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Diffuse knapweed


Diffuse knapweed (Centaurea diffusa) is a member of the genus Centaurea of the family Asteraceae. A highly competitive plant, diffuse knapweed has established itself in many areas throughout North America and is considered an invasive species. An invasive species is an organism introduced through human actions to an environment in which it is not native and due to competitive advantages threatens the stability and diversity of the local ecosystem [3]. Diffuse knapweed is very resistant to drought, can spread quickly and possesses other competitive advantages that have allowed it to establish and flourish in its introduced range [18].

Contents
1 Species description

1.1 Morphology
1.2 Dispersal
1.3 Range
1.4 General impact

2 Control

2.1 Biological control
2.2 Physical control
2.3 Chemical Control

3 human influence on invasion
4 Toward an integrated control strategy

4.1 Potential mechanisms of invasion

4.1.1 Allelopathy
4.1.2 Enemy release hypothesis
4.1.3 Superior resource competition
4.1.4 Summary

4.2 Future research

5 References

Species description

Morphology

Diffuse knapweed generally grows to heights between 4-24 inches. It possesses a highly branched stem and a large taproot. It has a rosette of leaves at its base and smaller leaves formed in an alternating pattern on the stem. The flowers are usually white or pink and grow out of urn-shaped heads located at the tips of the branches [18]. Diffuse knapweed assumes a short rosette form for one or more years until reaching a maximum size at which point it experiences rapid growth, flowers, releases seeds and dies. A single diffuse knapweed plant can produce approximately 18,000 seeds [7].

Dispersal

  • Agriculture: Impure seed and alfalfa contaminated with diffuse knapweed seed can cause the spread of diffuse knapweed [13].
  • Wildlife: By either eating the seeds or transporting the seeds in their fur wild animals can transport the seeds of diffuse knapweed [13].
  • Wind: By blowing the seeds out of their containers on the plant the seeds can be distributed over a short range. If the plant dries out and dies with the seeds still contained, it can roll for great distances like tumbleweed before the seeds are released. Wind is the primary means by which diffuse knapweed seeds are spread [7].
  • Water: Rivers and other waterways can carry seeds in their current for long distances before depositing the seeds onto the shore where they can germinate [13].

Range

The native range of diffuse knapweed includes Asia Minor, the Balkans and the southern portion of Russia. It is also frequently found in Romania, Turkey, Greece, Bulgaria, Syria and the Eastern coast of the Mediterranean. Diffuse knapweed is not invasive in its native range [7] and generally inhabits less then 1% of groundcover [8].

Diffuse knapweed was first identified in North America in 1907 when it was found in a Washington alfalfa field. The seeds had presumably been transported in an impure alfalfa seed shipment coming from somewhere in diffuse knapweed’s native range [1].

Diffuse knapweed has invaded at least 19 states in the US, including all contiguous states west of the Rockies . Additionally, diffuse knapweed is found in Connecticut, Massachusetts and New Jersey [18]. Portions of Western Canada have also been infested by diffuse knapweed.

The areas that diffuse knapweed has been able to establish in generally are plains rangelands or forest benchlands. Land that has been recently disturbed, by human or natural processes, is favored for the establishment of diffuse knapweed [17]. It has the potential to prosper in semi-arid and arid environments and seems to favor light, dry, porous soil. Areas with large amounts of shade or high levels of water discourage diffuse knapweed growth.

General impact

By 1998 Diffuse knapweed had infested over 12,640 km² in the western US and was increasing its range at a rate of 18% each year [7]. Diffuse knapweed can establish itself in grassland, scrubland and riparian environments. Once introduced into a novel, non-native, environment it causes reduced species diversity and can frequently establish a monoculture [7]. It has little or no use as feed for livestock as its thistles can damage the mouth and digestive tract of animals [13]. A study in 1973 concluded that ranches lost approximately 80 cents per acre ($20/km²) of diffuse knapweed due to decreased grazing area [13]. In an agricultural setting it can greatly reduce crop yield and purity. This reduction in crop yield can have disastrous economic effects on agricultural business. Diffuse knapweed also causes an increase in erosion rates by displacing species with larger root systems and a decrease in wildlife population.

Control

Effective control of diffuse knapweed requires a fusion of well executed land management, biological control, physical control, chemical control and reestablishment of the native species. Any method of control must insure that the root is removed or the plant will grow back. Additionally, native plant growth in areas where diffuse knapweed has been removed should be encouraged to prevent reestablishment.

Biological control

Biological control involves the introduction of organisms, usually natural competitors of the invasive species, into the invaded environment in order to control the invasive species. The biological control of diffuse knapweed has been primarily approached through the use of insects. Biocontrol is most effective when several species of biocontrol organisms are used [28].

