As with site effects, year effects are likely to be a complex array of interacting differences, including differences in rainfall, temperatures, weed challenges and herbivores. Our watering partly offsets the difference between the long-term mean and the rainfall lost during the fall drought of 2011/12, and the result was a priority effect much more similar to 2008 than unwatered 2011/12 . These results strongly suggest that much of the differences in results between these 2 years can reasonably be attributed to differences in fall rainfall . The three-way interaction arose because the effect of watering on priority effects differed significantly across sites. Early-season watering during a dry period strongly reduced the benefits of short-term priority for native perennial grasses over exotic annual grasses at Davis, moderately at Hopland and not at all at McLaughlin. We suggest again that the McLaughlin result was related to the overall lower growth at this cooler site, which may have reduced both competition and priority effects, and if so also reduced the effect of watering on this priority effect. These three sites were chosen to represent characteristic settings for grassland restoration in our area, namely former agricultural plots on alluvial clay loams . All three are in interior northern California plant communities within 125 km of each other on similar soils, growing rack and the three climates are within the range of projections for medium-term climate change. Recommended native seed mixes for all three are the same except for provenances .
Yet the differences across sites were enough to produce widely divergent covers of restored native grasses and widely divergent responses to our experimental manipulations.Scotch broom , French broom , Spanish broom , and Portuguese, or striated, broom were introduced into California in the mid-1800s for landscape planting, mine tailings stabilization, and roadside erosion control. The abundant bright yellow flowers were used medicinally, and the stems were cut for broom-making materials. The traits that were once considered useful and desirable are the same attributes that define brooms as aggressive and invasive noxious weeds. These traits include their tolerance for most soil conditions, ability to fix nitrogen and grow for most of the year, and copious production of long-lived seed. These attractive attributes for erosion control make the plants difficult to manage where erosion control is not a goal. Brooms grow rapidly and form dense stands that are inaccessible to and unpalatable to wildlife. The dense stems make regeneration of most other species difficult or impossible, and they also create a dangerous fire hazard. As plants grow, the inner stems die back, providing a highly flammable fuel. The California State Department of Food and Agriculture has listed brooms as a Class C pest species, that is, “troublesome, aggressive, intrusive, detrimental, or destructive to agriculture, silviculture, or important native species, and difficult to control or eradicate.”
The California Exotic Pest Plant Council has placed Scotch, French, and Portuguese brooms on their List A, Most Invasive Wildland Pest Plants, aggressive invaders that displace natives and disrupt natural habitats. Spanish broom is placed on List B, Wildland Pest Plants of Lesser Invasiveness, invasive pest plants that spread less rapidly, cause a lesser degree of habitat disruption, and may be widespread or regional .Scotch broom is an upright evergreen shrub that can grow to 12 feet tall but is more commonly 3 to 6 feet high . It reproduces by seed that germinate mainly in spring and autumn. Young plants usually do not flower until their third year. While plants live for up to 25 years , Scotch broom has an average life span of 17 years . Stems are numerous, erect, woody, green to brownish green, prominently ridged, and five-angled. Young stems remain green for about 3 years and actively photosynthesize . Leaves occur singly or in clusters, on short stalks, with scattered hairs above and soft hairs below the bright green groups of three leaflets. Single leaflets can be found on the growing tips. Each leaflet is oval to lance-shaped, 1⁄16 to 5⁄16 inch wide. The central leaflet is largest, up to 3⁄4 inch long. New leaves,produced in spring, are often lost during the dry summer months or other periods of stress. Plants may be leafless for most of the year . Flowers are pea-like, bright yellow, and sometimes with red markings in the center, and are borne on short stalks. They are solitary in the leaf axil and are about 1⁄2 to 1 inch long. April to June is the peak flowering time, but some flowers may appear sporadically throughout the year .
