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Noah Ross
Noah Ross

Arroyo Willow


The core range of the arroyo willow includes most of California, including the California Coast Ranges, Arizona, Klamath Mountains, Peninsular Ranges, Sierra Nevada, and Transverse Ranges.[3] It extends north into Washington, south into Baja California, and east into Idaho, Utah, Texas, and Coahuila (México).[2][4]




arroyo willow


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Arroyo willow is native throughout California and elsewhere in the western United States, below 7000 feet (2200 m). It is generally found in riparian areas within a variety of vegetation associations, including forests, chaparral and grasslands.7


In the Reserve, the fresh water sources that support arroyo willow are often partially or completely anthropogenic. At the Nature Center, the willows along the board walk receive fresh water from urban runoff from the homes north and east of Manchester Ave. Along the south side trail, willows are often encountered in gullies at the ends of residential roads, such as Rios and Santa Carina, which indicate drainage from nearby yards.


Arroyo willow is a dicot angiosperm in the willow family (Salicaceae). Members of this family, which includes poplars and cottonwoods, are important members of riparian communities. They are characterized by flowers that lack petals or sepals and which are densely clustered in catkins. Male and female catkins occur on different plants.44


Four other species of willow are found in the reserve:48 sandbar willow (S. exigua), black willow (S. gooddingii) red willow (S. laevigata) and Pacific willow (S. lasiandra). Arroyo willow is the most common and can be distinguished from other willows by characteristics of the mature leaf: the leaf length is less than 10X the leaf width, the leaf tends to be widest toward the tip, and the underside of the leaf is paler than the top side.50 A large tree by the Rios Ave trailhead may be a hybrid or a non-native species.100


Although arroyo willow may survive short droughts, persistence requires a reliable source of fresh water. It is dependent upon flooding for regeneration. In addition to enhanced production of seedlings, large limbs which are dislodged by the force of flooding may root and produce a new plant.11


Unlike the catkins of most species, willow catkins are primarily insect pollinated.499 Insects are attracted by nectaries, small glands within both male and female flowers that produce a sugary liquid.4 Seeds are wind-dispersed.


The family Salicaceae comprises two genera and 325 species of shrubs and trees commonly known as willows, cottonwoods (poplars), and aspens. They are all deciduous, fast-growing trees that can be found all over the world, but most abundantly in the cooler parts of the Northern Hemisphere. Their flowers have no petals or sepals, and are borne in a compact inflorescence called a catkin. Male and female flowers are produced by separate plants. Aside from two species of cottonwood, all Bay Area representatives are willows (genus Salix). To distinguish a willow from a cottonwood in the wintertime, look closely at the buds: willow buds are covered by a single scale, while the buds of cottonwoods are covered by several overlapping scales.


The arroyo willow is a riparian woodland regular, thriving along the edges of streams where it enjoys the moist soil it requires. This small tree grows to a maximum of about 10 meters in height, and has alternate, hairy, entire leaves that are lanceolate-elliptic to oblanceolate in shape. In the spring, the catkins appear before the leaves do.


The Chumash trimmed and used branches as poles to build huts. They used the bark for rope and chewed the bark to relieve toothaches (it contains salicylic acid which is found in aspirin). They also used the willow to make bows, tools and cradle boards to carry babies.


Arroyo willow riparian forest is a stream or lake associated community dominated by arroyo willow, often in dense, pure stands. This riparian zone is found in the wetest portions of streambanks and lakesides.


Another useful chemical first derived from the willow is Aspirin. Native Americans had many uses for this flexible plant, an infusion of either its bark or its flowers was used to cure a variety of ailments from fevers to itchiness to diarrhea. The inner bark was made into rope, the shoots used for baskets, and stakes provided structure for thatched houses.


A good place to see some of the native willows is in the inlet to Lake Lagunita. All the species, except the narrow-leaved willow, have leaves with shiny green upper surfaces and are silvery gray beneath. When the wind blows in the willows the pattern of contrasts is very characteristic. The male trees have catkins composed of numerous tiny staminate flowers which, if examined with a lens, are seen to consist only of the stamens plus the hairy bract from which the stamens emerged. The female trees have similar catkins whose numerous flowers consist only of a pistil plus bract. The fruit is a two-valved capsule with many hairy seeds.