Some of the more commonly utilized biocontrol agents are:

  • Lesser knapweed flower weevil (Larinus minutus). Individuals of this species lay their eggs on the seed heads of both diffuse and spotted knapweed species. When the larvae emerge from the eggs they feed upon the seeds of their host plant. As the females of this species can create from 28 to 130 eggs [2] and each larva can consume an entire seed head, an adequate population of Larinus minutus can decimate entire stands of knapweed. The adult weevils feed upon the stems, branches, leaves and undeveloped flower buds [36]. It is native to Greece and is now found in Montana, Washington, Idaho and Oregon [7].


  • Knapweed root weevil (Cyphocleonus achates). Knapweed root weevils lay approximately 50 to 70 eggs on either diffuse or spotted knapweed. As the name suggests the larvae produced burrow into the root where they metamorphose into its adult form. At this point it will tunnel trough the root to the surface where it will feed on the leaves of the knapweed plant [2]. It is native to Austria, Greece, Hungary and Romania and has been introduced to Idaho, Montana, Washington and Oregon [7].

Physical control

Physical control of diffuse knapweed primarily comprises of cutting, digging or burning to remove the plants.

  • Cutting. While cutting the aboveground portion of diffuse knapweed will greatly decrease the spread of seeds it does not remove the root. With only its root still intact, diffuse knapweed can survive and continue to grow. For a program of cutting to be effective it must be long term so that the effect of reduced seed spreading can be realized [7].
  • Digging. Digging removes both the aboveground portion and the root of diffuse knapweed has shown to be very effective, as if the plant is properly disposed of it can neither regrow nor spread its seeds [13]. The largest problem with digging knapweed is that it is extremely labor intensive. Additionally, the recently vacated soil should be planted with a native species to avoid knapweed reintroducing itself in the disturbed soil.
  • Burning. Setting fire to a crowd of knapweed, if the fire is sufficiently severe, can successfully destroy the above ground and belowground sections of diffuse knapweed. However, precautions must be taken to first ensure that the fire is properly contained and that a new plant community is established to prevent the reintroduction of diffuse knapweed [13].

Chemical Control

Chemical control involves the use of herbicides to control diffuse knapweed. The herbicide Tordon (picloram) is recognized as the most effective, but it is common to use multiple herbicides in order to reduce strain on local grasses [3]. The herbicides 2,4-D, dicamba, and glyphosate are also effective for control. Regardless of which herbicide is used it is important that it be applied before the knapweed plants have released there seeds in order to be most effective [13].

human influence on invasion

One of the first influences humans had on diffuse knapweed was to inadvertently introduce it to North America. Diffuse knapweed was reached a level of success and coverage that dwarfs what it experiences in its native range [5].

Diffuse knapweed is known to establish more easily and effectively in recently disturbed environments [36]. Disturbed environments generally present low environmental stress as more resources are available then are being used [18]. These available resources often allow the establishment of an invasive in an ecological community. The concentration of diffuse knapweed in such an area is often linked to the level of soil disturbance [18]. Human disturbances often lead to less species diversity in a community. In turn less species diversity can lead to unused resources which allow invasive species to more readily establish [26]. Areas such as fallow land, ditches, rangelands, residential and industrial districts and roadsides are all disturbed habitats where diffuse knapweed frequently establishes [36]. Additionally the removal of foliage and other ground cover increases the likelihood that seeds will come in contact with the soil and germinate.

The largest impact of humans on diffuse knapweed is certainly due to our efforts in controlling and eradicating its invasive populations. The several methods outlined in the control section represent a small sample of literally hundreds of approaches being tried with varying levels of effectiveness. Besides reducing the spread of diffuse knapweed we are also providing selective pressure against the individuals that cannot withstand a certain method of control. Selective pressure, given sufficient time, can cause the adaptation or evolution of invasive species [22] such as diffuse knapweed. If an individual diffuse knapweed plant survives control efforts because of a trait it possesses its progeny will make up a greater portion of the population then the plants that succumbed to the control.

Toward an integrated control strategy

To successfully control diffuse knapweed we must first develop an understanding of the mechanism that allows it to be invasive. If we were able to isolate the reason for its invasiveness, control methods designed to specifically target the effectiveness of that mechanism could be developed. Additionally, precautions designed to minimize the invasibility of at risk environments could be carried out [26].

Potential mechanisms of invasion

The ability for a plant to effectively establish and spread in a novel environment is generally due to a mechanism or trait that grants it superior competitive ability over the native species in its new environment. Common mechanisms attributed to invasiveness include allelopathy, the enemy release hypothesis and superior resource competition.

