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of succession,but a conceptual <br /> model for weed management can <br /> be developed based on the general Designed -► Controlled <br /> causes of succession: site availability, disturbance go colonization <br /> differential species availability and Weedy Desired <br /> differential species performance. plant plant <br /> community community <br /> The role of disturbance <br /> In order for succession to occur,a Controlled <br /> site or niche must be available for performance <br /> desirable species and unavailable for <br /> undesirable ones.Disturbance cre- <br /> ates available sites. Thus,succession Figure 3.Integrated weed management must be based on the three gen- <br /> can,in part,be controlled by altering eral causes of succession:site availability,differential species availability <br /> the size, severity, frequency and and differential species performance.These correspond to management <br /> patchiness of disturbance in a manner actions:designed disturbance,controlled colonization and controlled <br /> favorable to desirable species. species performance.The appropriate combination of these causes of <br /> Historically,weed management succession can alter the plant community in a favorable direction. <br /> strategies have included designed <br /> disturbance,such as cultivation, are created and become established, anisms causing succession(distur- <br /> timed grazing,burning and herbicide species performance must be al- bance,colonization and performance) <br /> applications.However,in an eco- tered to favor desirable species over must be considered as a package.De- <br /> logically-based weed management weeds. Controlled species perfor- signing disturbances to either create <br /> system,the disturbance is used to alter mance includes using methods to sites for desirable species alone,con- <br /> the processes driving succession in a alter growth and reproduction of spe- trolling colonization without making <br /> desirable direction which minimizes cific plant species,thus contributing sites available,or increasing their <br /> the need for continuous high-energy to a desirable shift in the plant com- relative performance without making <br /> inputs.The usefulness of any distur- munity.This requires understanding them available,is unlikely to shift the <br /> bance will depend on the range site, the factors that influence competitive plant community in the desired direc- <br /> plant community type,invading weed balance,such as grazing,disease, tion.Designing successful rangeland <br /> species,history of the site and climate. resource availability, allelopathy(the weed management strategies will re- <br /> Once sites are available for de- inhibition of growth in one species quire carefully integrating techniques <br /> sirable species,they must be occu- by chemicals from another species), aimed at addressing each of the three <br /> pied before the weeds can establish. predators,growth rates and their general causes of succession,Man- <br /> This"controlled colonization"will complex interactions. Shifting the agement strategies must be carefully <br /> intentionally affect succession. chosen to ensure that one technique <br /> Processes that must be exploited Site: an area that meets the is complementary to another. Once <br /> are seed dispersal and vegetative plant's requirements for suc- this is achieved,conversion from <br /> reproduction. Introductions of de- cessf it establishment, growth a weedy plant community to a de- <br /> sirable species must be enhanced, sirable one can occur. <br /> while those of the weeds must be and reproduction. When developing an ecologi- <br /> limited.Procedures which shift cally-based weed management <br /> seed banks are also important in plant community from weedy to de- plan,options can be placed in <br /> controlling colonization.Factors af- sirable plants requires understanding categories of designed disturbance, <br /> fecting establishment or encouraging the stages in the weed's life cycle controlled colonization and con- <br /> germination and seedling survival that are most vulnerable to stress or trolled species performance. Care- <br /> may also be used to favor desirable control and understanding those stag- fully consider and test each technique <br /> species.Using techniques to prevent es and procedures in the desirable for effectiveness in directing plant <br /> weed encroachment or altering envi- species'life-cycles that can enhance succession and determine if the pro- <br /> ronmental or managerial conditions their performance. In many cases, posed procedures complement one <br /> to exploit dispersal mechanisms or controlling species performance re- another. Integrated weed manage- <br /> germination requirements will favor quires repeated applications, such as ment systems can be designed,tested <br /> establishment of desirable species. repeated grazing. and documented using this ecologi- <br /> This puts the emphasis on encour- This conceptual model forms the cally-based conceptual model. <br /> aging the desired species rather than ecological basis for developing inte- Several schematics using ecologi- <br /> simply controlling weeds. grated rangeland weed management cally-based weed management plan- <br /> When sites for desirable species strategies(Figure 3). The three mech- ning are shown for spotted knapweed <br /> 3 <br />