Halting the Spread of Japanese Knotweed

           Wetland ecosystems are some of our most diverse, most valuable, and most threatened natural systems. They offer a variety of ecosystem services, including water purification, erosion protection, and habitat for a wide variety of species. Though they are under attack from a variety of angles, one force in particular is especially aggressive and poorly understood: invasive plants. Invasive plants often lack natural enemies, diseases, and herbivores, and are very strong competitors, pushing native plants aside and taking over large swathes of natural territory. Of particular concern to riparian ecosystems is Japanese Knotweed (Fallopia japonica, European name or Polygonum cuspidatum, North American name). (Stone, 2010) After being introduced as an ornamental plant, Japanese knotweed escaped cultivation and is now present across much of the United States, Canada, and Europe. (Stone 2010, Hollingsworth 2000) It is a notorious invasive plant that is able to spread rapidly through riparian ecosystems, completely replacing native vegetation with a tall, thick monoculture that harbors little biodiversity.

            It is crucial that resources are devoted to studying this threatening plant, if there is to be any hope for the ecosystems that it invades. In 2000, Michelle Hollingsworth published a paper postulating that all of the Japanese Knotweed plants in Britain were clones of a single progenitor. (Hollingsworth, 2000) Though it was later proven that at least some groups of knotweed are fertile, the majority of knotweed spread does take place through clonal propagation, with a new colony being able to grow from a single stem. (Aguilera et al., 2009) Japanese knotweed drastically reduces the biodiversity of habitats that it invades and, as Aguilera et al. state, plots of native vegetation have been observed as having anywhere from 1.6 to 10 times more diversity than stands of Japanese Knotweed. (Aguilera et al., 2009)

This reduced diversity also impacts local animal communities, as this invasive plant does not host many herbivores in its new environment. A study by John C. Maerz et al. revealed that frogs show reduced foraging success in stands of Japanese knotweed when compared to adjacent stands of native vegetation. This is likely due to the fact that knotweed does not host many native arthropods, which help form the base of local food webs. With such organisms absent, much of the animal diversity in the area is lost, in addition to the plants that were overwhelmed by the knotweed. (Maerz et al. 2004) Japanese knotweed has a strong advantage that makes it a successful competitor: in addition to its capacity for rapid clonal growth, it has a large network of deep underground rhizomes that allow for nutrients to be transferred from one area of the colony to another where they are most needed. It was found that severing these rhizomes while leaving the rest of the plants intact significantly impaired their growth. (Aguilera et al., 2009) Finally, Lecerf et al. have proven that Japanese Knotweed is capable of altering ecosystem structures in the streams it borders by selecting for new assemblages of species that are capable of breaking down and utilizing its leaf litter. (Lecerf et al. 2007)

            Past research has clearly established the negative effects of Japanese knotweed on valuable native ecosystems. It has also been proven to be a powerful invasive species, with a variety of adaptations conducive to rapid spread. In the future, control methods for this species must be found. In a recent study, Shaw et al. outlined the process and impacts of implementing biological control methods in the UK, and discussed how such methods could be used as examples for other members of the European Union. Through observation and experimentation with a variety of fungi and insects from the knotweed’s native range, it was found that a species of psyllid, Aphalara itadori might serve as a suitable biological control agent if it were released. (Shaw et al. 2011) Since this herbivorous insect has coevolved with the knotweed in its native habitat, it has an affinity for eating knotweed leaves and greatly weakening the plant. Further research of this nature must be conducted.