Allelopathy

A subsection of the novel weapons hypothesis, allelopathy is a mechanism where a plant secretes chemicals that cause deleterious effects on neighboring organisms [5]. Diffuse knapweed releases a concoction of chemicals from its roots that contain substances, such as 8-Hydroxyquinoline, which have negative effects on plants.

It has been shown that under experimental conditions plants, including other knapweed species, exposed to the chemicals secreted by diffuse knapweed showed a significant decrease in productivity, growth and germination [32]. One of the greatest hurdles facing this theory is the question of whether the same degree of effectiveness would be found in a complex natural setting. A multitude of factors such as soil composition, microbial community, root density, microclimate, groundwater levels, and plant densities could all have substantial influences on the effectiveness of allelopathy [8]. Also the absence of some naturally occurring minerals in laboratory settings could have the effect of making the root membranes more permeable therefore increasing the level of allelopathic chemicals absorbed and artificially raising the observed effect of allelopathy [27]. These possibly misleading issues have led many to discount the applicability of allelopathy as an important invasive mechanism [8]. In order to reach an absolute conclusion on the effectiveness of allelopathy the biotic and abiotic factors that govern the spread and effectiveness of allelopathic chemicals in the invaded region must be known.

Enemy release hypothesis

The enemy release hypothesis proposes that an invasive plant will experience increased competitive ability because of a decrease in natural enemies [5]. The 2002 article by Ryan M. Keane and Michael J. Crawley [15] states three main points in support of the ERH. Firstly plant populations are highly regulated and controlled by the effects of their natural enemies. Secondly these enemies have a greater effect on native plants in comparison to invasive plants. Lastly a plant experiencing less pressure from enemies will have a minimal resource investment in resistance and tolerance and experience amplified competitive ability. However, the species native to the environment will continue to devote resources to resistance and tolerance. Although we generally consider the ERH to pertain to the invasive plant’s escape from natural enemies such as herbivores or pathogens, it can also apply to a difference in abiotic factors between the native and novel range. Certain abiotic factors, such as a more favorable level of nutrients, could promote increased productivity in the invasive species. Additionally, changes in abiotic conditions can diminish or even reverse the effect of enemies [10].

Situations that could prevent an invasive from benefiting from the ERH include organisms similar to native competitors existing in novel environment or native competitors being introduced with the invading species. In each of these cases an organism would be present in the novel environment to suppress the invasive species, which would negate any benefit from the ERH [15].

Although the ERH is a fairly simple and well supported argument for why introduced species become invasive, there are opposing reports on its applicability to various invasive species. Many experiments conducted to test for the effect of ERH have found both results that support the ERH and results that dispute the ERH [10]. It has been found in some cases that species from the novel environment do in fact become effective regulators of an introduced species [26]. One of the proposed reasons is that the invasive species will have reduced genetic diversity because of the founder effect. If the invasive species has a low diversity of defense mechanisms it may be more susceptible to enemies then the native species [10]. In this case if the introduced plant becomes invasive it is likely due to a factor besides the ERH. Because the ERH is not always applicable it must be known that this heightened level of fitness is due to the absence of natural enemies before we attribute the increased vigor of an invasive species to the ERH.

Superior resource competition

Superior resource competition can also have a substantial effect on the success of an invasive species. An introduced plant’s first interaction with its non-native environment will involve the competition for limited resources [31]. If the invasive species happens to have evolved a greater ability to more efficiently gather or use some resource than its competitors in the novel environment then we could expect the invasive to be more productive than the natives. Another manifestation of superior resource competition could be an adaptation that allows the invasive species to better exist in the novel environment then the native inhabitants. For example, an invasive species that is better adapted to recover from fires then any species in its novel environment will be more productive in a fire prone setting. Traits that are greatly advantageous to establishing and spreading in a novel environment would be expected to be shared by most invasive plants. Although there is no absolute list of characteristics that lead to invasiveness, trends in the structures and life cycles of invasive plants definitely exist. Rapid growth, quick adaptation to environmental stress, large dispersal radius and the ability to reproduce both sexually and asexually are considered common traits for invasive plants [3]. However, these traits do not mandate that a plant will become invasive. Plants with some, or all of these traits commonly vary widely in there levels of invasiveness [26].

Summary

Thus, the success of diffuse knapweed must be attributed to a combination of several mechanisms [5]. Its invasiveness is due to a mix of allelopathy, ERH and superior resource competition. However, the most importance must be attributed to the ERH since diffuse knapweed, while a very effective invasive species in its novel environment, is non-invasive and doesn’t establish monocultures in its native range. It is the differences, biotic and abiotic, between its novel and native surroundings that cause it to be invasive.

