Ecologically-based Healthy Plant Weed Communities: Management by Roger L. Sheley, Tony J. Svejcar, Bruce D. Maxwell, and James S. Jacobs*

This MontGuide is designed to help landowners make economically and ecologically sound weed management decisions. It explains how plant communities develop, how weeds invade and how you can work toward MT199909 AG reprinted 3/2004 developing a useful plant that is relatively weed-resistant.

ost rangeland managers and land owners have M focused their weed man- agement efforts on controlling weeds and have paid limited atten- tion to the existing or resulting plant community. However, it is becom- ing increasingly clear that weed management decisions must be based on environmental and ecolog- ical principles as well as economic ones. Weed management educa- tion should provide land managers with the principles and concepts on which to base their decisions, rather than provide a simple prescription for weed control. This publication presents a conceptual, ecologically- based framework to aid in making economically and ecologically sound weed management decisions. Figure 1. A healthy, weed-resistant plant community consists of a diverse Land-use objectives must be de- group of species occupying all the niches (sites) and using all the veloped before weed management resources in the system, keeping them from weeds. decisions can be made. An inte- grated weed management plan can land are determined. Simply killing of species diverse enough to occupy be designed once the uses of the weeds is an inadequate objective in all the niches (Figure 1). Desirable most situations, especially for large- plants capture a large proportion of *Sheley is Extension noxious weed scale infestations. The generalized the resources in the system, keeping specialist, Maxwell is assoc. pro- objective should be to develop a the resources away from weeds. The fessor of weed and Jacobs healthy plant community that is rel- soil resources, particularly moisture, is post-doctoral research associ- atively weed-resistant while meet- are most limiting in the shortgrass ate, all in the Department of Land ing other land-use objectives—for prairies and intermountain regions Resources and Environmental Sci- instance, forage production, wildlife where we must pay more attention ences, Montana State University- development, recreational to developing plant communities Bozeman. Svejcar is supervisory land maintenance or natural area that effectively use the soil resourc- range scientist, USDA-ARS, East- conservation. es over time and space. ern Oregon Agricultural Center, A healthy, weed-resistant plant A weed-resistant plant commu- Burns, OR 97220. community consists of a collection nity may include an early emerging

