Trees of Santa Barbara: A Conservatory of Global Biodiversity

Previous volumes of Trees of Santa Barbara by Maunsell Van Rensselaer (1940, 1948), and Katherine Muller, Dick Broder, and Will Beittel (1974) documented the diversity of Santa Barbara’s rich arboreal legacy. Those volumes and Will Beittel’s Santa Barbara’s Street and Park Trees (1972) set the stage for this latest revision of Trees of Santa Barbara, by Robert Muller, published in 2005 by the Santa Barbara Botanic Garden. This excerpt from the author’s Introduction has been adapted here with permission. Following it is a page from the book on one of the many species of Erythrina grown in Santa Barbara.

Horticultural development in Santa Barbara has depended greatly on its citizens and the love of plants that they brought with them from other regions. The area’s natural beauty attracted many settlers to the area. Certainly the setting of Santa Barbara on the narrow coastal shelf below the Santa Ynez Mountains and the picturesque views of the Channel Islands created a balmy ambience that was difficult to resist. However, equally important was the combination of climate and soils that favored a variety of plant species found in few other places in the world.

Southern California enjoys a mediterranean climate, which is generally characterized by ocean-moderated temperatures and annual precipitation of ten to forty inches that occurs primarily during the winter months. In Santa Barbara, the average annual precipitation for the period from 1867 to 1989 was 17.9 inches, and rains occur primarily during the winter and spring months of November through April. Daily maximum temperatures for the period 1951 to 1980 average 76° F during July, the hottest month, and daily minimum temperatures averages 43° F in December and January, the coldest months of the year. Although temperatures as high as 105°F have been recorded, on average, Santa Barbara experiences temperatures above 90° F only two days per year. Similarly, the lowest temperature recorded was 29° F, and, on average, only one day per year experiences temperatures as low as 32° F.

These data are averages from climatic stations located near the coast. Local topography strongly influences actual temperatures experienced. For instance, the neighborhoods known as the Riviera, the Mesa, and the “banana belt” of Hope Ranch, all at slightly higher elevations, are essentially frost-free. On the other hand, cold air drainage in lowland areas away from the coast, such as Hidden Valley and Goleta Valley, results in frequent winter frost. These microclimatic differences can strongly influence the suitability of species for some locations.

The coastal outline of Southern California is another feature that significantly influences the climate of Santa Barbara. The Catalina eddy is created by strong atmospheric movement down the West Coast. When the coastline turns to the east starting at Point Conception, the north-south axis of this air movement carries its trajectory out to sea, resulting in an eddy that reaches as far south as Santa Catalina Island. This counterclockwise circulation brings warm, moisture-laden air that condenses on salt particles to create the “marine layer” of which Santa Barbarans are so aware. The fog and overcast skies of the Catalina Eddy persist during the hot, dry summer months and reduce the drying potential of the intense summer sun.

The soils of Santa Barbara are seldom limiting to plant growth. Almost all of the soils along the coastal shelf from Goleta to Summerland are of sedimentary origin, having been derived from either old marine terraces or alluvium that washed down from the marine sandstones and mudstones of the Santa Ynez Mountains. The low rainfall has done little to leach nutrients from these soils and much of the area is reasonably alkaline or, at most, modestly acidic. These relatively benign soils support a vast range of plant requirements and have helped foster the diversity of species grown here. Santa Barbara has successfully grown trees from the tropical regions of the world, the subalpine zones of Tasmania and Asia, the grasslands of eastern Africa and South America, and, of course, the mediterranean-climate regions of Europe, South Africa, Australia, and South America.

The many species of trees in Santa Barbara are ambassadors from nature of their native lands. Some have unique biological features, such as cherimoya (Annona cherimola), whose flowers increase in temperature by as much as 6° F when they open, thereby releasing a variety of odors that attract pollinating beetles and flies. Others reflect the cultures of native peoples from many different regions of the world. The seeds of the broad-leaved lucky bean tree (Erythrina latissima) were worn in necklaces and charms to ward off evil by native peoples of southern Africa. The bark of Hindu laurel (Cocculus laurifolius) contains alkaloids that were used by indigenous peoples of the Malay Peninsula to poison their arrows and darts. Other species are simply best known for their natural beauty.

Santa Barbara’s streets and parks are an outdoor conservatory for the display and conservation of an important element of the biotic wonder of our world. Many of its trees come from restricted distributions and are subject to the devastating effects of conflicting land use policies. Others are indeed endangered and exist primarily or solely in horticulture. As a conservatory, Santa Barbara’s streets and parks present an unsurpassed opportunity to educate its citizens and visitors about the wonders of the natural world.