To demonstrate that the ERH applies to diffuse knapweed it is essential to show that the absence of natural enemies has a significant positive effect on its success [10]. One way to show this is to observe the effect of introducing some of diffuse knapweed’s natural enemies into its novel environment. If diffuse knapweed, which generally thrives in its invaded environment, is significantly inhibited through the introduction of natural enemies it can be concluded that diffuse knapweed is more competitive in the absence of its natural enemies. A recent effort at biocontrol of diffuse knapweed in Idaho’s Camas County effectively reduced 20,000 acres (80 km²) of knapweed to minimal levels through the release of the lesser knapweed flower weevil and the knapweed root weevil [4]. Since both of the insects released are natural competitors of diffuse knapweed and since this and other similar efforts at biocontrol have been successful there is significant evidence that diffuse knapweed benefits from the absence of its natural enemies.

Another aspect of diffuse knapweed’s success relies on the effect of its allelopathic chemicals in its novel environment. Although there is still debate concerning the effectiveness of allelopathic chemicals in the field [27], the evidence of allelopathic effects demonstrated in a laboratory setting and its propensity to establish monocultures support the importance of allelopathy to diffuse knapweed’s success [13].

Curiously diffuse knapweed’s allelopathic chemicals were shown to have a deleterious effect on the North American competitors but were beneficial to its native competitors [32]. While diffuse knapweed’s native competitors are able to compete more effectively in the presence of allelopathic chemicals, the novel competitor’s fitness is decreased. This situation provides an example of the effectiveness of the allelopathy mechanism benefiting from the ERH. The increased effectiveness of allelopathic chemicals cause diffuse knapweed to experience less competitive pressure and as a result is able to establish more predominantly in this new area.

Another connection between allelopathy and the ERH is the fact that concentrations of allelopathic chemicals were found to increase when diffuse knapweed was planted in North American soil as opposed to Eurasian soil [32]. This effect is probably due to the absence of unfavorable soil conditions or soil microorganisms that exist in its native environment. As a result the allelopathic chemicals will be able to reach higher concentrations, spread farther and therefore be more effective [8]. By effecting more neighboring plants the favorable changes in soil condition contribute to the success of diffuse knapweed.

Besides the advantages that diffuse knapweed gains from the ERH and allelopathy it also possesses several characteristically invasive traits. One factor leading to the superior resource competition of diffuse knapweed is its ability to exist in drought conditions [18]. This advantage allows diffuse knapweed to devote its resources to competition while its neighbors are conserving resources to survive. The high number of seeds produced by diffuse knapweed is also a common trait of invasive plants. A higher density of knapweed will not only increase the concentration of allelopathic chemicals in the soil but will also restrict the nutrients available to native plants. Unfortunately very little research has been conducted to determine the relative competitive ability between diffuse knapweed and its novel competitors. However, tests conducted on the effect of diffuse knapweed on North American grasses in the absence on allelopathic chemicals demonstrated that the fitness of these grasses declined in the presence of diffuse knapweed [6]. Regrettably we cannot decide if diffuse knapweed is for general purposes a better competitor from this data alone. Comparisons of the deleterious effects between these and other pairs of competitors to arrive at a conclusion.

Diffuse knapweed is successful in its novel range primarily because the organisms and conditions that prevent it from becoming invasive in its native environment are absent. It follows that the introduction of species from its native habitat would be an effective method of control. However, the introduction of a non-native organism has the potential to result in another invasive species outbreak. Therefore any method of biological control must be preceded by analysis of possible effects.

Future research

One of the greatest results from the experiments conducted with diffuse knapweed is the knowledge used to more effectively control its invasive populations. The losses, both economical and ecological, to diffuse knapweed are staggering [13]. The most important experiment that should be conducted on diffuse knapweed would compare the effectiveness of the many methods of control under varying conditions. This experiment would hopefully discover which methods of control are most effective under what conditions. This knowledge could be used to select the right method of control for a given condition thus more effectively and efficiently controlling diffuse knapweed.

Other potentially useful experiments include:

- A study of allelopathy in a field environment would be useful but would be difficult to carry out because of the lack of control [8]. However, if this experiment could be completed it would help to answer the debate over the effectiveness of allelopathy in a natural environment [27].

- A study comparing the diffuse knapweed plants in its native environment to those found in the novel environment in terms of quantity of allelochemicals produced, quantity of seeds, resource requirements and other aspects influencing its success. This experiment would identify if evolutionary changes have occurred in either of the diffuse knapweed populations.

- An experiment comparing the effects of enemy release on diffuse knapweed and the competitors in its novel environment. This research would tell if the effect of enemies is greater on exotic species, in this case diffuse knapweed, or native competitors. If enemy release had a greater effect on the native species then on diffuse knapweed it would provide additional evidence in support of the ERH [15].

References

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