A-11 condition, whereas the condition of Successional stage and direction those composed mostly of Figure 2. Alien late-successional species have been Early Middle Late considered excellent. (Climax) invasive weeds are so aggressive they Rangeland managers have used Climate throw this suc- grazing management as their major cessional system tool. Their objective has been to Plant community into disorder. The balance stocking rates so that graz- Grazing Successional only component ing is a force equal and opposite to tendency in this model that natural successional tendency when WEEDS can shift the plant the range is in good or excellent community in a condition. Adjustment of stocking Poor Fair Good Excellent desirable direction rates is a continual process based is successional on short-term climatic variation and Range condition tendency. compensating for minor fluctuations. Much of range manage- species, such as the shallow-rooted munity is comprised of fast-growing, ment has focused on techniques to al- Sandberg’s bluegrass, which uses short-lived species, typically annual low increased stocking rates without the resources that are available in and biennial plants. As succession causing retrogressive changes in the the upper soil profile early in the progresses, these species alter the site plant community. Many techniques, growing season and during periods enough to allow the establishment such as fencing, salting and watering of light precipitation. As the season and colonization of short-lived pe- systems, are aimed at encouraging progresses, species which initiate rennial plants. Soon these short-lived uniform forage utilization through growth later and continue growth perennial species dominate and alter increased animal distribution. Man- further into the season are needed the site in favor of long-lived pe- aging rangelands using grazing sys- to use available soil resources from rennial plants, eventually producing a tems has worked well in many cases, moderate soil depths. Finally, the di- stable, climax plant community. but only in the absence of invasive verse plant community may include In land once dominated by Idaho alien weeds. a deep taprooted, very late maturing fescue and bluebunch wheatgrass, the species, such as alfalfa or big sage- plant community immediately after a When weeds invade brush. These species are capable of disturbance might be composed pri- When alien or noxious weeds invade extracting resources from deep in the marily of annual and biennial mus- native rangeland, they throw the suc- soil profile and throughout much of tards and a few legume species. As cessional pattern into disorder (Figure the growing season. Although little is succession proceeds, that community 2). Many of these weeds evolved in known about the role of many spe- might become dominated by short- the eastern hemisphere where a long cies within the plant community, it lived perennial species, such as bot- history of intensive disturbance has is generally accepted that maximum tlebrush squirreltail, Thurbers needle- selected for very competitive species. diversity is optimum for energy flow grass or sticky geranium. Eventually, The weeds have usually been intro- through the system as well as nutrient succession may progress to a climax duced without the natural enemies and water cycling. Once a desired community, co-dominated by Idaho that help control their abundance in plant community has been chosen, fescue and bluebunch wheatgrass. their place of origin. These factors an ecologically-based weed manage- Most land managers agree that allow alien weeds to dominate native ment plan can be developed. succession occurs. However, there species. The weeds become climax is some disagreement as to whether or stable plant communities. How do The succession model succession has a fixed end-point for a we manage rangelands dominated by To understand ecologically-based particular site, or whether succession these aggressive weeds? rangeland weed management, we varies depending on assorted envi- According to the ecological model must remember the basic ecological ronmental and managerial circum- on which range management has principle on which rangelands have stances. The processes and mecha- been based for the past 60 years, the been managed during the past 60 nisms causing succession are not yet only component shifting plant com- years. This principle states that plant well understood. munities in a desirable direction is communities change over time until Rangeland managers have con- the natural successional tendency. they reach a final and stable compo- dition-classed, monitored and man- Ecological weed management sys- sition, called climax. This process aged rangelands based on succession tems must be developed that are of change is called succession. Sec- for decades. Plant communities based on our understanding of the ondary succession occurs after a site dominated by early-successional causes of succession. As mentioned, has been disturbed. Immediately species have been considered in poor we know little about the mechanisms after a disturbance, the plant com-

2 3 of succession, but a conceptual model for weed management can Designed Controlled be developed based on the general disturbance colonization causes of succession: site availability, differential species availability and Weedy Desired differential species performance. plant plant community community The role of disturbance In order for succession to occur, a Controlled site or niche must be available for performance desirable species and unavailable for undesirable ones. Disturbance cre- ates available sites. Thus, succession Figure 3. Integrated weed management must be based on the three gen- can, in part, be controlled by altering eral causes of succession: site availability, differential species availability the size, severity, frequency and and differential species performance. These correspond to management patchiness of disturbance in a manner actions: designed disturbance, controlled colonization and controlled favorable to desirable species. species performance. The appropriate combination of these causes of Historically, weed management succession can alter the plant community in a favorable direction. strategies have included designed disturbance, such as cultivation, are created and become established, anisms causing succession (distur- timed grazing, burning and herbicide species performance must be al- bance, colonization and performance) applications. However, in an eco- tered to favor desirable species over must be considered as a package. De- logically-based weed management weeds. Controlled species perfor- signing disturbances to either create system, the disturbance is used to alter mance includes using methods to sites for desirable species alone, con- the processes driving succession in a alter growth and reproduction of spe- trolling colonization without making desirable direction which minimizes cific plant species, thus contributing sites available, or increasing their the need for continuous high-energy to a desirable shift in the plant com- relative performance without making inputs. The usefulness of any distur- munity. This requires understanding them available, is unlikely to shift the bance will depend on the range site, the factors that influence competitive plant community in the desired direc- plant community type, invading weed balance, such as grazing, disease, tion. Designing successful rangeland species, history of the site and climate. resource availability, allelopathy (the weed management strategies will re- Once sites are available for de- inhibition of growth in one species quire carefully integrating techniques sirable species, they must be occu- by chemicals from another species), aimed at addressing each of the three pied before the weeds can establish. predators, growth rates and their general causes of succession, Man- This “controlled colonization” will complex interactions. Shifting the agement strategies must be carefully intentionally affect succession. chosen to ensure that one technique Processes that must be exploited Site: an area that meets the is complementary to another. Once are seed dispersal and vegetative plantʼs requirements for suc- this is achieved, conversion from reproduction. Introductions of de- a weedy plant community to a de- sirable species must be enhanced, cessful establishment, growth sirable one can occur. while those of the weeds must be and reproduction. When developing an ecologi- limited. Procedures which shift cally-based weed management seed banks are also important in plant community from weedy to de- plan, options can be placed in controlling colonization. Factors af- sirable plants requires understanding categories of designed disturbance, fecting establishment or encouraging the stages in the weed’s life cycle controlled colonization and con- germination and seedling survival that are most vulnerable to stress or trolled species performance. Care- may also be used to favor desirable control and understanding those stag- fully consider and test each technique species. Using techniques to prevent es and procedures in the desirable for effectiveness in directing plant weed encroachment or altering envi- species’ life-cycles that can enhance succession and determine if the pro- ronmental or managerial conditions their performance. In many cases, posed procedures complement one to exploit dispersal mechanisms or controlling species performance re- another. Integrated weed manage- germination requirements will favor quires repeated applications, such as ment systems can be designed, tested establishment of desirable species. repeated grazing. and documented using this ecologi- This puts the emphasis on encour- This conceptual model forms the cally-based conceptual model. aging the desired species rather than ecological basis for developing inte- Several schematics using ecologi- simply controlling weeds. grated rangeland weed management cally-based weed management plan- When sites for desirable species strategies (Figure 3). The three mech- ning are shown for spotted knapweed