Fruit are flat, brown or black, and hairy on the margins. Mature pods are 1 to 21⁄2 inches long and 5⁄16 to 1⁄2 inch wide . The pods ripen during summer and burst open to eject the seed. Some pods curl up after seed are ejected. Seed are green to yellowish-brown when ripe. Each pod contains 5 to 20 shiny, rounded or slightly flattened seed that are 1⁄8 inch long and 1⁄16 inch wide .Portuguese, French, and Spanish brooms can be similar to Scotch broom in size, color, and appearance of flowers and stems . French broom has obvious and more persistent trifoliolate leaves than Scotch broom. Leaflets are largely oval and about 1⁄4 inch long. Leaflets appear waxy above and slightly hairy below. Spanish broom is often leafless or nearly leafless, especially later in the season . Portuguese broom closely resembles Scotch broom in terms of leaf patterns and is often mistaken for it.Unlike Scotch broom, the flowers of Spanish and French brooms occur in clusters. Spanish broom flowers appear as racemes at the end of the twigs; French broom clusters are found in the axial of the branches. Spanish broom flowers are noticeably fragrant; French broom flowers are less fragrant. Spanish broom has round stems; French broom stems are ridged but not sharply angled. Portuguese broom is round in cross-section with 8 to 10 very fine ribs along the surface.As the name implies, broom species were used historically for broom production. Broom plants were also used for thatching, fence rows, and cattle fodder. The flowers were used as a hop substitute in beer making, and the seed have been used as a coffee substitute. The woody plant was used for tanning leather, and very large, old stems were used for veneer. Even the fibrous stems were used to make cloth . The leaves, bark, and flowers were gathered for their medicinal properties. Broom contains alkaloids and hydroxytyramine and should not be ingested in any form. While various parts of the plant are said to be cathartic, diuretic, and emetic, large doses can cause vomiting, purging, and low blood pressure and can also speed up the heartbeat. Large doses have been reported to cause fatal poisoning. Advanced stages of toxicity can cause complete respiratory collapse .Scotch broom is native to the British Isles and central and southern Europe. It was initially introduced into the United States as an ornamental and later to control erosion and stabilize coastal dunes. Today Scotch broom is estimated to infest more than 600,000 acres in California . French and Spanish brooms are native to the Mediterranean region and the Canary Islands. They were first introduced into California around 1850 as ornamentals . French broom is the most widespread of the three species, hydroponic rack system reported in at least 23 counties in California. Spanish broom is less widely distributed but appears to be expanding it range. At higher elevations, cold winter temperatures limit them.Brooms can successfully invade pastures, fields, and grasslands. All brooms grow best in dry, sandy soils in full sunlight, but they also grow well in a variety of soil textures and a wide pH range . A common feature of the invasive brooms is an aggressive root system characterized by a taproot that can exceed 2 feet in length and an extensive, many branched, shallow lateral root system. As with most legume species, brooms are able to fix nitrogen from the atmosphere.
This allows them to grow on otherwise poor quality soils .Brooms are often found along roadsides in nearly pure stands where soils are frequently disturbed. Equipment and contaminated road surfacing materials can spread seed. While Scotch, French, Portuguese, and Spanish brooms do not establish well in highly shaded areas, they initially invade open sites such as logging roads, landings, skid trails, and harvest areas. A mature Scotch broom plant can produce about 15,000 seed per year. On average, less than 5 percent of the seed are lost to insect predation, and 98 percent of surviving seed are viable . Scotch broom, like other pioneer species, uses a variety of tactics to spread. About 40 percent of the seed in any year germinate immediately after dispersal, and another 25 percent germinate the second year . The majority of the remaining seed have an impervious seed coat that requires scarification for germination to occur. Movement along waterways, transportation in gravel and road surface materials, or exposure to low-intensity grass or brush fires can stimulate germination of these seed .Seed will germinate for many years even after the parent plants are removed. Seed stored in glass jars at Kew Gardens in England remained viable after 81 years . In the field, seed were found to germinate at least 4 years after removal of the seed . In the Sierra Nevada, few animals eat Scotch broom seed. In coastal conditions, quail and grouse can be relatively effective seed predators, digesting enough of the seed to effectively kill them. Increased soil disturbance can increase infestation in most sites, but it can also increase predation by quail and grouse, possibly because seed are easier for the birds to find . Scotch broom also sprouts readily. Plants that are cut during the growing season can grow back to the original size in 1 to 2 years .Integrated pest management is an ecosystem-based strategy that focuses on long-term prevention of pests through a combination of techniques such as biological control, pesticide use, habitat manipulation, and modification of cultural practices. Pesticides are used only after monitoring indicates that they are needed according to established guidelines, and treatments are made with the goal of removing only the target organism. Pest control chemicals are selected and applied in a manner that minimizes risks to human health, beneficial and non-target organisms, and the environment. Through IPM practices, we can develop a strategy for controlling Scotch, French, Portuguese, and Spanish brooms. Heavy shade is an effective long-term treatment. Broom is moderately shade intolerant, needing nearly full sunlight to thrive. It will grow in partial shade, but the plants grow slower. French broom appears to be more tolerant of shade than Scotch or Spanish brooms . Competition from grasses and other vegetation may control broom seedlings; removal of broom should be followed by propagation of preferred plants. Although about a dozen potential insect biological control agents have been identified and are under investigation, no releases have yet been approved.Hand-pulling of young plants is an effective strategy when the infestation is relatively small. Hand-pulling is easiest when the soil is moist. It is most effective before the plants produce seed, in the first 2 years following germination. Large plants can be mechanically removed using specialized tools. Hoeing very young plants can also be effective. Both of these techniques cause some soil disturbance that can lead to reinvasion. Broom seed geminate only in the top 21⁄2 inches of the soil . Soil disturbance should be avoided as much a possible, as it can bring these long-lived seed close enough to the surface to germinate. Only about 10 percent of Scotch broom stems will re-sprout when lopped near the base during the driest period of the season, which generally extends from the end of July until the first rains in October. Lopping at other times can lead to vigorous sprouting. For the most effective results, lop within 3 inches of the soil surface .Mowing is also more effective when Scotch broom is under drought stress. Since drought stress and high fire danger occur together, care should be taken to avoid causing sparks with the equipment. Mowing close to the ground results in the least amount of sprouting.