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| Photo: Adam Gonnerman (CC BY 4.0) |
Introduction and Objective
Going into the initial survey of available literature for this paper, I did not have high expectations. While the field of agroforestry focuses on the integration of woody perennials into diverse types of agricultural practices, and autumn olive (Elaeagnus umbellata) is a type of shrub that has potential in such a system, its status as an ‘invasive species’ limits practical application. While peer-reviewed literature on the uses of autumn olive is limited generally, I found in particular that studies delving into the potential for this plant dropped off even more after the 1980s. One application in particular, that being the use of Elaeagnus umbellata as a nurse plant for black walnut trees, was effectively discontinued after 1989, only being picked up again more recently in this century, and then only in a cursory fashion.In this paper I will review the history and status of Elaeagnus umbellata along with research that explores the use of this shrub in agricultural production.
Taxonomy, Description and Natural History
Elaeagnus umbellata is a perennial shrub in the oleaster family, part of the order Rosales. It is native to parts of northeast Asia, including China, Korea, and Japan.It is also prevalent in Pakistan and northern India. In its native range it it is widespread and readily found in open and disturbed areas. In Japan, for example, it often colonizes riparian zones. It was brought to the United States in 1830, and through the 19th century and into the 20th it was planted to serve as windbreak and wildlife cover (Oliphant et al., 2016; Black et al., 2005). It has also been introduced in Europe and Australia (Pasiecznik & Rojas-Sandoval, 2008)E. umbellata was first introduced to the UK in 1829, one year prior to its introduction in the United States, and it was in this same general time period that it was also brought into continental Europe. In the US it has been employed in conservation efforts, as already indicated. State and federal agencies as well as private groups actively promoted it for erosion control and in strip mine reclamation projects. It’s also been marketed as an ornamental, although it is unclear whether this remains the case today (Pasiecznik & Rojas-Sandoval, 2008).
Description & Characteristics
Elaeagnus umbellata, as a perennial woody shrub, does not naturally take the form of a tree with a single trunk, although with proper pruning it can be trained in such a fashion with limited success. Without intervention it grows as as a large shrub, reaching 3.5 to 5 meters (11.5 to 16.4 feet) in height, and roughly up to 6 meters (almost 20 feet) in diameter (Black et al., 2005). Shrubs that reach this size do not lend themselves well to animal forage and need to be coppiced in order to provide tender, young shoots for livestock (Judy, 2017). The height and spread of individual specimens have been observed to inhibit the growth of other plants, including native varieties in reclamation areas (Oliphant et al., 2016). At the same time, this straining effect in competition for light has been shown to be beneficial to walnut trees in terms of fostering straighter trunks and better upward growth (Clark & Williams, 1979).
In northern India the shrub flowers in April, with fruiting taking place in July and August. In China it flowers between April and May and fruits in July to August. Meanwhile, in North America it fruits closer to August with the berries ripening in August/September. Fruit can remain on the plant well into late winter. One of the earliest plants to put out leaves in the spring, it is known in Illinois to break dormancy in mid-March (Pasiecznik & Rojas-Sandoval, 2008).
Referred to as ‘autumn olive’ in the eastern United States, a term that will be used interchangeably for the remainder of this paper with the scientific name Elaeagnus umbellata, it is known in other regions by such names as autumn elaegnus, asiatic oleaster, umbellate oleaster, aki-gumi and Japanese silverberry (Black et al., 2005). For marketing purposes, Dustin Kelly refers to the product as ‘autumn berries’ (Kelly, 2021).
The autumn olive has characteristics that make it particularly effective at colonizing otherwise inhospitable land. Although it responds to cold stratification, a winter season is not strictly necessary for it. Once it sprouts, E. umbellata grows quickly and is tolerant of both drought and salty winds. It can handle soil that ranges from alkaline to acid, and while it is not a legume, it does form nitrogen-fixing root structures in symbiosis with Frankia bacteria (Ahmad et al., 2008). This later characteristic means it can thrive even in poor, mineralized soil with little organic matter present. This was a key factor in the intentional use of this plant for reclamation of disturbed sites and in tests of autumn olive specimens as nurse plants in an agroforestry system (Black et al., 2005).
