Russian Olive Fruit Production in Shelterbelt and Riparian Populations in Montana

Russian olive (Elaeagnus angustifolia) became a common ornamental plant in the southwestern United States in the early part of the 20^th century and escaped cultivation in all southwestern U.S. states by the early 1950s (Stannard et al. 2002). Russian olive was introduced in the 1930s in the Great Plains of the U.S. for soil conservation. Few native trees are found in open, windswept areas of the northern Great Plains, and planted Russian olive windbreak populations provide shelter to humans and livestock. However, Russian olive is invasive in riparian areas throughout the western United States (Nagler et al. 2011). Riparian populations of Russian olive prevent recreational and agricultural use of riparian areas and threaten native populations of cottonwood and willow trees (Lesica and Miles 2001). These nitrogen-fixing trees have the potential for cascading, ecosystem-wide negative impacts, including altered beaver population dynamics, restructured food webs in invaded watersheds, and altered nitrogen cycling in both local terrestrial and aquatic habitats (Lesica and Miles 1999, Pearce and Smith 2001, Mineau et al. 2011). The effectiveness and cost of invasive species control depends on the spatial distribution of populations (Richardson and Bond 1991, Epanchin-Neill and Hastings 2010) and persistence of biological control organisms is deeply affected by resource availability (De Clerck-Floate and Bourchier 2000). Therefore, differences between shelterbelt and invasive riparian populations may affect approaches to population removal and control of expansion. Here we summarize the different demographic characteristics of desirable shelterbelt populations and invasive riparian populations of this species in order to make predictions about costs and efficacy of control.

Table 1.

Features that distinguish shelterbelt from riparian Russian olive populations.

Planted shelterbelt populations of Russian olive are even-aged and rarely spread into immediately adjacent croplands and rangelands (Stannard et al. 2002). Invasive riparian populations are spreading along riparian corridors; few limits to expansion of these populations have been found (Lesica and Miles 1999, Nagler et al. 2010). Less well-studied are the populations of Russian olive that do not fit into the planted-windbreak invasive-riparian dichotomy: planted riparian populations and opportunist populations in drainages and wet pastures. Planted populations along rivers share most characteristics with invasive riparian populations (Table 1) and often blend with them. Opportunist populations are usually small and isolated.

Russian olive has prolific fruit production, with thousands of oblong fruits per tree late in the fall (Katz and Shafroth 2003). These fruits either remain on the trees over winter or are dropped. Fruits may be vertebrate- or water- dispersed and seeds are long-lived. Trees initiate fruit production after four years of growth but do not reliably produce fruit until they are ten years of age (Lesica and Miles 1999). Trees also stump- and root- sprout after fire, mechanical injury, or other disturbances.

Planted shelterbelt populations are relatively small (tens to hundreds of trees) and isolated, as plantings tend to be near farmsteads and settlements (Table 1). These populations are even-aged, as they are generally planted in a single-entry occurrence with little natural regeneration. While Russian olive trees in these populations rarely expand beyond planted population boundaries (Stannard et al. 2002), the distance of seed movement from these populations is unknown. There are no published data on the spatial distribution of shelterbelt populations in Montana.

Riparian populations are large, connected (sometimes isolated), with mixed age structure (Table 1). A spatial data set of Russian olive patches along the Yellowstone River (Combs and Potter 2011) reveals no stretches with Russian olive trees absent at the 5 km scale between Fairview and Miles City, MT (> 200 km). In contrast, Lesica and Miles (1999) mapped the number of Russian olive trees in 5 km segments along the Milk River in Montana. They found long stretches of river without any mature trees (≥ 25 km), however, around settlements tree numbers increased and remained well above zero (~5–100) for long distances (≥ 70 km). The Milk River is a tributary of the Missouri River with a median volume of 20,000 acre-feet, and...