Sandbar Willow

Compared with the tall, stately cottonwoods, sycamores and willows that reign over most of San Elijo’s wetlands, sandbar willows (Salix exigua) are dwarfs, easily overlooked, even with their silvery foliage. However, they often play a key role in the first chapter of a wetland. They are pioneer species, the first trees to inhabit a marsh or shore after a significant disturbance. They arrive as tiny airborne seeds and reproduce vegetatively, quickly forming dense patches of small willows that stabilize the terrain, and provide shade, protection and food for the initial animal inhabitants. Both the pioneering seeds and subsequent vegetation are intolerant of shade and are ultimately overgrown as other species become established.

The sandbar willows in the Reserve were planted in the late 1990’s, following the physical removal of an invasion of giant reed (Arundo donax). The willows have done their job and are slowly giving way more permanent plants.

Description ^{4,5,11,34,306}

Sandbar willow (Salix exigua) is a small, winter deciduous, shrubby tree, usually less than 15 feet tall (4.5 m). It is most often found in clonal clusters with many erect stems growing from the from horizontally spreading roots. The slim trunks have ascending, flexible branches. Linear leaves are long and narrow – to five inches (13 cm) long, generally more than 10 times longer than wide.  Leaf shape alone, distinguished sandbar willow from other willows in coastal San Diego ⁵⁰. Leaves grow on very short petioles or none; leaves are symmetrical and tapered at both ends; margins are generally smooth or remotely toothed. New growth is silvery with sleek, appressed hairs. Hair density diminishes with maturity allowing more green color to appear, but a grayish color remains on both sides.

Flowers occur just before, or as, spring leaves emerge, generally Feb. into April⁴⁶⁸. Sandbar willows are dioecious, having male and female flowers on different trees. Tiny flowers are clustered into dense cylindrical clusters, catkins. Flowers lack petals and sepals. A rounded bract at the base of each flower, protects the developing reproductive organs; the bracts are woolly with shaggy white hairs. There are one or two nectaries with secretions that attract pollinating insect.

The male flower consists of two stamens with greenish filaments and yellow anthers producing bright yellow pollen. I have yet to find a female plant among our sandbar willows.

Seeds are less than 1/32 inch (1 mm) long, with long, silky white hairs attached at one end which facilitate wind transport. Seeds have a very short dormant period and must find moist, open ground within a week.

Classification ^2,7,44

Sandbar willow is a dicot angiosperm in the willow family (Salicaceae). The willow family has only two genera. In addition to Salix, the Populus genus contains cottonwoods, poplars and aspen. There are approximately 30 species of native willow currently recognized in California.^7,50  Species in this family are characterized by unisexual flowers, with male and female catkins occurring on different plants.  Flowers lack obvious petals and sepals and are densely clustered in catkins. Both genera contain important members of riparian communities.

Four other species have been found in the reserve:⁴⁸  arroyo willow (Salix lasiolepas), black willow (S. lucida), red willow (S. laevigata) and Pacific willow (S. lasiandra). Sandbar willow can easily be distinguished from the others by the fact that the leaf is linear, at least 10X longer than wide and is pale on both surfaces.⁵⁰

Many recent studies recognize two varieties of S. exigua: var. exigua, and var. hindsiana; others lump do not distinguish varieties,¹¹  and older studies occasionally consider them separate species.³⁴ Local studies^8,48,468 identify our variety as var. hindsiana.  The varietal distinction depends partly on characteristics of the female flower, and lacking female plants, we are not able to confirm this.

Ecology ^5,11

Although most members of the willow family are highly suited for the variable conditions of wetlands,⁵⁰⁴ sandbar willows are the acknowledged pioneer species – particularly adapted to locate and rapidly colonize newly disturbed, moist areas. The abundant, tiny seeds, with their parachutes of silky hairs are well adapted to travel into new areas. They require both a moist substrate and bright light to germinate, increasing the chance that they sprout in a favorable, disturbed, habitat. Their vegetative propagation quickly produces dense stands of young trees, stabilizing the soil and producing shade, shelter and food for associated organisms Since sandbar willows are shade-intolerant, they are ultimately replaced by more permanent wetland species as the area recovers.

The sandbar willows along the Pole Road were initially planted in the late 1990’s following the disruptive removal of non-natives. In the intervening 25 years, a lot has happened, especially the Restoration project (2019-2021) that changed circulation patterns throughout the Central Basin. However, stands of sandbar willows still remain and, as predicted, are being overgrown by native plants such as mule fat  (Baccharis salicifolia), and Mexican elderberry (Sambucus mexicana)– as well as a selection of newly arrived non-native “weeds”.

Human Uses ^34,75

Willows have been used interchangeably by a large number of Native American tribes for a large number of purposes²⁸². However,  sandbar willow appears to have been preferred for weaving small, ceremonial figures simply called “split-stick animals”. In California, these were first discovered in Newbury Cave, near the Mohave River in southern California, and were accompanied by hunting tokens such as crystals, pictographs and feather bundles, all suggesting the “magic” articles of a hunting society about 1500 years ago. Since then, split-stick animals have been found in about 20 caves from the southwestern U.S.

Sandbar willow produces the long, slim, flexible and tough branches necessary for the bending and wrapping needed for a stick-figure. Native Americans are known to have coppiced this willow in order to maintain a supply of such shoots. (Other civilizations coppiced other species of willow to maintain a supply of shoots for wicker.)

With the help of images, Campbell: ⁷⁵ (p.346) describes his efforts to split and wrap a sandbar willow twig into a figure. Did he succeed? Was it a mountain sheep or a deer? He doesn’t commit himself, letting “the spirits judge”.

Interesting Facts ²²

Willows serve as hosts to a boggling array of gall-forming insects, insects that modify plant tissue, producing a tumor-like structure, the gall, that harbors the developing larvae, providing food and protection. The sandbar willow is host to a sand fly that effects the stem, producing a swollen portion along the stem, that is a smooth surfaced and tapered at either end. This shelters one or more sawfly larvae. The larvae pupate within the gall, emerging as adults through a small exit hole they chew near the top of the gall.

A gall species usually infects only one or a few related host species and its gall is unique to that insect-plant combination. For instance, another sawfly in the same genus as the sandbar willow sawfly, produces stem galls on the common arroyo willow. In addition, on arroyo willow we often see red bead galls on leaves (a mite) and cabbage-like bud galls (a midge). A different bud gall, also a midge, is often seen on coyote brush (Baccharis pilularis). Considering all plant hosts, the oaks have the most different species of galls. Willows, tied with creosote bush (Larrea tridentata), are a distant second.