borman, Michael M.; Pyke, David A.

Citation Borman, M. M., & Pyke, D. A. (1994). Successional theory and the desired plant community approach. Rangelands, 16(2), 82-84.

Publisher Society for Range Management

Journal Rangelands

Download date 28/09/2021 01:20:41

Item License http://rightsstatements.org/vocab/InC/1.0/

Version Final published version

Link to Item http://hdl.handle.net/10150/638989 82 RANGELANDS 16(2), April 1994

Successional Theory and the Desired Plant Commun- ity Approach

Michael M. Borman and David A. Pyke

The Bureau of Land Management (BLM) has been con- climax or potential natural community, provided that sidering use of a Desired Plant Community (DPC) ap- severedisturbances are eliminated. This conceptis widely proach as the basis for managing and evaluating BLM- taught and is presented in currently used textbooks of administered lands. Concern has been raised that this rangeland management (e.g., Stoddart et al. 1975, Heady approachmay not be scientificallysound or that it may be 1975, Holechek et al. 1989). inappropriately used by managers tojustify unacceptable The traditional rangeland dynamics model used in rangeland conditions.The BLM currentlyevaluates range- determining condition classes in the United States was land condition and trend based on the successional con- developedby Dyksterhuis (1949, 1958), was based on the cept that plant communities progress toward a species Clementsian successional theories,and wasaccepted by compositionsimilar to undisturbed communitiesif envi- federal land management agencies inthe late 1960s. This ronmental conditions remainthe same and if disturbance successional approach for defining rangeland condition is reduced or eliminated.This successional concept was classes was strongly criticized because it did not accu- influencedgreatly by successional theoriesdeveloped by rately reflectthe health of grazed rangeland. Forexample, Frederick E. Clements in the early 1900s. the species composition of shrub-steppe communities The Clementsian-based successional theory as a model grazed by cattle declines in herbaceous plantsas woody forvegetation change isinconsistent with currentecolog- plants dominate the site. Once woody plants become ical thought regarding succession on many rangelands. dense, the elimination of livestock grazing will not be Smith (1988) and Laycock(1991) referenced anumber of sufficient to allow herbaceous plants to fully recover to ecologistswho have suggested that traditionaltheories of their previous levels in the community. On some sites, plant succession leadingto a climatic climaxare scientif- woody plantsreplace herbaceous plantsregardless ofthe ically inadequate for semiarid and arid rangeland eco- livestockgrazing because suppression of fires allows the systems. There is not yet an agreement as to an alterna- more competitivewoody species todominate. Because of tive theory that may be consideredappropriate; however, several problems associated with inaccurate condition theories involving multiple steady states and state-and- classes formulated from the traditional rangeland suc- transition processes appearto be gaining acceptance. cession model, the Rangeland InventoryStandardization The purposeof this paperis to briefly describe current Committee (1983) recommended using a potential natu- conceptsof vegetation dynamicsin semiarid communi- ral community(PNC) rather than climax as the basis for ties relative to traditional theory and to try to place the comparison. This approachcompares currentvegetation Desired Plant Community approach into the context of to the potential community the site can accommodate current theories. Knowledge gaps and the potential of while consideringpast modificationsof the site. Thus, a this approachfor vegetation management and evaluation site maydevelop into oneof many potentialcommunities will also be discussed. depending on the type and severity of the disturbanceon Current Concepts: Vegetation Dynamics of Semiarid thesite. Yet, this approach retains the conceptof a single Communities steady state that the communitywill achieve if disturban- eliminated. The Potential Forecasts of the likelihood that a site will successfully ces are Natural Community recoverafter a disturbanceare based on models of vege- approach currently is being used by several federal land tation dynamics for similar sites. Much of our current management agencies, including the BLM, to describe basis for judging rangeland dynamics is based on the rangeland condition. theory, as described by Clements, that a successional Regardless of the traditional condition class approach both are rooted in the trajectory will lead to a single stable plant community, chosen, strongly Clementsian theory that succession is a predictable, linear, bi-directional Authors are rangelandecologist and senior rangelandecologist, National process. In otherwords, succession progresses towards Biological Survey/Bureau ofLand Management, PacificForest & Basin Range- land Systems CooperativeResearch & Technology Unit; and assistantand a climaxor potentialcommunity if disturbancesare elim- associate professors, Department of Rangeland Resources,Oregon State inated and from the climax or com- University,Corvallis 97331. regresses potential The authors wishto thank WayneElmore, Pat Johnson,Bill Krueger,Steve munity if disturbances continue. The traditional range- Leonard, and an anonymous reviewer for reviewing an earlier draft of this manuscriptand making helpful suggestionsfor improvement. land succession model and the Potential Natural Com- RANGELANOS 16(2), April 1994 83 munity model have largely been accepted for rangeland lead to alternativestable plant communities. Within the management because of the desireto find asingle objec- western United States, Laycock (1991) proposed a state- tive standard forassessing theimpacts of grazing animals and-transition model for the sagebrush-steppe ecosys- (Smith 1988). tem that combines transitionsresulting from grazing, fire, Alternative theories of vegetation dynamics take an and climatic conditions that result in the development of individual species approach to the development of com- six relatively stablecommunities. munities based on the early work of Gleason (1926). Thesealternatives rely on the species' life-history,on the The Desired Plant Community Concept interactions among the individuals that constitute the The philosophical concepts which form the basis of population,and on the interactionsbetween the individ- rangeland condition assessment are now in a period of ual and its environment. Disturbancebecomes a part of considerable ferment and change. Rangelands often the ecosystem and several stable communitieshave the suffer degradation because oftheir relatively low produc- potentialto develop after disturbancesare eliminated or tion per unit area, some traditional management practi- reduced. The nature, frequency,and intensityof the dis- ces, and because of the erratic climatic influenceswhich turbancedifferentially impacteach plant species, there- drive their biological productivity.It is very important to fore the communityformed after a disturbancedepends have objective methods to assess their productivity and on the abilitiesof the species to survivethe disturbance or the effectiveness of management applied to the lands to replace themselves through reproductionafter the dis- (Foran et a!. 1986). turbance. The likelihood of a species surviving or replac- The Society for Range Management formed a Task ing itself after a disturbancedepends on the species ger- Group in 1989 on Unity in Concepts and Terminology mination characteristics, competitive ability, growth, phen- which has recommended that objectives for multiple use ology, and on its geneticvariability and plasticityrelated management on rangelands be defined in terms of a to the myriad of environmental factors it may face. In Desired PlantCommunity (DPC) foreach ecologicalsite short, forecastingthe dynamics of a communityfollowing (Task Group on Unity in Concepts and Terminology a disturbancerequires the knowledgeof the physiologi- 1991). These same recommendations are applicable to cal and demographic responses of the individualsthat ecosystem management that is being advocated by the constitute the interacting populations of species that federal land management agencies in the USA. The form the community. recommendation was approved by the Board of Direc- The individualistic approach to vegetation dynamics tors, Society for Range Management, in July 1991. Of the recognizes that morethan one potential communitycan several plant communitiesthat are capable of occupying result following a disturbance. Acceptance of this idea an ecological site1, the Desired Plant Community is has led to the recognition that multiple stable states or defined as the one that has been identified through a communitiesmay exist for any given site (Holling 1973, management plan to best meet the plan's objectives for May 1977,Walker etal. 1981). In a recentreview of range- the site. It is selected to meet the needs and values of land successional models, Westoby et al. (1989) outlined people who have legitimate interests in land manage- the limitationsand exceptions tothetraditional approach ment. The value of the concept is that the desired com- that have led them to advocate that many stable plant munityis clearly defined interms of human values, includ- communities have the potential to exist on any given ing economicand social considerations, and not in terms landscape. They have proposed an alternativestate-and- of a "pristine" condition which may or may not have transitionmodel fordescribing rangelands and for apply- existed at some arbitrary time in the past. Vegetation ing management prescriptions.States are relatively sta- management status should be reported in terms of sim- bleassemblagesofspeciesthatdeveloponasitedepend- ilarity to (i.e., condition) and trend toward or away from ing on the timing, intensity, and severityof disturbances. the selected community. The SRM Task Group recom- Transitions, also referred to as thresholds (c.f., Friedel mended that the effectiveness of a vegetative community 1991), are actions that result in newstates (communities) in protecting the site against accelerated erosion be of species assemblages. Transitionsare characterized by assessed independently of the use of the site. This the following: (1) unpredictable natural events, such as assessment should be called a Site Conservation Rating fires or changing climatic conditions,or human-induced (SCR). The point at which an unacceptable level of ero- uses ofthe ecosystem, suchas farming or grazing;and (2) sion beginsshould be called the Site Conservation Thres- changes in states are not reversible on a practicable man- hold (SCT). Sites with erosion in excess of the threshold agement time scale without human intervention (e.g., arti- would be assigned a Site Conservation Rating of "unsus- ficial restoration). tainable" or "unprotected" and those with lower rates of The state-and-transition model appears to be approp- 'Ecological Site isdefined as akind ofland with specific physical characteris- riate for semiarid communities of North America (Lay- tics which differs from other kinds of land in its ability to produce distinctive cock 1991), Australia, and South Africa (Westobyet al. kinds and amounts ofvegetation and its responseto management. The ecologicalsite conceptis and willremain a useful tool forassessing theproductive 1989). This approach has been successfullyapplied to capability of