Signs of Fall 3: Russian Olive!

Northern plains of Colorado

(Click to listen to an audio version of this blog!) …. Russian olive

We have done quite a bit of driving since we moved to Greeley a year ago. Greeley seems to be 40 or 50 miles away from almost anywhere interesting!  So we’ve logged some miles just getting from our home over to the foothills, or to the mountains, or to the Denver zoo or out to the airport.  We’ve also gone on two very long, road trips in the past year.  In May, we drove from Greeley to Seattle (and back), and in July we drove from Greeley to Glacier National Park in northwest Montana (and back).

So, over the past year, we have viewed our surroundings and the extended landscapes all the way to the northwest coast from the vantage point of our car windows for more hours than we would care to add up! Along with the occasional mountain and forest vistas, we have seen some amazingly open spaces: expansive, irrigated corn fields, golden wheat fields, green hay fields, dusty natural gas extraction fields, vast, open-pit mining sites, rangeland with grazing cattle, and even some remarkably pristine shortgrass prairie and sagebrush steppes.

In all of these flatland landscapes, one plant has almost always been present often on the edges of the open expanses. Any place where there is a chance of moisture accumulation (like in the un-mowed margins of irrigated fields, or along the banks of irrigation ditches or almost any stream) there are dense patches or long, thin lines of grey-green, shrubby trees that are often punctuated by tall, dark-green cottonwoods or thickets of light-green willows. These silvery, short trees are Russian olives!

Russian olive. Photo by Famartin, Wikimedia Commons

Russian olive (Elaeagnus angustifidia) is also known as “oleaster,” or “wild olive,” or “silver berry.” It is an exotic invasive plant originally from southern Europe and central and western Asia that was introduced to North America primarily as an ornamental tree in the late 1800’s. It was subsequently planted extensively across the central United States as a windbreak component of government sponsored soil reclamation and erosion control projects in the 1930’s and 1940’s. It is now found all across North America but is especially abundant in central and western states particularly across the Great Basin and along the streams of the Great Plains.

A number of states (including California, Nebraska, Wisconsin and Wyoming) have classified Russian olive as “invasive” (i.e. an alien plant that can establish itself and spread extensively in an ecosystem in which it is not native). Several other states (including New Mexico, Utah and Colorado) have declared Russian olive to be a “noxious” weed (i.e. an invasive plant that is injurious to public health, agriculture, recreation, wildlife or property). The “noxious” designation allows authorities to quarantine the plant and take significant steps to eradicate it from their ecosystems. In Colorado, it is feared that Russian olive will soon become the dominant tree of the state’s riparian forests outcompeting and excluding native willows and cottonwoods.

Why has Russian olive become so perniciously invasive? The reasons are written into its biology and ecology!

Russian olive trees, I. Beal, Flickr

Russian olive can grow in almost any type of soil and thrives even when the soil it is very alkaline or heavily saline. Once established, a Russian olive seedling or sapling has a very high rate of survival (in one study, Russian olive trees planted on the Great Plains had a 96% survival rate). Russian olives grow best in relative moist soils and are especially found in true wetlands, wetland meadows, seasonally moist habitats, along riverbanks, lake and ocean shores, fence rows, roadsides, and irrigation ditches, but they also can become established in the moist, shady microhabitat under dense stands of native trees. For example, on dry terraces above rivers across the Great Plains, if there had been a standing cottonwood forest, the forest floor conditions greatly favor the establishment of invasive Russian olives.

Russian olives grow from sea level to elevations over 8000 feet, and tolerate an incredibly broad range of environmental temperatures (from -50 degrees F to 115 degrees F). Seedlings are both shade tolerant and sun tolerant (if they have sufficiently high levels of soil moisture). Russian olive is also fire resistant and is not extensively grazed upon by native animals or livestock. Also, beaver do not readily take Russian olive trunks or branches for their dams.

Russian olives produce large numbers of very tough, highly resistant seeds that can persist (and accumulate) in an ecosystem for years before germination. These seeds are in fruit that are consumed by a large number of birds and mammals, and passage of the seeds through the digestive systems of these consumers stimulates their germination.

European starling. Photo by D. Daniels, Wikimedia Commons

In a study in Oregon, large numbers of European starlings were observed eating Russian olive fruit during the day. These birds then night-roosted in the branches of larger trees (like black cottonwoods and Siberian elms) and defecated their seed enriched feces onto the forest floor beneath them. The resultant Russian olive seedlings then grew rapidly in the moist, well shaded microhabitat under the cottonwoods and elms eventually choking out the previously established, native trees.

Russian olives can also spread via root shoots and stump and stem sprouts. Their roots can penetrate up to forty feet into their soil profile and can rapidly spread across a significant area via well-developed laterals. Under good conditions a Russian olive seedling can grow 4 or 5 feet in its first year and reach 8 to 12 feet in height after its second.

Russian olive roots also make symbiotic connections with nitrogen fixing bacteria (Frankia species) thus providing the trees with a readily available source of nitrogen. Colonization of disturbed habitats (a frequent strategy of Russian olives) is greatly facilitated by the tree’s ability to generate its own nitrogen.

Russian olive flower. Photo by Lazaregagnidze

Flowering (an early to mid-summer event) and fruit production begin, on average, when the trees are seven years old. Honeybees pollinate the Russian olives and use the gathered nectar and pollen to generate honey. Fruit from the pollinated flowers matures from August to October

Stands of Russian olives form dense canopies and  generate significant areas of deep shade. Considering the intense summer sun and extreme heat in the Great Plains and Great Basin habitats favored by this tree, one would expect this shaded habitat to be highly favorable for wildlife. Studies, however, have shown that this potential benefit is far outweighed by the negative impact of the Russian olive on native plants, and that areas with significant Russian olive growth actually support a lower abundance and reduced richness of animal species.

So why are the Russian olives so abundant and so ubiquitous out here in the west? They can grow anywhere, especially anywhere that is moist. They can grow from seeds that are widely dispersed by birds and mammals, or from their rapidly spreading roots. They can grow in the full sun (if there is sufficient moisture), or in the shade often under native trees. They generate their own nitrogen via symbiotic, root bacteria. They can out-compete taller trees (like cottonwoods, for example) by growing rapidly up under their protective shade and by generating even deeper shade, thus preventing these less shade-tolerant tree species from effectively reproducing.

They seem designed to do nothing but grow and thrive! Watch for the stands of silvery, green leaves from the windows of your car as you drive past our busy landscapes! There is so much going on out there!