Humid temperate climates are ideal for E. umbellata and it is tolerant of a range of rainfall. However, while it is drought tolerant, this is not the case for dry periods lasting more than a season. High temperatures in the summer and very low temperatures in the winter pose little problem for autumn olive (Pasiecznik & Rojas-Sandoval, 2008).
Non-Native Range
Autumn olive enjoys a wide distribution in the eastern United States, from Maine all the way down to Florida, and west as far as Kansas. It can also be found in Ontario, Canada and even Hawaii. On the northern extreme it does appear to be limited by the extreme cold, although one cultivar has been identified as hardy to zone 6 (Munger, 2003).
As noted above, there are concerns that as an invasive with significant height and width it can keep down regeneration of native species, it is not known to colonize within woodlands. It favors the periphery of wooded areas as well as places that have been distrubed by deforestation, mining, and other human activity. It is only moderately shade tolerant, and thus is found most commonly in open canopy areas, fields, and pastures (Edgin & Ebinger, 2001; Munger, 2003). Research appears to indicate that it is unlikely to become an invasive understory species, given its shade intolerance (Brym et al., 2011), although there is potential for the shrub to establish itself in wooded areas where the loss of trees to emerald ash borer and dutch elm disease leave open canopy (Dornbos et al., 2016).
As noted above, autumn olive can thrive in riparian zones. These areas are naturally disturbance-prone and tend to provide the open space that the plant requires, and as such it dominates in many Japanese rivers (KOHRI et al., 2011).
This species is also spreading across Europe. France, Italy, Belgium, and the Azores have autumn olive listed as an invasive species. Oddly enough, it does not seem to be invasive in the United Kingdom. Though it was introduced in 1829, it wasn’t observed in the wild for nearly a century and a half afterwards. In mixed planting trials with black walnut in the UK it did not naturally regenerate (Pasiecznik & Rojas-Sandoval, 2008).
Tropical climates can be amenable to E. umbellata as well, as it is naturalized in Costa Rica. There is spreads in disturbed sites and along roadways (Pasiecznik & Rojas-Sandoval, 2008).
Invasive Qualities
From its arrival in the early 1800s, autumn olive has been used as an ornamental shrub and in reclamation projects, particularly on the sites of former mines in the Appalachians (Oliphant et al., 2016).
Means of Distribution
Autumn olive plants produce large numbers of fruit, and in North America these ripen between September and October. Birds in particular consume them through the winter and subsequently scatter the seeds through droppings. There are well-established populations throughout the eastern United States, largely due to intentional planting for conservation purposes followed by natural dispersal. The shrubs do not spread through roots, although once established they will grow back from the roots when cut down. Autumn olive is listed as a noxious weed in West Virginia, and in New Hampshire it is labeled as a prohibited invasive species (Black et al., 2005).
Disturbance of sites and the knock-on impact from that on the local ecology and available resources provides an opening for autumn olive to invade a region. This can be seen especially on Appalachian mine sites where soils are low in organic matter and nitrogen, being composed largely of rock fragments. As a nitrogen-fixer autumn olive has a strong competitive advantage over other plant species in such nutrient poor soils. The lack of competition leaves them ample room to flourish in such environments. The intentional propagation of these shrubs as part of mine site reclamation projects into the 1980s gave them a solid foothold in such regions. As already noted, prolific masting and wide dispersal by birds and other animals are key components of this plant’s success in spreading (Oliphant et al., 2016).