ecosystems. Ecologicalsites should be the basisfor classification ofrangelands for the purposesof inventory,assessment, and extrapola- arid and semiarid rangelands where the interaction be- tion of management practices. tween different types of disturbance and climate can 84 RANGELANDS 16(2), April 1994 erosion would be assigned a rating of "sustainable"or trend are interpretations of field data characterizing "protected."Any communitywith plant cover that is cap- attributesof the plant/soil system. The monitoringdata ableof maintaininglower erosion rates than thethreshold collected should provide a record of trends in these would be a candidate for the Desired Plant Communityat attributes even though the values placed on them may the site. The Desired Plant Community could include change over time (Smith 1988). introduced plant species. Any plant community that The Desired Plant Communityapproach appears to be resultsin erosionin excess ofthe thresholdlevel is unde- compatiblewith the concepts of multiple steady states sirableand should not be selected. and state-and-transition models and should be seriously A site where erosion exceeds the threshold may, considered as a method for addressing multiple use and through management, improveand reach a satisfactory ecosystem management objectives. Basic and applied condition. However, if erosion remains in excess of the research will be needed to provide the theoretical basis threshold long enough, presumably the site will lose a for the principle of "sustainability" as is implied in the significant part of its former productivity and become, for term "Site Conservation Threshold",and todevelop prac- practical purposes, a new ecological site. In that case, a tical procedures for application. new Site Conservation Thresholdand new arrayof possi- ble Desired Plant Communities must be defined. Literature Cited It must be understood that Site Conservation Thres- Dyksterhuls, E.J. 1949. Condition and management of rangeland on 2:104-115. hold has not yet been definedfor any given site, with the based quantitativeecology. J. Range Manage. of Dyksterhuls, E.J. 1958. Ecological principles in range evaluation. exception some preliminarywork by theSoil Conserva- Bot. Rev. 24:253-272. tion Servicein Arizona. It is atthis time merely a proposal. Foran,B.D., G. Bastin,and K.A. Shaw. 1986. Range assessment and We must learn how to define it and how to identify the monitoringin arid lands:the useof classification and ordination in parameters to measure or estimate it. The threshold,and rangesurvey. J. of Environ. Manage. 22:67-84. the Frledel,M.H. 1991. Range condition assessment andthe concept of parameters used to measure it, would be specific to thresholds:A viewpoint. J. Range Manage. 44:422-426. the ecological site. Initially, Site Conservation Ratings Gleason,H.A. 1926. The individualisticconcept ofthe plant associa- will be developed for communitieson specific sites tion. Bull. Torr. Bot. Club 53:1 -20. by H.F. 1975. McGraw-Hill,N.Y.. individuals with sufficient experience at these sites and Heady, Rangeland management. Holechek,J.L., R.D. Pleper,and C.H. Herbel. 1989. Range manage- communities to know what community attributes offer ment. Prentice Hall, EnglewoodCliffs, N.J. protection against accelerated erosion. Holling, C.S. 1973. Resiliency and stability of ecological systems. Smith and others Annu. Rev. Ecol. Syst. 5:25-37. (1987) have recommended that range- W.A.1991. Stablestates and thresholdsof condition land Laycock, range managers take the responsibilityto decidethe type on North American rangelands:A viewpoint. J. Range Manage. of vegetation which most closely meets management 44:427 -433. goals (including the fundamental of site May, R.M. 1977. Thresholds and breakpoints in ecosystemswith a goal protection) of stablestates. Nature269:471 -477. and use that as their standard multiplicity vegetation type for condi- Rangeland Inventory StandardizationCommittee. 1983. Guidelines tion (Smith 1988). In the Desired Plant Community and terminologyfor rangeinventories and monitoring.Soc. Range approach, condition would be based on those attributes Manage., Denver, Cob. pertinentto projecteduses ofthe land. Factors otherthan Smith, E.L. 1987. DoesRISC offersecurity? p. 19-23 In:G. Secrist, and K. Eshelman (coordinators) Rangeland MonitoringWorkshop potentially narrowly defined management objectives Proc. Jan. 12-16, 1987, Golden, Cob. USD1, Bureau of Land should be considered. An example would be the potential Management. for weeds, particularly noxious weeds, to invade a com- Smith, E.L. 1988. Successional conceptsin relation to rangecondi- munity that otherwisesatisfies tion assessment. In: P.T. Tueller (ed.) Vegetationscience applica- management objectives. tions for rangelandanalysis and management.Kluwer Academic Given more than one option, the community with the PubI., Boston. lower potential for noxious weed invasion would be Stoddart, L.A., A.D. Smith, andT.W. Box.1975. Range management selected. (Third Ed.). McGraw-Hill,N.Y. A concernwith Task Group on Unity In Concepts and Terminology. 1991. New the Desired PlantCommunity approach direction in range condition assessment. Rep. to the Board of to rangeland monitoring is that it may lead to moving Directors, Soc. Range Manage., North Platte, Nebr.July 1991. targets as the values of society and managers change Walker, B.H., D. Ludwig, CS. Holling, and R.M. Peterman. 1981. ofsemi-arid savanna J. Ecol.69:473-498. overtime. Anotherconcern is that agencies may attempt Stability grazingsystems. to use the Westoby, M., B. Walker, and I. Noy-MeIr. 1989. Opportunistic man- approach to coverup the lack of improved agement for rangelands not at equilibrium. J. Range Manage. rangeland condition. However, rangeland condition and 42:266-274.