One of the realities of today’s global travel and transport is that plants introduced to new areas may become aggressive colonizers, invading disturbed and natural landscapes alike. Fortunately, this occurs only with a small number of the many plants that have been intentionally or accidentally introduced; yet the impacts of those few invaders can be substantial. The reasons for these invasive tendencies are varied, but generally result from lack of natural controls (eg, competitors, herbivores, plant diseases). Aggressive competitive abilities of exotic species may result in loss of native species with lesser competitive ability, loss of habitat that supports a diverse array of native species, and fundamental change in ecosystem function.

Most often, successful invasion occurs on lands that have been disturbed by grazing, agriculture, or other human activity, and usually the invading species are herbaceous. However, a number of trees have been found to be invasive in California’s landscapes. One of the more common of Santa Barbara’s trees, the California pepper (Schinus molle), is also an aggressive invader of landscapes throughout much of Southern California. Several species of Tamarix are aggressive invaders of riparian habitats in desert lands and have significantly lowered the water tables of these regions. Tamarix aphylla, the tamarisk found at Coal Oil Point on the West UCSB Campus, is the least aggressive of these species; however, its potential threat as an invasive species is currently being reevaluated. In the Santa Barbara region, other invasive species include tree of heaven (Ailanthus altissima), black locust (Robinia pseudoacacia), and blue gum (Eucalyptus globulus).

The threats of introduced species raise important questions regarding plant introduction programs. During the days of Sexton, Franceschi, Orpet, and others, exotic species were given little thought except for their horticultural value and ease of propagation. We now recognize that plant introduction must be undertaken with more care. It is particularly important to recognize that many of the features that are the basis for horticultural selection (eg, ease of propagation, success in a variety of environments) are also important attributes of successful invaders. Screening of potential invasive exotic species has become a high priority for the horticultural industry and must be a consideration for any new trees introduced to Santa Barbara’s urban forest.


Erythrina

Coral trees

Flowering Erythrina coralloides with distinctive greenish orange bark in  midwinter. Author’s photographs

Flowering Erythrina coralloides with distinctive greenish orange bark in midwinter. Author’s photographs

Erythrina is a tropical genus of 112 species, mostly from the Americas, but also from Asia and Africa. The bright red flowers, long corolla tubes, and production of nectar are adapted to pollination by hummingbirds, although some Malaysian species have long, stout peduncles that serve as perches for sunbirds. Several species in the genus produce curare-like alkaloids that can be highly toxic, inducing paralysis and even death. However, these toxic effects usually require direct introduction into the blood stream and cannot be induced by ingestion. The name comes from the Greek erythros (red), in reference to the color of the flowers.

Erythrina coralloides  

Trifoliate leaves with deltoid leaflets

Trifoliate leaves with deltoid leaflets

Naked coral tree, flame coral tree

Native to the mountains of eastern Mexico, at elevations from 2,500 to 8,500 feet.

Habit: A deciduous tree with a spreading crown; to 30 feet tall.

Densely clustered flower racemes with elongate banners

Densely clustered flower racemes with elongate banners

Inflorescence: Bright red, cone-shaped, axillary racemes at the ends of upturned branch tips; flowers somewhat densely clustered on the rachis; banner narrow, long, to 3 x 1⁄2 inches, recurved only at the tip so that most of the floral parts are hidden; overall, the flowers tend to stand away from the rachis giving the inflorescence the appearance of a ballet dancer’s tutu; deciduous when flowering. Flowers February to April.

Fruit: Pod dull gray brown, to 8 inches long, with an elongated apex, swollen around each seed; seeds red with a black line near the point of attachment, 1⁄2 x 1⁄4 inch.

Twigs: Stout, smooth, greenish gray.

Bark: Smooth, greenish to pinkish gray, with shallow, widely spaced, corky furrows and white corky lenticels.

Dull, dark gray seed pods with red seeds

Dull, dark gray seed pods with red seeds

Leaves: Alternate, trifoliate; leaflets heart-shaped, 2–5 inches long, usually less broad, dull light green on both sides; petioles 3–5 inches long, quite slender.

Notes: Erythrina coralloides makes a striking display in late winter when it comes into full flower before the summer leaves have developed. The species has been confused with E. americana; the latter, however, has a waxy network on the undersides of the leaflets. It has also been referred to erroneously as E. poianthes. The species name, coralloides, means coral red.

Locations: One tree is in the lawn in upper Orpet Park. Another is in Franceschi Park along Mission Ridge Road. Several more are on the campus of the University of California, Santa Barbara, between Kerr Hall and North Hall.