Impact on Ecosystem
Autumn olive can easily outcompete native plants on poor, disturbed soils. Putting out leaves in early spring, before many other plant species begin to leaf out, they obtain a head start on season growth (Pasiecznik & Rojas-Sandoval, 2008). Further, they have been observed overtopping young planted trees, limiting their access to light and slowing their growth. The shrubs at one post-mine location continued to dominate 4 years into a project to reforest the area with native trees. When established at such locations it spreads with relative ease to adjacent land, such as a pastures and unmanaged fields (Oliphant et al., 2016). Research has also indicated that the presence of autumn olive populations in riparian zones can result in increased nitrogen in runoff, which in turn alters the biochemistry of waterways (Singh et al., 2019). As is seen with artificial fertilizer, such nitrogen-shedding into waterways can result in adverse effects, such as algae blooms that deplete water of oxygen and other resources, choking out fish and fouling it for use by aquatic and non-aquatic species of plants and animals.
Management and Removal
Natural ManagementAs would be expected of any species in its place of origin, there are natural competitors and predators for E. umbellata. In its native range throughout northeast Asia, autumn olive is susceptible to attack from various species of fungi. In particular, the species-specific rust Aecidium elaeagni-umbellatae (a pathogenic fungi of the order Pucciniales) has been identified as one that attacks Elaeagnus umbellata in China (Huang et al., 2004). Aside from that, several other fungi have been found on specimens of the genus Elaeagnus. Over 20 arthropods have also been found to consume plants of this genus, including autumn olive (Zheng et al., 2006). It is possible that such fungi and other pests from the native range of E. umbellata could be used as biocontrol agents. However, the introduction of additional non-native species to North America opens the possibility that they too will be invasive, causing unexpected damage to native plants as well as agricultural crops, along with other unforeseen consequences.
Physical and Chemical Intervention
Different approaches for direct intervention to eliminate autumn olive are available, with varying degrees of effectiveness. While the chief of these will be reviewed here, it is important to bear in mind that such methods are difficult to apply at scale due to cost and logistical challenges (Oliphant et al., 2016).
Physical/Mechanical
The most straightforward method of removing autumn olive would appear to be mowing or cutting such plants. However, they resprout vigorously, so physical methods alone will probably not be effective. Repeated and routine cuttings are also ineffective without also treating stumps and resprouts with herbicide (Munger, 2003).
Hand pulling young seedlings can be effective, particularly in moist soil.This entails a great deal of literal manual labor and assumes that such seedlings can be located in sufficient numbers to make a difference while they are still young. While they put out leaves earlier than most native shrubs and can be fairly easy to identify in early spring in the United States, close examination of the ground is required to single out the smallest specimens (Pasiecznik & Rojas-Sandoval, 2008). Once they have a fair amount of growth it becomes untenable to remove them from the roots manually, at least not without enormous effort.
Fire
In prairies infested with E. umbellata and other invasive plant species a study was carried out over a period of 25 years. The prairie was expanded and the invasives removed through cutting and controlled burning. Bear in mind that prairie fires are a natural occurrence for which native species are largely prepared Thus, it may be appropriate in some circumstances to use such methods to control or entirely remove populations of autumn olive (Pasiecznik & Rojas-Sandoval, 2008). A similar method would not be advisable in wooded areas or near permanent structures where a conflagration can get out of control.
Chemicals
Among the herbicides that have been found useful in controlling populations of autumn olive are dicamba, glyphosate, 2,4-D, and triclopyr. Used alone or in combination these can be effective in foliar and bark applications, as well as on resprouts. Summer foliar herbicide application has been shown to be the most effective at achieving total mortality (Byrd et al., 2012). However, use of biocides has been found most effective only in small areas and with repetition over the course of five years, which is resource-intensive and can prove unaffordable to sustain. Over larger areas there has only been moderate success in controlling populations (Pasiecznik & Rojas-Sandoval, 2008), and again escalating cost becomes a concern at scale (Oliphant et al., 2016).