2 3 infested rangeland in Figure 4. Two Figure 4. Some spotted knapweed management strategies. levels of infestation are examined and two successional weed manage- Plant community Controlled Plant community ment plans are tested for each sit- prior to weed Designed Controlled species after weed uation. These examples show how management disturbance colonization performance management integrating various weed manage- Broadleaf Biological Biological 70% seed species 98% spotted ment systems direct successional herbicide control control knapweed, 20% cheatgrass/ processes, resulting in different suc- 2% cheatgrass/ bluegrass Mowing cessional patterns and usefulness to bluegrass Cultivation Mowing 10% spotted knapweed range managers. The plant commu- Non-selective Drill seeding Early spring herbicide nity after implementation depends grazing R 95% cheatgrass/ 50% spotted upon the weed management system Flooding and Broadcast bluegrass, knapweed draining seeding Fertilization 5% spotted and the plant community prior to 30% native knapweed weed management. Climatic vari- plants Fertilization Sheep ation introduces an uncontrolled 20% cheatgrass/ grazing bluegrass 65% native random element that can influence grasses Grazing Grazing R Broadleaf the short-term and long-term out- herbicide 30% cheatgrass/ bluegrass come. Weed management actions Irrigation Broadleaf Reduce soil 5% spotted should be based on your land use herbicide fertility knapweed objectives, desired degree of energy Burning Irrigation Irrigation inputs and economics. R 20% quackgrass Preventing 50% native The successional weed manage- weed plants ment model presented here allows introductions 20%bluegrass, Burning 10% spotted for integration of currently avail- knapweed able tools. With conceptual models of this type, there are seldom large This chart lists the various choices that could be made in devising a strategy to comprehensive research projects manage spotted knapweed infestations. Follow the track from hypothetical situations that have tested all possible options in the left hand column through treatment options that design disturbance, control for a particular plant community. colonization and control species performance to find the expected results in the right Development of successional weed hand column. "R" refers to repeated aplications. management plans will require use of existing research information, management experience and mon- itoring of successes and failures to adjust future plans.

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The U.S. Department of Agriculture (USDA), Montana State University and the Montana State University Extension Service prohibit discrimination in all of their programs and ac- File under: Weeds tivities on the basis of race, color, national origin, gender, religion, age, disability, political beliefs, sexual orientation, and marital and family status. Issued in furtherance of coopera- A-11 (Range and Pasture) tive extension work in agriculture and home economics, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Douglas L. Steele, Vice Provost Reprinted March. 2004 216-2,000-0304-MG and Director, Extension Service, Montana State University, Bozeman, MT 59717.