E. Malinich, N. Lynn-Bell & P.S. Kourtev. 2017. “The effect of the invasive Elaeagnus umbellata on soil microbial communities depends on proximity of soils to plants.” Ecosphere 8(5):e01827. doi: 10.1002/ecs2.1827
Invasive plants plague many parts of the US, from roadside environments to natural ecosystems. Research on one invasive plant, the autumn olive (Eleagnus umbellata), indicates that the soil microbial community changes based on proximity to the plant. Long-term changes in soil microbial communities might negatively impact restoration efforts.
Why invasive species are a problem
An invasive species is a plant, animal or fungus that is introduced to a native ecosystem, and subsequently spreads and proliferates to the point where it interferes with normal ecological processes in that system. Some invasive plants you may be familiar with include kudzu (Pueraria spp.; widespread throughout the southeastern US), Japanese knotweed (Fallopia japonica; often found along roadsides or in disturbed areas throughout the US and parts of Europe), phragmites (some strains of Phragmites australis; often invades wetlands and saltmarshes) and purple loosestrife (Lythrum salicaria; disrupts waterflow and crowds out native wetland plants).
Some invasive plants originated as ornamental plants, and some, like kudzu and autumn olive (Eleagnus umbellata) were widely planted by farmers in the past, based on recommendations by the USDA to prevent erosion and other problems. However, after introduction these plants are often classified as noxious weeds, and some states now restrict their import or planting to prevent them from spreading into more areas and threatening more ecosystems.
Why are invasive plants considered to be such a problem? According to Wikipedia, common traits of invasive species include: “Fast growth, rapid reproduction, high dispersal ability, phenotypic plasticity (the ability to alter growth form to suit current conditions), tolerance of a wide range of environmental conditions (ecological competence), [and] association with humans and prior successful invasions”. This suite of characteristics allows some individuals from the established invasive species to out-compete neighboring native species and grow into large monocultures (many identical plants growing in close proximity; picture a cornfield). These monocultures can disrupt important ecosystem functions (e.g. providing habitat and food to native wildlife, preserving water flow and quality, promoting diversity). Unfortunately, once invasives become established in an ecosystem, it is very difficult and expensive to remove them and restore the ecosystem to its “pristine”, pre-invaded state.
Soil: It’s not just dirt
Most elementary school children learn that plants need sunlight, air, water and soil to grow. In nature, soil is critical to healthy plant growth, since it serves to anchor plants and holds the water critical for plants to grow. Soil also contains all the necessary nutrients plants need to thrive – this includes the big three (nitrogen, phosphorus and potassium – the N-P-K on fertilizer bags) and other trace nutrients like calcium, sulfur and magnesium. But have you ever wondered where those nutrients come from in natural systems?
Since plants in natural environments grow perfectly happily without the additions of synthetic fertilizers, these nutrients must come from somewhere. The answer: the soil microbial community. The soil microbial community is what scientists call all the microscopic organisms found in soil, including fungi, tiny worm-like nematodes, small single-celled eukaryotic organisms (like amoebas) and bacteria. In fact, soil is teeming with life – a teaspoon of soil contains more organisms than there are people on earth! These microbes are critical in recycling dead plant matter (fallen leaves, dead roots) and other soil microbes into available nutrients. Without the soil microbial community, plants can’t grow. Dramatic changes to the soil microbial community can have long-term consequences for the plants in an ecosystem. If you’re interested in learning more about life in soil, check out this great infographic by the FAO!
Autumn olive (Eleagnus umbellata) – a common invasive in the Northeastern and Midwestern US
Autumn olive is a good example of an invasive plant, as it grows readily across much of the northern US. Autumn olives are readily recognized by their two-tone leaves (green on top, silvery on the underside), and their fragrant white-yellow blossoms which develop into red fruits speckled with small silver spots (see Figure 3 & Figure 4). These fruits are actually edible, though quite tart. They make a tasty jam (see this recipe), which might be one of the few upsides of this proliferous shrub.
According to Malinich et al., “Autumn olive is a highly invasive woody shrub that has the potential to affect both soil nutrient cycles and microbial communities as either a consequence or mechanism of invasion. Autumn olive was introduced to the United States from Eurasia in the 1800s. Since then, the shrub has been labeled a noxious pest and its growth has been prohibited in four states. Autumn olive grows on a wide range of soils, but is especially competitive in nutrient-poor, disturbed sites.” One reason autumn olive plants are able to thrive in nutrient-poor soils is their ability to form a symbiotic relationship with a species of bacteria (Frankia spp.) that is able to fix atmospheric nitrogen gas into a plant-usable form (peas and beans do something similar).
Research shows: Autumn olive influences the soil around it
A study by Malinich et al. published in Ecosphere this year begins by reminding us that previous studies have shown that invasive plants alter the microbial community and chemistry of soils, which can affect aboveground plant growth and ecosystem function. Healthy ecosystems need healthy soils, but if soils are altered by invasive plant action, the ecosystem may no longer be able to function normally. Malinich et al. decided to study autumn olive because previous research has shown that woody shrubs like autumn olive don’t grow in dense monocultures like other invasives (e.g. phragmites or kudzu) do. This makes autumn olive a great candidate on which to conduct research to investigate the effects of non-uniformly distributed autumn olive plants on the surrounding soil microbial community.
In this experiment, the authors made grids in autumn olive-growing areas and collected soil samples in each square of the grid. The research question the authors were attempting to answer was: Is the composition and function of the soil microbial community affected by proximity to autumn olive, or by local density of autumn olive?
To answer this question, researchers extracted DNA from soil samples, and then used special molecular tools to compare the soil communities from different soil samples. One tool researchers used in this study is a “denaturing gradient gel” (DGGE), where copied DNA from each sample is loaded into small slots in the special gel. An electric current is applied to the gel and differently sized DNA pieces separate, creating bands in the gel. The patterns of the bands in the DGGE gel allowed the authors to compare the microbial community compositions among soil samples.
After analyzing the results, the researchers concluded that autumn olive clearly had an effect on the composition of soil microbial communities, specifically that how close a sample was to an autumn olive plant was related to changes the composition of the soil microbial community. However, this effect is localized – samples taken farther away from plants show less influence on the microbial community composition. The authors conclude that their findings suggest that even a few autumn olive plants can have a measurable effect on microbial communities in the soil. The researchers did note that while the community composition appears to change in soils closest to autumn olive plants, the community’s function does not change measurably.
Other invasive plants have also been shown to change soil communities. The authors mention that perhaps this special strategy is partly the reason invasive plants are so successful. For example, invasive plants may be able to foster special communities of soil microbes that make soil conditions optimal for the invasives, to the detriment of native plants. However, researchers don’t know exactly how or to what extent these plants are able to change the microbes in the soil – something that could be critical to understand when trying to restore invaded ecosystems to their former state. Often, autumn olive management means cutting down the shrubs and spraying any remaining stumps with herbicides to kill the plants’ below-ground components. However, the root system is then still in place, and could still impact the soil community and affect soils for decades after removal. This study suggests that changes to the soil microbial community composition could potentially remain long after invasive plants have been removed, possibly affecting ecosystems and restoration efforts for quite some time.
It is not particularly clear what can be done about this issue, but hopefully more future research on ecosystem restoration efforts will focus on changes in soil communities over time, in addition to common aboveground measures of ecosystem function. Future research might consider whether “seeding” invaded soil under restoration with small cores of soil taken from healthy ecosystems could support native plant establishment and growth. This type of information can have a big impact on future restoration efforts, so that we can effectively remove invasive plants and their legacies from our natural ecosystems. Restoring ecosystems is critically important to support native wildlife and insects.