Uses and Benefits
OrnamentalAs ornamental plants that bear flowers and fruit, having silvery leaves, autumn olive can be seen as having a positive social impact in that sense (Pasiecznik & Rojas-Sandoval, 2008). Further, they attract birds in winter months for their berries, which also can be enjoyable for people to observe. The value of this shrub goes well beyond aesthetics, though.
Soil Quality
Nitrogen-fixing and tolerant to salt winds, autumn olive has been used in Japan for fixation of coastal sand dunes. It is also planted there along with black pine (Pinus thunbergii) to improve soil quality. In mountainous areas of that country it is used to secure eroded areas and promote further vegetation growth (Pasiecznik & Rojas-Sandoval, 2008). This shrub’s tolerance of poor soils that are unsuitable for other commercial crops makes them ideal for disturbed areas (Black et al., 2005). Autumn olive can hold soil in place along roadsides and in otherwise barren fields while also contributing significant amounts of nitrogen to soil, which in turn promotes the growth of other types of plants.
Nutrition
While having no real value for its wood, the fruit of the autumn olive is edible, and can be described is sweet to acidic. The red berries can be kept at room temperature for up to 15 days, and they are high in protein. In India a bush can yield about 650 grams of fruit in 2 to 3 pickings (Pasiecznik & Rojas-Sandoval, 2008). The berries of this plant offer an excellent source of nutrition, including vitamins A, C, and E, as well as flavonoids and essential fatty acids (Sabir & Riaz, 2005; Ahmad, 2006). Additionally, autumn olive berries have high antioxidant enzyme activities and nonenzyme components (Fordham & Wang, 2007). As such it can serve as a healthy supplemental food source for people in resource-poor regions.
Autumn olive is also being investigated for its potential use in pharmaceutical compounds, due to the berry being exceptionally high in the antioxidant carotenoid lycopene and several other carotenoids. While the human body does not produce lycopene, it is available from a handful of plant sources, chief of which in western diets is the tomato. Lycopene can also be found in watermelon, papaya, and pink grapefruit. Studies point to lycopene reducing the incidence and severity of prostate cancer, and it may also reduce the risk of a few other types of cancer as well as cardiovascular disease (Black et al., 2005; Sabir & Riaz, 2005) .
Human Consumption
Autumn olive can be cultivated for food, as should be readily apparent from the analysis above. Since the berries grow along the stems, the fastest way to harvest them manually is by running a hand along the stem and letting them fall into a basket held in the other hand. Although this brings some debris with it, it is arguably easier to clean that out than to pick one berry at a time (Pesaturo, 2019). In one of the studies reviewed for this paper a commercial blueberry-harvesting machine was used to securely reap the fruit, indicating that labor-intensive manual harvesting is not necessary where such machinery can be obtained. Potential markets include using the fruit to produce jam, syrup, or preserves. Fundamental to making products from the berries the preliminary process of boiling and mashing the berries, then running them through a food mill to remove the pits. With the now pitless mash it is possible to make products such as jams, jellies, and syrup (Pesaturo, 2019). It would also be possible to extract the lycopene for use as part of a dietary supplement (Black et al., 2005). On a personal note, I purchased a jam and a jelly produced using autumn berries (Kelly, 2021) and found them comparable to any other berry-based jams or jellies.
Wildlife Cover and Food
Ripening in late autumn, elaegnus umbellata is available to birds and other wildlife as a food source, and after leaf fall the red fruit become especially conspicuous, attracting more attention from wildlife in the winter months (KOHRI et al., 2011). Autumn olive was originally introduced and promoted by government agencies and private groups throughout the United States with the intention of attracting wildlife and securing areas against soil erosion, and it certainly serves for both of those purposes. Aside from birds, both migratory and residential, the fruit and at times the foliage is browsed by raccoons, skunks, possums, black bears, and deer (Pasiecznik & Rojas-Sandoval, 2008). With increased forage and cover, seasonal hunting opportunities will conceivably be improved.