In order to halt the spread of this plant, it is essential that we pursue a multitude of options for its control and eventual eradication. A variety of studies should be undertaken: more can be learned about the reproductive and vegetative systems of the plant, to better understand how it spreads, where it uses nutrients the most, and which nutrients would prove most limiting. By targeting a key reproductive process, or by limiting the influx of important nutrients, we could slow or halt the spread of this plant. In addition to that, learning how the plant distributes water and nutrients within itself could lead to more effective herbicide development and application techniques. If indeed nutrients are shared throughout a colony via rhizomes, then perhaps a specific herbicide could be rapidly shared in the same way. Greater study into the genetics and reproductive strategies of the plant are warranted; if it is true that most of the knotweed present are clones of a single organisms, then they could be vulnerable to a specific biological control agent or disease. Monocultures have been known to be susceptible to a particular pathogen or pest in many historical cases. Better understanding of the plant’s vasculature and rhizomes might lead to more efficient ways to control the plant by hand, if a certain vulnerable point could be found. Finally, more investigation into biological control agents is needed. The plant does have natural enemies, as has been proven in previous studies. The highest priority should be placed on determining which of these natural enemies would be sufficient to control or eliminate the weed, while at the same time being safe and specific enough for release into our environment. Japanese Knotweed has been proven in many studies to spread rapidly, degrade the ecosystems it invades, and drastically reduce biodiversity. Future studies designed to find its specific weaknesses will be key in halting its spread and preserving some of our most valuable wetland ecosystems.  

Works Cited:

Aguilera, A., Alpert, P., Dukes, J., & Harrington, R. (2009). Impacts of the invasive plant Fallopia japonica (Houtt.) on plant communities and ecosystem processes. Biological Invasions, 12(5), 1243-1252. doi:10.1007/s10530-009-9543-z

Hollingsworth, M., & Bailey, J. (2000). Evidence for massive clonal growth in the invasive weed Fallopia japonica (Japanese Knotweed). Botanical Journal of the Linnean Society, 133(4), 463-472. doi:10.1006/bojL2000.0359

Lecerf, A., Patfield, D., Boiche, A., Riipinen, M., Chauvet, E., & Dobson, M. (2007). Stream ecosystems respond to riparian invasion by Japanese knotweed (Fallopia japonica). Canadian Journal of Fisheries and Aquatics Sciences, 64, 1273-1283. doi:10.1139/F07-092

Maerz, J., Blossey, B., & Nuzzo, V. (2004). Green frogs show reduced foraging success in habitats invaded by Japanese knotweed. Biodiversity and Conservation, 14, 2901-2911. doi:10.1007/s10531-004-0223-0

Shaw, R., Tanner, R., Djeddour, D., & Cortat, G. (2011). Classical biological control of Fallopia japonica in the United Kingdom - lessons for Europe. Weed Research, 51, 552-558. doi:10.1111/j.1365-3180.2011.00880.x

Stone, K. (2010). Polygonum sachalinense, P. cuspidatum, P. × bohemicum. Retrieved November 12, 2015, from http://www.fs.fed.us/database/feis/plants/forb/polspp/all.html

Plant Microbiomes May Improve Crop Yields

A key question that needs to be addressed by a variety of disciplines in the near future is how to feed the world’s rapidly growing population. In many parts of the world, agriculture is struggling to keep pace with an increasing population that is consistently devoting a smaller fraction of its time to food production. A promising area of research is attempting to help solve this problem: plant microbiomes. Just as humans have communities of microbes in our mouths, intestines, and many other parts of our bodies that perform essential functions on our behalf, so too do plants have an intricate community of fungi and bacteria that live in, on, and around their root system in an area called the rhizosphere. Much like the biosphere is the area around the surface of the Earth on which life can persist, the rhizosphere is the area on and around a plant’s root that hosts an especially diverse community of microbes.

Understanding how soil microbiomes take shape will be invaluable to future crop breeders. Manipulating the rhizosphere by crop scientists has the potential to improve plant health, increase yields, decrease needs for additional fertilizers, reduce diseases, and improve the overall health of the soil. A team of researchers recently explored this topic [1]. HilleRisLambers et al. put forward different theories for community assembly, and in addition to helping us understand plant microbiomes this research has the potential to benefit the entire discipline of ecology by helping determine what factors affect assembly of new communities. Veresoglou et al. conducted an exploration of microbial roles in, and fungal influences on, nitrogen fixation. They showed how much there is still to learn about what exact roles soil organisms play in the nitrogen cycle, and how different groups usually benefit associated plants nearby by competing with, limiting, or enhancing each other [2]. In addition, Bakker et al. synthesized information such as that presented above and explained how such knowledge will benefit crop breeders. They noted that crops may be improved by taking advantage of the many services provided by microbes, which include hormone production, improved nutrient uptake, enhanced stress tolerance, facilitation of plant immunity, and alteration of plant functional traits and tissue chemistry. [3]