Although we have five willows in the family Salicaceae (willow family), in the genus Salix, listed in the Vascular Plant List, I have only found four at the Preserve. Three of these are fairly common and one is uncommon at JRBP.


Common features of the willows are: woody plants, shrubs or trees; deciduous, all leaves are lost every year; dioecious (two houses), male and female flowers are on separate plants; flowers lack sepals and petals, but have scale-like bracts (a structure at the base of each flower) and nectar glands associated with each flower; flowers are arranged in catkins, a spikelike inflorescence of many unisexual flowers, usually pendent; fruit is a capsule with many seeds, each seed is covered with fine or cottony hairs and these can be seen floating in the air in late winter through summer. Habitat: moist areas.


Willows not found recently at JRBP but on a historical plant list: Salix sitchensis Bong. - Sitka willow - Shrub, small tree; twigs yellowish or reddish brown covered with silky hairs lost with age; upper leaf surface dull green, with a few or many silky hairs that are lost with age, lower blade densely silky hairy; stipules small, or lacking; bud scale margin fused; inflorescence appearing just before or with the leaves; flower bract tawny or brown; stamen 1; ovary silky. Synonym S. coulteri.


References: Hickman, J.C. (Editor). (1993). The Jepson Manual: Higher Plants of California. University of California Press, Berkeley, California. West-Bourke, Diane. (1985). Entomophily and anemophily in three sympatric willows: Salix hindsiana, S. lasiandra, and S. lasiolepis. M.A. thesis, Department of Biological Sciences, San Jose State University.


The association between host-plant mineral nutrients and insect herbivory is complex, idiosyncratic, and dynamic. Because of this we employed an unusual statistical method to evaluate the relationship between tenthredinid sawfly herbivory and concentrations of foliar nutrients in arroyo willow,Salix lasiolepis. We discovered that among 20 willow clones and between two years arroyo willow varied in the size (absolute concentrations) and shape (relative concentrations) of five mineral nutrients (Ca, Mg, N, P, and K). Similarly, the densities of five sawfly species varied in size and shape among clones and between years. In 1984, relative densities, or shape, of the sawfly community was associated with relative concentrations, or shape, of foliar nutrients in 20 willow clones. In contrast, in 1985 absolute densities, or size, was associated with absolute concentrations, or size, of foliar nutrients. This shift from a shape to a size association between herbivory and foliar mineral nutrients may reflect the large difference in winter precipitation and sawfly densities between years: precipitation and sawfly densities were much greater in 1985 than in 1984.


A large-scale mesocosm study was conducted to determine if vegetation with willow trees enhances biodegradation and to evaluate the mechanisms of natural biodegradation of weathered petroleum compounds under field conditions. The mesocosms were designed to model conditions at a former oil field where mid-range petroleum distillates were used as a diluent for pumping crude oil contaminated the soil and groundwater at the site with petroleum compounds. Ten mesocosms were constructed at the field site using un-impacted soil and diluent-impacted groundwater from the site. Five of the mesocosms were planted with Arroyo Willow trees native to the field site and the other five served as controls without trees. Since these willow trees are phreatophytes, their roots are capable of consuming water from the water table. A previous study was conducted using these mesocosms, however the willow trees then were in poor condition. In this study, fertilizer was added to the mesocosms to promote healthy growth of the willows. Fertilizer was added equally to mesocosms with and without willow trees to avoid introducing bias. Groundwater was circulated through the mesocosms for two 109 to 126 days runs, while the total petroleum hydrocarbon (TPH) concentrations of the groundwater were measured periodically. Dissolved oxygen concentrations were also monitored in each of the mesocosms to determine if the willow trees had any impact on oxygen transfer to the subsurface.


In the first run without nutrient amendments the trees did not enhance biodegradation. All the mesocosms started with an average TPH concentration of 6.3 mg/L and ended with a concentration of 1.0 mg/L. After this first run, nutrient amendments were added to all the mesocosms, resulting in healthy trees with robust growth. With healthy willow trees, the planted mesocosms resulted in a statistically significant increase in long-term biodegradation of dissolved-phase petroleum compounds. The planted mesocosms resulted in 29 percent more degradation. These results agree with prior lab studies using bench-scale microcosms with media from the former oil field which indicated that TPH concentrations after 100 days were lower in containers with willows or lupines compared to controls without plants. Microtox toxicity decreased for both planted and control mesocosms, showing no toxic root exudates or by-products. 041b061a72


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