Agroforestry Implementation
SilvopastureGreg Judy, a self-styled ‘regenerative rancher’ in mid-Missouri, has written favorably about the use of Elaeagnus umbellata to heal soil in pastures and to feed livestock (Judy, 2017). While he does not advocate for planting autumn olive, he has observed that on land that has been made poor through excessive haying, this shrub fosters new growth. Beneath the trees green grass grows, while just outside the dripline poor soil and inedible weeds are all that’s present. Rather than cut them all down and paint the stumps with herbicide, he now coppices every specimen in order to maintain the branches and leaves tender and within reach of livestock. Every part of the plant is non-toxic, and while cows can forage on them, pigs and goats in particular relish the autumn olive.Meanwhile, the nitrogen-fixing microbes in the soil thrive around this shrub, providing a means for the soil to come back to live, sustaining fescue, orchard grace, and legumes (the latter of which also in turn contribute to soil health as nitrogen-fixers.
Given its status as an invasive, it would be ill-advised to attempt sowing Elaeagnus umbellata with a view towards restoring soil health in pastures or as forage, but if they are already present, there is no immediate need to remove them if they are actively maintained, and they can be used to some advantage.
Food Forest and Forest Farming
Since 2011, Dustin Kelly has been foraging Autumn Olive berries around Urbana, Illinois for the purpose of making food products (Kelly, 2021). He sells jellies and jams created using what he refers to as ‘Autumn Berries’ online. While he does not intentionally plant them, he makes use of them to turn something of a profit. As they will not grow well in shaded areas, favoring instead disturbed, degraded locations with full or partial sunlight, they might not be strictly considered part of a ‘food forest’ as such. However, they do commonly grow in liminal spaces between field and forest, at the woodline. An astute forager out looking for mushrooms or mint will no doubt be able to locate these bright red berries and harvest an ample supply in the correct season. Further, their status as woody perennials at least puts autumn olives within the realm of agroforestry if they are actively managed as part of a larger system that includes annuals and possibly other woody perennials. Limited as invasives, as with silvopasture (or simply, ‘pasture’) applications, this is more of a matter of managing an existing population than promoting a new one.
Mixed Walnut/Autumn Berry Production
Research has been done on the use of E. umbellata as a nurse plant in walnut production. As a juglone-tolerant species that is also a nitrogen-fixer, autumn olive is capable of not only coexisting with walnut groves, but also providing essential nutrition that promotes walnut growth and production.
In 1967, black walnuts were planted at a site in Indiana, and two years later autumn olive was interplanted along with them. European alder and black locust were also planted there. In 1973 the black locust trees had to be removed because they were overtopping the black walnuts. Between 1975 and 1978, when the study concluded, the walnut trees grown with autumn olive were taller and broader than those with other co-plantings. Furthermore, walnuts grown with nurse trees (whether autumn olive or one of the other varieties) grew straighter and required less pruning than those in the control group (Clark & Williams, 1979).
Another test was carried out in the 1970s as well, in five plantations located in Missouri, Illinois, and Indiana. The results from this study demonstrated that walnut trees grown with autumn olive were taller after 9 years than those growing without the shrub. Two plantings in particular in Illinois showed a 134% increase in height among walnuts grown with autumn olive over those without. During the growing season, soil and air temperatures were lower, and soil moisture was lower as well, than among pure walnut stands. Meanwhile, there was markedly greater soil nitrogen among the mixed plantings (Funk et al., 1979).
Nitrogen concentration is not high solely in the soil around mixed walnut/autumn olive plantings. Nitrogen content is also much higher in walnut leaves from such mixed plantings when compared to walnut leaves growing on trees without autumn olive shrubs in the vicinity. And, it isn’t just higher nitrogen that can be found in such leaves, but also greater levels of phosphorous, potassium, calcium, and magnesium. This higher concentration of other minerals in walnut leaves from mixed plantings is likely due to the metabolic improvement brought about with higher levels of available nitrogen. The end result is a longer growth period in which better growth takes place (Ponder, 1983). Black walnut trees that benefit from weed control survive better and grow taller and wider than those without, and the addition of autumn olive plantings only increases the benefit of such treatment to walnut trees (Ponder, 1988).