An important question is how rhizospheres of different field crops across the country are related to each other, and upon what those relationships are based [4]. Is soil type the main factor, or does taxonomy play a larger role? What about geographic location? To determine the answer, researchers led by Jason Peiffer conducted a study in which they grew 26 different maize (corn) genotypes in 5 separate fields: one in Illinois, one in Missouri, and three in towns on the shore of Cayuga Lake, including Ithaca. They compared the bacterial composition of the rhizosphere to that of the nearby bulk soil for each of the plots, and, using sequencing of specific genes, they also examined the taxonomic composition of each of the different maize cultivars’ rhizospheres.

Peiffer et al. found that soils from the three New York plots had the most groups of related microbial taxa, despite the fact that the soils each had very different physical and chemical characteristics, leading the researchers to speculate that climate has a strong influence on the assembly of microbes present in the soil. They also noted that the microbes of the rhizosphere exhibited a great deal more taxonomic variety than those in the nearby bulk soil. Finally, they concluded that the particular cultivar of corn being grown had a small, but significant, effect on the microbial community around each type of plant. The researchers admitted that associations between cultivar and microbiome had been found to be stronger during an earlier greenhouse experiment, and that naturally occurring variation in field experiments, including soil type and preexisting communities, may dull the observed effects of relationships between crop and microbial genotypes. [4] The fact that there is an association between rhizosphere microbiota and cultivar type will have important implications for future work in the field, if crop managers can take a cultivar’s ideal microbiome into account and seek to promote the best mix of microbes in and around the plant and its soil.

Future research will likely continue to explore the relationship between different genotypic varieties of plants and the microbiomes that grow along with them. The interactions between different microbes in the soil, and especially the influences that myccorhizal fungi and nearby bacteria have on each other, should be explored more in depth to determine if nutrients, such as nitrogen, can be incorporated into the soil more quickly through natural means, hopefully reducing our strong dependence on artificial fertilizers. The assembly of microbial communities, their effects on their plant hosts, and our ability to influence them, should all be studied in further depth as well.

The ecological implications of these studies are quite significant. Our supplies of artificially fixed fertilizers, fossil fuels, arable land, and healthy soil are all limited. As our population continues to grow and a large percentage of it remains food insecure, it is vital that we develop sustainable agriculture solutions to make the most efficient use of our diminishing resources. Just as human microbiomes have been found to have many far-reaching affects on our health in the last few years, so too have plant microbiomes been found to provide a great many services to their hosts, from nutrient fixation and absorption to disease resistance. These studies and others like them seek to better understand the plant microbiome so that we can foster the development of healthier, more productive, and more sustainable crops in the future.

Works Cited:

[1] HilleRisLambers, J., Adler, P., Harpole, W., Levine, J., & Mayfield, M. (2012). Rethinking Community Assembly through the Lens of Coexistence Theory. Annual Review of Ecology, Evolution, and Systematics, 43, 227-248. doi:10.1146/annurev-ecolsys-110411-160411

[2] Veresoglou, S., Chen, B., & Rillig, M. (2011). Arbuscular mycorrhiza and soil nitrogen cycling. Soil Biology and Biochemistry, 46, 53-62. Retrieved October 19, 2015, from www.elsevier.com/locate/soilbio

[3] Bakker, M., Manter, D., Sheflin, A., Weir, T., & Vivanco, J. (2012). Harnessing the rhizosphere microbiome through plant breeding and agricultural management. Plant and Soil, 360(1), 1-13. doi:10.1007/s11104-012-1361-x

[4] Peiffer, J., Spor, A., Koren, O., Jin, Z., Tringe, S., Dangl, J., . . . Ley, R. (2013). Diversity and heritability of the maize rhizosphere microbiome under field conditions. PNAS, 110(16), 6548–6553-6548–6553. doi:10.1073/pnas.1302837110