Sahara mustard (Brassica tournefortii) is one of the top non-native invasive plants of concern for land managers in Clark County, Nevada. It is regarded as one of the most invasive wildland pest plants in the Mojave Desert and has been targeted for control by the Clark County Cooperative Weed Management Area, the Bureau of Land Management Las Vegas Field Office (BLM), and the Lake Mead National Recreation Area (NPS). This plant is capable of dominating entire desert landscapes throughout Southern Nevada if no control actions are taken. If uncontrolled, this plant could
• displace native wildflowers common in the springtime that enhance visitor enjoyment of public lands and provide valuable forage for the desert tortoise;
• destroy habitat for rare species, making them vulnerable to becoming federally listed; and
• form dense thickets that could also increase the frequency, intensity, and size of desert fires making the weed more difficult to control and increasing the threat to native plant communities, the desert tortoise, and other wildlife.
Sahara mustard typically establishes large populations first in highly disturbed areas (e.g. roadsides, below the high-water mark at Lake Mead, gravel/borrow pits, old agricultural fields), from which it spreads into wildland areas (e.g. rangeland, wilderness, desert tortoise habitat). This species is presently abundant in some highly disturbed sites, but does not occur in, or is only in the early stages of invading, most wildland areas in southern Nevada. Federal land management agencies must act immediately to control this species before it becomes too widespread and abundant to effectively manage.
Sahara mustard is an annual plant, which depends on seeds to survive from year to year, and to disperse to and colonize new sites. Accordingly, effective management of Sahara mustard requires a two-part approach: (1) reducing seed production in large established populations; (2) and eradication of small new populations soon after they appear and before a significant soil seedbank can establish. Currently, highly disturbed areas contain both large established and small new populations of Sahara mustard, whereas most wildland areas contain only small new populations.
There is likely to be no single control method for Sahara mustard that is appropriate for all situations. Large populations require control methods that can be cost-effectively applied over large areas. However, these large-scale control methods may have undesirable side-effects on non-target native plants. Accordingly, it may be most feasible to control large populations in highly disturbed areas that have low native plant cover and diversity, and low conservation value in general. In contrast, small populations can be controlled with more precise methods, which may be necessary in wildland areas where minimizing collateral damage to native plants is often a priority.
The ultimate purpose of control efforts is to reduce or eradicate populations of Sahara mustard, while maintaining or increasing the abundance and diversity of native annual plants. It is not hard to kill individual plants, either by pulling them from the ground or spraying them with herbicide. However, killing plants often have other undesirable side-effects. For example, if control methods cause soil disturbance, germination rates may increase in the soil seedbank, thus increasing Sahara mustard populations during subsequent years (Brooks unpublished data). Also, control strategies that only kill a fraction of the germinated cohort may allow surviving plants to thrive and produce more net seeds per unit area than in un-thinned areas (Trader and Brooks in prep). Thus, the ideal control method for Sahara mustard would be one which significantly reduces seed production of the treated cohort, does not improve site conditions for future generations, and has a net positive effect on the abundance and diversity of native plants.
In this study we will evaluate the effects of control methods for Sahara mustard appropriate for use in both highly disturbed and wildland areas. First, we will conduct small-scale herbicide trials to identify the most effective herbicide types, application rates, and application timings for reducing populations of Sahara mustard. Second, we will conduct large-scale comprehensive trials to compare the effectiveness of the herbicide treatment we identify in the first experiment, with other mechanical, heat, and cultural treatments. This second experiment will also evaluate the net effects of various treatments on the abundance and diversity of native plants.
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