While walnut is often grown in pure stands or as isolated specimens, perhaps due in part to the presence of juglones that suppress many other types of plants, the foregoing research demonstrates that walnut trees can benefit from mixed plantings in certain circumstances. During the first several years following planting growth is measurable enhanced among walnuts in mixed plantings with autumn olive. There is an improvement in microclimate, the walnuts are naturally trained to grow straighter to reach for the available light (a favorable competition with autumn olive), increased nitrogen availability, and an improvement in weed control. While autumn olive can crowd out other species, the walnut seems to actually do better in terms of consistent upward growth in response to the proximity of autumn olive plants that compete for light. Additionally, protection from the wind is observed to be provided by autumn olives encompassing walnut plantations. (Clark et al., 2008).
Research on the use of Elaeagnus umbellata in walnut groves appears to have been discontinued in the 1980s, likely with the identification of autumn olive as an invasive species and its designation as a noxious weed in some places. It has thus been largely ruled out as a potential nurse species (Pedlar et al., 2006). While its prolific reproduction cannot be ignored, along with the risk of spread to areas where such plants would not be desirable, it is possible to conceptualize a design that incorporates autumn olives with walnuts. Given that many other species are not juglone-tolerant, and that autumn olive works as a nitrogen fixer, a mixed planting of walnut and autumn olive for commercial production would be feasible. The primary crop would most certainly be the walnuts, as there exists a ready market for them. The fruit of the autumn olive, however, could also function as a fine secondary crop. It would be a niche product more likely to appear at farmers markets rather than to be delivered for industrial use, but as we saw above, jams and jellies are among the possible products derived from the berries.
Further, as wildlife cover and forage for wildlife, walnut groves with autumn olive plantings could promote an environment conducive to seasonal hunting.
Conclusion
EvaluationAs a designated invasive species in North America, E. Umbellata should not be actively cultivated in an agroforestry system where it is not already present. Managing an existing population, however, presents certain opportunities in agroforestry design that can take advantage of the hardiness of this species, with its nitrogen-fixing capacity and abundant berry production.
In a pasture setting, as we have seen, a managed existing population of autumn olive shrubs can provide food to livestock while also enriching degraded soil. Regular foraging and stomping by livestock will keep the shrubs under control under normal circumstances, although they can also be coppiced if the need arises to produce more tender and accessible shoots. If never allowed to go fully to seed this plant will not spread, as it does not propagate through shoots, but only through seed.
While planting autumn olive among walnut trees is unadvisable due to the tendency of E. umbellata to spread through seeding to other areas, it is not beyond the scope of reason to make use of an existing population of this shrub by planting walnuts alongside them. With proper cutting the autumn olive shrubs will not block sunlight to young walnuts, and with time their competition for light will benefit the quality of growth experienced by the walnut trees, as we have seen in this review.
Ideally, what we would want to see is the development of a cultivar that produces fewer berries while still providing the benefit of nitrogen fixation. One with no berries would be ideal, though of course it would have to be manually propagated in perpetuity through cuttings in other to maintain the benefit of its use. At the same time, removing the berries from the picture also takes away a key benefit to having this shrub, which is its nutritional value for wildlife and humans alike.
Final remarks
Further experimentation with cultivars that have less invasive qualities could produce a variety that is particularly useful in agroforestry applications. In the meantime, the pandora’s box has already been opened and the autumn olive is unlikely to disappear from North American landscapes due to the enormous expense and difficulty of complete eradication. Therefore, I conclude that it is in the best interests of agroforesters to learn to work with existing populations, exploiting them in ways that do not indulge this species’ propensity to reproduce.
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